Dose Reduction in MDCT: Technique + Technology

Dose Reduction in MDCT: Technique + 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

Radiation exposure to US population
Medical 0.54 mSv per capita Total 3.6 mSv per capita Medical 3.0 mSv per capita Total 6.2 mSv per capita © UW and Kalpana M. Kanal, PhD, DABR K Kanal

Effective Dose Contributions
Medical Exposure 1990 2007 Effective Dose Contributions © UW and Kalpana M. Kanal, PhD, DABR slide courtesy: Dr M. Mahesh, Johns Hopkins

Background vs. Medical Radiation
Now about equal: 3m Sv 1/2 medical radiation from CT

Dramatic Growth in CT 1990 - 13 million CT scans

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 / 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: 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)

CT scans raise risk of cancer, report cautions
N Eng J Med 2007, 357: Thursday, November 29, 2007 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... © UW and Kalpana M. Kanal, PhD, DABR

Relative Risk from a CT To individual:
Lifetime risk of cancer: 25% (1 in 4 ) Added risk: 0.05% (1 in ) To population: 62M CT scans year in USA Without CT: M will die of cancer With CT: M will die of cancer (additional 31K)

CT Radiation Increasing
% 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 CT Brenner 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 Publish dose of each exam
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

Breast shield with air gap

Gonadal Shield Breast shield with air gap

Thyroid Shield Breast shield with air gap

Dose Reduction Techniques: Filters
Specialized Noise Reduction Filters General noise reduction filter: thin sliced acquisitions for neuro 3D modes such as reformat or MIP Cardiac noise reduction filters Filters will allow for an estimated reduction exceeding 30% Most other manufacturers have noise reduction filters as well. IQ Enhance (where applicable) Cardiac noise reducing and edge preserving filters C1, C2, or C3 are available for Cardiac helical exams. These filters allow you to reduce the dose up to 30% on top of ECG modulation dose reduction while preserving the same image quality Some manufacturers call them adaptive filters.

Dose Reduction Techniques: Filters
The bow-tie filter : graphite that sits between the X-ray tube and patient Results in reduction in CT dose.

Dose Reduction Techniques: Penumbra
Unused X-ray (penumbra) 70 % 95% Wider beam coverage improves Z axis dose efficiency . Umbra is the used radiation. The Z axis dose efficiency an indicator of how much beam falls off the ends of the detector. In a single slice system, the slice profile is determined by a source collimator and the detector intercepts all of the beam. Hence, the dose efficiency is 100%. Note that the wider slices are more dose efficient. Beam tracking keeps the collimated beam centered over the over the detector to minimize penumbra a lower dose will be delivered if you Rx using 20mm collimation vs 10 mm for a head ie., As the length of the area to be scanned increases (to about 300mm), this will diminish. 10 mm width 70% of beam utilized 40 mm width 95% of beam utilized

Dose Reduction: Technique
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 !)
Even thins look good –overdosed ?

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) Vendor specific term
Standard deviation of CT numbers within a ROI in a water phantom Vendor specific term Typical NI: High NI (low dose):

The Physics

Department of Radiology University of Washington
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 Radiology University of Washington SCBT/MR Scientific Session, 2006; AJR 2008

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

Patient Centering: Image Noise in ATCM
SD noise inc mA boost Cent’d % 0% 4 cm % 68% 6 cm % 100% Elevated 4 cm Elevated 6 cm Centered 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
Large patient- DFOV 50 more than 350 lbs but with larger tube and ASiR looks great

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

Renal donor Becker 39384942 100 KV- very low dose but nice study DLP 260
J. Platt

100 kV Scanning: Small Patients
Decreases dose 43% BMI < 25, weight < 160 lbs. May need to increase mA or decrease NI Himes kV Aorta on 425 lb patient– 140 makes axials look good but aorta but may be less bright DLP 2600

140 kVp Scanning: Large Patients ?
Very high dose Use only in unusual cases Techs must get radiologist’s permission to use 140 kVp Himes kV Aorta on 425 lb patient– 140 makes axials look good but aorta but may be less bright DLP 2600 50 mSv !

Rotation Time and Dose Linear direct relationship
Decrease in time = decrease in dose 0.35 vs sec

Dose Reduction: Technology
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 Chest Abdomen Pelvis

Effective Dose Example: If a chest CT has a DLP of 350 mGy
effective dose is - 350 X = 5.9 mSv

mSv Range for Body CT Chest: Abdomen Abdomen + Pelvis 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 ( ) 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. Lockhart, Lee M. Mitsumori, Bill H. Warren, Theodore J. Dubinsky, James H. Caldwell University of Washington Department of Radiology 84

Negative Whole Chest Gated CT
May et al, AJR, July 2009 Standard of Care: 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

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 ? Filter back projection is complicated but make simplifications to make an image , raw data in the formula to make an image

Iterative Reconstruction
ASiR Adaptive Statistical Iterative Reconstruction

ASIR : Different Assumptions
FBP ASIR Point Focal Spot Point Detector Point Voxel Pencil Beam Perfect Sample Line Integral Simple Calculation Simplicity Real Focal Spot Real Detector Cubic Voxel Broad Beam Statistical Model Physics Model Complex Computation Image Quality . Today to make the formula work the focal spot is a point, no physical size, also the detector is a point, when we look at x-ray measurement we look a pencil beam (infinitely small thin pass, no physical dimension in width) with ir you are marrying a bundle(in ir we measure a bundle over an area in a volume) for fbp we ignore statistics (electronic noise and photon statistics) with fbp you treat it as perfect which makes it nosier, with IR you understand patterns and treat them differently which allow easier correction x-ray flux A Better Model of Reality !

