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Published byHilary Osborne Heath Modified over 9 years ago
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Seeram Chapter 13: Single Slice Spiral - Helical CT
Oh no, not more physics…
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Spiral CT Incentives for development New technology required
Shorter study times Improved 3D imaging New technology required Slip ring Allows continuous gantry rotation
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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
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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
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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
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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
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Faked Image Respiration variations from group to group can cause
Anatomy omissions Slice-to-slice misregistration Inaccurate 3D images Step-line contours
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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
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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
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Helical Reconstruction Complication
Patient moves as gantry rotates No two fan beams at same z coordinate “z” direction
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As Gantry Rotates, Fan Angles Repeat
Distance between repetitions is movement of table during one rotation “z” direction
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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
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Reconstruction Performed for Single Location
Fan beam only at one orientation at slice location But other orientations needed for reconstruction “z” direction
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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
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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
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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)
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Disadvantage of Interpolation
Can increase effective slice thickness Calculation averages data measured at many z values “z” direction
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Redundant Data All rays sampled twice in 360o of rotation
Duplicate data called “Complimentary”
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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
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Redundant Data * Can reduce slice thickness averaging substantially by using only 180o worth of data 180o rotation 360o rotation
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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
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Spiral CT Challenges Requires special interpolation reconstruction
More computing-intensive
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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
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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
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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
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Spiral CT Advantages No gaps in data acquisition
slice can be reconstructed for any axial position Patient motion artifacts reduced
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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
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Table Moves During Helical Scanning
table increment during one rotation Slice Pitch = slice thickness Slice thickness Table Increment
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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
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Single-Slice Detectors
Many detectors rotate around patient Single row in z-direction Slice thickness determined by collimation Slice Thickness Z-Axis
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Single Slice CT: Changing Slice Thickness
Thin Slice Thick Slice Z-Axis Z-Axis
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Pitch = 1 Pitch = 1 means slices abut one another
table motion during one rotation Slice Pitch = slice thickness
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Pitch >1 Pitch > 1 means gap in slices
table motion during one rotation Slice Pitch = slice thickness
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Pitch <1 Pitch < 1 means overlap in slices
Can improve visualization of objects table motion during one rotation Slice Pitch = slice thickness
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Spiral vs. Conventional CT & Patient Dose
Dose is strongly dependent on pitch Please explain. Inquiring minds wanna know
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Pitch = 1 equivalent dose to non-spiral
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Pitch >1 lower dose for spiral if table increment per rotation > one slice thickness
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Pitch <1 higher dose for spiral if table increment per rotation < one slice thickness
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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)
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Developments Multi-slice CT Real-time CT fluoro Better 3D imaging
CT Angiography CT Endoscopy
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