Post on 27-Aug-2018
1Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Effect of Scan Parameters in CardiacImaging with MDCT
Mahadevappa Mahesh, M.S., Ph.D.
The Russell H. Morgan Department of Radiologyand Radiological Science
Johns Hopkins University, Baltimore, MD
47th Annual Meeting, Seattle, WA (Yr 2005)
Outline
• Introduction
• Fundamentals of Cardiac CT Imaging
• Temporal and Spatial Resolution
• Pitch, Geometric Efficiency,…
• Effect of scan parameters on image quality
• Conclusions
Key Issues in Cardiac Imaging with CT
• Fast imaging - High temporal resolution tofreeze cardiac motion and avoid artifacts
• Fine imaging - High spatial resolution toresolve small lesions in any plane
• Radiation dose - the consequences of CTimaging
Essentials for Cardiac Imaging
• High temporal resolution is key for imagingcoronary arteries located close to heart musclesthat show strong movement during cardiac cycle
• Rapid movement is present during systole phase
• Imaging should be performed during diastole phase
• Image acquisition and reconstruction are to besynchronized accurately with heart movement
Diastolic Phase versus Heart Rate• Least cardiac motion is
observed during diastolic phase• Diastole phase narrows with
increasing heart rate• Desired temporal resolution for
motion free cardiac imaging− ~ 250 ms for heart rates ~ 70 bpm− ~ 150 ms for heart rates ~ 100 bpm
• Motion-free imaging duringother phases requires temporalresolution ~50 ms
40 60 80 100 120
600
400
200
0
Heart rate (bpm)
ms Exposuretime
Diastole
500 ms
250 ms
100 ms
Essentials for Cardiac Imaging
• Most proximal coronary segments(RCA, LAD) require high sub-millimeter isotropic spatial resolution
• Sufficient contrast-to-noise ratio isrequired to resolve small and low-contrast structures such as plaques
• Low-contrast resolution with limitedradiation exposure at shortestexposure time is key
LADRCA
2Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
High Quality Coronary CTA images
Axial Coronal Sagittal
• Cardiac imaging is a high demandingapplication of CT
• Temporal, spatial and contrastresolution are to be optimized andalso radiation exposure are to be limited
Cardiac Images from 16 section MDCT*
*Mahesh M, Clini Cardio Vasc Img Textbook, pp 1-77, 2004
Temporal Resolution
Approaches for stopping heart motion
• Acquire all data fast enough to stopcardiac motion
• Achieved in MDCT by− Prospective ECG triggering
− Retrospective ECG gating
Prospective ECG Triggering
Conventional Axial “ Partial Scan ” (Step and Shoot)
ECG
moving couch-top
PresetDelay
Temporal resolution 200 – 250 msecRadiation dose minimized Limited data set
X-ray ON X-ray ONPresetDelay
3Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Retrospective ECG Gating
ECG
Continuous recording of spiral scan and ECG
Time / Pos.moving couch-top
Temporal Resolution 200 - 250 msecRadiation dose higher thanprospective triggering
Partial vs Segmented Reconstruction
TR: <100 msec
Time/position
Segmented reconstruction
Continuous spiral scan
TR: 200 msecRetrospective reconstruction (partial scan:
180o + fan angle)
Factors affecting Temporal Resolution
• Gantry rotation speed• Image Reconstruction
− Prospective triggering− Retrospective gated
• Partial or segmental
• Pitch• Post-processing algorithms
Retrospective Reconstruction
• Segmented reconstruction− Different segments of projection
data from same phase of cardiaccycle at successive heartbeats used
• Partial scan reconstruction− continuous segment of projection
data at single heartbeat
Partial Scan Reconstruction
• Data from prescribed time range during onecardiac cycle is selected for reconstruction
• 200 to 270 ms temporal resolution isachieved for 0.4 s gantry rotation
• Works best with very low pitch (p<0.25)
Multisegment Reconstruction• Each heart cycle provides segment of data
required for partial scan reconstruction• For ‘M’ segments (‘M’ heart cycles), maximum
temporal resolution is− ‘TR/2M’ where TR is gantry rotation time− 100 ms with 2 segments and 50 ms with 4 segments
for TR of 0.4 sec
• For M segment, temporal resolution varybetween TR/2 to TR/2M
4Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Temporal Resolution:Partial vs Segmented ECG gated reconstruction
Courtesy Toshiba
Half-scan reconstructionTemporal resolution: 250 msec
Segmented reconstructionTemporal resolution: ~105 msec
Temporal Resolution
• Depends on gantry rotation time- 0.5 - 0.37 second with 64 section MDCT scanners
• Up to 80 - 250 ms achieved through partial scansor sub-segment data reconstruction
• Improves with sub-segment data reconstruction,but spatial resolution decreases and motionartifacts increases
Spatial Resolution
Coronary Angiography vs CT Angiography
Hoffmann et.al., AJR: 182, March 2004
Right coronaryartery showing
calcification
Coronary Angiogram CT angiography
Volume rendered CT angiogram of rightcoronary artery acquired at 16x0.75 mm and
0.42 sec rotation time
Factors affecting Spatial Resolution
• MDCT detector array designs• Section thickness/Section collimation/
Effective section thickness• Pitch• Reconstruction Increment• Reconstruction algorithms• Patient motion … Single row detector CT
(SDCT)Multiple row detector CT
(MDCT)
X-ray Tube
Tube Collimator
Collimated Slice
DetectorCollimator
1-Row Detector
8-Row Detector
SDCT versus MDCT
*Mahesh M, RadioGraphics, 22: 949-962, 2002
5Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Detector Element Arrays* in 4-section MDCT scanners
Uniform
Non-uniform
Hybrid
20 mm
16 x 1.25 mm
32 mm15 mm 15 mm
4 x 0.5
15 2.5 1.5 52.51.51
20 mm
Z-axis*Mahesh M, RadioGraphics, 22: 949-962, 2002
How are detector elements used in MDCT?