ASiR ASIR is more computationally intensive
With today’s faster processors: Increased time not noticeable

Low Contrast Detectability
50% ASiR at half dose = full dose FBP. Full dose: mGy Half dose: mGy

Why not 100% ASIR? “Plastic or Waxy” look of 100% ASIR images Reduce ASiR 30% to 50% blend with FBP

ASIR: none vs. 100% None 100% ASIR
Deneau HA anuerysm small FOV, HI Res ( .9 Pitch ) and 100 kV with NI of DLP 268 Aorta HU 754 0 and 100 % ASiR sets None 100% ASIR

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

Noise Index and ASIR % Mitsumori NI 40, 0% NI 40, 30% NI 30, 0%

NI 40, ASIR 50% (190 – 114) / 190 = 40% dose reduction

Which Is the ASIR Image: 40% Lower Dose ?
VCT on left with DLP of 798 and HD on right with DLP of 378– VCT a little better but does it justify twice the dose ?

Impact of 40% ASIR, NI = 30 DLP = 815 DLP = 414 mSv = 11.4 mSv = 5.8
One study at 815 DLP and the other at NI ASiR 30 % DLP = 815 mSv = 11.4 DLP = 414 mSv = 5.8 J. Platt

Impact of 30% ASIR 40% dose reduction
O and 30 % ASiR sets look fine hence overdosed DLP 880 40% dose reduction

Crohn’s: Low Dose (ASIR 40%)
Low dose CT, DLP 388, with first dx of IBD in 70 year old DLP 388 = 5.4 mSv J. Platt

Rules of Thumb If routine images look good (2.5 to 5mm)
Cut dose by 30%: NI 30-36 Add ASIR: – 50% If thin slice imagers look good (0.625 mm) Cut dose by 50%: NI 36 – 40 Add ASIR: – 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: Chest No ASIR, NI 30 30 % ASIR, NI 36
30 % ASiR too much for HRCT- plastic- use 0% ASiR instead or could reduce dose further No ASIR, NI 30 30 % ASIR, NI 36

ASIR: Chest 30 % ASiR 30 % ASiR too much for HRCT- plastic- use 0% ASiR instead or could reduce dose further

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: Technique
Noise index and Automatic Tube Current Modulation (ATCM) for mA kVp and body size Rotation time Centering Body Shields Z – axis control

Cardiac CT: Dose control techniques ASIR: The Theory ASIR the practice How we started Evolution of ASIR practice Cardiac ASIR ASIR: The Impact

The Future: Model Based Iterative Reconstruction (MBIR)
ASiR MBIR 12 mAs FBP Paul Kinahan

FBP vs. ASIR vs. MBIR FBP ASIR MBIR
120 kVp, variable mAs (NI=36), pitch mm, BMI = 34 PAUL KINAHAN

Change is inevitable…. ….except from a vending machine. Groucho Marx

FBP vs. ASiR vs. (MBIR) Simple Advanced Powerful FBP MBIR ASiR
Raw Data Mathematics Real Focal Spot Real Detector 3D Voxel Actual Beam Physics Model Point Focal Spot Point Detector Point Voxel Pencil Beam Point Focal Spot Point Detector Point Voxel Pencil Beam Perfect Sample Statistical Model Advanced Statistical Model Historically, CT technological advancement has been driven by hardware. However, image reconstruction is a critical part of the process. As a matter of fact, the images that radiologists see are the output from the reconstruction engine, not directly from the detector reading. For the past 30+ years, every vendor used a class of reconstruction algorithm call filtered backprojection (FBP). To make the mathematics tractable, many simplifications were made. These include the assumption that the focal spot is an infinitely small point, the detector is also an point located at the detector cell center, the reconstructed voxel has not shape or size, and the measured signal contains no error due to photon statistics or electronic noise. These assumptions are necessary from a mathematics point of view, but the end result is a compromise in image quality. This is due to the fact that these assumptions do not represent the true status of the CT system. In the past 9 years, GE has invested heavily in this area. We teamed up with top researchers from three major universities (ND/Purdue/Michigan) to developed a different class of reconstruction algorithm: model-based iterative reconstruction (MBIR). This algorithm accurately models both the CT system optics (geometric information) as well as statistics. The application of these algorithm can result in significant improvement in terms of spatial resolution, noise and dose reduction, and temporal resolution, as will be illustrated later. The drawback of this approach is the significant computational complexity. Even with today’s state-of-the-art computer platform, the image generation speed still does not meet the routine clinical workflow. To overcome this difficulty, we developed an adaptive statistical iterative reconstruction algorithm (ASIR) that simplifies the system optics modeling during the reconstruction. As a result, a significant increase in computational speed has been achieved. Because ASIR still full models the system statistics, we achieve noise and dose reduction as will be illustrated later. Stop – we keep going until no visual change in image. How many times do you iterate – not relevant - Optimal IQ Lower Noise Hi Resolution More Computation Simple Fast High Noise Better IQ Low Noise Lower Dose

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