NCRP 133

 

NCRP REPORT No. 133

RADIATION PROTECTION FOR PROCEDURES PERFORMED OUTSIDE THE RADIOLOGY DEPARTMENT Recommendations of the NATION NA TIONAL AL COUN COUNCIL CIL ON RADIATION RADIATION PROTECTION AND MEASUREMENTS

 Issued August August 31, 2000

 Na  Nati tion onal Co Coun unci ciltl A on Rad Ra dia iati tion Prot Pr otec ecti tion on an and dand Me Meas urem emen ent ts 7910 79 10 Wal oo oodm dmon ont ven venue ue / on Be Beth thesd esda, a, Ma Maryl rylan d asur 208 20814 14-3 -3095 095

 

LEGAL NOTICE  This Report was prepared by the National Council on Radiation Protection and Measurements (NCRP). The Council strives to provide accurate, complete and useful information in its documents. However, neither the NCRP, the members of  NCRP, other persons contributing to or assisting in the preparation of this Report, nor any person acting on the behalf of any of these parties: (a) makes any warranty or representation, express or implied, with respect to the accuracy accuracy,, completeness or usefulness of the information contained in this Report, or that t hat the use of any information, method or process disclosed in this Report may not infringe on privately owned rights; or (b) assumes any liability with respect to the use of, or for f or damages resulting from the use of any information, method or process disclosed in this amended nded 42 U U.S .S.C. .C. Report, under the Civil Rights Act of 1964, Section 701 et seq. as ame  Section 2000e et seq. (Title VII) or any other statutory or common law theory governing liability l iability..

Library of Congress Cataloging-in-Publication Cataloging-in-Publication Data Radiation protection for procedures performed outside the Radiology Department. De partment.   p. cm. -- (NCRP report ; no. 133)   Includes bibliographical references and index.   ISBN 0-929600-66-5   1. Radiation--Safety measures. 2. Diagnosis, Radioscopic--Eval Radioscopic--Evaluation. uation. I. National Counc Council il on Radiation Protection and Measu Measurements. rements. II. II. Series RA569.R266 2000 616.9’89705--dc21

00-045218

Copyright Copyri ght © Nat Nation ional al Cou Counc ncil il on Rad Radiat iation ion Prot Pr otec ecti tion on and and Me Meas asur urem emen ents ts 20 2000 00  All rights reserved. This pu publication blication is protec protected ted by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyrightowner, except for brief quotation in critical articles or reviews.

For detailed informa information tion on the availabili availability ty of NCRP documents see page 92.

 

Preface This Report is one in the series of NCRP documents developed under the auspices of Scientific Committee 46, the NCRP scientific program area committee concerned with operational radiation safety. This is the eleventh report  in a series dating back to 1978. This Report provides practical recommendations on how to protect workers while performing x-ray procedures outside the radiology department. Some of these procedures have the potential to cause exposures to workers which exceed occupational dose limits. Serving on Scientific Committee 46-11 were:

Douglas R. Shearer Shearer,, Chairman Rhode Island Hospital Providence, Rhode Island

 Members Libby F. Brateman University of Florida   College of Medicine Gainesville, Florida

Robert C. Murry, Jr. Dallas, Texas

Donald P. Harrington Stat St ate e Uni Unive vers rsit ity y of of New New Yor York k Stony Brook, New York

Ray Rossi* Univ Univer ersi sity ty of of Colo Colora rad do Health Science Center Denver,, C Denver Colorado olorado

Mary Ellen Masterson-McGary Holmes Regional Medical Center Melbourne, Florida

* Deceased iii

 

iv / PREFACE  Liaison William R. Hendee (1989–1995) Medical College of Wisconsin Milwaukee, Wisconsin

 NCRP Secretariat James A. Spahn, Sp ahn, Jr., Senior  Senior Staff Scientist (1989–1998) Scientist (1989–1998) Eric E. Kearsley, Staff Scientist (1998–1999) Scientist (1998–1999) Jonelle K. Drugan, Visiting Staff Scientist (1999–2000) Cindy L. O’Brien, Managing Editor Editor

  The Council wishes to express its appreciation to the Committee Co mmittee members for the time and effort devoted to the preparation of this Report.

Charles B. Meinhold  President

 

Contents Preface  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii 1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

2. Radiologic Examinations Outside the Radiology Department . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Cardiac Catheterization Catheterization Laboratory Laboratory . . . . . . . . . . . . . . . . 2.1.1  Angiography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Coronary Angioplasty . . . . . . . . . . . . . . . . . . . . . . 2.1.3 Electrophysiology Studies . . . . . . . . . . . . . . . . . . .

5 5 8 8 9

Miscellaneous 2.2 2.1.4 Operating Room . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 2.2.1 Urological Procedures . . . . . . . . . . . . . . . . . . . . . . 2.2.1.1 Cystoscopy . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1.2 Nephrostomy Tube Placement Placement . . . . . . . . 2.2.1.3 Ston Stone e Removal Removal . . . . . . . . . . . . . . . . . . . . . 2.2.2 Orthopedic Procedures Procedures . . . . . . . . . . . . . . . . . . . . . 2.2.2.1 Hip Pinning and the Orthopedic Placement of Rods and Nails . . . . . . . . . 2.2.2.2 Fracture Reduction Reduction . . . . . . . . . . . . . . . . . 2.2.3 Operating Room Cholangiography Cholangiography . . . . . . . . . . . . 2.2.4 Operating Room Room Angiography . . . . . . . . . . . . . . . 2.2.5 Placement of Permanent Pacemaker Pacemaker Lines . . . . . 2.2.6 Endoscopy Examinations Examinations . . . . . . . . . . . . . . . . . . . 2.3 Miscellaneous Examinations Examinations and Location Location . . . . . . . . . . 2.3.1 Facet Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Catheter Placement Placement . . . . . . . . . . . . . . . . . . . . . . . 2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 9 9 9 10 10 11

3. Sources of Exposure to Workers . . . . . . . . . . . . . . . . . . . 3.1 Total Radiation Exposure Exposure During the Procedure Procedure . . . . . . 3.1.1 Equipment Selection, Quality Control, and Preventive Maintenance Maintenance . . . . . . . . . . . . . . . . . . . . 3.1.2 Establishment of Appropriate Radiological Technical Procedures Procedures . . . . . . . . . . . . . . . . . . . . . .

14 14

 v

11 11 11 11 12 12 12 12 12 13

15 15

 

 vi /

CONTENTS

3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.2.5 3.1.2.6

Tube Voltage . . . . . . . . . . . . . . . . . . . . . . 15 Tube Current Current and Exposure Time . . . . . 16 Filterss and Grids Filter Grids . . . . . . . . . . . . . . . . . . . 17 Projections and Patient Thickness . . . . . 17 Image Receptor . . . . . . . . . . . . . . . . . . . .  19 Minimization of Radiation Field Size . . .  .   19

3.2 Personal Protection Protection Tech Techniques niques . . . . . . . . . . . . . . . . . . . 22 3.2.1 Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2.2 Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  .  23 3.2.3 Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25  25 3.2.4 Reduction of X-Ray Field Size . . . . . . . . . . . . . . . . 26 3.2.5 Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.2.6 Decreasing the Total Amount of Radiation Used in a Procedure . . . . . . . . . . . . . . . . . . . . . . . 29 3.3 Equipment Evaluation, Maintenance and Monitoring. .  .  31 3.3.1  Acceptance Testing Testing . . . . . . . . . . . . . . . . . . . . . . . . 31 3.3.2  Quality Cont Control rol . . . . . . . . . . . . . . . . . . . . . . . . . . ..   31 3.3.3 Equipment Maintenance Maintenance . . . . . . . . . . . . . . . . . . . . 32 4. Personal Monitoring   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  .  33 4.1 Ratio Rationale nale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..   33 4.2 Who Should Be Monitored? Monitored? . . . . . . . . . . . . . . . . . . . . . . . 33 4.3 Meth Methods ods of Monitorin Monitoring g Workers Workers . . . . . . . . . . . . . . . . . . . 33 4.3.1 Film Badges Badg es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34 4.3.2 Thermoluminescent Dosimeter Badges Badges . . . . . . . . 35 4.3.3 Optically Stimulated Luminescence Luminescence Devices . . . . 35 4.3.4 Pocket Ionization Chambers . . . . . . . . . . . . . . . . . 35 4.3.5 Digital Personal Dosimeters Dosimeters . . . . . . . . . . . . . . . . . 36 4.4 Individual Responsibility for Personal Monitoring Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..   36 4.5 Where Should Personal Personal Dosimeters Be Worn? Worn? . . . . . . . . 37 4.5.1 Procedures Not Requiring a Lead Apron or Other Shielding Devices . . . . . . . . . . . . . . . . . . . . 37 4.5.2 Procedures Requiring a Lead Apron or Other Shielding Devices . . . . . . . . . . . . . . . . . . . . . . . . .  37 4.5.3 Pregnant Workers Workers . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.5.4 Procedures During Which Extremities are Exposed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..   39 4.6 Personal Monitoring Records Records . . . . . . . . . . . . . . . . . . . . . . 39 4.7 Periodic Evaluation of Workers’ Doses Doses . . . . . . . . . . . . . . 39 5. Administrative Responsibilities  . . . . . . . . . . . . . . . . . . .  41

