Seeram Chapter 13: Single Slice Spiral - Helical CT

1. CT Seeram Chapter 13: Single Slice Spiral - Helical CT

2. Spiral CT Incentives for development Need for shorter study times Improved 3D imaging capabilities New technology requirement 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 (stop/start) Inter-group delays 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 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. Spiral CT Challenges Requires special interpolation reconstruction More computing-intensive

11. 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

12. Data Acquisition Challenges No single defined slice slice localization more difficult Different slice volume geometry conventional: cylinder spiral: wafer with radial crack see figure 6-5 page 114 Slight increase in effective slice thickness slice thickness influenced by fan beam thickness speed of table motion

13. 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

14. Helical Reconstruction Complication Patient moves as gantry rotates No two fan beams at same z coordinate

15. As Gantry Rotates,Fan Angles Repeat Distance between repetitions is movement of table during one rotation

16. Reconstruction Performed for Single Location Fan beam only at one orientation at slice location But other orientations needed for reconstruction

17. 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

18. Spiral Reconstruction Algorithms Uses interpolation for input projection data output slice attenuation data Slice can be calculated at any position from raw projection data

19. Interpolation Estimates value of function using known values on either side

20. Disadvantage of Interpolation Can increase effective slice thickness Calculation averages data measured at many z values

21. Redundant Data All rays sampled twice in 360o of rotation Duplicate data called �Complimentary�

22. Redundant Data All rays actually measured in 180o of rotation 360o compared to 180o covers 2X thickness (�z�)

23. Redundant Data Can reduce slice thickness substantially by using only 180o worth of data

24. 180o Reconstructionfor 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

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 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

29. Table Moves During Helical Scanning

30. Pitch = 1 table motion during one rotation Slice Pitch = --------------------------------------- slice thickness

31. Pitch >1 table motion during one rotation Slice Pitch = --------------------------------------- slice thickness

32. Pitch <1 table motion during one rotation Slice Pitch = --------------------------------------- slice thickness

33. Spiral vs. Conventional CT & Patient Dose Dose is strongly dependent on pitch

34. Pitch = 1 equivalent dose to non-spiral

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

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

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

38. Developments Real-time CT fluoro Better 3D imaging CT Angiography CT Endoscopy