CT SCAN basics-1

Computed Tomography (Basics)

Siemens Medical Systems

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 1

The Discovery of X-Rays... 100 years ago, Wilhelm Conrad Roentgen, a German scientist, discovered x-rays...

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 2

Look Inside The Human Body... This allowed people for the first time to be able to view the anatomy of the human body noninvasively

« But anatomic structures were superimposed « And soft tissue couldn’t be differentiated Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 3

CT Broke the Barrier... In 1972, two scientists Hounsfield and Ambrosepresented the first clinical CT image ...

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 4

The Human Body Slice by Slice... So we could see tomographic anatomy & density differences ♠ But it was time consuming (10 min. / image) ♠ And the resolution needed to be improved Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 5

What Does a CT Look Like?

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 6

The First Siemens CT Scanner...

SIRETOM (in 1974) Acquisition time 7 min., image matrix 80x80 pixels, scan field 25 cm , spatial resolution 1,3 mm (4LP/cm) Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 7

The Progress in Image Quality...

SIRETOM (1974) Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 8

SOMATOM Plus 4 UFC (1996)

What Does a CT Look Like? From the outside... ♣ ♣ ♣ ♣ ♣

Gantry Table Generator Console Computer

Gantry

Generator Table

Console Computer Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 9

What Does a CT Look Like? From the inside...

Tube

♣ Tube ♣ Detector ♣ DAS* DAS Detector * Data Acquisition System Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 10

How Does CT Work? X-ray X-ray generation generation Data Data acquisition acquisition Recon Recon && postprocessing postprocessing

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 11

Image Generation - The “Slice” X-rays pass through a collimator therefore only penetrating an axial layer of the object, called a "slice"

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 12

Image Generation - The “Voxel” The slice is artificially divided into small volume elements called "voxels" with a square base, inside which the attenuation is measured as a constant value. And in plane, the picture elements are called “pixels” Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 13

Image Generation - The “Matrix” The attenuation of radiation values is measured, encoded and transferred to a computer.

35 36 34 39 33 31 34 33 35 32 31 78 80 85 90

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 14

CT Image Generation - A/D/A* The numerical matrix is converted into a black and white image in a corresponding gray scale. 35 36 34 39 33 31 34 33 35 32 31 78 80 85 90

*Analog - Digital - Analog Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 15

Criteria for CT Image Quality CT Image Quality

Spatial resolution

Artifacts

Contrast Detectability

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 16

Spatial Resolution... The ability to resolve High Contrast Objects, (also called “High Contrast Resolution”)

This is influenced by system geometry, and determines image definition Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 17

Definition... Image definition means the sharpness of an object relative to surrounding tissue. It depends on:

Definition

Image display Scan time Algorithm Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 18

Operating mode Slice thickness

Contrast Detectability... When Small Contrast Differences are crucial (also called “Low Contrast Resolution”)

This is influenced by image definition & noise Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 19

Noise? Noise is superimposed on the image and results in a "grainy" impression, as is the case with poor TV reception. 206 mA

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 20

60 mA

Noise... Image “noise” is determined by the number of x-ray quanta that reach the detector and then contribute to the image. It depends on: Noise Image display

mAs

Operating mode

kV Algorithm

Patient size

Slice thickness Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 21

Artifacts... The various structures or patterns that appear in a CT image, but are not found in the original object.

They depend on: Scan time Slice thickness Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 22

Artifacts Patient Operating mode

...

Influences on CT Image Quality?

CT Image Quality System

User Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 23

Patient

System Efficiency... 100 mAs

100 mAs

The complete system design determines how efficiently x-rays are finally converted to electrical signal as the detector output, after passing through the patient. The generator, tube, geometry, filtration, collimation and detector design all play a role. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 24

mAs... Tube current and scan time, determine dose. Noise Rule Ruleof ofthumb: thumb: The Thehigher higherthe thedose, dose, the thelower lowerthe thenoise. noise.

high

1

noise

dose

* Noise reduced by factor 1.4 while dose is doubled. low mAs low Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 25

high

mAs... Low mAs

Image 1: Low mAs value high noise level Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 26

High mAs

Image 2: 4 times the mAs valuehalf the noise level

Recommendations for mAs... In the case of a soft tissue study, it is most important to keep noise to a minimum by using higher mAs. The lower the noise level, the easier it is to recognize structures with minute differences in density. But for bone or lung studies, higher mAs is not necessary. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 27

Tube Voltage - kV The dose level, depends very strongly on the voltage applied to the tube.