 

CONTENTS

 / vii

 Appendix A. Medical X Rays, Their Biological Biol ogical Effects, and Philosophy of Radiation Protection  . . . . . . . . . . . .  A.1 Inte Interact ractions ions of Medical Medical X Rays Rays with Tissue Tissue . . . . . . . . . .  A.2 Quantities and Units Relevant to Ionizing Radiation . .  A.2.1 Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  A.2.2  Absorbed Dose . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44 44 47 48 48

 A.2.3 Equivalent Dose . . . . . . . . . . . . . . . . . . . . . . . . . . .  A.2.4 Effective Dose . . . . . . . . . . . . . . . . . . . . . . . . . . . .  A.3 Biological Effects of Ionizing Radiation . . . . . . . . . . . . .  A.3.1 Deterministic Effects Effects . . . . . . . . . . . . . . . . . . . . . . .  A.3.2 Stochastic Effects Effects . . . . . . . . . . . . . . . . . . . . . . . . .  A.3.3 Herit Heritable able Effects Effects . . . . . . . . . . . . . . . . . . . . . . . . . .  A.4 Sources and Magnitude of Ionizing Radiation Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  A.5 Risk and Risk Assessment . . . . . . . . . . . . . . . . . . . . . . .  A.6 Philosophy of Radiation Protection Protection . . . . . . . . . . . . . . . .  A.7 Dose Limits and Their Bases . . . . . . . . . . . . . . . . . . . . .

49 50 50 50 51 52

 Appendix B. The X-Ray Imaging Process  . . . . . . . . . . . . . B.1 Image Production Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2 Image Receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2.1 Screen Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2.2 Grids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2.3 Image Intensifier . . . . . . . . . . . . . . . . . . . . . . . . . . B.2.4 Equipment Use and Design . . . . . . . . . . . . . . . . . B.2.4.1 Portable and Mobile Radiographic Equipment . . . . . . . . . . . . . . . . . . . . . . . . B.2.4.2 Fluoroscopy with Image Intensifier . . . .

61 61 64 64 65 67 68

B.2.4.3 Fluoroscopic Procedures in the Operating Room Room . . . . . . . . . . . . . . . . . . . B.2.4.4 Pulsed and Digitally Filtered Fluoroscop Fluor oscopy y ....................... B.2.4.5 Cinefluorography . . . . . . . . . . . . . . . . . . . B.2.5 Spot Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2.5.1 Spot Films with with Image Intensification . . B.2.5.2 Digital Spot Films . . . . . . . . . . . . . . . . . . B.2.5.3 Spot Films with No Image Intensification . . . . . . . . . . . . . . . . . . . . . B.2.5.4 Seria Seriall Radiography Radiography . . . . . . . . . . . . . . . . .

53 53 56 57

68 69 69 70 71 71 71 71 72 72

B.2.5.5 Digital Subtraction Angiography . . . . . . 72 Glossary  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

 

 viii /

CONTENTS

References  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..   79 The NCRP  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..   82 NCRP Publications  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..   92 Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

 

1. Introduction

Modern medical practice is making increasing use of x-ray imaging systems for the diagnosis and treatment of disease. Procedures range from relatively simple diagnostic procedures such as the chest radiograph to exceedingly complex and time consuming  interventional procedures such as vascular interventional radiology and coronary angioplasty. The total collective dose to workers in the medical professions in the late 1970s was in excess of  400 person-Sv, person-Sv, while the total collective dose from all occupa occupational tional radiation exposure was 2,300 person-Sv (see Tables 1.1 and 1.2) (NCRP, 1989a). With the increase over the past 20 y in the use of  radiation in medical procedures, the collective dose is now likely to be higher and to make up a greater fraction of the total occupational collective dose. Historically, the majority of x-ray Historically, x- ray imaging procedures procedure s have been performed by individuals with specialized training and within the controlled environment of a radiology department. There are an increasing number of physicians using x rays whose expertise is not in radiology radiolog y. Examples include the expanded use of fluoroscopy fluor oscopy during orthopedic surgery, fluoroscopically guided electrophysiology,, and cardiac cathe ogy catheterization. terization. This Report R eport addresses procedures procedu res which require x-ray imaging outside of the radiology department. This Report does not address exposures from nuclear medicine or radiation therapy. This Report is intended for the use of clinical staff who conduct medical procedures, radiation protection staff, and those responsible for developing relevant employee education and training programs. Some examples of areas where employees may potentially be exposed are given in Table 1.3. Every medical facility operator should be responsible for incorporating the information contained in this Report into local operational and educational programs. The radiation protection training trainin g of individuals who are responsible for the performance of these procedures vary widely, as do individual attitudes about exposure to radiation. For all the above reasons, this Report contains an overview of the nature of the biological effects of ionizing radiation, a discussion of basic medical 1

 

2 /

1. INTRODUCTION

TABLE 1.1— Doses

to radiation workers workers exposed to low-LET a and high-LET radiations. radiations.b

Occupational Category

 Annual Collective H E (person-Sv)c,d

Industrial personnel   (other than nuclear fuel cycle)

390

Nuclear power plant personnel   Low-LET   High-LET

550 0.6

DOE personnele   Low-LET   High-LET

160 64

Uranium miners   Low-LET   High-LET

12 100

Uranium mill and fuel fabrication personnel

6

Well loggers

30

U.S. Navy   Low-LET   High-LET

48 3

Flight crews and attendants

170

Medical staff (other than PHS)f 

420

Governmentg

60

Other workers

200

Education and transportation Rounded total a

50 2.3 × 103

LET = linear energy transfe transferr. b Kumazawa et al. (1984). c 1 person-Sv = 100 person-rem. d  Effective dose equivalent  ( H   H E) is the quantity currently used by regulatory bodies in the United States (1998).  Effective dose  ( E  E) is the quantity NCRP recommends be used in the future (NCRP, (NCRP, 1993a). e U.S. Department of Energy personnel. f  U.S. Public Health Service personnel. g Includes personnel from the U.S. Department of Defense, Veterans  Administration, Public Health Service, National Institutes of Health, and the National Aeronautics and Space Administration.

 

1. INTRODUCTION

TABLE 1.2— Summary

 / 3

of mean annual effective dose equivalent and

collective effective dose equivalent to monitored medical workers. workers.a Number of Workers (thousands)

Mean H E (mSv)

Collective H E (person-Sv)

Dentistry

259

0.2

60

Private medical   practice

155

1.0

160

Hospital

126

1.4

170

44

0.5

20

584

0.7

410

Occupation

Otherb Total a

NCRP (1989a); Kumazawa et al. (1984). b “Other” includes chiropractic medicine with 15,000, podiatry with 8,000, and veterinary medicine with 21,000 potentially exposed workers.

TABLE 1.3— Areas

where radiation exposures to staff staff may occur. occur.

