Seeram Chapter 13: Single Slice Spiral - Helical CT

1. CT Seeram Chapter 13: Single Slice Spiral - Helical CT Oh no, not more physics…

2. Spiral CT • Incentives for development • 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 • 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 = real data point • Uses interpolation for • input projection data • output slice attenuation data • Slice can be calculated at any position from raw projection data coordinate of interest Interpolated data

16. Interpolation When x = 50, y = 311 When x = 80, y = 500 What will be the value of y when x=58? • Estimates value of function using known values on either side 500 (x2,y2) ? (x1,y1) (x,y) 311 50 80 y = x1+ (y2 – y1)* (x – x1) / (x2 – x1) 58 y = 311 + (500-311)* (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. 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 rotation360o rotation

21. 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

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 TableIncrement

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 TableIncrement

30. Single-Slice Detectors • Many detectors rotate around patient • Single row in z-direction • Slice thickness determined by collimation SliceThickness 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