mdct technology - aapm: the american association of ... · kanal 20 regarding multiple detector ct...

MDCT TechnologyMDCT Technology

Kalpana M. Kanal, Ph.D., DABRKalpana M. Kanal, Ph.D., DABRAssistant ProfessorAssistant Professor

Department of RadiologyDepartment of RadiologyUniversity of WashingtonUniversity of Washington

Seattle, WashingtonSeattle, Washington

ACMP Annual Meeting 2008 ACMP Annual Meeting 2008 -- Seattle, WASeattle, WA

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Educational ObjectivesEducational Objectives

�� Historical Development of CTHistorical Development of CT�� Why has the frequency of CT increased?Why has the frequency of CT increased?�� MultiMulti--Row Detector CT and PitchRow Detector CT and Pitch�� Advantages of MultiAdvantages of Multi--Row Detector CTRow Detector CT�� Automatic Exposure Control in CTAutomatic Exposure Control in CT�� Cardiac CT Cardiac CT �� Future of CTFuture of CT

KanalKanal 33Dr. Mahesh, John Hopkins, MD,Dr. Mahesh, John Hopkins, MD, AAPM Handout.AAPM Handout.

64 slice 64 slice scannersscanners

Dual Source Dual Source CTCT

20042004-- 20052005

Historical Development of CTHistorical Development of CT

Slow scan time Fast scan time

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Historical Development of CTHistorical Development of CTSeventh Generation CT (late 1990Seventh Generation CT (late 1990’’s s –– 2000)2000)

MultiMulti --Row Detector SystemsRow Detector Systems

Radiographics 2000: 20Radiographics 2000: 20--17871787--18061806

Kalendar WA, Computed Tomography, Kalendar WA, Computed Tomography, Second Edition, 2005, pg 58.Second Edition, 2005, pg 58.

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Historical Development of CTHistorical Development of CTPET/CT Scanners (2001)PET/CT Scanners (2001)

Kalendar WA, Computed Tomography, 2Kalendar WA, Computed Tomography, 2ndnd ed., 2005, pg 74.ed., 2005, pg 74.

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Historical Development of CTHistorical Development of CTDualDual--Source CT (2005)Source CT (2005)

� A scanner using two X-ray sources and two detectors at the same time

� Enables scanning a heart at any heart rate and at the lowest radiation dose possible

www.medical.siemens.comwww.medical.siemens.com

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Fast Scanning, 0.3 secFast Scanning, 0.3 sec

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Why has the frequency of CT exams Why has the frequency of CT exams increased?increased?

�� New New technologytechnology

�� Faster scansFaster scans

�� New exams New exams done with CTdone with CT

�� More More screening screening examsexams

UNSCEAR 2000

More like 60%

ICRP Publication 87 (2001)

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SingleSingle--Row Detector versus MultiRow Detector versus Multi --Row Row Detector CT Detector CT


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SingleSingle--row Detector vs. Multirow Detector vs. Multi --row Detector row Detector CTCT

Seeram, Computed Tomography, 2Seeram, Computed Tomography, 2nd ed.,ed., 2001, pg 258.2001, pg 258.

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Multiple Detector ArraysMultiple Detector Arrays

Bushberg, et al. The Essential Physics of Medical Imaging, 2Bushberg, et al. The Essential Physics of Medical Imaging, 2ndnd ed., p. 341.ed., p. 341.

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Slice Width SelectionSlice Width Selection

http://www.impactscan.org/slides/impactcourse/helical_and_multi-slice_principles/img17.html

GE 64 MDCT GE 64 MDCT (40 mm (40 mm detector)detector)

64 x 0.625 mm64 x 0.625 mm

32 x 1.25 mm32 x 1.25 mm

16 x 2.5 mm16 x 2.5 mm

8 x 5 mm8 x 5 mm

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32/40/64 Slice CT Systems32/40/64 Slice CT Systems

http://www.impactscan.org/slides/ukrc2005/img11.html

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�� Two pitch definitions seen in MSCTTwo pitch definitions seen in MSCT

�� Collimator pitch = Collimator pitch = table travel_____table travel_____xx--ray beam widthray beam width

�� Detector pitch = Detector pitch = table traveltable travel

detector widthdetector width

�� Collimator pitch = Collimator pitch = Detector pitchDetector pitch

NN

Pitch in MultiPitch in Multi --slice CTslice CT

c.f. http://www.impactscan.org/slides

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MultiMulti --Row Detector CT Row Detector CT -- AdvantagesAdvantages

�� Same acquisition in shorter times (fewer motion artifacts)Same acquisition in shorter times (fewer motion artifacts)


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�� Reconstruction in different slice widths Reconstruction in different slice widths

http://www.impactscan.org/

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�� Efficient use of xEfficient use of x--ray beam with increasing slicesray beam with increasing slices�� Reduction of radiation dose Reduction of radiation dose