Within the Radiology Department General radiography Tomography Computed tomography Mammography General fluoroscopy  Angiography Interventional procedures

Outside the Radiology Department Private office Trauma suites Orthopedic rooms Mobile radiography areas Mobile C-arm fluoroscopy Cystoscopy unit Cardiac catheterization laboratory Emergency rooms Operating rooms Intensive Care units Coronary Care units Gastrointestinal fluoroscopy Reconstructive oral surgery (dental service department) Endocrinology-bone Endocrinolog y-bone mineral   densitometry Lithotripsy unit

 

4 / 1. INTRODUCTION imaging, and techniques for minimizing radiation radiatio n exposure to medical workers. Within the context of the reports of the National Council on Radiation Protection Protecti on and Measurements (NCRP), the terms “shall” and “should” are used with strictly-defined meanings.  Shall  indi Shall indicates a recommendation that is necessary or essential to meet the currently acceptedrecommendation standards of radiation protection. Should protection.  Should indicates an advisory that is to be applied when  indipracticable and is equivalent to “is recommended” or “is advisable.” When these words occur in the text in such a manner as to refer to a recommendation, they are italicized. The term “qualified “qualified expert” expert” is used throughout this Report. It is defined as (1) for radiation protection, a person having the knowledge and training to measure ionizing radiation, to evaluate radiation safety techniques, and to advise regarding radiation protection needs (for example, persons certified in an appropriate field by field by the  American Board of Radiology, Radiology, the American Board of Medical Physics, or the American Board of Health Physics, or persons otherwise determined to have equivalent qualifications); and (2) for x-ray imaging equipment performance evaluations, a person having in addition to the qualifications of Item 1 above, training and experience in the physics of medical x-ray imaging (for example, persons certified in Diagnostic Radiological Radiolo gical Physics by the American Board of Radiology or Diagnostic Imaging Physics by the American Board of Medical Physics or persons persons deter determined mined to ha have ve equivalent equivalent qualifications). The reader is referred to the Glossary for definitions of other specialized terms in this Report. Section 2 describes the procedures and equipment used to perform medical procedures, procedu res, as well as the exposures that are ar e likely to result from performing these procedures. Section 3 describes the sources of exposure to workers and the recommended techniques of  personal protection. Section 4 discusses the techniques of personal monitoring. Recommendations for how to evaluate exposures and where to wear the dosimeter are made. The responsibilities of management are summarized in Section 5. Appendix A provides information on the sources of radiation, the biological effects of  radiation exposures, exposu res, the quantities and units, dose limits for workers, and the system of radiation protection. Appendix B provides a primer on the x-ray imaging process.

 

2. Radiologic Examinations Outside the Radiology Department This Section discusses radiographic and fluoroscopic studies performed outside the traditional radiology department. The equipment used (fixed or mobile), the nature of the procedure, the number of films taken, and the possibility of exposure to workers in these areas are considered (see Tables 2.1 and 2.2). The level of  radiation exposure varies with the nature of the procedure and the specific exposure parameters employed employe d ( e.g  e.g., the screen-film combination, types of radiographic grids or other imaging equipment, calibration, source-to-image distance, etc.). Some of the procedures described here will result in relatively low exposures to personnel who routinely perform them using standard precautions. Performance of such “low-exposure” procedures will result in an accumulated annual unshielded dose of less than 1 mSv [the annual effective dose ( E)  E) limit for individual members of the public]. Other procedures, performed using standard techniques, are not likely to result in an accumulated unshielded exposure exceeding exceedin g an annual average effective dose of 25 mSv, although routine performance of  these procedures procedure s will result in annual a nnual exposures in excess e xcess of 1 mSv. mSv. For the purposes of this Report, such procedures are said to result in “medium exposure.” “High exposure” means any accumulated unshielded exposure resulting in an annual effective dose greater than 25 mSv. mSv. As described describ ed below below,, some procedures pro cedures have the t he potenpoten tial to exceed the annual effective dose limit of 50 mSv.

2.1 Cardiac Catheterization Laboratory

The cardiac catheterization laboratory usually contains complex, multi-angular fluoroscopic and cinefluorographic equipment that present the potential for high  accumulated radiation exposure and doses to personnel exceeding 50 mSv y–1. The primary 5

 

 2.1—Common TABLE 2.1— Common

 6   /  

radiographic techniques techniques and their exposure potential. Exposure Potential

Equipment

Procedure

Location

Fixed or mobile

 Angiography or DSA d

Cath lab

Fixed or mobile Fixed or mobile Mobile Mobile Fixed or mobile Fixed or mobile Fixed or mobile Mobile Mobile Mobile

Cystoscopy Nephrostomy tube placement Hip pinning  Reduction of fracture Cholangiography Peripheral angiography Catheter placement Carotid arteriography Pediatric chest or abdomen  Abdomen

OR, outpatient OR OR ER, OR OR OR  V  Various arious OR Newborn ICU ER, Recovery room

 Average  Average Number of Filmsa

Number of Procedures per Week  Lowb Not More Than

Highc More Than

50–150

0

0

3 2 4 3 2 1 1 1 1 1

0 0 0 7 0 1 20 1 30 1

10 15 10 185 15 40 500 40 750 35

a

Techniques vary from one institution to another another,, depending on the selection of screen-film combination, type of radiographic grids, grids, type of imaging equipment, calibration, etc. Source to image distance is generally 1 m. b

If this procedure is the only type performed by the employee, the employee’s annual effective dose should not exceed the annual effective dose limit for members of the public (1 mSv) provided the employee does not perform more than the stated number of procedures per week. c Performing Performi ng more than the stated number of procedures per week is likely to result in an annual effective dose to employees in excess of  25 mSv. d DSA = digital subtractio subtraction n angiography angiogra phy..

2   .  R A  D I    O L   O  G I    C  E X  M A  I    N A  T  I    O  N  S   O  U T   S  I   D E T  H E  R A  D  O I   L   O  G Y  D E P  T   .

 

TABLE 2.2—  2.2—Common Common

fluoroscopic techniques and their exposure potential. Exposure Potential

Equipment

Fixed or mobile Mobile Mobile Fixed or mobile Fixed or mobile Mobile Mobile Mobile Mobile Mobile Fixed or mobile Fixed or mobile Mobile Mobile Mobile

Procedurea

 Angiography or DSA d Coronary angioplasty Electrophysiology Line placement   (pressure measurements) Cystoscopy Nephrostomy tube placement Lithotripsy ESWLd Hip pinning  Reduction of fracture ERCPd Cholangiography Pacemaker lines Facet block (anesthesiology) Transplant biopsy

Location

Duration

Number of Procedures per Week  Lowb Not More Than

Highc More Than

Cath lab Cath lab Cath lab Cath lab, CCU, ICU

20 min 20 min 20 min 2 min

0 0 0 0

 1  1  1 20

OR, outpatient OR OR OR ER, OR ER, OR Endoscopic suite OR Cath lab, CCU, OR OR CCU

5 min 15 min 3 min 30 min 5 min 2 min 5–20 min 1 min 15 min 1–2 min 40 min

0 0 1 0 0 0 0 1 0 1 0

 7  2 13  1  7 20  7 41  2 27  1

a Exposure b

rate at patient’s skin is approximately 50 mGy min –1. If this procedure is the only type performed by the employee, the employee’s annual effective dose should not exceed the annual effective dose limit for members of the public publi c (1 mSv) provided the employee does not perform more than the stated number of procedures per month. c Performing more than the stated number of procedures per week is likely l ikely to result in an annual effective dose in excess of 25 mSv. mSv. d DSA = digital subtraction angiography; ERCP = endoscopic retrograde cholangiogram pancreatography; ESWL = extracorporeal shock-wave lithotripsy.

 

8 / 2. RADIOLOGIC EXAMINATIONS OUTSIDE THE RADIOLOGY DEPT. reason to perform cardiac catheterization in adults is for the diagnosis of coronary artery disease. In the pediatric age group, the major indication is congenital heart disease. In the cardiac catheterization laboratory, the highest potential for radiation exposure to health care workers comes from interventional procedures such as angioplasty and stent insertion. The major source of exposure to personnel working in this area is radiation scattered from the patient.

2   .1   C  A   R D I   A   C   C  A  T  H E T  E  R I   A  Z  T  I    O  N L  A  B  O  R A  T   O  R Y 

 /    7 

2.1.1

 Angiography

Coronary angiography consists of inserting a catheter and, while observing by fluoroscopy, guiding it along and injecting contrast material into the coronary arteries and imaging the results. Stenosis or blockage of one or more of these vessels can lead to death of part of the heart muscle (myocardial infarction or “heart attack”). Visualization of such injuries can be followed by effective treatment with surgery or coronary angioplasty. Coronary angiography is a procedure that presents the potential for a high degree of radiation clinical staff, resulting in annual effective dose greater greexposure ater thanto 25the mSv mSv. . In this diagnostic procedure, cineangiography is used in angulated projections which can expose the operator to a higher dose than when wh en the x-ray equipment is used in the standard posterior-anterior position.

