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

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Presentation on theme: "CT Seeram Chapter 13: Single Slice Spiral - Helical CT Oh no, not more physics…"— Presentation transcript:

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

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

4 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

5 Conventional Tube Rotation Cables only allow ~ 360 o rotation Sequential scanning steps 1. Gantry must accelerate from full stop to constant operating speed required for data acquisition 2. Data acquired during constant speed rotation 3. Gantry decelerated from constant operating speed to full stop 4. Table & Patient indexed to next scanning position Interscan Delay Interscan Delay cycle time above which is not constant scanning

6 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

7 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

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

9 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

10 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

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

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

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 Position at start of rotation Position of interest

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

15 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

16 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

17 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? ? y = ( )* (58-50) / (80-50) (x 1,y 1 ) (x 2,y 2 ) (x,y) y = x 1 + (y 2 – y 1 )* (x – x 1 ) / (x 2 – x 1 )

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

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

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

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

22 180 o Reconstruction for Spiral Scanning Substantially reduces effective slice thickness Better z-axis resolution Increases image noise Image based on only 180 o instead of 360 o of data Redundant data reduces noise

23 Spiral CT Challenges Requires special interpolation reconstruction More computing-intensive

24 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

25 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

26 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

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

28 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

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

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

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

32 Single Slice CT: Changing Slice Thickness Z-Axis Thin Slice Z-Axis Thick Slice

33 Pitch = 1 table motion during one rotation Slice Pitch = slice thickness  Pitch = 1 means slices abut one another

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

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

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

37 Pitch = 1 equivalent dose to non-spiral

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

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

40 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)

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


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