ct seeram chapter 13: single slice spiral - helical ct oh no, not more physics…
TRANSCRIPT
CT
Seeram Chapter 13:
Single Slice Spiral - Helical CT
Oh no, not more
physics…
Spiral CT
Incentives for developmentShorter study timesImproved 3D imaging
New technology requiredSlip ringAllows continuous gantry rotation
Conventional (Non-spiral) CTTube rotates once around patient
Table stationarydata for one slice collected
Table increments one slice thicknessRepeat
Tube rotates opposite direction
Conventional Tube Rotation Cables only allow ~ 360o 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 rotation3. Gantry decelerated from constant operating
speed to full stop4. Table & Patient indexed to next scanning
position Interscan DelayInterscan Delay
cycle time above which is not constant scanning
Non-spiral Intergroup DelayScans grouped for single breath holdInter-scan delay causes long studyBecause of delay, studies may require >1
groupReduced scanner throughput
Limitations of Conventional (non-spiral) Scanning
Long exam timesInter-scan delays
Table motionInter-group delays
Breathing
Limitations for angiographyFew scans made during
maximum contrast enhancement
Faked Image
Respiration variations from group to group can causeAnatomy omissions
Slice-to-slice misregistration
Inaccurate 3D images Step-line contours
Volume Scanning
Also calledSpiral Volume CT (SVCT)Spiral-helical scanning
Data collected continuouslyTable moves continuouslyTube traces spiral path with respect
to patient
Requirements for Volume data AcquisitionContinuous tube rotation
requires slip ring technologyProvides electricity to rotating components
Continuous couch movementIncrease in tube heat capacity & cooling
rate requirementsNo inter-scan tube cooling
Helical Reconstruction Complication
Patient moves as gantry rotatesNo 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 ChallengesProjection data not confined to single sliceStreak artifacts
appear with “standard” or “conventional” (non-spiral) reconstruction
caused by motionspecial algorithms required
Position at start of rotation
Position at start of rotation
Position of interest
Reconstruction Performed for Single LocationFan beam only at one orientation at slice
locationBut other orientations needed for reconstruction
“z” direction
Calculating Fan Beams at Odd Locations using Interpolation
Use 2 beams in correct direction closest to slice locationCalculate beam attenuation by interpolating between adjacent
beams
“z” direction
Spiral Reconstruction AlgorithmsUses interpolation for
input projection dataoutput slice attenuation
data
Slice can be calculated at any position from raw projection data
= real data point
coordinate of interestInterpolated
data
InterpolationEstimates value of function using known
values on either side
When x = 50, y = 311When x = 80, y = 500
What will be the value of y when x=58?
?
y = 311 + (500-311)* (58-50) / (80-50)
(x1,y1)(x2,y2)
(x,y)
y = x1+ (y2 – y1)* (x – x1) / (x2 – x1)58
8050
500
311
Disadvantage of Interpolation
Can increase effective slice thicknessCalculation averages data measured at many z values
“z” direction
Redundant DataAll rays sampled twice in 360o of rotationDuplicate data called “Complimentary”“Complimentary”
Redundant DataAll rays actually measured in 180o of rotation360o compared to 180o covers 2X thickness (“z”)
Distance moved during single 360o rotation
*
Redundant DataCan reduce slice thickness averaging
substantially by using only 180o worth of data
180o rotation360o rotation
*
180o Reconstructionfor Spiral Scanning
Substantially reduces effective slice thicknessBetter z-axis resolution
Increases image noiseImage based on only 180o instead of 360o of
dataRedundant data reduces noise
Spiral CT Challenges
Requires special interpolation reconstructionMore computing-intensive
Data Acquisition ChallengesNo single defined slice
slice localization more difficultDifferent slice volume geometry
conventional: cylinderspiral: wafer with radial crack
Slight increase in effective slice thicknessslice 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 requirementslarger buffer required if data
acquired faster than can be sent to computer
Spiral CT AdvantagesShorter acquisition times
no inter-scan delaysshorter study timesentire organs / volumes
scanned togetherBetter throughputBUT: Larger demands on tube
Much less cooling time
Spiral CT AdvantagesNo gaps in data
acquisitionslice can be
reconstructed for any axial position
Patient motion artifacts reduced
Spiral CT = Faster Scanning: Advantages
Less potential for motionLess effect of varying respiration
spiral scan done in single breath holdLess effect of shifting anatomy between
slicesImproved contrast protocols possible
faster scanning; less dilutionmore uniform contrast concentration
Greater accuracy for multiplanar & 3D images
Table Moves During Helical Scanning table increment during one rotation
Slice Pitch = ---------------------------------------
slice thickness
Slicethickness
TableIncrement
Table Moves During Helical Scanning
Slicethickness
TableIncrement
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
Single-Slice DetectorsMany detectors rotate
around patientSingle row in z-directionSlice thickness
determined by collimation
Z-Axis
Single Slice CT: Changing Slice Thickness
Z-Axis
Thin Slice
Z-Axis
Thick Slice
Pitch = 1
table motion during one rotation Slice Pitch = --------------------------------------- slice thickness
Pitch = 1 means slices abut one another
Pitch >1
table motion during one rotation Slice Pitch = --------------------------------------- slice thickness
Pitch > 1 means gap in slices
Pitch <1
table motion during one rotation Slice Pitch = --------------------------------------- slice thickness
Pitch < 1 means overlap in slices Can improve visualization of objects
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 ObservationsNon-spiral phantoms may not be sufficient to
test spiral performance
Performance characteristics comparedSpatial resolutionImage uniformityContrastNoiseSlice sensitivityDoseartifacts
Study showed subtle decrease in abdominal axial resolution (not clinically significant)
Developments
Multi-slice CTReal-time CT fluoroBetter 3D imagingCT AngiographyCT Endoscopy