2.1.2

Coronary Angioplasty  Angioplasty 

Coronary angioplasty angiopla sty is a therapeutic procedur procedure e to open blocked arteries by either inflating a small balloon inside the artery, compressing and fracturing the obstruction, or the use of rotating, cutting blades to remove the obstruction. During conventional coronary angioplasty angio plasty,, cineangiography is infrequent, infr equent, but prolonged prolong ed fluoroscopy in severely angulated positions increases the dose to the operator. Coronary angioplasty has the potential to result in a high degree of radiation exposure and may result in annual doses greater than 25 mSv. As in all radiographic procedures, the exposures are higher when working close clos e to the x-ray source, but diminish rapidly when working wor king farther away away.. The usual arterial arte rial accesses are the femoral artery in the groin and the brachial artery in the shoulder. The latter places the operator closer to the x-ray source.

 

2.2 OPERATING ROOM

2.1.3

 / 9

 Electrophysiology  Electrophysiolog y Studies

Electrophysiology is the study of the electrical properties of biological tissue. Its clinical application in cardiology is to study the electrical conduction conduction pathway pathwayss of the heart which support support the heart's pumping action. In the cardiac catheterization laboratory, fluoroscopic control is used to position catheters in the heart to measure electrical activity and to map electrical conduction pathways. This technique has increasing usefulness because abnormal conduction pathways, which may lead to life-threatening cardiac arrhythmias, can be controlled by ablation ablatio n of specific tissues. Abla-

tion therapy thera py,, which began as a surgical proce procedure, dure, now is being per per-formed by catheter-directed methods, which are rapidly becoming  the therapy of choice for destroying destroying the abnormal electrical conduction pathways. These procedures usually require only posterior-anterior fluoroscopy, although oblique or angulated views may sometimes be used. The recent successful treatment of some cardiac arrhythmias by radiofrequency ablation will increase the use of fluoroscopy for this type of therapy. While angulated views are not generally necessary, long fluoroscopic times and the occasional use of angulated views could inadvertently result in the annual occupational dose limits being exceeded. 2.1.4

 Miscellaneous

Other fluoroscopic procedures include pressure measurements in the right and left heart, placement of pulmonary artery lines for long-term pressure measurements, and both implantation and replacement of pacemaker wires in the right ventricle. These examinations are relatively simple and generally require minimal fluoroscopic imaging. They do not require angulation or cineangiography. They are sometimes performed in the cardiac catheterization laboratory, but are more frequent in intensive care units or other clinical care areas. The occupational radiation exposures from these procedures are not likely to cause doses to exceed 25 mSv y–1. 2.2 Operating Room 2.2.1

Urological Procedures

2.2.1.1  Cystoscopy Cystoscopy.. Cystoscopy is a urological procedure during  which an endoscope endosco pe is placed into the th e urinary bladder. bladder. The bladder

 

10 / 2. RADIOLOGIC EXAMINATIONS OUTSIDE THE RADIOLOGY DEPT. is then examined by direct visualization and, in some cases, radiographically graphicall y after administration administration of contrast media followed by further radiological or fluoroscopic examination of the ureter and collecting system of the kidneys. Cystoscopy may be performed in the operating room, but now is usually performed in outpatient facilities. The examination can be accomplished accom plished with either fixed or mobile equipment. The cumulative personal exposure is medium and not likely to result in doses greater than 25 mSv y –1 because the examination is performed perfo rmed in a set position with a small number of films. However, placement of internal ureteral stents increases the fluoroscopic exposure, and therefore, effective dose to both the

patient and the staff. 2.2.1.2  Nephrostomy Tube Placement. Placement. Nephrostomy tube placement is a fluoroscopic procedure that is sometimes performed in the operating room. It consists of placing a needle into the collecting system of the kidney, to drain an obstructed system or to place tubes through a stricture so that drainage is normal. This is primarily a fluoroscopic procedure that requires at least 15 min of exposure. Staff exposures are usually in the “medium” range. However How ever,, these accumulated doses can exceed 25 mSv when angulation is

necessary for appropriate nephrostomy tube placement. The operator should be exposed only during the initial needle placement. When films are taken, they are usually few in number and use standard techniques. In some patients, repeat examinations are needed to determine proper nephrostomy tube placement. 2.2.1.3  Stone Removal. Removal. Stone formation in the urinary tract is a common pathological problem. Many of these stones pass spontaneously,, but if the stones are too large tto neously o pass, intervention may be necessary.. Stone removal necessary re moval using laser l aser,, ultrasound ultrasou nd or hydromechanhydro mechanical methods may be indicated. There are two other techniques

which involve radiation exposure for removing stones: lithotripsy and extracorporeal extracor poreal shock-wave lithotripsy lithotr ipsy.. Fluoroscopy and a nd radiography may be used to determine where to focus the shock waves, which are used to break up the stones and to evaluate the success of treatment. In a typical lithotripsy procedure for kidney stone ablation, biplane fluoroscopy is used for about 3 min. Percutaneous nephrostomy allows access to the kidney for stone removal. After this procedure, it is frequently necessary to place a catheter stent in a ureter, which is the tube connecting the kidney to the urinary bladder bla dder.. All of the above are performed perfo rmed under fluoroscopic control and may take 30 to 45 min of exposure. Occupational

 

2.2 OPERATING ROOM

 / 11

x-ray exposure is usually medium but can result in annual effective dose greater than 25 mSv if oblique positioning of the x-ray equipment is used. 2.2.2

Orthopedic Procedures  Procedures 

Orthopedic procedures involve radiography and fluoroscopy and are usually performed with mobile equipment in the emergency department or operating room.

2.2.2.1  Hip Pinning and the Orthopedic Placement of Rods and  Nails.. Hip pinning and the placement of rods and nails take place  Nails during orthopedic procedures. Fluoroscopic use may take several minutes and involves medium exposure to clinical staff. Effective dose to personnel can exceed 1 mSv y–1, often because of the need for angulation of the intensifier and x-ray source. The procedure generally includes three or more films. 2.2.2.2  Fracture Fracture Reduction. A Reduction. A second category of orthopedic procedure involves fracture reduction under fluoroscopy. This may take

place in the With operating roomduration or, increasingly, in the emergency department. a typical of approximately 2 min, the annual effective dose to personnel is not likely to exceed 25 mSv, i.e.,, medium. i.e. 2.2.3

Operating Room Cholangiography Cholangiography

Mobile or fixed equipment is used for intraoperative cholangiography,, which is performed raphy perfor med during surgery after af ter removal of o f the gallbladder to identify stones in the liver and bile ducts that may have been missed during surgery. This involves one to two films usually following the standard technique. There may also be a; brief fluoroscopic examination. The operator’s exposure is low the annual effective dose should be below 1 mSv. 2.2.4

Operating Room Angiography

 Angiographic examinations are usually us ually confined co nfined to a dedicated dedicat ed angiographic suite, but there may be a need for abbreviated studies at the time of vascular surgery to confirm the integrity of a repair. This “single film arteriogram” arteriogr am” uses a mobile or fixed x-ray unit and one or more injections of contrast material directly into the artery.

 

12 / 2. RADIOLOGIC EXAMINATIONS OUTSIDE THE RADIOLOGY DEPT. This is a low-exposure procedure, resulting in an annual effective dose that does not exceed 1 mSv mSv.. 2.2.5

 Placement of Permanent Permanent Pacemaker Pacemaker Lines Lines

 A procedure that is sometimes based in the operating room or in a critical care unit is the placement of permanent permane nt pacemaker lines. Fluoroscopic time should be less than 15 min; the occupational radiation exposure is medium  resulting in an annual effective dose above 25 mSv.

