1. Computed Tomography Basics

2. Limitations of Radiography
3D body rendered in 2D
Structures superimposed on image
Structure of interest viewed through underlying / overlying structures
Multiple views used to adequately visualize structure.
X-ray
Beam
Patient
Film

3. Limitations of Radiography
Density (gray shade) dictated by total attenuation encountered by beam
Thin highly-attenuating objects have same density as thicker low-attenuating object.
X-ray
Beam
Patient
Thin dense
object
Thick less
dense object
Image

4. Radiography vs. CT
Both based on differential attenuation of x-rays passing through body
Radiography
“Shadowgraph” using x-ray light source CT
Cross-sectional image
Image computed from pencil beam intensity measurements through only slice of interest

5. X-Ray vs CT
View anatomy without looking through underlying / overlying structures
improves contrast
Tightly collimated beam
minimizes scattered radiation
Very small contrast differences visible
Conventional
X-ray Beam
CT X-ray
Beam

6. Conventional (not Computed) Tomography
Philips Polytome
Predated CT by decades
Application Examples
IVP
Inner ear

7. Body Section Tomography
Not CT
Body Section Imaging
Goal
keep plane of interest in focus
blur all other plans
enhances contrast
Popularity decreased because of CT
MRI

8. Conventional Tomography Blurring
Image produced on film
Objects above or below fulcrum plane change position on film & thus blur

9. Conventional vs Axial Tomography
Conventional Cut
CT Axial Cut

10. Limitations of Conventional Tomography
Overlying / underlying structures blurred, not removed
5-10% subject contrast difference required for objects to appear different
many anatomic systems don’t have this much subject contrast

11. CT Advantages Excellent low-contrast resolution sensitive detectors
small beam size produces little scatter
Much better than film

12. CT Disadvantages Poorer spatial resolution than radiography
Higher dose
Artifacts at abrupt transitions
bone / soft tissue interfaces
metallic objects

13. CT Early Units 4 minute scans 5 minute reconstruction 80 X 80 matrix
1 slice
5 minute reconstruction
80 X 80 matrix
head only
water bag fit tightly around head

14. Data Acquisition
cross sectional image reconstructed from many line transmission measurements made in different directions
Tube
Detector

15. Projection Measurements
Radiation detector generates a voltage proportional to radiation intensity

16. Image Reconstruction Minicomputer does its thing
Analog to Digital (A to D) conversion

17. CT - Improvements
All CT generations measure same transmission intensities in many directions
Improvements
Protocol for obtaining many line transmissions
# of line transmissions obtained simultaneously
Detector location
Overall acquisition speed

18. 1st Generation Translate / Rotate

19. 3rd Generation CT Wide angle fan beam
Rotational motion only / no translation
detectors rotate with tube
30o beam
Many more detectors

20. Z-axis orientation perpendicular to page
3rd Generation CT
Z-axis orientation perpendicular to page
Patient

21. 3rd Generation (Non-spiral) CT
Tube rotates once around patient
Table stationary
data for one slice collected
Table increments one slice thickness
Repeat
Tube rotates opposite direction

22. Spiral CT (late 1980’s) Continuous rotation of gantry
Slip ring technology
Patient moves slowly but continuously through gantry
No dead time as gantry reverses
Much faster

23. Z-axis orientation perpendicular to page
Spiral CT
Z-axis orientation perpendicular to page
Patient

24. Data Aquisition Slice by slice Volume acquisition One slice at a time
data for an entire volume collected
patient moves in axial direction during scan
tube traces spiral-helical path through patient
Before ~ 1990
Since ~ 1990

25. Spiral CT Advantages volume data acquisition in single breath hold
no delay between slices
improved 3D imaging
improved multi-planar image reformatting
Special applications
bone mineral content
radiation treatment planning
CT angiography

26. Multi-slice CT (2000’s) Multiple rows of fan beam detectors
Wider fan beam in axial direction
Table moves much faster
Substantially greater throughput

27. Multi-slice CT Advantages
Scans at much greater speed OR
Allows scanning of same volume with thin slices
Makes possible additional clinical applications