160.X Rays in Medicine

Physics Factsheet www.curriculum-press.co.uk

Number 160

X Rays in medicine What are X-rays? • • •

They are electromagnetic (e.m.) waves of very short wavelength and high frequency. They travel at the speed of light in vacuo (meaning in a vacuum) at c = 3.00 × 108 ms-1. They are very penetrating and so can be used in medical diagnosis, to ‘see’ inside the body.

If the frequency of the X-rays is known, their wavelength can be calculated (and vice versa), using the wave equation c = f λ Exam hint:- Watch out for frequencies being given in units like MHz or GHz.

Photon energy E=hf The energy of an X-ray photon is proportional to the frequency, according to the equation E = hf. The higher the frequency, the higher the energy of the X-rays. The energy of an X-ray photon may be given in eV (electron volts), instead of in joules.

How are X-rays generated? high voltage supply anode

Intensity v Energy graph (i) The intensity* of the X-rays produced can be increased by increasing the accelerating voltage in the machine. (*Intensity is defined as power per square metre across the Xray beam). It’s approximately proportional to V2 (so doubling the voltage more than doubles the intensity –it’ll be roughly 4 times larger).

cathode

(ii) The beam energy is approximately proportional to the current through the cathode in the machine. (Doubling the current makes the beam energy roughly double).

lead housing

There is always a compromise between getting a sharp X-ray image (high contrast) and the need to reduce the radiation dose to the patient- supplying more and more energy to the X-rays causes problems for the patient.

x-rays emmitted

Electrons are emitted from a heated cathode. Some of the kinetic energy gained by the electrons as they are accelerated away from the cathode is converted to X-ray radiation when they smash into the anode.

The dangers of X-rays X rays are an ionising radiation. They can cause the molecules in cells to become ionised. These ions can cause cell damage, by killing the cell outright, or by changing how the cell reproduces itself, or by causing mutations (changes to the cell’s DNA). Rapidly dividing cells are most at risk from X-rays.

X-ray machines accelerate electrons with voltages of up to 100kV. Less than 1% of the kinetic energy of the electrons is converted to X-rays. The rest of their kinetic energy becomes internal energy (heat) in the anode. This means that ways are needed to keep it cool.

How are X-ray images produced? Images are produced by differential absorption. X-rays pass easily through some tissues but are attenuated / absorbed by others. The X-rays which pass through the body are detected, usually by photographic paper or film and an image is produced. The greater the amount of X-rays that reach the film, the brighter that portion of the image will be. The greater the difference in absorption by different tissues, the better the contrast of the X-ray image will be.

Damaging effects of X-rays: Stochastic effects If the effect is stochastic, the chance of the damage occurring depends on the dose of X-rays the victim has received. The higher the dose the greater the chance of the damage occurring. With stochastic effects, the severity of the damage does not depend on the size of the dose. Cancer is a stochastic effect.

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Physics Factsheet

160. X Rays in medicine Non-stochastic effects

Exam Hint:- You may have to split the calculation into two stages, perhaps working out the ì x power value first. Again, the intensity should decrease, if it doesn’t, check you raised I0e to a negative power.

The higher the dose, the greater the severity of the effect is. Radiation burns are a non-stochastic effect.

Somatic effects Only the person who receives the X-rays is affected. The effects are not genetic and therefore not hereditary.

The attenuation coefficient is a measure of how much a material absorbs X-rays. It will be high for bone, and low for soft tissues. As a general rule, the greater the density of a material, the higher its attenuation coefficient will be.

Genetic effects These are caused by genetic damage and are therefore seen in the offspring of the person who received the X-rays.

X-ray imaging relies on different tissues having different attenuation coefficients. The greater the difference in the attenuation coefficients between two materials is, the better the contrast between them on an X-ray image.

Precautions taken to protect patients include: • Ensuring parts of the body not being X-rayed are shielded • Exposing the patient for as short a time as possible • Ensuring the patient receives the minimum dose of X-rays that will still ensure high quality images are produced.

The half value thickness is the thickness of material needed to reduce the intensity of the X-rays to half their initial value.

Dose measurement

Radiographers stand behind a screen to reduce their exposure to Xrays. The half value thickness of this screen depends on the energy of the X-rays and the material of the screen.

Because of the potential for harm, the dose given to patient (and the radiographer) must be accurately known. Dose can be defined as absorbed dose or effective dose.

Question 2

The absorbed dose is a measure of the dose per unit mass (i.e. per kg). It is measured in grays (Gy). If 1 kg of matter absorbs 1 J of ionising radiation, the dose absorbed is 1 gray (so 1Jkg-1 = 1Gy)

Should the screen used by a radiographer have a high or low value of half value thickness? Answer: It should have a low half-thickness value so that a thin layer of the material can absorb most (or all) of the Xrays. The half value thickness of lead is typically a few mm. Generally, the denser a material is, the lower its half value thickness.

The effective dose takes into account the fact that different kinds of ionising radiation cause different amounts of damage to cells. Different ionising radiations have different ‘scaling factors’ called quality factors. Effective dose is measured in sieverts (Sv). 1Sv = 1Jkg

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The quality factor for most X rays is 1, whereas for more heavily ionising alpha particles, it is 20. This means that the same absorbed dose of alpha particles would produce an effective dose 20 times larger than X-rays.