2.2.6

 Endoscopy Examinations Examinations  

Endoscopic examinations such as endoscopic retrograde cholangiogram pancreatography and fiberoptic bronchoscopy are becoming increasingly common. They may be performed in the radiology department or in a separate endoscopy suite. These examinations combine endoscopy endo scopy with fluoroscopy fluorosco py.. Catheters are placed by direct  vision and fluoroscopy of the pancreatic and common bile ducts and its branches. Commonly, 5 to 20 min of fluoroscopy time is needed, resulting in medium radiation exposure. The annual effective dose from this procedure probably will not exceed 25 mSv. 2.3 Miscellaneous Examinations and an d Loc Locatio ation n 2.3.1

 Facet  F acet Block Block 

The facet block procedure may be performed by members of the radiology, orthopedic or anesthesiology departments. It involves placing a needle into the patient's patie nt's back in order to anesthetize deep nerves. The procedure may be performed in the radiology department but is often performed with mobile equipment. The procedure utilizes 1 or 2 min of fluoroscopy and has medium  exposure potential. 2.3.2

Catheter Placement  Placement 

Many films for each patient patie nt may be taken to check the positions of various arterial and venous catheters used for nutritional support or long-term antibiotic and chemotherapy administration. The imaging of these catheters is done in a variety of locations outside

 

2.4 SUMMARY

 / 13

the x-ray department including the patient room, dialysis unit, recovery room, operating room, and intensive care unit. The radiation exposure to personnel is low. 2.4 Summary

 Important characteristics of the most common procedures are summarized in Tables 2.1 and 2.2. Although the majority of these common procedures produce little radiation exposure to health care workers, procedures that involve interventional techniques

and the evaluation of peripheral and coronary circulation have been identified as potential sources of significant exposure. The dose associated with each procedure, however, is highly dependent on the technique used and the number of steps performed. Annual Annual occupational effective dose limits can be exceeded if it is necessary for someone to hold a patient and the individual is exposed to the primary x-ray beam. Extra care should be taken to avoid such high-exposure situations. situat ions. Although the primary beam may produce only a localized exposure of the hands and arms, the resulting dose is still many hundreds of times higher than that occurring as a result of scattered radiation from the patient. Protective gloves shall   be used to reduce the accumulated dose to the hands. 1  As shall noted in subsequent sections of this Report, recognition of the potential for exposure is the first step in a systematic program for radiation protection.

1

Leaded surgical gloves do not always offer adequate protection. The proper protective equipment needed depends on exposure parameters including, but not limited to, duration of exposure.

 

3. Sources o off E Ex xposure to Workers

The amount of radiation to which workers are exposed depends on: (1) the total radiation exposure necessary to perform the exam-

ination, and (2) personal protection techniques employed during  the examination. In general, reducing the patient dose results in reduced operator dose. Workers, however, however, should take further steps ste ps to limit their personal exposure. Specific techniques to reduce total radiation levels and worker exposure are discussed below and in Section 4.

3.1 Total Radiation Exposure During the th e Proc Proced edur ure e

There are many different considerations that influence the total radiation exposure required to perform a medical procedure. Some of these are purely medical and include an evaluation of the need for the examination or treatment, possible contraindications, and the choice of the most appropriate procedure. Such decisions are based on patient needs and professional judgment. A qualified  expert should  should  be consulted when new techniques or significant changes in technique are contemplated. Technical considerations that influence the radiation exposure required to perform the medical procedure include: • approp appropria riate te equip equipmen mentt select selection ion and and accep acceptan tance ce testi testing  ng  • ongoin ongoing g qualit quality y assura assurance nce prog program ramss and regu regular lar qual quality ity control testing, including staff training in equipment operation and radiation safety • establish establishment ment and impleme implementati ntation on of appropri appropriate ate radiolog radiologiical technical procedures Dose reduction is highly desirable, although care must be taken that it is not done at the expense of unduly compromising image quality. 14  

3.1 TOTAL TOTAL RADIATIO RADIATION N EXPOSURE DURING DURING THE PROCEDURE

3.1.1

 / 15

 Equipment Selection, Quality Control, and Preventive  Maintenance

To obtain an image of optimal quality for a particular application at the lowest dose, the selection of appropriate x-ray equipment is critical. critical . This requires input from imaging physicians, medical physicists, engineers, and technologists. Following installation, an ongoing quality control and preventive maintenance maintenance program that monitors system performance is necessary to maintain image quality and ensures that doses remain as low as possible while still being able to obtain the necessary clinical information.

It is particularly important that the performance of the entire imaging chain be optimized to produce the required image at the lowest possible dose. The elements of an effective quality control program are discussed in NCRP Report No. 99, Quality Assurance  for Diagnostic Imaging (NCRP, Imaging (NCRP, 1988). 3.1.2

 Establishment of Appropriate Radiological Radiological Technical Technical  Procedures

Technique factors for a given examination should should be  be selected to optimize diagnostic information and minimize patient dose. Factors showing the settings to be used for specific types of study and patient sizes are generally generall y developed by a team that includes imaging physicians, other qualified experts, and radiological technologists. Variables include tube potential (voltage), current, exposure time, added filtration, use of grids, type of image receptor, source-to-image receptor distance, and anatomic projections. 3.1.2.1  Tube Voltage. In Voltage. In general, the use of a higher peak kilovoltage (kVp) with lower tube current (milliamperes) reduces patient dose and scatter to workers, but a higher kilovoltage also reduces image contrast. Therefore, careful decision making is required to

arrive at the best tube potential (voltage) for a given application. Figure 3.1 schematically illustrates the relative exposure levels resulting from an abdominal film obtained at 70 kVp and at 120 kVp. The tube current current and exposure exposure time is adjusted to produce the same film density densit y in both situations. situatio ns. In Figure 3.1 and all subsequent similar figures, the density of the black dots corresponds to the amount of scattered radiation present at different locations in the room outside the primary beam projection at the patient. Darker areas represent higher radiation levels. The 70 kVp beam is less penetrating than the 120 kVp beam. Therefore, to achieve the same film darkening, the dose at the

 

16 / 3. SOURCES SOURCES OF EXPOSURE TO WORKERS

Fig. 3.1.  Radiation exposures to attending personnel for a typical abdominal film at (a) 70 kVp and (b) 120 kVp. The film density is the same in both cases. Each coordinate box represents a 50 × 50 cm area. The density of the dots is proportional to the calculated air kerma at each location, that is, the darker areas represent areas having higher exposures. Lighter areas represent lower exposures. exposures.

surface of the patient near the x-ray tube is much higher for the 70 kVp procedure, procedure, an and d produces produces much more total total scatter than the higher energy beam. Clearly, personnel standing near the bedside for the 70 kVp procedure receive a significantly higher dose than they would for the 120 kVp procedure. 3.1.2.2 Tube Current and Exposure Time. Time. For For a specific tube potenpo tential, the patient exposure is proportional proporti onal to the product of tube current and exposure time (milliampere seconds). Values for these parameters vary widely depending upon the type of radiological study. For example, during radiography, the tube current is extremely high, but the exposure time is short (typically much less than 1 s). The total exposure to both patient and workers is quite

 

3.1 TOTAL TOTAL RADIATIO RADIATION N EXPOSURE DURING DURING THE PROCEDURE

 / 17

low unless many radiographs are taken. In most radiographic procedures, workers are outside the room or behind the protective barriers during the exposure. During fluoroscopy, the tube current is quite low, low, but the exposure exposu re times can be very long. Cumulative dose do se to personnel can be quite high. Typical dose rates to air (i.e. ( i.e.,, air kerma), exposure times, and cumulative dose to air at 1 m for representative medical imaging techniques are shown in Table 3.1. To limit patient and worker dose, exposure rate and time should should   be kept as low as possible possib le to obtain a useful image. During Duri ng fluoroscopy fluorosc opy,, the operator must take special care to ensure that the minimum exposure time is used. To accomplish this, the imaging physician

should  operate the exposure switch intermittently when new or should  updated information is required rather than maintaining continuous exposure during the procedure. Exposures should should only  only be made while the imaging physician physi cian is actually look looking ing at the monitor. monitor. New technology,  e  e.g .g.., pulsed fluoroscopy systems, digital storage, etc., can also help to reduce the dose to patients and workers wor kers by limiting  the exposure time. One effective digital storage technique is to retain the last displayed image in computer memory (last image hold). This allows the operator to study the image at leisure with no additional exposure to the patient and workers. Filters and Grids. Grids. On most x-ray units, filters are added in 3.1.2.3  Filters the beam path near the x-ray tube to absorb nonpenetrating, low-energy x rays which otherwise increase the patient's dose without contributing to the image. Filters effectively make the average energy of the x-ray beam higher. Adding more filtration reduces the dose to the patient and reduces the the scattered radiation, so that the dose to workers is lowered. Grids are often placed between the image receptor and the patient to improve image quality. Achieving the same density on the film, however, requires about two to four times more patient and worker dose than if a grid were not present. Prudent selection of the composition and thickness of filters and the type of grid (if  any) for a given procedure is required. The grid can often be removed for some procedures or even portions of a particular procedure. 3.1.2.4 Projections and Patient Thickness Thickness.. The projections (i.e. (i.e.,, the path of the beam through the patient) required for a procedure affect patient and worker dose. If the x-ray beam must penetrate thick body parts before befor e reaching the image receptor r eceptor,, more radiation will be required to create a useful image. As a result, both patient

 

1  TABLE 3.1—  3.1—Typical Typical

air kermaa rates  rates,, exposure times and cumu cumulative lative air kerma from routine imaging procedures.

