dr. gunn ct lecture
TRANSCRIPT
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Computed Tomography:Physics considerations for
Quality and Dose
Martin Gunn
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Outline
Frequency of CT
Bioeffects of radiation
Radiation dose in the ER
Image noise and radiation dose
kV and intravenous contrast Shielding
Z -Overscanning
Protocol design Prediction rules and utilization
Special considerations
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USA CT Procedures / Year
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
CT
Scans(millions)
Hospital Non-Hospital
Annual Growth > 10% / year. Pop growth < 1% / year
NCRP Scientific Committee 6-2, 2008
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CT Scanners Per Million Population
CT Scanners /million (OECD)
20.62005
15.91995
10.81990
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% of ED Evaluations involving CT
Broder and Warshauer, Emergency Radiology Sept 2006 13: 25-30
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CT Utilization in the ER 2000-2005
Broder and Warshauer, Emergency Radiology Sept 2006 13: 25-30
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Radiation Exposure in BWH
Brigham and Womens Hospital, BostonMA
Longitudinal study looking retrospectivelyat 22 years of data.
190,712 CT exams in 31,462 patients.
Mean 6.1 CTs (54 mSv), max 132CTs (1375 mSv).
15% > 100 mSv 4% > 250 mSv
1% > 400 mSv
Sodickson et al, American Society of Emergency Radiology Annual Meeting, Oct 2008, Houston TX
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BWH Longitudinal CT Survey *
Max 1/15 (6.7%)Max 1/8 (12.5%)
Mean 1/509 (0.2%)Mean 1 / 320(0.3%)
1% of patients >1/62 (1.6%)
1% of patients > 1/38 (2.6%)
3% of patients >
1/100
7% of patients > 1
/ 100
CANCER
MORTALITY
CANCER
INCIDENCE
* Sodickson A, American Society of Emergency Radiology Annual Meeting, Oct 2008, Houston TX
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Diagnostic Accuracy
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Increasing Utilization of CT
Diagnostic accuracy Cx spine, appendicitis, renal colic, multi
rule out.
Replacement of othermodalities:
Volume of CT > study it replaces.
Renal CT vol. > IVU vol. for renal colic
Increased availability
Clinicans and Staff: ? Reduced tolerance for diagnosticuncertainty or delay.
More rapid patient throughput
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BEIR VII Report 2005
Supports Linear No Threshold (LNT)Risk Model
Risk model for cancer development:
1 person / 1000 would develop cancer from10 mSv (CT Abdomen / Pelvis)
Committee to assess health risks from exposure to low levels of ionizing radiation, NationaResearch Council (2005) Health risks from exposure to low levels of tadiation: BEIR VII phas
2, National Academies, Washington DC.
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Relative Biological Risk of Cancer
0 50 100 250150 200
1.01
1.02
1.03
1.04
1.05
1.06
1.00
1.07
BIER VII ReportEffective Dose (mSv)
Lifetimeattributable
risk(LAR)
Linear No Threshold(LNT)
Linear ThresholdHormesis
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The following organizations believe that the currentevidence supports the Linear No Threshold (LNT)model of radiation induced cancer and hereditarydisease.
International Commission on Radiation Protection (IRCP) United National Scientific Committee on Effects of Atomic
Radiation (UNSCEAR) Radiation Protection Division of the UK Health Protection Agency
(formerly NRPB).
National Council on Radiation Protection (NCRP) (USA) National Academy of Science (USA). Environmental Protection Agency (USA).
International Organizations Supporting LNT Theory
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Hormesis
Greek: hormaein: to excite.
Low levels of radiation exposure have a
beneficial effect, lowering the rate ofcancer compared to no exposure. Theory is that a small radiation dose up-
regulates DNA repair mechanisms, adaptiveresponse.
Supported mostly by plant, protozoal andfungal studies, a few mouse studies, and a
few human observational studies. Nearly all studies have serious problems.
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Hormesis: Position ofNational Academy of Sciences (BEIR VII)
BIER VII: The assumption that any
stimulatory hormetic effects from lowdoses of ionizing radiation will have asignificant health benefit to humans that
exceeds potential detrimental effectsfrom the radiation exposure isunwarranted at this time.
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Evidence of Radiation Risks
Studies of humans exposed toradiation.
Mostly from Atomic bomb survivors fromHiroshima and Nagasaki.
Radiation Effects Research Foundation (RERF) and theAtomic Bomb Casualty Commission (ABCC)
Insufficient statistical power at lowradiation doses (< 50-100 mSv).
Linear response above these levels.
Cellular and animal studies used forlower levels.
Latency problem: some cancers take20-30 years to develop.
Lif ti Att ib t bl Ri k f C D th d
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Brenner et al, NEJM 2007 357: 2277
Lifetime Attributable Risk of Cancer Death andAge: Abdominal CT
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Brenner et al, NEJM 2007 357: 2277
Lifetime Attributable Risk of Cancer (10mGy)
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Availability Utilization.
Need fordiagnostic certainty
Concerns about
radiationRegulation
What can we do?1.Use technology to reduce radiation exposure.2.Image patients appropriately
3.Track per scan and per patient radiation dose.
