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CT Seeram Chapter 13: Single Slice Spiral Helical CT Oh no, not more physics… Spiral CT Incentives for development Shorter study times Improved 3D imaging New technology required Slip ring Allows continuous gantry rotation 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 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 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 1. Interscan Delay cycle time above which is not constant scanning 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 Limitations of Conventional (nonspiral) Scanning Long exam times Inter-scan delays Table motion Inter-group delays Breathing Limitations for angiography Few scans made during maximum contrast enhancement Faked Image Respiration variations from group to group can cause Anatomy omissions Slice-to-slice misregistration Inaccurate 3D images Step-line contours 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 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 Helical Reconstruction Complication Patient moves as gantry rotates No two fan beams at same z coordinate “z” direction As Gantry Rotates, Fan Angles Repeat Distance between repetitions is movement of table during one rotation “z” direction 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 Reconstruction Performed for Single Location Fan beam only at one orientation at slice location But other orientations needed for reconstruction “z” direction 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 Spiral Reconstruction Algorithms Uses interpolation for input projection data output slice attenuation data Interpolated data Slice can be calculated at any position from raw projection data = real data point coordinate of interest Interpolation Estimates value of function using known values on either side When x = 50, y = 311 When x = 80, y = 500 500 What will be the value of y when x=58? ? 311 y = x1+ (y2 – y1)* (x – x1) / (x2 – x1) y = 311 + (500-311)* (58-50) / (80-50) (x1,y1) (x2,y2) (x,y) 50 80 58 Disadvantage of Interpolation Can increase effective slice thickness Calculation averages data measured at many z values “z” direction Redundant Data All rays sampled twice in 360o of rotation Duplicate data called “Complimentary” * Redundant Data All rays actually measured in 180o of rotation 360o compared to 180o covers 2X thickness (“z”) Distance moved during single 360o rotation * Redundant Data Can reduce slice thickness averaging substantially by using only 180o worth of data 180o rotation 360o rotation 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 Spiral CT Challenges Requires special interpolation reconstruction More computing-intensive 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 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 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 Spiral CT Advantages No gaps in data acquisition slice can be reconstructed for any axial position Patient motion artifacts reduced 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 Table Moves During Helical Scanning table increment during one rotation Slice Pitch = --------------------------------------slice thickness Slice thickness Table Increment 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 Single-Slice Detectors Many detectors rotate around patient Single row in z-direction Slice thickness determined by collimation Z-Axis Single Slice CT: Changing Slice Thickness Thin Slice Thick Slice Z-Axis Z-Axis Pitch = 1 Pitch = 1 means slices abut one another table motion during one rotation Slice Pitch = --------------------------------------slice thickness Pitch >1 Pitch > 1 means gap in slices table motion during one rotation Slice Pitch = --------------------------------------slice thickness Pitch <1 Pitch < 1 means overlap in slices Can improve visualization of objects table motion during one rotation Slice Pitch = --------------------------------------slice thickness Spiral vs. Conventional CT & Patient Dose Dose is strongly dependent on pitch Please explain. Inquiring minds wanna know Pitch = 1 equivalent dose to non-spiral Pitch >1 lower dose for spiral if table increment per rotation > one slice thickness Pitch <1 higher dose for spiral if table increment per rotation < one slice thickness 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) Developments Multi-slice CT Real-time CT fluoro Better 3D imaging CT Angiography CT Endoscopy