Modality

 Air Kerma Rate to the Patient Surface Closest to the  X-Ray Tube Tube

Exposure Time for a Procedure

 Air Kerma Rate at  Approximately 1 m  Approximately

Cumulative Air Kerma at Approximately 1 m

Fluoroscopy

10 to 100 mGy min–1

Relatively long,   multiple minutes

10 to 100 µGy min–1

250 µGy (50 µGy min–1 for 5 min)

Cine angiography

0.1 to 1 Gy min–1

Relatively short, tens   of seconds

0.1 to 1 mGy min–1

100 µGy (600 µGy min–1 for 10 s)

Ra Radi dio ogra raph phic ic--spo spot film film

18 Gy min–1

 V  Very ery short, 6 weeks

—

Late erythema

15

~6 – 10 weeks

—

Dermal necrosis (first phase)

18

>10 weeks

—

Dermal atrophy (first phase)

10

>14 weeks

—

Dermal atrophy   (second phase)

10

>1 y

—

Telangiectasia

12

>1 y

—

>15

>1 y

—

Dermal necrosis (late phase)

a

Wagner and Archer (1995). b Ellipsis indicates no peak value for that effect.

Occupational radiation dose limits are based on dose levels which essentially eliminate deterministic determinis tic effects. These dose limits are also designed to make the probability of stochastic health effects small in comparison with their spontaneous occurrence and to make any risk incurred from working with radiation comparab comparable le to the risks of being employed in other safe industries (Sections  A.1.5 and A.1.6).  A.3.2  Stochastic Effects

These effects, which are generally delayed or latent in nature, are defined as ones in which the probability of occurrence, rather than the severity of the effect, is a function of radiation dose

 

52 /  APPENDIX A   A.3—Other Other TABLE A.3—

deterministic effects from x radiation.a Single-Dose Threshold

Effect

Onset

(Gy) Parotid gland function   (saliva)

>2

Prompt

2–10

Prompt

~6 (~3 soft-tissue dose)

May not be detected until after puberty

Very high

>6 months

Parotiditis Bone growth deficit Bone necrosis Behavioral   maldevelopment

~0.5

a

Wagner and Archer (1995).

without threshold. Cancers (solid malignant tumors and leukemia) and heritable (genetic) effects are regarded as the main stochastic effects from exposure to ionizing radiation. Heritable effects were considered most important in the 1950s and 1960s. Currently, the risk of stochastic effects for the whole population is 7.3 percent per sievert, made made up of five percent per sievert sievert for fatal cancer, cancer, one percent per sievert for nonfatal cancer, and 1.3 percent per sievert for severe hereditary effect (ICRP, 1991).  Among the many factors which influence the stochastic effects

of exposure to ionizing radiation are the total dose received, the rate at which the dose was received, the age at exposure, the type of radiation, the type of tissue irradiated, and gender. It must also be noted that stochastic effects at normal occupational dose levels are estimated from well documented effects at much higher dose levels and dose rates. At low levels, those effects are indistinguishable from effects that occur naturally or are induced by some other factor.  A.3.3  Heritable Effects

These appear in descendants of the exposed individual and are stochastic in nature. Such effects result from alterations in the reproductive cells, which can lead to inherited defects in the offspring. Detectable radiation-induced cellular mutations have been observed in animals whose reproductive cells received an absorbed

 

 A.5 RISK AND RISK ASSESSMENT ASSESSMENT

 / 53

dose of more than 100 tto o 200 mGy (NAS/NRC, ( NAS/NRC, 1990; NCRP, NCRP, 1993b) of ionizing radiation. Initially it was thought that any such radiation-induced changes were irreversible, and radiation protection standards were written to safeguard the genetic pool of the population from radiation damage. More recent information has shown that DNA repair is possible. It should be noted that, to date, radiation-induced genetic defects have not been proven in humans.  A.4  A. 4 Sour So urce ces s and an d Magn Ma gnit itud ude e of Ioni Io nizi zing ng Radiation Exposure

Everyone is exposed to ionizing radiation. radiation . Some individuals will be exposed to a wide variety of such sources, while others only to a few. The sources include those of natural and artificially-produced origin. Natural sources include cosmic radiation, terrestrial radiation from naturally occurring radioactive sources in the ground, radionuclides naturally present in the body, and inhaled and ingested radionuclides of natural origin such as radon as shown in Figure A.4. Radon is is estimated to be responsible responsible for the largest perpercentage of radiation dose to humans. huma ns. When human exposure to natural sources increases as a result of human action, deliberate or otherwise, the natural sources are known as enhanced as enhanced;; an example is a person’s increased exposure to cosmic rays as a result of air travel (NCRP, (NCRP, 1987a). 19 87a).

 Artificiall y-produced sources of radiation include sources such  Artificially-produced as x rays and radiopharmaceuticals in medicine, consumer products containing radioactive materials such as some smoke detectors or static eliminators, electronic consumer products, electricity generation using nuclear fuels, and such episodic events as atmospheric nuclear weapons testing. The magnitude of exposure from these sources for an average member of the United States population is reviewed in NCRP Report No. 93 (NCRP ( NCRP,, 1987a). A summary s ummary is given give n in Table A.4 and Figure A.4.  A.5  A. 5 Risk Ri sk and an d Risk Ri sk Asse As sess ssme ment nt

Risk is a part of life, and exists in all common daily activities. The nature of the risk and the potential consequences are dependent upon the type of activity. Individuals choosing to engage in a

 

54 /  APPENDIX A  TABLE A.4— Estimated

total effective dose equivalent per year for an average average member of the the population population in the United United States a and nd

Canadaa from various sources of natural background radiation.b Total Effective Dose Equivalent per Year (mSv y –1)c Source

Lung

Gonads

Bone Surfaces

Bone Marrow

Other Tissuesd

Total

wT

0.12

0.25

0.03

0.12

0.48

1.0

Cosmic

0.03

0.07

0.008

0.03

0.13

0.27

Cosmogenic

0.001

0.002

—

0.004

0.003

0.01

Terrestrial

0.03

0.07

0.008

0.03

0.14

0.28

Inhalede

2.0

—

—

—

—

2.0

In the body

0.04

0.09

0.03

0.06

0.17

0.40

Rounded   total

2 .1

0.23

0.05

0.12

0.44

3.0

a

The annual effective dose equivalent for Canada is about 20 percent lower for the terrestrial and inhaled components components.. b NCRP (1987b). c 1 mSv = 100 mrem. d This is an approximation derived by assuming that the other tissues had the same dose equivalent rate as the gonads; this adds 0.17 mSv to the annual effective dose equivalent. e Derived from calculations of ICRP Publication 32 (ICRP, 1981;

NCRP, 1987a). Note that, for exposure of the lungs to radon and radon decay products, wT is assumed to be 0.08 (not 0.12).

specific activity are making the decision, consciously or not, that they will receive a benefit from the activity that outweighs the associated risk. The benefit associated with a specific activity should be more than the risk associated with it. In daily life, choices that involve the comparison of benefits and risk are made almost continuously,, although they are so familiar they are not given much continuously thought. For example, driving an automobile to work has a risk of  injury associated with it, yet few would abandon aban don this mode of transportation based on that level of risk as the perceived benefit of con venient transportati transportation on far outweighs the risk. In medical applications, there is a risk from a surgical procedure, but in most cases, the benefit outweighs the risk.  For the clinical management of the patient the benefit of information from an x-ray examination is weighed against the potential

 

 A.5 RISK AND RISK ASSESSMENT ASSESSMENT

 / 55

Fig. A.4. The percentage contribution of various radiation sources to the annual effective dose equivalent to the average nonsmoking member of the United States population (NCRP, 1987a).

risk of radiation-induced health effects. Because the diagnostic benefit of a properly performed radiologic procedure can be expected to far outweigh the potential risk from the associated

x-ray exposure, this assessment rarely needs to be done. The information received by the physician from a radiologic examination about the clinical condition condit ion of the patient is an essential part of the practice of modern medicine. For medical workers the occupational risk of radiation exposure should be compared with the benefits of  the job and the cost of further reduction. How individuals perceive the risk associated with a given activity versus the actual risk associated with that activity is of  considerable interest. In many cases, the perceived level of risk is not consistent with the actual level of risk. Examples of risks for a number of common activities ac tivities are provided in Table A.5. In evaluating the information in this Table, it should be kept in mind that in some cases there is a direct cause and effect where the cause of  death is obvious, e.g obvious, e.g.., drowning. In the case of radiation exposures associated with medical imaging procedures, this relationship is less apparent and must be calculated. [See NCRP Report No. 115

 

56 /  APPENDIX A  TABLE A.5— Estimated

loss of life expectancy expectancy from health risks. risks.a Estimate of Days of Life Expectancy Lost, Average

Health Risk Smoking 20 cigarettes per day

(calculated)

2,370

(6.5 y)

Over Ov erwe weig ight ht (by (by 15%) 15%)

(c (cal alcu cula late ted) d)

77 777 7

(2 (2.1 .1 y)

 All accidents combined combined

(real)

366

(1 y)

 Auto accidents

(real)

200

 Alcohol consumption consumption (United States average)

(calculated)

130

Home accidents

(real)

74

Drowning

(real)

41

(calculated)

51

(calculated)

23

Natural background radiation (excluding radon)

(calculated)

9

Medical diagnostic x rays   (United States average)

(calculated)

6

Lifetime occupational radiation dose 10 mSv y–1 for 47 y 4.5 mSv y–1 for 47 y   (the average for workers workers in   the nuclear industry)

 All catastrophes (earthquake, etc.)

