1. CT Physics
V.G.Wimalasena
Principal
School of radiography

2. Slice / Cut
The cross sectional portion of the body which is scanned for the production of CT image is called a slice.
The slice has width and therefore volume.
The width is determined by the width of the x-ray beam

3. Matrix The image is represented as a MATRIX of numbers.
Matrix :- A two dimensional array of numbers arranged in rows and columns.
Each number represents the value of the image at that location

4. VOXEL
Each individual element or number in the image matrix represents a three dimensional volume element in the object, called a VOXEL

5. PIXEL
The VOXEL is represented in the image as a two-dimensional element called PIXEL - (picture element)

6. CT numbers The numbers in the image matrix are called CT numbers.
Each pixel has a number which represents the x-ray attenuation in the corresponding voxel of the object

7. Visual image & Gray Scale
To obtain a visual image, the CT numbers are assigned different shades of gray on a gray scale.
Each shade of gray represents the x-ray attenuation within the corresponding voxel

8. CT Image

9. Phases of CT imaging Scanning the patient Data Acquisition
Tube or tube and detector move
Multiple attenuation measurements are taken around the object
Image reconstruction
Image Display
Image archival (recording)

10. Scanning the patient - Evolution of CT scanners
1st Generation
2nd Generation
3rd Generation
4th Generation

11. DATA ACQUISITION Basic components
X-ray tube
Collimators
Detector/s
Collimated x-ray beam traverses the object and enters the detector.
The attenuation is determined from the difference between incident intensity and transmitted intensity
X-ray tube
Collimators
Detector

12. Translate rotate system
In the basic CT system the x-ray tube and detector are translated linearly so that the beam scans the object (eg. A disc of uniform density)

13. Ray, Ray sum, View & Attenuation Profile
Ray – Imaginary line between Tube & Detector
Ray Sum – Attenuation along a Ray
View – The set of ray sums in one direction
The attenuation for each ray sum when plotted as function of its position is called an attenuation profile
Ray sums
Ray
View
Attenuation profile

14. Attenuation profile of different structures
Attenuation of objects with different densities will change the attenuation profile
Object with low attenuation
Object with high attenuation

15. Attenuation profile at different positions
In a translate –rotate CT, after a view is recorded, the tube and detector rotate a small angle and the entire process is repeated until many views have been recorded for the same slice

16. Views & attenuation profiles for a slice1 2 4 3

17. Image reconstruction
The image is created by reflecting the attenuation profiles back in the same direction they were obtained
This process is called BACK PROJECTION
BACK PROJECTION

18. Drawbacks in Back projection (Artifacts - Star shape & streaks)
The resultant image closely resembles the original object
But it shows star shaped patterns around objects and streaks
These are called ‘Star’ and ‘streak’ artifacts

19. Formation of Star artifact and streaks
Consider a scan of a single high density object suspended in air

20. The attenuation profile for this object has a single impulse signal

21. The back projections take the form of a stripe through the center of the object

22. Back projections are crated for each profile

23. Addition of the attenuation profiles create an image with star and streak artifacts
Final back projection
To be continued – CT Complementary 3