Dose Reduction in MDCT: Technique + Technology William P. Shuman MD, FACR Department of Radiology...

Dose Reduction in MDCT:Dose Reduction in MDCT:

Technique + TechnologyTechnique + Technology

William P. Shuman MD, FACR

Department of Radiology

University of Washington

Conflict of Interest Statement

• I administer a grant from GE Healthcare which supports clinical investigation of cardiac CT.

• No support for equipment or salaries

Acknowledgements

• I thank my colleagues Dr. Kalpana Kanal, Dr. Lee Mitsumori, and Dr. Paul Kinahan for their input.

• I also thank Dr. Joel Platt of the University of Michigan for some of the image slides.

Radiation Induced Risk

• Heritable genetic

• Developmental fetus

• Cancer linear non threshold model

• A-Bomb data (150 mSv), dirty radiation

© UW and Kalpana M. Kanal, PhD, DABR

Radiation exposure to US populationRadiation exposure to US population

Medical 0.54 mSv per capitaTotal 3.6 mSv per capita

Medical 3.0 mSv per capitaTotal 6.2 mSv per capita

US 1982US 2006

K Kanal

© UW and Kalpana M. Kanal, PhD, DABR

Medical Exposure

Effective Dose Contributions

slide courtesy: Dr M. Mahesh, Johns Hopkins

1990 2007

Background vs. Medical Radiation

• Now about equal: 3m Sv

• 1/2 medical radiation from CT

• 1990 - 13 million CT scans

• 2000 - 46 million CT scans

• 2006 - 62 million CT scans

Dramatic Growth in CT

Arch Intern Med 169; Dec, 2009 (p.2071)

Insurance Claims Data

Projected Risks from CT

• 70 milion CT’s done in 2007

• 29,000 future cancers could result

• Biggest contributors:• CT of Abdom and Pel

• CT of chest

• CT of head

• CT pulmonary angio for PE

Projected Risks from CT

• One third in pts. betw. 35 and 54• 15% younger than 18• 66% in females• Some scans up to 50 mSv• Conclusion:

• Large contributions to total cancer risk• Dose reduction efforts warranted

Arch Intern Med 169 ; Dec, 2009 (p.2078)

Radiation Dose Assoc. with CT

• 1119 consec. scans in 4 SF hosp’s

• Dose varied widely within and among instituitions for similar exams.• Up to 13-fold variation

• 1 / 270 women Cor. CT age 40 - Cancer

• 1 / 8100 - Head CT:

Radiation Dose Assoc. with CT

• For 20 y/o risks doubled

• CT radiation doses are higher and more variable than usually quoted

• Greater monitoring / standardization needed

Arch Intern Med 169; Dec, 2009 (p.2049)

Editorial: Cancer Risks and CT

• Of 72 million scans, 2% were high dose: 20-50 mSv

• From CT scans performed in 2007, 15,000 deaths from induced CA

• “Public Health Time Bomb…”

• What to do?

Editorial: Cancer Risks and CT

• Improve protocols and standardization• Get rid of 13 fold variation

• Lower dose per scan significantly

• Inform Pts. About radiation risk• 1 CT = 300 CXR’s

CBS Evening News with Katie Couric (12/14/09)

“CT Scans Harmful”?

Seattle Times 12/15/09 (p.A12 in print)

© UW and Kalpana M. Kanal, PhD, DABR

CT scans raise risk of cancer, report cautions

Millions of Americans, especially children, are needlessly getting dangerous radiation from "super X-rays" that increase the risk of

cancer...

Thursday, November 29, 2007

N Eng J Med 2007, 357:2277-84

Relative Risk from a CT

• To individual:

• Lifetime risk of cancer: 25% (1 in 4 )

• Added risk: 0.05% (1 in 1000 - 2000)

• To population:

• 62M CT scans year in USA

• Without CT: 13.778M will die of cancer

• With CT: 13.809M will die of cancer

• (additional 31K)

CT Radiation Increasing

• 1.5 - 2% of cancers in US from CT

• Physicians are uninformed on risks

• 3 steps to improve:

• Optimize each CT for dose

• Alternatives to CT when possible

• Appropriateness of each CTBrenner et al, NEJM 2007

Overuse of CT in Some Patients

• 15% had more than 100 mSv

• Follow up CT• Stone CT• CT Enterography• Rule out PE

Sodickson et al, Radiology 2009

CT Risk vs. Benefit

• If CT is felt to be medically necessary, radiation risk is both accepted and minimized

What Shoud Radiology Do ??