Soft tissues do not show up on X-ray images very well. To image the digestive system, a patient will have a ‘barium meal’. Barium is opaque to X-rays, so it artificially increases the contrast of the digestive tract against the background of surrounding tissue.

Question 1 Suggest how the radiographer ensures that they are protected against damage from X-rays.

Question 3 Suggest why soft tissues do not show up well on X-rays images.

Answer: Precautions to protect the radiographer include leaving the room or standing behind a lead shield whilst Xrays are being taken and monitoring the radiographer’s exposure to X-rays over time.

Answer: Reasons for this include: • Too little attenuation – most X-rays just pass through • Too little difference in attenuation compared to other nearby tissue so contrast would be too low • Any bone between the X-ray machine or the detector and the soft tissue would absorb the X-rays and would therefore ‘block’ the soft tissue.

Attenuation and half value thickness X-rays in materials (like a human body) are attenuated by absorption and scattering. Not surprisingly, the intensity of the X-rays transmitted through a material depends on the thickness of the material. This is the value x in the equation: I = I0 e – µ x

Common uses of X-rays in medicine: Bone X-rays are very common. These can be useful in order to: ƒ diagnose broken bones or dislocated joints. ƒ guide surgery ƒ look for arthritis and other bone and joint diseases, including bone cancer.

I0 is the initial intensity of the X-ray beam, before it passed through the material. ì is the attenuation coefficient,

Exam Hint: Equations with e often look scary- but you just need to plug numbers into your calculator. As always, practice using it beforehand is a must! Don’t forget the minus sign for the power. (You’ll know if you’ve missed it out because you’ll get a figure suggesting the intensity increased, not decreased!)

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Physics Factsheet

160. X Rays in medicine Fluoroscopy and CT (or CAT) scans use X-rays

Adominal and pelvic CT scan

Fluoroscopy enables moving (real-time) X-ray images to be viewed. Instead of photographic film, a fluorescent screen is used. An image intensifier is used to brighten the image. This reduces the X-ray radiation dose that would otherwise be required, and means the radiographers no longer need to work in dim surroundings attempting to work out what was shown by the previously dim images. A TV-style monitor allows the radiographer to view proceedings safely in another room. Fluoroscopy has a range of medical applications, including implanting heart pacemakers and operations on the intestines. Computerised axial tomography uses X-rays to image a ‘slice’ of the body. An X-ray beam rotates around the body and the passage of the beam through the body is picked up by thousands of detectors.

Not surprisingly, there are pros and cons of CAT scans. The image is of really high quality due to the many thousands of detectors, but the cost of the scanner is phenomenal, and patients receive a higher dose of radiation.

A computer calculates how much attenuation has been caused by the different bits of the body, and uses the information to produce a detailed image.

3. (a) absorbed dose = energy absorbed / mass = 12/60 = 0.2 Gy (9, including one mark for correct unit) (b) effective dose = quality factor × absorbed dose = 1 × 0.2 = 0.2 Sv 9 (c) absorbed dose = effective dose / quality factor = 0.2 / 10 = 0.02Gy 9 = absorbed dose × mass (of patient) = 0.02 × 60 = 1.2J 9

Practice Questions

total energy

1. Explain the following: (i) why the tube is evacuated, (ii) the function of the high voltage supply. (iii) why the anode is rotated by the motor at a few thousand r.p.m. (4 marks) 2. (a) Suggest why a radiographer will ask if a female patient if they could be pregnant before they use X-rays on the patient. (3 marks)

2. (a) X rays are particularly harmful to a developing foetusv, by causing mutations and/or affecting cell division v. As the cells in the foetus are rapidly dividing so are most susceptible to damage. 9 (b) A clearer image is obtained, which helps the radiographer. 9 The patient receives a lower dose of X-rays. 9 (c) (i) X-rays will be suitable (no mark) because bones have a much higher absorption coefficient than surrounding soft tissue v so a high contrast image will be obtained 9 (ii) X-rays will not be suitable (no mark) because there will be insufficient difference in attenuation between the muscle and other surrounding soft tissue v the presence of nearby bones could also prevent the muscles appearing on the image OR muscles have such low attenuation that X-rays will hardly be affected by their presence. 9

(b) An image intensifier could be used in X-ray medicine. Explain how this benefits both radiographer and patient. (2 marks) (c) Explain whether X-rays are suitable for each of the following applications. (i) Diagnosing a broken bone (2 marks) (ii) Diagnosing a muscle injury (2 marks) 3 (a) Calculate the absorbed dose if a 60kg patient absorbs X rays with a total energy of 12J.Give the correct unit (2 marks). (b) Given a quality factor of 1 for X rays, calculate the effective dose in Sv. (1 mark) (c) Take the quality factor of neutrons to be 10. What energy would have to be supplied to the 60kg patient by these neutrons to give the same effective dose calculated in (b)? (2 marks)

1 (i) The tube is evacuated so nothing impedes the flow of electrons from the cathode. 9 (ii) The high voltage accelerates electrons towards the anode and the target. 9 (iii) It reduces the build up of heat in just one place.9 Due to all the electrons hitting just one part of the anode / which could damage the anode 9 N.B. The anode could also be kept cool by circulating water or oil, or by the use of cooling fins (as with car or motorbike engines).

Acknowledgements: This Physics Factsheet was researched and written by Ian Francis The Curriculum Press,Bank House, 105 King Street,Wellington, Shropshire, TF1 1NU Physics Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136

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