CT Dilemmas
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CMS PQRI Test Measures 2008:
T144: COMPUTED TOMOGRAPHY (CT)
RADIATION DOSE REDUCTION Percentage of final reports for CTexaminations performed withdocumentation of use of appropriate
radiation dose reduction devices ORmanual techniques for appropriatemoderation of exposure.
CMS 2008 PQRI Test Measure Specificationhttp://www.cms.hhs.gov/PQRI/Downloads/PQRI2008TestMeasureSpecifications.pdf
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New technologies.
Getting more for
less.ALARA
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Tube Current Modulation
b d l
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Tube Current Modulation
Longitudinal Tube CurrentModulation
Angular tube current modulation
Combined (Angular-Longitudinal)
Tube Current Modulation
Cardiac CT:
ECG synchronized tube currentmodulation.
Prospective cardiac gating.
Longitudinal Tube Current
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Longitudinal Tube CurrentModulation
Tube Current
Varies the tube current
(mA) along the z-axis
Different mA / dose
applied to differentregions
Scout series used tocalculate mA along z-axisto yield a pre-determined
setting for image qualityGE = Noise Index . 0 380
A l T b C M d l i
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Angular Tube Current Modulation
Radiation output (mA) is adjusted tominimize dose in lower density profiles of
the patients. Occurs during each tube rotation.
mA
C bi d D M d l ti
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z axis of scan
Tub
ecurre
nt(mA)
Combined Dose Modulation
Fixed mA
Dosetoo
highwithfixedmA
Dose too low with fixed m
D S i
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Dose Savings:
1. McCullough CH Radiographics 2006; 26: 503-5122. Hausleiter et al, Circulation. 2006;113:1305-1310
3. Shuman et al, Radiology 2008;248:431-437
Retrospective< 77%3ProspectiveTriggering
Fixed mA< 40%2RetrospectiveGating
Fixed mA0-45%1Combinedmodulation
Comparedto:
SavingTechnique
ECG G t d T b C t M d l ti
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ECG Gated Tube Current Modulation
High TubeCurrent
Low TubeCurrent
Tube current reduced during parts of the cardiaccycle when data not used for coronary CTA isobtained.Beam is on during the whole acquisition.
ECG Gated Current Modulation
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ECG Gated Current Modulation
mA
Prospective ECG Triggering
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Prospective ECG Triggering
Tube on Tube off
X-ray beam is on about 25% of the R-R interval.Step and shoot technique.Predicts timing of next R-wave.Mean dose 6.2 mSv (2.3-11.9 mSv)1
Shuman et al, Radiology 2008;248:431-437
Table movement
Prospective ECG Triggering
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Prospective ECG Triggering
Prospective Gated CCTA
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Prospective Gated CCTA
Partial Scan
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Partial Scan
Tube is turned off for part of the rotation toavoid exposure to radiosensitive organs.
Occurs during each tube rotation.
Tube on for about 232 degrees.
Fig C: Vollmer and Kalender, Eur Radiol. 2008 Aug;18(8):1674-82
Dose Distribution
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Bismuth ShieldingZ Over-scanning
Bismuth Shielding
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Bismuth Shielding
Noise Distribution with Bismuth Shielding
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Noise Distribution with Bismuth Shielding
Adapted from Vollmar and Kalender, Eur Radiol. 2008 Aug;18(8):1674-82
Z- Over-scanning
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Primary beam exposure in areas aboveand below the scan range.
Ends of the helix.
Wider detector arrays and higher pitches.Overscan
Top axial slice Bottom axial slice
Z Over scanning
Adaptive Collimation
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Adaptive Collimation
Top axial slice Bottom axial slice
Collimator Collimator
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Adjusting the ScanParameters:
kVp / Dual Energy CTmA,
Effective mAs or Noise IndexReconstruction kernel.
Display window.Reconstruction thickness
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Changing the kVp and DECT
kVp: Iodine k Edge and Contrast
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kVp: Iodine k Edge and Contrast
Attenuation of x-ray by contrast is affectedby the mean energy (keV) of the photon.
This is lower than the kVp of the beam
With increasing kVp, photon energy increases andattenuation decreases.
At lower kVp, there is greater attenuation due toiodine, as more photons are close to the k-edge ofI (33.2 keV)
Studies have shown an increase in contrastenhancement of vessels (CNR) with decreasingkVp (140 120 100 80.)
kV: Polychromatic X-ray beam
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kV: Polychromatic X ray beam
140Photon energy (keV)
kVp
Photonnumber k-edge of I
33.2
Same Patient, Different kVp
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100 kVp120 kVp
CTDIvol = 419 CTDIvol = 362
, p
kVp and Dose: Exponential
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p p
80 100 120 140
0
20
40
60
80
100
Relativ
eCTDI(%)
120 kVp 140 kVp = 1.4 x in CTDI
120 kVp
80 kVp = 2.2 x
in CTDI
Dual Energy CT
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gy
Dual energy scanning (typically 80and 140kVp):
Dual source (two tubes at differentkV)
Single source with rapid kV switching.Sandwich Detector.