(real)

3.5

a

Cohen (1991); Cohen and Lee (1979).

(NCRP, 1993b) for a complete discussion of risk estimates for radiation protection.]  A.6  A. 6 Phil Ph ilos osop ophy hy of Radi Ra diat atio ion n Prot Pr otec ecti tion on

The goal of radiation protection is to limit human exposure to ionizing radiation to the extent that the likelihood of occurrence of  somatic and heritable effects is considered to be acceptably low in relation to the cost of further risk reduction and the benefit gained from the activities that involve the exposure. Specific objectives of radiation protection include: (1) preventing  the occurrence of severe radiation-induced deterministic effects by

 

 A.7 DOSE LIMITS AND THEIR THEIR BASES

 / 57

adhering to dose limits that are below the threshold dose levels for deterministic effects; effe cts; and (2) limiting the risks of stochastic effects, i.e., fatal cancers and heritable effects, to an acceptable level in i.e., comparison with nonradiation risks. Achievement of these objectives in a clinical environment results from observing the following  general principles: • any applica application tion of radia radiation tion must be justi justified fied,, that that is, is, it it must have a positive net benefit • the applica application tion must be opti optimiz mized, ed, that is, is, all all expos exposures ures must be kept ALARA, without compromising the medical utility of the examination • doses doses to work workers ers must must not exc exceed eed the the estab establis lished hed limi limits ts  A basic philosophical principle of the use of ionizing radiation emphasizes keeping exposures at levels ALARA. This premise has been accepted by advisory and regulatory agencies and requires that radiological personnel accept the responsibility of optimizing  the exposure to their patients and limiting exposure of any other individuals involved in a radiological procedure. Implementation of  the ALARA principle is achieved by application of the basic principles of radiation protection and a thoughtful approach to all work involving exposure to ionizing radiation (NCRP, 1990).

 A.7  A. 7 Dose Do se Limi Li mits ts and an d Thei Th eir r Ba Base ses s

Because of the potential biological effects associated with radiation exposures, it is important to establish dose limits for both workers and the general public. The limits for radiation workers have been set with the goal of ensuring that the risks from their exposures to ionizing radiation do not exceed those generally accepted by workers exposed to other types of industrial and occupational stresses. The limits for radiation doses are based on the knowledge that deterministic effects do not occur at low doses, and that the predicted risk of stochastic effects (i.e. (i.e.,, incidence of fatal cancer among exposed workers of severe genetic effecshould, effects ts to their offspring), together with fatal and nonradiation accidents on the average, not exceed the average risk of accidental death among  workers in so called “safe” industries. The limits are also established on the basis that the doses received by radiation workers will be kept ALARA. ALARA. Examples Examples of fatal accident accident rates in various various industries in the United States for calendar years 1976, 1986, and 1997 are presented in Table A.6. “Safe” industries are generally

 

58 /  APPENDIX A   Fatal atal TABLE A.6— F

accident rates in various industries: 1976, 1986, and 1997. Mean Rate 1976a (10–4 y–1)

Mean Rate 1986b (10–4 y–1)

Mean Rate 1997c (10–4 y–1)

 All groups

1.4

1.0

0.4

Trade

0.6

0.4

0.2

Manufacturing

0.9

0.6

0.3

Services

0.9

0.5

0.1

Government

1.1

0.8

0.2

Transportation and   public utilities

3.1

2.7

1.2

Construction

5.7

3.3

1.4

Mines and quarries

6.2

5.0

2.4

 Agriculture

5.4

5.2

2.0

a

NSC (1977). b NSC (1987). c NSC (1998).

regarded as those in which the annual fatal accident rate per 10,000 workers is less than one (NCRP, 1993a). It should be noted that these rates have been steadily falling. This may have conse-

quences for radiation protection limits; i.e. i.e.,, although the estimated risks from radiation exposure may not change, acceptable exposure levels may be adjusted to keep pace with the less hazardous work environment. Radiation limits pertinent to the protection of occupationally exposed workers are known as dose limits. Currently recommended values va lues for dose limits are given in Table A.7 (NCRP, (NCRP, 1993a) and reflect the recommendations of the NCRP as put forth in NCRP Report No. 116, Limitation 116, Limitation of Exposure to Ionizing Radiation (NCRP ation  (NCRP,, 1993a). The basic basi c limit is age in years times 10 mSv. mSv. The short-term effective dose limit for whole body occupational exposure is 50 mSv y–1. It is believed and supported by scientific evidence that with these levels there is little likelihood of either adverse somatic or heritable effects. On the basis of data gathered by epidemiological studies of people exposed at high doses, the lifetime risk of fatal cancer for radiation workers has been

 

 A.7 DOSE LIMITS AND THEIR THEIR BASES

 / 59

estimated to be 4 × 10–2 Sv–1 and the risks for nonfatal cancer detriment and for severe heritable effects have each been estimated to –2

–1

be 0.8 × 10 Sv . Occupationally exposed workers can generally be divided into two groups: those who are frequently frequentl y exposed in the course of their everyday duties and those who are not. An example of the former is an x-ray technologist who routinel routinely y operates radiation-producing  radiation-produci ng  equipment; an example of the latter is a nurse who occasionally assists a patient during an x-ray examination. With respect to limiting exposure and dose, the institution may establish administrative controls that are different for these two groups. It is currently recommended by the NCRP that the total equivalent dose limit (excluding medical exposure) for the embryo/fetus not exceed mSv per month The during the entire gestation to period (Table A.7)0.5 (NCRP, 1993a). control of exposures the embryo/fetus is accomplished by limiting the dose to pregnant women who are engaged in radiation work. If necessary, appropriate shielding should be employed. For the female radiation worker wearing a personal monitoring device, it is possible to determine her average exposure and so regulate her duties to limit radiation to the embryo/fetus if necessary. In most circumstances, however, no regulation of duties is required.

 

60 /  APPENDIX A  TABLE A.7— Summary

of recommendations. recommendations.a,b

 A. Occupational ex exposures posuresc 1. Effe Effect ctiv ive e dos dose e llim imit itss a. Annual b. Cumulative 2. Equivalent dose annual limits for tissues and organs a. Lens of eye b. Skin, hands and feet

50 mSv 10 mSv × age

150 mSv 500 mSv

B. Guidan Guidance ce for for emerge emergency ncy oc occup cupati ationa onall exposurec [See Section 14 of NCRP Report No. 116 (NCRP, 1993a)] C. Public Public exposu exposures res (an (annua nual) l) 1. Effect Effective ive dose dose lim limit, it, con contin tinuo uous us o orr frequent exposureb 2. Effect Effectiv ive e dose dose llimi imit, t, infre infreque quent nt ex expos posure ureb  3. Equiva Equivalen lentt dose dose limi limits ts for for tis tissue suess and organs organsc  a. Lens of eye b. Skin, hands and feet 4. Remedial action for natural sources: a. Effective dose (excluding radon) b. Exposure to radon decay products D. Education and training training exposures exposures (annual)c 1. Effective dose limit 2. Equivalent dose limit for tissues and organs a. Lens of eye

1 mSv 5 mSv 15 mSv 50 mSv >5 mSv >7 × 10–3 Jh m–3 1 mSv 15 mSv

b. Skin, hands and feet

50 mSv

E. Embryo/ fetus exposures (monthly)c 1. Equivalent dose limit

0.5 mSv

F. Negligible individual dose (annual) (annual )c

0.01 mSv

a

NCRP (1993a). b Excluding medical exposures. c Sum of internal and external exposures but excluding doses from natural sources.