• Leadship in:

• Dose Awareness

• Dose Reduction

Dose Awareness

• Education• All in Radiology Dept.

• All Referrers

• Publish dose of each exam• Put in report + med. Record

• ? National Registry

• Monitor exam doses regularly

CT Dose Reduction

• The Easy Stuff:• Control Z axis scan length

• Minimize number of phases

• Externally applied body shields

• Noise reduction filters

Control Z-Axis

• Z-axis length creep

• “Throw in” a chest or pelvis

• Regions of overlap• Neck/chest• Lower chest/upper abdomen

Control Z Axis: Shrink to Fit Pt.

• Challenge your technologists

• Control patient breathing and moving

• Especially avoid breast or pelvis

Minimize Number of Phases Used

• Limit to only needed:• 4 phase liver vs. 3 phase

• Use new protocols:• CTU – split bolus

Externally Applied Shields

• Breast

• Gonadal (males)

• Thyroid

Breast Shield

• Bismuth shield

• Decreases breast dose

• Up to 40% ( ! )

Breast Shield

• Increased noise in ant. chest

• Proper positioning:

• Slight stand off from chest (blanket)

• All females age 12 - 50

Gonadal Shield

Thyroid Shield

Specialized Noise Reduction Filters

• General noise reduction filter:

• thin sliced acquisitions for neuro 3D modes such as reformat or MIP

• Cardiac noise reduction filters

Dose Reduction Techniques: Filters

Dose Reduction Techniques: Filters

Results in reduction in CT dose.

The bow-tie filter : graphite that sits between the X-ray tube and patient

95%70 %

Unused X-ray

(penumbra)

10 mm width70% of beam utilized

40 mm width95% of beam utilized

Dose Reduction Techniques: Penumbra

Dose Reduction: TechniqueTechnique

• Noise index and Automatic Tube Current Modulation (ATCM) for mA

• kVp and body size

• Rotation time

• Centering

CT Dose and Image Quality

• How do we balance ??

• What are the trade offs ??

Dose Too Low (noise!)

Dose Too High (looks too good !)

Dose and mA

• Linear relationship

• Decrease mA to reduce dose linearly

• mAs incorporates gantry rotation time

Tube Current mA Modulation

• Fixed tube current• Modulated tube current

• XY modulation• Z modulation• XYZ

• Automated Modulation (ATCM)• Noise index

Automated Tube Current Modulation

• Based on density in AP and Lat scouts

• Patient dose controlled not by mA but by manual setting of the Noise Index

• Standard deviation of the image noise

Image Quality: Noise

• Noise index (NI)• Standard deviation of CT numbers

within a ROI in a water phantom

• Vendor specific term

• Typical NI: 10 - 20

• High NI (low dose): 30 - 40

1Noise IndexDose

The Physics

Development of a Noise Index Table Demonstrating Interrelationships

Among Noise Level, Reconstruction Slice Thickness, and Radiation Dose in

64-slice CT

Kalpana M. Kanal, PhD, Brent K. Stewart, PhD, Orpheus Kolokythas MD, William P. Shuman, MD