Single helical acquisition. Can generate 80kVp, 140 kVp and
virtual 120kVp, and non-enhanced ima es.
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Graser et al Eur Radiol. 2008 Aug 2 Epub
8080 kVpkVp / 400/ 400 mAsmAs 140140 kVpkVp / 96/ 96 mAsmAs
SimSim UnUn--enhenh
SimSim 120120 kVpkVp
IodineIodine Iodine +Iodine + UnenhUnenh
Radiation Dose and DE CT
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No silver bullet
Dose from single DE CT ~ multiplephase single energy (SE) CT1,2
Single phase DECT acquisition is higher
dose than single phase SECT.Need studies comparing image noise,
number of phases, and diagnosticaccuracy.
1.Chandarana et al, Radiology. 2008 Sep 23. Epub ahead of print.2.Chae EJ et al, Radiology. 2008 Sep 16. Epub ahead of print.
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Increased Noise Index /Reduced Effective mAs
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Reduced Effective mAs
DLP 853
mA 4392.5mm Recon
DLP 325
mA 2445mm Recon
Stab wound to left flankNI = 15.4 NI = 22.0
Reducing the mAs
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Radiology 2003; 229:575580
140 kVp, 170mA, 136mAs 140 kVp, 100mA, 80mAs
CT KUB for Renal Calculi, single and 4 channel CTscanners with fixed mA
Are Lower Dose Techniques Accurate?
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AJR 2008; 191:396-401
Dose of IVU = 2.6 mSvLow dose CT 0.7-2.1 mSv
Routinely used CT Abd Pelv = 8-16 mSv
Sensitivity = 0.966Specificity = 0.949
Ultra-Low Dose CT Colonography
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3D Colonoscopy
ptical Colonoscopy
Surgical Spec
140 kVp; and 10 mAsTotal radiation exp. (prone + supine)1.7 mSv (M) and 2.3 mSv (F).
Optical colonoscopy:9 Ca2 polyps in 15 ptsRemaining 12 patients normal
Ultra-low-dose CT:
Detected all carcinomas10 / 12 polyps (sens 83.3%).Missed 2/6 < 5mm polyps.
Eur Rad 2003 Jun;13(6):1297-302
Noise and Windowing
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WW 3000 WL550 WW 340 WL60
Bone Plus Algorithm
Reconstruction Kernel
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Standard Bone Plus
Slice Reconstruction Thickness
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2.5 mm 0.625 mmDouble Image Noise
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Reduce Phases.
Reduce phase overlap.Reduce scan range.
Center the patient.Reduce Follow-up Exams.
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Reduce Phases: Adrenal Washout
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AJR 2000;175:14111415
OR DO MRI!!
Reducing the Scan Range
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Segmented approachSegmented approachPanPan--Scan ApproachScan Approach
Overlap regions:Overlap regions:
Wasted radiationWasted radiation
Positioning:Body / Profile Size and Symmetry
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Noise increases with increasing
phantom diameter.Also increases in humans, but slightly
differently, due to a number of factors
(asymmetry, tissue type, intrinsiccontrast of fat.
X-ray attenuation increases
exponentially with body diameter
Noise level doubles every 4-8 cm
increase in effective body diameter.
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Arms at side
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Arms up
Arms at side
Standard approachStandard approachTotal Body ApproachTotal Body ApproachmA
Patient Size
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Iterative Reconstruction
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Original way to reconstruct CT data.Replaced by Filtered Back Projection
Latest statistical iterativereconstruction techniques produce:
Less noisy images with significantly lowerradiation dose.
Less beam hardening artifact.
Currently limited by computer power.
Iterative Reconstruction
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2.5 mmImages courtesy of GE Healthcare
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Quality Improvement
How to Approximate Effective Dose
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eDLP FactorRegion
0.019Pelvis
0.015Abdomen
0.017Chest
0.0054Neck
0.0023Head
EUR 16262 EN-European Guidelines on Quality Criteria for Computed Tomography May 1999).
= DLP x k = 0.017x 547.37
= 9.3 mSv
http://faculty.washington.edu/aalessio/doserisk/index.html
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Repeat CT for Renal Colic
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5,564 examinations performed on 4,562patients. 61% women (mean age, 45.5 y)
38% men (mean age, 44.7 y) 3% (44) patients of pediatric age (
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* Does not include examinations performed at other sites
AJR 2006; 186:1120-1124Other examinations have a proven efficacy
Quality Control
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Collect Dose Data on All Scans
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Effective Dose (mSV) = 0.016 x DLP
0.017 x 1710.95 = 29.08 mSv
Head 0.0023, Neck 0.0054, Chest 0.017, Abdomen 0.015, Pelvis 0.019
Other ways to reduce dose
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Not doing a CT!Ultrasound, MRI, x-ray, or no imaging.
Using prediction rules to determinethe need for imaging.
Wells criteria for CT PA for PE, NewOrleans criteria for minor head injury,
Mann-Wilson C Spine CT rules.
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