1. Seeram Chapter 13: 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. Conventional Tube Rotation
Cables only allow ~ 360o rotation
Sequential scanning steps
Gantry must accelerate from full stop to constant operating speed required for data acquisition
Data acquired during constant speed rotation
Gantry decelerated from constant operating speed to full stop
Table & Patient indexed to next scanning position
Interscan Delay
cycle time above which is not constant scanning

5. Non-spiral Intergroup Delay
Scans grouped for single breath hold
Inter-scan delay causes long study
Because of delay, studies may require >1 group
Reduced scanner throughput

6. Limitations of Conventional (non-spiral) Scanning
Long exam times
Inter-scan delays
Table motion
Inter-group delays
Breathing
Limitations for angiography
Few scans made during maximum contrast enhancement

7. Faked Image Respiration variations from group to group can cause
Anatomy omissions
Slice-to-slice misregistration
Inaccurate 3D images
Step-line contours

8. Volume Scanning Also called Data collected continuously
Spiral Volume CT (SVCT)
Spiral-helical scanning
Data collected continuously
Table moves continuously
Tube traces spiral path with respect to patient

9. Requirements for Volume data Acquisition
Continuous tube rotation
requires slip ring technology
Provides electricity to rotating components
Continuous couch movement
Increase in tube heat capacity & cooling rate requirements
No inter-scan tube cooling

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

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

12. Data Acquisition Challenges
Projection data not confined to single slice
Streak artifacts
appear with “standard” or “conventional” (non-spiral) reconstruction
caused by motion
special algorithms required
Position at start of rotation
Position at start of rotation
Position of interest

13. Reconstruction Performed for Single Location
Fan beam only at one orientation at slice location
But other orientations needed for reconstruction
“z” direction

14. Calculating Fan Beams at Odd Locations using Interpolation
Use 2 beams in correct direction closest to slice location
Calculate beam attenuation by interpolating between adjacent beams
“z” direction

15. 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

16. y = x1+ (y2 – y1)* (x – x1) / (x2 – x1)
Interpolation
Estimates value of function using known values on either side
When x = 50, y = 311
When x = 80, y = 500
What will be the value of y when x=58?
500
(x2,y2) ?
(x1,y1)
(x,y)
311 50 80
y = x1+ (y2 – y1)* (x – x1) / (x2 – x1) 58
y = ( )* (58-50) / (80-50)

17. Disadvantage of Interpolation
Can increase effective slice thickness
Calculation averages data measured at many z values
“z” direction

18. Redundant Data All rays sampled twice in 360o of rotation
Duplicate data called “Complimentary”

19. Distance moved during single 360o rotation
Redundant Data *
All rays actually measured in 180o of rotation
360o compared to 180o covers 2X thickness (“z”)
Distance moved during single 360o rotation

20. Redundant Data *
Can reduce slice thickness averaging substantially by using only 180o worth of data
180o rotation 360o rotation

21. 180o Reconstruction for Spiral Scanning
Substantially reduces effective slice thickness
Better z-axis resolution
Increases image noise
Image based on only 180o instead of 360o of data
Redundant data reduces noise

22. Spiral CT Challenges Requires special interpolation reconstruction
More computing-intensive

23. Data Acquisition Challenges
No single defined slice
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

24. Requirements for Volume data Acquisition
New reconstruction algorithms required for spiral weighting
Larger detector data memory requirements
larger buffer required if data acquired faster than can be sent to computer

25. Spiral CT Advantages Shorter acquisition times no inter-scan delays
shorter study times
entire organs / volumes scanned together
Better throughput
BUT: Larger demands on tube
Much less cooling time

26. Spiral CT Advantages No gaps in data acquisition
slice can be reconstructed for any axial position
Patient motion artifacts reduced

27. Spiral CT = Faster Scanning: Advantages
Less potential for motion
Less effect of varying respiration
spiral scan done in single breath hold
Less effect of shifting anatomy between slices
Improved contrast protocols possible
faster scanning; less dilution
more uniform contrast concentration
Greater accuracy for multiplanar & 3D images

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

29. Table Moves During Helical Scanning
Slice thickness determined by collimation
Table motion per revolution determined by table speed
Coverage = table increment X # rotations
table motion during one rotation Slice Pitch = slice thickness
Slice
thickness
Table Increment

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

31. Single Slice CT: Changing Slice Thickness
Thin Slice
Thick Slice
Z-Axis
Z-Axis

32. Pitch = 1 Pitch = 1 means slices abut one another
table motion during one rotation Slice Pitch = slice thickness

33. Pitch >1 Pitch > 1 means gap in slices
table motion during one rotation Slice Pitch = slice thickness

34. Pitch <1 Pitch < 1 means overlap in slices
Can improve visualization of objects
table motion during one rotation Slice Pitch = slice thickness

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

36. Pitch = 1
equivalent dose to non-spiral

37. Pitch >1
lower dose for spiral if table increment per rotation > one slice thickness

38. Pitch <1
higher dose for spiral if table increment per rotation < one slice thickness

39. Spiral vs. Conventional CT & Other Observations
Non-spiral phantoms may not be sufficient to test spiral performance
Performance characteristics compared
Spatial resolution
Image uniformity
Contrast
Noise
Slice sensitivity
Dose
artifacts
Study showed subtle decrease in abdominal axial resolution (not clinically significant)

40. Developments Multi-slice CT Real-time CT fluoro Better 3D imaging
CT Angiography
CT Endoscopy