4-section scanners collect4 simultaneous channels of data
Switching Array
Detectors
20 mm
4 x 1.25 mm
Detector elements in 16 section MDCT scanners*
24 mm
16 x 0.75 mm4 x 1.5 mm 4 x 1.5 mm
16 x 0.625 mm
4 x 1.25 mm
20 mm
4 x 1.25 mm
32 mm
16 x 0.5 mm12 x 1 mm12 x 1 mm
GE - Lightspeed 16
Siemens - Sensation 16Philips - Mx8000 IDT
Toshiba - Aquilion 16
*Mahesh M, Clini Cardio Vasc Img Textbook, pp 1-77, 2004
Detector elements in >32 section MDCT scanners
40 mm
64 x 0.625 mm
28.8 mm
32 x 0.6 mm4 x 1.2 mm 4 x 1.2 mm
40 mm
40 x 0.6 mm6 x 1.25 mm 6 x 1.25 mm
32 mm
64 x 0.5 mm
GE - Lightspeed 64
Siemens - Sensation 64
Toshiba - Aquilion 64
Philips - Brilliance 40
Pitch: Definition, Confusion…
Pitch redefined for MDCT
I - Table feed (mm/rotation)W - Beam width (mm)
I
T
W
Beam Pitch =I
W
Detector Pitch =IT
T - Single DAS channel width (mm)N - Number of active DAS channels
Beam Pitch =Detector Pitch
N=
IN*T
= Pitch†
† IEC Part 2-44, 2003
6Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Dose in MDCT varies as:
• Pitch >1 implies extendedimaging and reduced patientdose with lower axialresolution
• Pitch <1 implies overlappingand higher patient dose withhigher axial resolution
Dose 1Pitch†
(mAs/rotation)∝
Why Cardiac CT protocols use Low Pitch†?
Time
Z-po
sitio
nZ-
posi
tion Helical
scandirection
Slope: Table feed speed
• Higher pitch produces gaps
• High quality 3D withminimal artifacts requiresdata overlap
• Hence pitch is low, andradiation dose is high
• Typical pitch: 0.20 - 0.4
Data gapswith higher pitch
Single Segment Reconstruction
• Retrospective ECG-gating with single segment(partial scan) reconstruction requires limiting thepitch dependent on heart rate
− For ex: For heart rates 45-100 bpm, with TR 0.5 s,TQ 250-360 ms, P = 0.375 to 0.875
N - Number of active DAS channelsTR - Gantry Rotation Time (ms)TRR - Time for one heart beat (ms)TQ - Partial scan rotation time (ms)
Multiple Segment Reconstruction
• Pitch is further restricted by the number ofsegments used in reconstruction
− For ex: For heart rate 60 bpm, with TR 0.4 s, N = 16 andM = 2, P = 0.21
− For ex: For heart rate 60 bpm, with TR 0.4 s, N = 16 andM = 3, P = 0.15
N - Number of active DAS channelsTR - Gantry Rotation Time (ms)TRR - Time for one heart beat (ms)M - Number of subsequent heart cycles
Effect of Pitch on Dose and Image Quality
P = 0.83CTDI = 37 mGy
P = 1.48CTDI = 20.6 mGy
45% lower
P = 0.64CTDI = 47.8 mGy
30% higher
Section Collimation (SC) vs Section Width (SW)
• Section collimation is total beamcollimation divided by number of activedetector channels
• Section width is the true thickness ofreconstructed image, measured asFWHM of slice sensitivity profile
• SW has to be larger than or equal to SC
• Affected by collimation, pitch,reconstruction algorithm, z-filter …
Slice Sensitivity Profiles:conventional and spiral
acquisitionSW ≈ 1.3 (±0.2) SC
7Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Reconstruction Interval (RI)• Defines degree of overlap between axial scans• Independent of section collimation or section width• Overlapping results in large number of images but
ensures optimum lesion display and improves MPRand 3D images without increasing patient dose
• Large RI yields fewer images but provides sub-optimum lesion detection
Reconstruction Interval
• For routine applications including small structuresdetection a 30% section overlap is sufficient
• For MPR and 3Ds, at least 50% overlap is desirable• Theoretic optimum is smaller than half the section
width with minimal added value in clinical practice• Limited by reconstruction time, number of images
to interpret and storage space
Effect of Reconstruction Interval
SW 0.5 mm, RI 0.3 mm301 images
SW 0.5 mm, RI 5.0 mm19 images
Effect of Reconstruction Interval
SW 0.5 mm, RI 0.3 mm301 images
SW 0.5 mm, RI 0.5 mm184 images
Effect of Reconstruction Algorthims:Spatial Resolution and Image Noise Trade-offs
Spatial Resolution
Smooth Medium SharpReconstruction Filters
©Johns Hopkins 32 slice MDCT