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�� Thinner slices for better zThinner slices for better z--axis resolutionaxis resolution

�� Isotropic ImagingIsotropic Imaging

Improved Spatial Improved Spatial ResolutionResolution

All sides of the voxel All sides of the voxel have equal dimensionshave equal dimensions


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Better Multiplanar Reconstruction & 3D DisplayBetter Multiplanar Reconstruction & 3D DisplayMultiMulti --Row Detector CT Row Detector CT -- AdvantagesAdvantages


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Regarding Multiple Detector CT Regarding Multiple Detector CT scanners:scanners:

0%

43%

29%

14%

14% 1)1) A scanner with a 16A scanner with a 16--detector array employs a total of 16x16 detector array employs a total of 16x16 detectorsdetectors

2)2) Detectors used are the nonDetectors used are the non--radioactive xenonradioactive xenon--gas detectorsgas detectors

3)3) The slice width is determined by width of detectors in slice The slice width is determined by width of detectors in slice

thickness dimensionthickness dimension

4)4) The slice width selected can be smaller than the physical The slice width selected can be smaller than the physical size of the detector size of the detector

5)5) The spatial resolution is worse than that of a singleThe spatial resolution is worse than that of a single--detector detector CT scanners CT scanners

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AnswerAnswer

The slice width is determined by width of detectors in The slice width is determined by width of detectors in slice thickness dimensionslice thickness dimension

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MultiMulti --Row Detector CT Row Detector CT -- Advantages Advantages Cardiac CTCardiac CT

Kalendar WA, Computed Tomography, 2Kalendar WA, Computed Tomography, 2ndnd ed., 2005, Video.ed., 2005, Video.

20 mm detector

(Pitch ~0.25)

6.2 cm in 5 sec

40 mm detector

(Pitch ~0.25)

12.5 cm in 5 sec

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For a 16For a 16--detector row CT scanner, the xdetector row CT scanner, the x--ray beam ray beam width is 20 mm, detector width is 5 mm and the width is 20 mm, detector width is 5 mm and the table travel per rotation is 20 mm. Calculate the table travel per rotation is 20 mm. Calculate the

detector pitch and collimator pitch? detector pitch and collimator pitch?

0%8%15%62%15% 1.1. 1, 41, 4

2.2. 4, 14, 1

3.3. 5, 205, 20

4.4. 4, 44, 4

5.5. 20, 520, 5

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AnswerAnswer

�� 4, 14, 1

�� PitchPitchdd = = 20 20 = 4 = Detector pitch= 4 = Detector pitch

55

�� PitchPitchxx = = 20 20 = 1 = Collimator pitch= 1 = Collimator pitch

2020

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Dose ReductionDose ReductionCurrent Modulation or AEC in CTCurrent Modulation or AEC in CT

�� Modern CT scanners are capable of modulating the mA Modern CT scanners are capable of modulating the mA (current) during the scan(current) during the scan

�� Rationale: it takes fewer photons (lower mA) to penetrate Rationale: it takes fewer photons (lower mA) to penetrate thinner tissue, and more xthinner tissue, and more x--ray photons (higher mA) to ray photons (higher mA) to penetrate thicker projections through the bodypenetrate thicker projections through the body

�� BenefitsBenefits

�� Consistent image qualityConsistent image quality

�� Dose reduction potentialDose reduction potential

�� Reduced tube loadingReduced tube loading

�� Extended scan runsExtended scan runs

�� Reduction in photon starvation artifactReduction in photon starvation artifact

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Automatic Exposure Control (AEC) in CTAutomatic Exposure Control (AEC) in CT

�� Patient size AEC: higher mA used for larger patientsPatient size AEC: higher mA used for larger patientsPatient size AEC

www.isteve.com www.yuzbasicafer1.comMHRA Report 05016, February 2005

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Automatic Exposure Control (AEC)Automatic Exposure Control (AEC)

�� ZZ--axis AEC: higher mA used at more attenuating axis AEC: higher mA used at more attenuating positions along the zpositions along the z--axis (patient)axis (patient)

Z-axis AEC

Radiographics, 2006, 26:503-512MHRA Report 05016, February 2005

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Automatic Exposure Control (AEC) in CTAutomatic Exposure Control (AEC) in CT

�� Angular AEC: the degree of modulation depends on the Angular AEC: the degree of modulation depends on the asymmetry of the patientasymmetry of the patient

Angular AEC

MHRA Report 05016, February 2005 Radiology, 2004, 233:649-657

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Dose ReductionDose ReductionCardiac ECG dose modulationCardiac ECG dose modulation

�� It is possible to vary the mA based on RIt is possible to vary the mA based on R--R interval phase %R interval phase %

�� This can lead to a dose decrease of up to 50% in low stable This can lead to a dose decrease of up to 50% in low stable heart ratesheart rates

