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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


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


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


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


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


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


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


Effect of Reconstruction Interval



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


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


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


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


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

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