1. Single Slice Spiral - Helical CT
Oh no, not more physics…

2. Spiral CT Incentives for development New technology required
Shorter study times
Improved 3D imaging
New technology required
Slip ring
Allows continuous gantry rotation

3. Conventional (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

4. Volume Scanning Data collected continuously Table moves continuously
Focal spot traces spiral path with respect to patient

5. Helical Reconstruction Complication
Patient moves as gantry rotates
No two fan beams at same z coordinate
“z” direction

6. As Gantry Rotates, Fan Angles Repeat
Distance between repetitions is movement of table during one rotation
“z” direction

7. Data Acquisition Challenges
Projection data not confined to single slice
Streak artifacts
caused by motion
special algorithms required
Position at start of rotation
Position at start of rotation
Position of interest

8. Calculating Fan Beams at Odd Locations using Interpolation
No complete data set for any single z location
Use 2 closest beams in correct orientation
Calculate beam attenuation by interpolating between adjacent beams
“z” direction

9. Spiral Reconstruction Algorithms
Uses interpolation for
input projection data
output slice attenuation data
Slice can be calculated at any position from raw projection data
= real data point
coordinate of interest
Interpolated data

10. Disadvantage of Interpolation
Trust me, interpolation is a guess
Can increase effective slice thickness
Calculation averages data measured at many z values
“z” direction

11. Data Acquisition Challenges
No single slice defined by acquisition geometry
slice localization more difficult
Different slice volume geometry
conventional: cylinder
spiral: wafer with radial crack
Slight increase in effective slice thickness
slice thickness influenced by
fan beam thickness
speed of table motion

12. Table Moves During Helical Scanning
table increment during one rotation Slice Pitch = slice thickness
Slice
thickness
Table Increment

13. Table Moves During Helical Scanning
Slice thickness determined by collimation
Table motion per revolution determined by table speed
table motion during one rotation Slice Pitch = slice thickness
Slice
thickness
Table Increment

14. Single-Slice Detectors
Many detectors rotate around patient
Single row in z-direction
Slice thickness determined by collimation
Slice Thickness
Z-Axis

15. Beam positions when tube directly above patient
Pitch = 1
Pitch = 1 means patient moves exactly one slice thickness per revolution of tube
table motion during one tube rotation Slice Pitch = slice thickness
Beam positions when tube directly above patient

16. Beam positions when tube directly above patient
Pitch <1
Pitch < 1 means patient moves less than a slice thickness during one tube rotation
Can improve visualization of objects
table motion during one tube rotation Slice Pitch = slice thickness
Beam positions when tube directly above patient

17. Beam positions when tube directly above patient
Pitch >1
Pitch > 1 means patient moves further than slice thickness during one tube rotation
table motion during one tube rotation Slice Pitch = slice thickness
Beam positions when tube directly above patient

18. Beam position when tube directly below patient
Pitch >1
No gap in image coverage when viewing study
table motion during one tube rotation Slice Pitch = slice thickness
No Gap
Beam position when tube directly below patient

19. Spiral vs. Conventional CT & Patient Dose
Dose is strongly dependent on pitch
Please explain. Inquiring minds wanna know

20. Pitch = 1
equivalent dose to non-spiral

21. Pitch >1 lower dose for spiral Table moves faster
Table increment per tube rotation > one slice thickness

22. Pitch <1 higher dose for spiral Table moves slower
table increment per tube rotation < one slice thickness

23. Pitch & Dose Dose inversely proportional to pitch
Pitch = 0.5 => Dose doubles
Pitch = 2 => Dose cut in half