Basics of CT

1. ‫تعلم‬ ‫تكن‬ ‫مالم‬ ‫وعلمك‬
‫هللا‬ ‫فضل‬ ‫وكان‬
‫عظيما‬ ‫عليك‬

2. Basics Of Computed
Tomography (CT)
Lecture Six
Gamal Fathalla M.
Mahdaly
gamal_mahdaly@hotmail.com

3. Multislice CT
 1990 two detector arrays were used to
produce two spiral slices this technology is
now called multislice CT.
 Two distinguishing features;
(a) Instead of a detector array, multislice
CT required several parallel detector
arrays.
(b) X-ray tube with large heat capacity.

4. Seventh Generation CT: Multiple Detector Array
7th Generation CT made use of helical slip ring technology
Helical Slip-ring technology
developed (allows gantry to
rotate continuously without
wires)
Helical CT scanners acquire
data while table is moving

5. There are two characteristic features
of multi-slice CT system:
 1- Several parallel detector arrays
containing thousands of individual
detectors.
 2- Energizing such a large detector array
for quickly scanning large volumes
requires an x-ray tube with a very large
heat capacity.

6. MULTISLICE CT
 Two detector arrays (dual slices).
 Four detector arrays (four slices).
 Sixteen detector arrays (16 slices).
 Thirty two detector arrays (32 slices).
 Sixty four detector arrays (64 slices).

7. This multidetector array contains 16 rows of 912
individual detectors, each 1.25 mm wide (14,592
detectors).
The side strips are data detectors.

8. Dual detector array allows twice as much tissue
volume to be imaged with no loss of mage quality.

9. A four-detector array with a beam pitch of 2.0 covers
eight times the tissue volume of the single-slice spiral CT.

10. Four-detector array system:
A, Four single slices of 0.5 mm each. B, combine two single slices
to make 1 mm each. C, combine four single slices to make one
slice 2 mm thick.

11. Asymmetric eight-detector array.

12. The asymmetric eight-detector array provides a
choice of slice thickness by switching the DAS.

13. Additional pre detector collimation is used to
produce higher spatial resolution

14. The DAS selects combinations of detector
arrays for various slice thicknesses

15. A 16-detectr array collimated to a 20-mm
beam width resulting in a beam pitch of 1.0.

16. Multi-slice Pitch
Patient translation/360-degrees
Beam Pitch= ---------------------------------------
width of x-ray beam

17. Patient movement /360 degrees
Slice Pitch= -------------------------------------
Slice width

18. Beam pitch is patient couch movement
divided by x-ray beam width.
Slice pitch is not as important in CT as
beam pitch.

19. Improper collimation in multi-slice spiral CT can result in
excessive patient dose:
A, wide collimation, high dose. B, narrow collimation, inadequate
image. C, proper collimation, normal dose and quality.

20. Detector Apertures
2 x 1 mm
X-ray Beam Collimator
Tube Focal X-ray Spot
2 signals collected per gantry
rotation from 2 detectors
with 1 detector contributing
to each signal

21. Detector Apertures
2 x 2mm
X-ray Beam Collimator
Tube Focal X-ray Spot
2 signals collected per gantry
rotation from 2 detectors
with 1 detector contributing
to each signal

22. Clinical Applications
Speeding the scan up is valuable in:
 Examinations of trauma and pediatric
patients.
 Better phase differentiation in contrast
studies.
 Improving CT angiography.
 Development of functional CT & perfusion
imaging.

23. A, Carotid CTA was reconstructed from a volume-
acquired CT data. B, Volume-rendered image of a head
that could be constructed to face (surface rendering)
and then rotated.

24. An MIP reconstruction creates a three-
dimensional image from multi-slice 2D data sets
the result is a CT angiogram

28. Coverage, Spatial Resolution
and Radiation dose
 Greater number of slices allows more volume
coverage.
 Smaller detector size results in better Spatial
Resolution.
 Higher-Resolution Multislice Helical CT
increases the Radiation Dose.

29. Advantages of Helical vs Conventional CT
 Faster image acquisition.
 Quicker contrast follow up.
 Reduced patient dose at Pitch>1.
 Physiologic monitoring.
 Improved 3D imaging.
 Angiographic imaging.
 Fewer partial volume artifacts
 Fewer motion artifacts
 Freeze breathing.
 No Mis-registration.
 Increased output.
 Improved patient comfort
 Real time CT guided biopsy.

30. Any Question???.
Take your Tiiime!!! .
Again Any Question???.
Otherwise, I’m going to ask!!!.
Should I Ask???.

31. Have a nice day