Department of RadiologyUniversity of Washington

SCBT/MR Scientific Session, 2006; AJR 2008

Reconstruction Slice Thickness (mm)0.625 1.25 2.5 3.75 5

NI NI NI NI NI19.51 16.48 11.42 9.01 7.46 4.205 320.5%20.53 17.35 12.02 9.49 7.85 3.795 279.5%21.61 18.27 12.65 9.99 8.27 3.425 242.5%22.75 19.23 13.32 10.51 8.70 3.091 209.1%23.95 20.24 14.02 11.07 9.16 2.789 178.9%25.21 21.30 14.76 11.65 9.64 2.517 151.7%26.54 22.42 15.54 12.26 10.15 2.272 127.2%27.93 23.61 16.35 12.91 10.69 2.051 105.1%29.40 24.85 17.21 13.59 11.25 1.851 85.1%30.95 26.16 18.12 14.30 11.84 1.670 67.0%32.58 27.53 19.07 15.06 12.46 1.507 50.7%34.30 28.98 20.08 15.85 13.12 1.360 36.0%36.10 30.51 21.14 16.68 13.81 1.228 22.8%38.00 32.11 22.25 17.56 14.54 1.108 10.8%40.00 33.80 23.42 18.48 15.30 1.000 0.0%42.00 35.49 24.59 19.41 16.07 0.907 -9.3%44.10 37.27 25.82 20.38 16.87 0.823 -17.7%46.31 39.13 27.11 21.40 17.71 0.746 -25.4%48.62 41.09 28.47 22.47 18.60 0.677 -32.3%

43.14 29.89 23.59 19.53 0.614 -38.6%45.30 31.38 24.77 20.51 0.557 -44.3%47.56 32.95 26.01 21.53 0.505 -49.5%49.94 34.60 27.31 22.61 0.458 -54.2%

36.33 28.68 23.74 0.416 -58.4%38.15 30.11 24.92 0.377 -62.3%40.05 31.62 26.17 0.342 -65.8%42.06 33.20 27.48 0.310 -69.0%44.16 34.86 28.85 0.281 -71.9%46.37 36.60 30.30 0.255 -74.5%48.69 38.43 31.81 0.231 -76.9%

On GE VCT scanner, the range of NI is 0.5 - 50

Relative Dose

% Dose Difference

ATCM: Not Entirely Automatic

• Based on density in AP and Lat scouts

• Patient centering during scouts is critical

• Vertical offset can increased average tube current a lot

Matsubara et al, AJR 2009

Elevated 4 cm

SD noise inc. mA boost Cent’d 6.47 0 % 0%4 cm 8.40 30% 68%6 cm 9.22 43% 100%

Centered

Elevated 6 cm

Patient Centering: Image Noise in ATCM

J. Platt

Standardize Image Quality: NI

• Lower NI for smaller patients (30)• FOV < 34, BMI < 25

• Mid Range NI (36)• FOV 34 – 44, BMI 25-35

• Higher NI for larger patients (40)• FOV > 44, BMI > 35

BEWARE: This Can Be a 50 mSv Exam

Large Patient: Accept NI = 40

kVp and Dose

• kVp→ exponential impact on dose

• 120 to 100 kV → 43% decrease in dose

• 120 to 80 kV → 70% decrease in dose

• Variable: patient size/density

Iodine K Edge

• Iodine K edge : 33 Kev (70 kVp)

• Benefit in vascular imaging

• Lower kV = better iodine conspicuity

• Endoleak, CT Angio

J. Platt

100 kV Scanning: Small Patients100 kV Scanning: Small Patients

• Decreases dose 43%

• BMI < 25, weight < 160 lbs.

• May need to increase mA or decrease NI

140 kVp Scanning: Large Patients ?140 kVp Scanning: Large Patients ?

• Very high dose

• Use only in unusual cases

• Techs must get radiologist’s permission to use 140 kVp

50 mSv !

Rotation Time and Dose

• Linear direct relationship

• Decrease in time = decrease in dose

• 0.35 vs. 0.50 sec

Dose Reduction: TechnologyTechnology

• Cardiac CT: Dose control techniques• Adaptive Statistical Iterative

Reconstruction (ASIR): the Theory• ASIR: The Practice

• How we started• Evolution of ASIR practice• Cardiac ASIR

• ASIR: The Impact

Effective Dose

• MilliSieverts (mSv)

• Risk from tissue exposure to radiation

• Gender

• Age

• Modeling and estimates

• For a population not one person

Effective Dose

Calculating effective dose from DLP

Use a multiplier:

Region mSv

Head 0.0023

Chest .017

Abdomen .015

Pelvis .019

Effective Dose

Example:

If a chest CT has a DLP of 350 mGy

effective dose is -

350 X 0.017 = 5.9 mSv

mSv Range for Body CT

• Chest:• 7 – 18 mSv

• 12 – 36 mSv cardiac

• Abdomen• 10 – 14 mSv

• Abdomen + Pelvis• 15 – 35 mSv

Cardiac CT: A High Dose Exam ?