Water 20 cm

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 28

kV kV

Relative Relativedose dose

140 140 120 120

100% 100% 58% 58%

80 80

12% 12%

Tube Voltage - kV The higher the voltage, the more the radiation spectrum is shifted to a higher energy level, resulting in decreased radiation attenuation. This is most noticeable in bone and contrast media. 140 kV

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 29

80 kV

Algorithms... ... provide the recipe for mathematical image calculation Image definition Sharp

ULTRA HIGH HIGH STANDARD

SOFT

Smooth

SOFT DETAIL

Noise low Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 30

High

Algorithms An edge-enhancing (HighRes) algorithm produces good edge definition, but also a high noise level, while a smoothing algorithm produces a low noise level, but also poorer edge definition. For routine studies, a standard algorithm is normally recommended. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 31

Algorithms Soft Algorithm

HighRes Algorithm

Soft algorithms provide better contrast detectability with less noise. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 32

Algorithms Standard Algorithm

HighRes Algorithm

HighRes algorithms provide better spatial resolution, but with more noise. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 33

Selecting Slice Thickness Selecting a suitable slice thickness is a balance between edge definition and noise because of their mutually offsetting effects. AA thick thick slice slice means: means:

low noise better contrast resolution poorer edge definition partial volume artifacts

AA thin thin slice slice means: means:

high noise poorer contrast resolution better edge definition no partial volume artifacts

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 34

Slice Thickness... slice thickness

1/2

no. of x-ray quanta

50%

noise level

Image definition = spatial resolution

Noise High

High

Low

Low 1 2

1.4

3

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 35

5 10 mm Slice thickness

1 2 3

5 10 mm Slice thickness

Slice Thickness... 3 mm Slice

Thicker slices give less noise & better contrast detectability for soft tissue Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 36

10 mm Slice

Slice Thickness 5 mm Slice

1 mm Slice

Thinner slices give better spatial resolution for bony structures. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 37

Patient Size... Noise 400%

Rule Ruleof ofthumb: thumb: The Thenoise noiselevel leveldoubles doublesfor forevery every88cm cm increase increaseininpatient patientdiameter. diameter.

300% 200% 100% 30

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 38

34

38

42

cm Patient diameter 46

Patient Size... An attenuation by a factor of 2 results from each 4 cm increase in patient thickness, thus increasing the pixel noise. 45 cm

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 39

28.8 cm

Image Display - Windowing 3000

Blood

60 Spleen 40

0

Heart Adrenal Gland

Intestine

Bladder

Water

-100 Mamma -200 Fat -900 -1000

Tumor

Kidneys Pancreas

Bone

Liver

Air

Lung

Rule Ruleof ofthumb: thumb: The TheCT CTvalue valueof ofwater waterisis00and and air air-1000. -1000.The Therelative relativevalues valuesof of the theother othertissues tissuesare arecalculated calculated relative relativeto tothat thatof ofwater water

This is the so-called CT number in Hounsfield unit (HU) Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 40

Image Display - Windowing The range of CT density values is defined from -1000 to +3000, but the human eye can distinguish only 30 - 40 gray scales at best. Lung Window

Mediastinum Window

So, the window settings must be in accordance with the structures to be visualized Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 41

Image Display - Windowing Hounsfield unit

+3000

Gray scale display White

Window width W

Window center C

0 -1000

Black

CT Windowing

Window width (W): the density range represented within the gray scale. Window center (C): the center of the density range. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 42

Image Display - Windowing Narrow Window Width

Narrow window width: High-contrast image, but structures outside that window range may be inadequately represented or overlooked. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 43

Broad Window Width

Broad window width: Minor density differences appear homogeneous and may be masked.