 

 Appendix B The X-Ray Imaging Process B.1 Image Production

The goal of the imaging process is to (1) produce an image that can accurately depict the smallest objects (resolution), (2) obtain the best differentiation between tissues which may differ only slightly in x-ray attenuation (contrast), and (3) use the least radiation exposure to the patient needed to obtain the diagnosis. All three of these factors interact and usually one is improved at the expense of at least one or both of the others. The x-ray imaging  setup is depicted in Figures B.1 and B.2 for basic radiographic and fluoroscopic procedures. It is important for the operator to adjust the radiation field size to the body part being imaged and the image receptor so that no portion of the patient is irradiated by the beam

without producing a useful image. It is also important that the equipment be periodically inspected to ensure that the localizing  light field and radiation field coincides. Since the voltage may fluctuate, it is usually expressed in terms of the peak voltage (expressed in terms of kilovolts peak or kilovoltage). kilovolt age). The purpose of  the voltage is to accelerate the electrons (generated by heating the negative filament or cathode) across the evacuated x-ray tube where they strike the target or anode and produce x rays. As the  voltage is raised not only are more x rays produced but the x-ray beam is more penetrating. The higher the current [generally expressed in milliamperes (mA)], the greater the quantity quant ity of x rays produced. In the case of fluoroscopy units, the current amounts to only a few milliamperes; for fast radiographic exposures, it may range up to 1,000 mA. The total radiation produced is proportional to the product of the milliamperes milliamper es and the exposure time in seconds and is expressed in terms of milliampere seconds (mAs). The 61  

62 /  APPENDIX B

Fig   B.1.  Typical exposure situation in diagnostic radiography. The attenuation information along the straight-line paths of the x rays is

summed up (integrated), allowing the three-dimensional anatomy of the patient to be compressed onto a two-dimensional image. The thickness and composition of tissue that the x rays transit influence the optical density of the film at each point on the film.

radiation produced is allowed to pass through an aperture in the protective tube housing and then typically through an aluminum filter to remove the less penetrating radiation not used in image formation.. This x-ray beam then passes through the patient to form formation the image on some kind of image receptor such as a screen-film combination or image intensifier.  X rays emanate in all directions direction s from the anode of the x-ray tube. The x-ray tube housing is used to restrict the x-ray beam to that which is required to produce an image of the desired patient anatomy (the primary beam). The operator aligns the patient and x-ray beam so that the proper anatomy is irradiated in the desired orientation. Some x-ray tubes allow only a fixed x-ray field size,

 

B.1 IMAGE PRODUCTION

 / 63

Fig. B.2.  Typical exposure situations in fluoroscopy. The imaging  technique of transmission projection is similar to that depicted in

Figur Fi gure e B.1.

while others have a variable collimator attached. The variable collimator may have a light field that indicates the location of the useful radiation beam. Figure B.3 is a schematic diagram of an x-ray collimator,, showing the use of a mirror collimator mirr or to allow light li ght to indicate the t he size and location of the useful radiation beam. The relatively uniform x-ray beam which enters the patient is modified by the patient’s anatomy to produce an image on the image receptor. receptor. Some tissues absorb mo more re radiation, radiation , and some less; the x-ray image, therefore, represents the differential absorption and transmission of the useful beam by the patient. The image represents x-ray shadow projections of tissues in the patient and is a superimposition superimposition of the shadows shadows from all all overlying structures (Figure B.1).

 

64 /  APPENDIX B

Fig. B.3. Idealized schematic diagram of an x-ray collimator.

B.2 Image Receptors B.2.1  Screen Film

 Although the final image is captured on film in radiography, radiography, x-ray film alone is not very sensitive se nsitive to x rays. Usually the image is produced by absorbing an x-ray beam with a fluorescent screen which emits light to expose the film. These fluorescent screens sandwich the film inside a cassette (Figure B.4). After processing, the film becomes blackest (i.e. (i.e.,, transmits less light) where the highest exposure has occurred. The higher this blackening, the higher the optical density of the film. The use of a screen-film cassette combination instead of film alone permits decreased exposure time which minimizes motion blur and also decreases the dose to the patient by a factor of ~100. This is at the cost of decreased resolution due to the finite size of 

 

B.2 IMAGE RECEPTORS

Fig. B.4. Operation of a screen-film cassette.

 / 65

the light spots produced by the x-ray photons in the screen. The choice of the screen-film combination determines the absorbed dose to the patient and should be a balance between decreased dose to the patient and clarity of the image. The less x-ray exposure required to produce a given optical density on the film, the faster the  faster the image receptor, or the higher the speed speed.. Furthermore, the screen-film combination combination can enhance the contrast contrast caused  caused by differing x-ray transmission of different tissues. Screen materials such as rare earth and other phosphors provide excellent images at lower doses and require an x-ray dose of ~5 µGy to the face of the screen-film combination to produce acceptable blackening of  the th e film film.. B.2.2

Grids

Because thick body parts produce more scattered radiation (Compton scatter) which decreases contrast, a grid is usually

 

66 /  APPENDIX B employed to improve the image by removing as much scatter as possible. A grid consists of many thin lead strips separated by an interspace material. Placed between the patient and the image receptor,, the lead strips (“lines”) absorb receptor abso rb a large fraction fractio n of the scattered x rays because they are not traveling in a straight line from the focal spot. spo t. The lead strips may be parallel par allel to each other o ther,, as in a parallel grid, or they may be focused, i.e. i.e.,, directed toward the focal spot. Figure B.5 shows the operation of a focused grid. Often the grid is moved back and forth during the exposure to decrease the visual effect effect of the lines on the final image. Since a grid absorbs some of the radiation transmitted through the patient, its use requires an increase in the radiation exposure to the patient (the Bucky factor). Using a grid to increase image quality, therefore, increases the required radiation exposure and patient dose, which can result in more scattered radiation to nearby personnel for each exposure.

Fig. B.5.  The removal of incident radiation [primary ( – – –) plus scatter (· · ·)] by a grid.

 

B.2 IMAGE RECEPTORS

 / 67

 A grid must be used wi with th care. In certain procedures the t he use of  a grid improves image quality. Since the use of a grid increases radiation dose, the decision as to whether wheth er a grid is appropriate and what type of grid is to be used should should be  be made by a qualified expert. B.2.3  Image Intensifier

For fluoroscopic work, an image intensifier7 is used to increase the brightness of the image produced on a fluorescent screen which is viewed using a television or motion picture camera rather than integrated by a screen-film system (Figure B.2). The brightness is increased to such an extent that images image s which could previously only be seen by means of dark, adapted eyes are now plainly visible by viewing the output screen of the image inteninten sifier by mirror optics or by viewing a video image produced by the television camera. Although the image quality is theoretically degraded to some extent by this process, the brighter light level allows increased detail and contrast perception by the observer. Modern image intensifiers intensifier s use cesium iodide as the input phosphor which produces a high-quality image with an even greater dose reduction to the patient. The brightness of the image is often controlled by an ABC which adjusts the current and the tube potential potenti al (voltage) to provide adequate radiation input to the image intensifier. The image intensifier often has different modes in which less of the input screen is projected onto the output phosphor (Figure B.6). B.6). When a smaller portion portion of the input screen screen is used,

this is equivalent to magnification. magnificatio n. When the ABC is operating, the exposure rate to the patient will automatically increase incre ase as the magnification is increased. This is done without operator intervention. The operator should, however, be aware that (1) the exposure rate to the patient is always higher (but less tissue is exposed) at the smaller field size and (2) with heavier patients the exposure is always increased. Routine quality control under the supervision of  a qualified expert is essential to ensure that the exposure rate does not exceed those required for the medical imaging task. 7

The image intensifier is an evacuated tube containing a fluorescent phosphor that produces light, which is used to produce eletrons that are accelerated onto an output fluorescent screen. The increase in brightness is due to two factors: the extra energy given to the electrons by acceleration and the increase in the concentration of electrons gathered from the larger area of the input phosphor onto the much smaller area of the output phosphor.

 

68 /  APPENDIX B

Fig. B.6  Magnification with an image intensifier is achieved by applying different voltages to the electronic lens system. In magnification

mode, only the electrons emitted from a smaller diameter circle are focused onto the output phophor. Because a smaller portion of the image