• Reported doses vary from 1 – 36 mSv• Low pitch (.20 - .25)• High mA• Long Z axis (triple rule out)

• What can be done to control dose in cardiac scanning??

Cardiac Prospective Triggering

• Earls, JP, et al. Radiology 2008; 246:742

• Prospective triggering vs. Retrospective Gating

• 84 RG, 121 PG, not matched• PG – 83% dose reduction; statistically

significant improvement in image quality

Cardiac Prospective Triggering

• Shuman, et al. Radiology, August 2008

• Prospective vs. Retrospective Gating

• 50 RG, 50 PG, matched for clinical features

• PG – 77% dose reduction; statistically identical image quality

Prospective vs. Retrospective

• Fact of Life:

• Prospective triggering only works at regular heart rates below 75 BPM

• Requires Beta blockers in most patients

Prospective vs. Retrospective

• Fact of Life:

• Prospective gating only images 20% of the R-R interval, so functional information about heart motion, ejection fraction, valve motion, and wall thickening is not produced.

• Usually only need functional info in patients with known heart disease.

Dose from Whole Chest CT

• Shuman, et al. AJR , June 2009

• TRO Whole-Chest Long Z-Axis • Prospective vs. Retrospective 41 RG, 31 PT,

matched for clinical features

• PG – 71% dose reduction; statistically superior image quality

Clinical TRO case: good contrast in PA, Coronary arteries, and Aorta with low density in right heart

Mitsumori

Low-Risk Chest Pain Patients in the Emergency Department: Negative 64 Channel Cardiac CT May Reduce Length of Stay and Hospital Charges

Janet M. May, William P. Shuman, Jared N. Strote, Kelley R. Branch, David W. Janet M. May, William P. Shuman, Jared N. Strote, Kelley R. Branch, David W. Lockhart, Lee M. Mitsumori, Bill H. Warren, Theodore J. Dubinsky, James H. Lockhart, Lee M. Mitsumori, Bill H. Warren, Theodore J. Dubinsky, James H.

CaldwellCaldwell

University of Washington University of Washington

Department of RadiologyDepartment of Radiology

Negative Whole Chest Gated CT

• May et al, AJR, July 2009• Standard of Care: 25 hours, $7597• Neg CT + 3 Enz, ECG: 14 hours, $6153• Neg CT + 1 Enz, EGC: 5 hours, $4251

• CT in the diagnostic pathway may shorten ER LOS by 20 hours and lower charges by 44% for most patients with low risk non-specific chest pain

Dose Reduction: TechnologyTechnology

• Cardiac CT: Dose control techniques• Adaptive Statistical Iterative

Reconstruction (ASIR): the Theory• ASIR: The Practice

• How we started• Evolution of ASIR practice• Cardiac ASIR

• ASIR: The Impact

Noise Limits Dose Reduction

• What if we did everything we could to lower dose and….

• The Images are too noisy ?• When generated with Filtered Back

Projection (FBP) reconstruction technique

Noise Limits Dose Reduction

• Answer:

• find a different way to reconstruct low dose images so they look much less noisy

Filtered Back Projection

• The Math :

• Several assumptions - simplify reality• Saves time in calculations

• What if instead tried to model reality ?

max

min

),,()'(),,(),,(

),,(2

2

ddphw

yxL

Rzyxf

Adaptive Statistical Iterative

Reconstruction

ASiR

Point Focal SpotPoint Detector

Point VoxelPencil Beam

Perfect SampleLine Integral

Simple Calculation

Simplicity

Real Focal SpotReal DetectorCubic VoxelBroad BeamStatistical ModelPhysics ModelComplex Computation

Image Qualityx

-ra

y f

lux

.

ASIR : Different Assumptions

FBP ASIR

A Better Model of Reality !

• ASIR is more computationally intensive

• With today’s faster processors:•Increased time not noticeable

ASiR

Full dose: 25.08mGy Half dose: 12.42mGy

• 50% ASiR at half dose = full dose FBP.

Low Contrast Detectability

Why not 100% ASIR?

