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