70%70% 80%80%

mAmA

200200

550550

tt

interval used interval used for image for image reconstructionreconstruction

dose dose notnot given to patientgiven to patient

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0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250

Heart rate (BPM)

Exp

osur

e C

TD

I (m

Gy)

Segment mode, no ECG

240o

HR 30-74 BPM (One Segment)

Coverage during scan

Dose Reduction Dose Reduction -- ECG dose modulationECG dose modulation

Paul Kinahan and Kelly Branch, UWMC Paul Kinahan and Kelly Branch, UWMC

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0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250

Heart rate (BPM)

Exp

osu

re C

TD

I (m

Gy)

Segment mode, no ECG

Segment mode [65-85%], w/ECG

Segment mode [70-80%], w/ECG

240o

HR 30-74 BPM (One Segment), TR 175 msec

Needed coverage during scan

Dose Reduction Dose Reduction -- ECG dose modulationECG dose modulation

42% 42% decrease decrease in dosein dose

Paul Kinahan and Kelly Branch, UWMC Paul Kinahan and Kelly Branch, UWMC

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Regarding automatic tube current Regarding automatic tube current modulation in CT: modulation in CT:

0%

100%

0%

0%

0% 1.1. It gives inconsistent image qualityIt gives inconsistent image quality

2.2. It increases the scan time for the examIt increases the scan time for the exam

3.3. It generates more heat in the xIt generates more heat in the x--ray tuberay tube

4.4. The tube current is automatically adjusted according to The tube current is automatically adjusted according to patient sizepatient size

5.5. It causes an increase in photon starvation artifact It causes an increase in photon starvation artifact

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AnswerAnswer

The tube current is automatically adjusted according to The tube current is automatically adjusted according to patient sizepatient size

Reducing Cardiac Dose Reducing Cardiac Dose –– Prospective Prospective GatingGating

Mahesh, et al. Radiographics, 2007; 27:1495Mahesh, et al. Radiographics, 2007; 27:1495--1509.1509.Kanal

No functional information

Retrospective GatingRetrospective Gating

Mahesh, et al. Radiographics, 2007; 27:1495Mahesh, et al. Radiographics, 2007; 27:1495--1509.1509.Kanal

Kanal Shuman, et al. submitted for publication, 2007Shuman, et al. submitted for publication, 2007

Prospective vs. Retrospective GatingProspective vs. Retrospective GatingUniversity of Washington StudyUniversity of Washington Study

�� 77% lower radiation dose with prospective study77% lower radiation dose with prospective study

�� This study concluded that the This study concluded that the ““64 channel cardiac CT 64 channel cardiac CT performed with prospective ECG gating may have performed with prospective ECG gating may have similarsimilarsubjective image quality and substantially subjective image quality and substantially lowerlower patient patient radiation dose compared to retrospective ECG gatingradiation dose compared to retrospective ECG gating””

Shuman, et al. submitted for publication, 2007Shuman, et al. submitted for publication, 2007

6.2 6.2 ±± 2.0 2.0 26.4 26.4 ±±6.2 6.2 Effective Effective Dose (mSv)Dose (mSv)

Prospective Prospective GatingGating

Retrospective Retrospective GatingGating

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Regarding prospective ECG triggering Regarding prospective ECG triggering for cardiac CT exams for cardiac CT exams

0%

0%

100%

0%

0% 1.1. Projection data has to be collected through out the cardiac Projection data has to be collected through out the cardiac cycle cycle

2.2. The temporal resolution is about 350 msThe temporal resolution is about 350 ms

3.3. The radiation dose is reduced compared to retrospective The radiation dose is reduced compared to retrospective

ECG gatingECG gating

4.4. Functional information of the heart is available with this Functional information of the heart is available with this examexam

5.5. Increase in motion artifacts occursIncrease in motion artifacts occurs

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AnswerAnswer

The radiation dose is reduced compared to The radiation dose is reduced compared to retrospective ECG gatingretrospective ECG gating

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Future of CTFuture of CT


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Future of CTFuture of CTOther DevelopmentsOther Developments


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TAKE HOME POINTSTAKE HOME POINTS

�� MDCT scanners are becoming common and have several MDCT scanners are becoming common and have several advantages over SDCTadvantages over SDCT

�� CT utilization has increased significantlyCT utilization has increased significantly

�� AEC or Automatic Tube Current Modulation and ECG AEC or Automatic Tube Current Modulation and ECG modulation available on modern scanners which helps modulation available on modern scanners which helps reduce dosereduce dose

�� Cardiac CT is becoming popular with MDCT scannersCardiac CT is becoming popular with MDCT scanners

�� Always keep ALARA in mind!Always keep ALARA in mind!

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THANK YOUTHANK YOU

mdct technology - aapm: the american association of ... · kanal 20 regarding multiple detector ct...

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