Double Window Technique Hounsfield unit +3000

Gray scale display White

Window 1

0 Black

Window 2 -1000

CT Windowing

This is used for the display of two tissue types differing greatly in their density values, such as the lung & the mediastinum. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 44

Double Window Technique* To see both lungs and mediastinum within image simutanously

*Double window is not recommended for diagnosis. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 45

Image Display - Windowing Double Window

Image 1: Both lung, thorax wall & mediastinum visible Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 46

Lung Window

Image 2: Only lung visible

Mediastinum Window

Image 3: Only thorax wall & mediastinum visible

Extended CT Scale Normally, CT values can be measured from -1024 to +3071, but with SOMATOM Plus 4, this can be extended from -10240 to +30710 to visualize metals of high attenuation. Therefore, it is always possible to measure the real CT value no matter where and how the window is positioned.

Post operative femoral head replacement - the CT value is 6000 HU

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 47

Review vs. Magnification Reviewed

Review:

A zoom reconstruction from raw data to enhance sharpness of details Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 48

Magnified

Magnification:

a purely optical magnification of image data which may result in blurred appearance

Image Artifacts - Origins ...

Image Artifacts

Operator error

Motion Metal Partial volume

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 49

Technical defects Beam hardening

Image Artifacts - Appearance Image artifacts

Streak

Cupping Dark bar

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 50

Ring

Motion Artifact & Scan Time Motion artifacts Severe

Rule Ruleof ofthumb: thumb: The Theshorter shorterthe thescan scantime, time,the the less lesslikely likelymotion motionartifacts artifactsare are occur. occur. Moderate

Scan time (s) Short Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 51

Long

Motion Artifact & Correction Motion artifacts can be compensated for by the Motion Correction Algorithm (MCA)

w/o correction Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 52

w/ correction

Metal Artifact Metals, such as gold, absorb x-radiation almost completely, thus producing “radiation shadows”, leading to pronounced streak artifacts over the entire reconstructed image This can only be avoided via a gantry tilt that excludes the disturbing metallic objects from the slice plane. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 53

Partial Volume Effect Streak-like artifacts, also called partial volume artifact, occur most frequently in the bony structures at the base of the skull and the petrous bone region. 5 mm

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 54

That is because the very dense structures (bones) are only partially included in the slice, resulting in high contrast errors.

Partial Volume Effect 5 mm

2 mm

Selecting a thinner slice prevents such artifacts from occurring, since high contrast structures are less frequently partially included, but this inherently increases the noise level, thus degrading contrast resolution. Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 55

VAR - Volume Artifact Reduction 5 mm

2 x 2 mm

Combines several thin slices (which reduces the partial volume artifact) to provide a thicker slice (which reduces the pixel noise and offers good soft tissue discrimination). Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 56

Beam Hardening Effect The x-ray photons emitted from the x-ray tube do not all have the same energy. As they penetrate the irradiated object, the spectrum is shifted to higher energies - called “beam hardening”. In the image, streak artifacts or the so-called “cupping effect” can be seen.

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 57

Beam Hardening & Correction

w/o correction Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 58

w/ correction

Beam Hardening & Correction The “cupping” effect can be compensated for by means of “beam hardening correction” w/o correction

severe cupping Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 59

w/ correction

homogenuous CT values

Technical Defects The individual detector elements of a detector system may not produce the same signal for the same irradiation. When a detector element outputs an erroneous signal, ring artifacts appear. This can be eliminated by calibration, if not, call technical service! Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 60

Adaptive Filter For a non-circular object, x-ray attenuation is greater along the long axis than along the short axis, therefore directional noise is seen. w/o A.F.

Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 61

w/ A.F.

Artifacts & Corrections The reasons for artifacts are quite diverse. What we do is to perform corresponding corrections in order to avoid them. But sometimes artifacts can not be compensated for completely.

Nothing is perfect... But we keep on working to reduce them as much as possible! Source: CTC/CTM © Siemens AG, 1998 Medical Engineering Group, Computed Tomography CT Basics 62

For the Clinical Routine... Since the influence on HC resolution & LC resolution by changing parameters can be contradictory, it is necessary to differentiate between: ¨ A Soft tissue study (contrast detectability) ( >90% of routine studies, normally) ¨ A Bone study (spatial resolution) (