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

2. Disclaimer
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3. Pixels Represent segment of the anatomy scanned
Contain numerical information based on tissue density
Two-dimensional representation
Slice thickness turns pixels into voxels (volume elements)
Expressed in Hounsfield units
Hounsfield units are directly proportional to the attenuation coefficient of the tissue

5. Windowing and Leveling
The process of “windowing” in CT Defines the shades of gray of interest.

6. Water Cerebrospinal fluid Tumors Blood (fresh) Blood (coagulated) Brain (white matter) – 46 Brain (gray matter) – 40 Muscle/aorta – 50 Soft tissue – 70 ( liver, spleen, kidney, pancreas, etc. ) Bone (average) – 1000 Petrous bone Air Lung tissue – 400 Fat

7. Windowing and Leveling
Window level determines the center of the gray scale and is generally set at the average tissue density of the structures within the anatomy being scanned.
Window width is normally set to include other structures or pathology that may also be located in the scan plane.

8. Water Cerebrospinal fluid Tumors Blood (fresh) Blood (coagulated) Brain (white matter) – 46 Brain (gray matter) – 40 Muscle/aorta – 50 Soft tissue – 70 ( liver, spleen, kidney, pancreas, etc. ) Bone (average) – 1000 Petrous bone Air Lung tissue – 400 Fat

9. Detectors Receive the attenuated x-ray photons
For detection to occur the attenuated photon must:
Be captured by the detector chamber
“collide” with an atom of the detector material
“Collision” must produce an electromechanical conversion suitable for measurement
Be able to be amplified and transmitted

10. Types of Detectors
Gas Detectors
Solid-state Detectors

11. Gas Detectors Xenon Inherent post-patient collimation 1 mm wide
Air-tight chambers
Widely dispersed Xenon atoms
Pressurized 30 times normal atmospheric pressure
Sensitive to x-ray photons
Generate electrical impulses
Impulse is amplified and sent to ADC
Digital signals sent to array processor
NOT suitable for 4th generation CT scanners

12. Solid-State Detectors
Composed of numerous types of crystals
Scintillation detectors
Emit light in response to x-ray photons
Convert the light into electrical signals using a photodiode
Amplify the signals and transmit them to the ADC
Sensitive to incoming attenuated photons
Suitable for 3rd and 4th generation scanners

13. Hounsfield Units Also called CT number or an integer
An integer is assigned to each amplified electrical signal in the form of a Positive or Negative Whole number.
The stronger the signal the greater the value of the integer

14. ADC ADC (Analog to Digital Converter)
Signals reaching the ADC are in analog form.
Signals are converted to integers and sent to the Array Processor

15. Array Processor special purpose logical processing unit
used to perform rapid image reconstructions
Computer computations
solves all of the complex mathematical problems (reconstructive algorithms)
Reconstructions, retrospectives, post processing techniques

16. Reconstructive Algorithms
Filtered-back projection
Simple-back projection
Convolution
Convolution and filtered back projection are considered analytical reconstruction algorithm

17. Convolution Method Projection profiles are obtained
Logarithm of data obtained
Logarithm values are multiplied by a digital or convolution filter (kernel)
Filtered profiles are then back-projected
Filtered profiles are added
Results in “blur-free” images

18. Raw Data
Convolution filters can only be applied to raw data or (scan data).
Convolution filters can NOT use image data.

19. Image Data Image Data can be used for post-processing techniques:
3D reformations
MIP’s
Volume rendering etc.

20. Spiral CT The data is acquired in Volume rather than slice by slice.
Filtered-back projection cannot be used alone to generate images.
Filtered-back-projection with linear interpolation is used as the reconstruction algorithm in single-detector-row spiral.
(the exact process depends on the manufacturer)

21. Interpolation
defined as a mathematical technique used to determine the value of a function from known values on either side of it.
This is a mathematical estimation technique.
Linear interpolation is the simplest form of interpolation.

22. Linear interpolation equation
Y3 = Y1 + (x3-x1)(Y2-Y1)(x2-x1)
Multi-Detector rows
Filtered-back-projection using interlaced sampling, longitudinal interpolation by Z-axis filtering, or ban-beam reconstruction is used in multi-detector-row CT scanning.

23. Spatial Resolution -the degree of blurring in an image
This is regarded as “the measure of the ability of a CT scanner to discriminate objects of varying densities located close together, against a uniform background”.

24. Spatial Resolution Represented by: Point Spread Function (PSF)
Line Spread Function (LSF)
Modulation Transfer Function (MTF)

25. Geometric factors for Spatial resolution
Focal spot size
Detector response curve
Slice thickness
Focal distance
Iso-center (center of rotation of the gantry)
Detector and sampling distance

26. Contrast resolution
Contrast resolution, low-contrast resolution or tissue resolution is the ability of the CT scanner to demonstrate small changes in tissue contrast.

27. Factors which affect low-contrast resolution
Photon flux
Slice thickness
Patient size
Detector sensitivity
Reconstruction algorithm
Image display
Image recording
Quantum noise

28. Reconstruction Limitations
The ability of the scanner to perform various reconstruction process is determined by the scanning parameters chosen by the Technologist as well as equipment characteristics!

29. Parameters Scan field of view (SFOV)
Reconstructed field of view (RFOV) (DFOV)
Window settings
Matrix size
Slice thickness
Radiographic tube output
Scan time and rotational arch
Focal spot size