dr. gunn ct lecture

8/13/2019 Dr. Gunn CT Lecture

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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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Longitudinal Tube CurrentModulation

Angular tube current modulation

Combined (Angular-Longitudinal)


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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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



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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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dr. gunn ct lecture

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