1. Computed Tomography Physics, Instrumentation, and Imaging
Module G
Computed Tomography Physics, Instrumentation, and Imaging

2. Disclaimer
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3. SELECTABLE SCAN FACTORS

4. SFOV
Scan Field of View This is the size of the FIELD in the gantry aperture.
Tip: The field size is NOT the entire visible area you see within the aperture.
The size of the scan parameter tells the computer which data to use and which data to ignore.
The SFOV determines the number of detectors required to collect data for a particular procedure.
The SFOV should always be larger than the area of interest

5. SFOV
When the anatomical area of the patient is OUTSIDE the SFOV, out-of-field artifacts will occur. These artifacts can present as:
Streaking
Shading
Miss-assignment of CT numbers

6. SFOV Too Large a SFOV may also produce artifacts such as: Shading
Streaking at the skin surface

7. DFOV Display Field of View Reconstructed field of view (RFOV)
Determines the size of the image viewed on the monitor
DFOV plus matrix size determine the limitations of perceived detail
DFOV should be equal to or smaller than the SFOV NEVER LARGER

8. DFOV The DFOV also impacts image noise and resolution
Wider DFOV’s increase the quantity of the photons from which data is retrieved
Image noise is reduced …however….at the expense of resolution.

9. Patient Dose Is directly proportional to the mA (tube current).
Scanners have the ability to produce more than one set of reconstructed images using the same Raw Data.

10. Reconstruction Filters
Primarily responsible for ensuring that the scanned anatomy is accurately represented
Secondarily responsible for enhancing the spatial or contrast resolution of the final image
High-pass (sharp) filters- provide definitive borders and edges, used for high contrast areas, musculoskeletal (increases image noise)
Low-pass (soft) filters- do not define borders and edges to the same extent as High-pass filters. Used for low contrast areas such as brain, abd. etc.

11. Reconstruction Filters
Primarily responsible for accurate anatomy representation on the final image
Secondary responsibility for enhancing the spatial resolution
High pass filters
Low pass filters

12. Pixel Size Pixel size can most often be chosen
512 x 512 matrix has fewer pixels than a 1024 x 1024 matrix
Fewer pixels = faster reconstruction = decreased information to archieved
However ….. The larger the size of the pixel the less detail is presented on the image. Smaller the pixel the greater the detail.

13. Slice Thickness To reduce image noise more mAs is required.
More mAs = greater patient exposure
Thinner slices = greater detail (conventional CT)
In spiral CT the selection of thin slices DOES NOT increase radiation dose.

14. Pitch Radiation dose is affected by the pitch of the helical scan.
1 : 1 changed to 1.5 : 1 results in a 33% reduction in patient exposure.
May be significant when scanning pediatric patients.

15. kVp and mAs Newer scanners have “smart scan” or “smart mA”
The scanner detects the remnant radiation and will reduce the mA accordingly.
The scanner will not increase the mA from what the technologist chooses.

16. Sampling and scan time
The rate at which a CT machine samples data is selectable at purchase…..
This rate does affect the scan time select ability.
Remember not enough sampling causes artifact called aliasing.

17. IMAGE QUALITY

18. Contributing Factors to Image Quality
Spatial resolution
Contrast resolution
System noise
Linearity
Spatial uniformity

19. Spatial resolution
Defined as the “degree of blurring in an image” and “the measure of the ability of the CT scanner to discriminate objects of varying densities located close together, against a uniform background”
Spatial resolution can be represented by:
PSF – point spread function
LSF – line spread function
MSF – modulation transfer function

20. MTF Modulation Transfer Function (MTF)
- is the expression of the ratio of the fidelity of an image to the original object canned.
Optimal MTF = 1
Non Optimal MTF = 0
-MTF is measured using a line-pair Phantom with a series of descending size bars.

21. Spatial Resolution
Determined by the smallest pair of lines which can be seen on the MTF phantom images.
line pairs/mm or (lp/mm)
Optimal resolution for spatial frequency obtained from a CT scanner is 0.3mm.
Spatial resolution is also affected by geometric factors
What are the geometric factors from Module F?

22. Geometric Factors focal spot size detector response curve
slice thickness
focal distance
isocenter
detector and sampling distance

23. Contrast Resolution Also called low contrast or tissue resolution.
By definition, it is the ability of the scanner to demonstrate small changes in tissue contrast.
Contrast resolution is determined by the absorption of x-ray photons in body tissue….. characterized by Hounsfield units or Linear Attenuation Coefficients (CT numbers)
Why is CT more accurate than conventional Radiography?

24. Linear Attenuation Coefficients
CT numbers are the function of:
The energy of the photons themselves
AS WELL AS
the atomic number of the body tissue scanned and its associated mass density.

25. Contrast Resolution
The ability of a CT scanner to image low contrast objects is limited by:
Size of the Object
Uniformity of the object
System noise

26. System Noise …..is represented as “graininess” on the CT image.
Quantum Mottle or Quantum noise
Noise is representative of deviations from the uniformity of the image matrix in which low contrast CT numbers slightly above or below zero (0), are interpreted as 0.

27. Factors for Noise mAs Scan time kVp and filtration Slice thickness
Object size
Detector efficiency
Pixel size
algorithm

28. Noise Equation

29. Noise ↑ = increase ↓ = decrease
If slice thickness ↑, Noise ↓, spatial resolution ↓
If patient dose ↑, Noise ↓

30. Quality Assurance / Water bath phantom
Noise should be measured daily, usually by the technologist
ROI (region of interest) are used to compute the standard deviation of the CT numbers.
20 cm water bath phantom. Within the phantom there phantom ROI areas seen on the axial images of the water bath phantom.
The technologist can use the “scanner ROI function” to measure the CT numbers within the Phantom ROI area seen on the axial phantom image.
(Please see chapter 11 in Seeram’s for phanom images and additional information)

31. CT Water bath Phantom test
The “scanner ROI size” is adjustable and should cover an area the size of 100 pixels.
5 scanner ROI’s should be used
4 on the periphery of the phantom ROI
1 in the center of the phantom ROI
(Please see chapter 11 in Seeram’s for phanom images and additional information)

32. Quality Assurance / Linearity
Defined as the accuracy of the calibration for a CT scanner.
5 pin performance phantom is scanned
CT system records the values in the performance phantom; the Standard Deviation is calculated and plotted.
The relationship should be a straight line if the scanner is in good working order (Bushong, 1997)
(Please see chapter 11 in Seeram’s for phanom images and additional information)

33. Pixel Values / Spatial uniformity
Spatial uniformity is achieved if the pixel values are constant in every region of the scanned water bath phantom.
Spatial uniformity is usually tested using an internal software package.
Package allows plotting of CT numbers along any axis of an image.
Histogram is generated
Histogram value should be within = or – 2 standard deviations of the mean.