• “Plastic or Waxy” look of

100% ASIR images

• Reduce ASiR

• 30% to 50% blend with FBP

ASIR: none vs. 100% ASIR: none vs. 100%

None 100% ASIR

Varying ASIR %NI 40 asir 0% NI 40 asir 30% NI 40 asir 50%

NI 40 asir 100%NI 40 asir 70% NI 30 asir 0%

Mitsumori

Noise Index and ASIR %

NI 30, 0%

NI 40, 0%

NI 40, 30%

NI 40, 50%

NI 40, 70%

NI 40, 100%

Mitsumori

NI 40, ASIR 50%

(190 – 114) / 190 = 40% dose reduction

Which Is the ASIR Image: 40% Lower Dose ?Which Is the ASIR Image: 40% Lower Dose ?

DLP = 815 DLP = 815

mSv = 11.4mSv = 11.4

DLP = 414DLP = 414

mSv = 5.8mSv = 5.8J. Platt

Impact of 40% ASIR, NI = 30

Impact of 30% ASIRImpact of 30% ASIR

40% dose reduction

Crohn’s: Low Dose (ASIR 40%)Crohn’s: Low Dose (ASIR 40%)

DLP 388 = 5.4 mSvDLP 388 = 5.4 mSvJ. Platt

Rules of Thumb

• If routine images look good (2.5 to 5mm)• Cut dose by 30%: NI 30-36

• Add ASIR: 30 – 50%

• If thin slice imagers look good (0.625 mm)• Cut dose by 50%: NI 36 – 40

• Add ASIR: 30 – 50%

ASIR and UWa

• Routine Body• Started at NI 40, ASIR 50%

• Looked too “different”

• Changed to NI 36, ASIR 40%

• Most images excellent, but not all

• Small pts: too noisy

• Large pts: Too much dose

ASIR and U Wa

• Routine Body Imaging• Current:

• Small: FOV < 34 (BMI < 25)• NI 30, ASIR 40%

• Medium: FOV 34 – 44 (BMI 25 – 35)• NI 36, ASIR 40%

• Large: FOV > 44 (BMI > 35)• NI 40, ASIR 40%

ASIR and U Wa

• Chest

• Started at NI 40, ASIR 50%• Edge effect annoying• A little too “waxy”

• Current:• NI 36, ASIR 30%

ASIR: ChestASIR: Chest

No ASIR, NI 30No ASIR, NI 30 30 % ASIR, NI 3630 % ASIR, NI 36

ASIR: ChestASIR: Chest

30 % ASiR30 % ASiR

ASIR and U Wa

• Neuro

• Started at NI 36, ASIR 40%

• Happy

Rules of Thumb

• For Cardiac CT, use 50% ASIR in all cases to reduce noise in images• On the 750HD, use HD mode and

small FOV for acquisition

• Use cardiac Prospective Gating in most cardiac cases (85%)

No ASIR

50% ASIR

HD + 50% ASIR

Dose Reduction: TechniqueTechnique

• Noise index and Automatic Tube Current Modulation (ATCM) for mA

• kVp and body size

• Rotation time

• Centering

• Body Shields

• Z – axis control

Dose Reduction: TechnologyTechnology

• Cardiac CT: Dose control techniques

• ASIR: The Theory

• ASIR the practice• How we started

• Evolution of ASIR practice

• Cardiac ASIR

• ASIR: The Impact

FBP ASiR MBIR12 mAs

The Future: Model Based Iterative Reconstruction (MBIR)

Paul Kinahan

FBP ASIR MBIR

120 kVp, variable mAs (NI=36), 1.375 pitch. 0.625mm, BMI = 34 PAUL KINAHAN

FBP vs. ASIR vs. MBIR

Change is inevitable….

….except from a vending machine.

Groucho Marx

FBP vs. ASiR vs. (MBIR)

•ASiR •MBIR

•Advanced Statistical

Model

•Real Focal Spot

•Real Detector

•3D Voxel

•Actual Beam

•Physics Model

•Perfect Sample

•Point Focal Spot•Point Detector

•Point Voxel•Pencil Beam

•FBP

Simple Advanced Powerful

Raw Data

Mathematics

SimpleFast

High Noise

Better IQLow Noise

Lower Dose

Optimal IQLower NoiseHi Resolution

More Computation

•Point Focal Spot•Point Detector

•Point Voxel•Pencil Beam

•Statistical Model