Smoothing filters results in minor reductions in spatialresolution but require less dose for constant image noise
Effect of Slice Thickness on Image Noise
0.5 mm5.0 mm10 mm
Scan at thin collimation (raw data) for high spatialresolution, but reconstruct thick sections withoverlaps to improve image noise
8Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
High Contrast Spatial Resolution in Z direction
• Phantom- 0.5 mm spacing
• Scan modes- 32 x 0.5 mm
• Technique- 120 kVp, 50 mAs
0.6 mm 0.5 mm 0.4 mm 0.3 mm
©Johns Hopkins
Spatial Resolution
• Axial or In-plane Resolution− MDCT: 10 - 20 lp/cm (resolvable ~ 0.5- 0.25 mm)
• Z-axis Resolution - influenced by sectioncollimation− MDCT: 7- 15 lp/cm (resolvable ~ 0.7 - 0.3 mm)
Geometric Efficiency
CT Dose Index (CTDI) comparison:4 vs 16* vs 64 MDCT¶ scanner
Sensation 64: Increase in CTDIw of nearly 4-19% compared toSensation 16 as measured for head and body phantoms
* CTDIw(weighted average) normalized to 16x0.75 mm scan mode ¶Siemens
Geometric EfficiencyFocal spot Beam
collimator
Penumbra
SDCT
MDCT
• Amount of radiation excluded (penumbra)relative to the radiation collected by thedetectors in forming an image
• Penumbra caused by finite focal spot size− contributes to image and patient dose in SDCT
− contributes to patient dose but not to image in MDCT (over-beaming to ensure equal image quality)
Penumbra
Geometric Efficiency
• Decreases with thinner sections and fewerdetector elements
• Improves with thicker sections or with moreactive thin sectionsPenumbra
Strong effect withthin collimation
4 x 1.25 mm 4 x 2.5 mm
Effect decreases withthicker collimation
Penumbra
Effect diminishes furtherwith more detectors
16 x 1.25 mm
Penumbra
9Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Geometric Efficiency in MDCT
Dose in 4 slice scannersgrows markedly with thincollimation but less sofor 16 and 16+ scanners
Dose grows markedly withthin collimation and fewactive detector elements butless so for thicker collimationand more detectors
©Johns Hopkins
Advantage of Thin Sections
• In general, thin sections yields higher z-axisresolution, improves partial volume, but requireshigher tube current to reduce image noise, whichleads to higher doses (especially with few detectors)
©Johns Hopkins MDCT scanners
Effect of kV and mAs on Image Noise
Effect of X-ray Beam Energy (kVp)
135 kV with 8.0 SD37 mGy CTDI
120 kV with 10.0 SD29 mGy CTDI
~22% dose reduction
Effect of kV and mAs on Image Noise
Artifacts
10Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Pulsation Artifacts
• Common artifacts dueto cardiac pulsation
• Multiple segmentreconstruction isdesirable
Radiographics 2005
Banding Artifacts
Banding artifacts
Artifacts due to increased heart ratestarting at 51 bpm increased to 69 bpm
Radiographics 2005
Artifacts due to incomplete breath holding
Radiographics 2005
Axial images show no artifacts
Sagittal
Coronal
Streak Artifacts
Streak artifacts due to earlier stent placementvisible on thin MIP and MPR images
Metallis structure butno artifact visible on
axial images
CT Dose Modulation
Impact of Dose Modulation: Chest CTMDCT 64*
Radiation dose:
Lateral: 16% increase, AP: 25% reduction*Mahesh, Kamel & Fishman, Evaluation of ‘CareDose’ on Siemens Sensation 64 MDCT scanner
11Mahadevappa Mahesh, MS, Ph.D. Johns Hopkins University, Baltimore, MD
Impact of Dose Modulation: Abdominal CT MDCT 64*
Radiation dose:
Lateral: 28% increase, AP: 48% reduction*Mahesh, Kamel & Fishman, Evaluation of ‘CareDose’ on Siemens Sensation 64 MDCT scanner
Dose Modulation for varying Image Noise Index
Low SD - High dose184 mA
Medium SD & dose69 mA
High SD - Low dose48 mA
Toshiba
Conclusions
• Cardiac imaging is highly demanding application ofMDCT and is possible due to technological advances
• Understanding trade-offs between various scanparameters that affects image quality is key
• Cardiac CT has the potential to becoming reliabletool for noninvasive diagnosis and prevention ofcardiac and coronary artery disease
Future with CT