CT Physics by Shahid Younas, M.Sc. Medical Physics (Surrey)UK Senior Medical Physicist & RPA SKMCH&RC, Lahore, Pakistan

1. Shahid Younas Lecture 5
INTRODUCTION TO CT PHYSICS
Introduction CT Physics

2. Computed Tomography
A single transmission measurement through the patient made by a
single detector at a given moment in time is called a
A. Projection
B. ray
C. view
D. beam
E. voxel

3. Computed Tomography
A series of rays that pass through the patient at the same
orientation is called a
A. Projection
B. ray
C. view
D. beam
E. pixel

4. Computed Tomography
A beam geometry, in which all of the rays in a projection are
parallel to each other, is named as
A. Parallel
B. divergent
C. orthogonal
D. tangential
E. anti-parallel

5. Computed Tomography
In fan beam geometry, the rays at a given projection angle
A. diverge
B. converge
C. split
D. tilt
E. spread

6. Computed Tomography
If It is the intensity produced by x-ray source and Io is the intensity
detected by the CT detector, then, relation between It and Io is,

7. Computed Tomography
Which beam geometry was used in first generation CT
SCANNER?
A. Pencil
B. Fan
C. Narrow Fan
D. Wide Fan
E. Helical

8. Computed Tomography
Which one describes the advantage of first generation CT scanner?
A. efficient scattered rejection
B. proper usage of x-ray beam
C. huge change in x ray flux
D. higher quantum noise
E. patient friendly

9. Computed Tomography
Which beam geometry was used in second generation CT scanner?
A. Pencil
B. Fan
C. Narrow Fan
D. Wide Fan
E. Helical

10. Computed Tomography
Which group describes the third generation CT?
A. Rotate / Translate Pencil Beam
B. Rotate / Translate Narrow Fan Beam
C. Rotate / Rotate Wide Fan Beam
D. Rotate / Stationary
E. Helical

11. Computed Tomography
Ring Artifact is a result of ?
A. Pencil Beam
B. Narrow Fan Beam
C. Gain of detectors remains equal
D. Gain of detectors does not cancel out
E. Heel effect

12. Fifth Generation CT : Stationary / Stationary
5th Generation CT
Electron Beam CT Scanner

13. Fifth Generation CT : Stationary / Stationary
 Cine CT
 Electron Cone Beam Tomography (ECBT)
 does not use a conventional x-ray tube
 large arc of tungsten encircles the patient and lies directly
opposite to the detector ring.

14. Fifth Generation CT : Stationary / Stationary
Ref:

15. Fifth Generation CT : Stationary / Stationary
 X-rays are produced from the focal track as a high-energy
electron beam strikes the tungsten.
 No moving parts to this scanner gantry
 Primarily to cardiologists.
 They are capable of 50-msec scan times and can produce fast-
frame-rate CT movies of the beating heart.

16. Sixth Generation CT : Helical
6th Generation CT
Helical

17. Sixth Generation CT : Helical
Helical CT (inaccurately
called spiral CT) acquire
data while the table is
moving.
“Volume Scanning”

18. Sixth Generation CT : Helical

19. Sixth Generation CT : Helical
By avoiding the time required to translate the patient table, the
total scan time required to image the patient can be much shorter
(~ 30 seconds for the entire abdomen).
Consequently, helical scanning allows the use of less contrast
agent and increases patient throughput. In

20. Sixth Generation CT : Helical
The table movement in the z direction during the acquisition
will naturally generate inconsistent sets of data, causing every
image reconstructed directly from a volume data set to be
degraded by artifacts.
However, special reconstruction principles generates a planar set
of data for each table position – produce artifact-free images

21. Second Generation CT : Rotate Translate Narrow Fan Beam
Do you know which special
reconstruction technique is used
in Spiral CT?
interpolation techniques

22. Sixth Generation CT : Helical
 Software applications enable the clinical use of spiral CT
even for regions which are subject to involuntary
movements.
 The speed of the table motion relative to the rotation of
the CT gantry is a very important consideration and the
pitch is the parameter that describes this relationship.

23. Sixth Generation CT : Helical
 Pitch = table feed per rotation/collimation
 The larger the table feed, the faster (i.e. with fewer
rotations) a body region can be scanned.
 However, if the table feed is too large, image quality will
be impaired.

24. Sixth Generation CT : Helical
Advantages of spiral CT in clinical use
1. Complete coverage of organs in a single respiratory position.
2. Short scan times (resulting in fewer motion artifacts and a
lower contrast medium requirement)
3. Additional diagnostic information due to improved resolution
(thinner slices) and 3D visualization in routine operation

25. Slip Ring Technology
1990s, the design of 3rd and
4th generation.
A slip ring is a circular
contact with sliding brushes
that allows the gantry to
rotate continually, untied by
wires.

26. Sixth Generation CT : Helical

27. Sixth Generation CT : Helical

28. Sixth Generation CT : Helical

29. Seventh Generation CT : Multiple Detector Array
7th Generation

30. Seventh Generation CT : Multiple Detector Array
1. An approach to make better use of the x-rays.
2. Collimator spacing is wider and therefore more of the x-rays
are used in producing image data.
3. Slice thickness is determined by the detector size.
4. four contiguous 5-mm detector arrays and 20-mm c. spacing

31. Thanks