Optimizing Patient Radiation Dose

John Aldrich PhD FCCPMDepartment of RadiologyVancouver Coastal Health

University of British Columbia

Outline

Changes in Medical Imaging Dose, Effective Dose and Risk Medical Imaging Utilization Status of Patient Dose in BC Skin Doses Summary

Radiology

The use of x-rays in medical imaging has shown rapid growth in the last 10 years

This has been due to Shorter imaging times Increased indications for use

This has increased concern over the associated radiation dose to patients

Exam Type and Patient Dose at VGH

0

10

20

30

40

50

60

70

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Year

Dos

e P

erce

ntag

e

CT

GI/GU

General

Angio N

Angio G

Over the 11 year period the average dose to patients almost doubled

Aldrich et al. CARJ 2005;56:94-99

Benefit and Risk

In all medical interventions there is a balance between benefit and risk

In the vast majority of cases the benefit from diagnostic x-ray studies far outweighs any detriment

What do we mean by Dose?

Surface Dose (mGy)

The surface dose of interest is normally the dose to skin

Effective Dose (mSv)

Effective dose takes into account the radiation sensitivity of different organs.

DE

E

Surface Dose mGy

Effective Dose mSv

External Radiation

Patient Dose

The risk from radiation is measured by Effective Dose, a quantity which combines the radiation dose to the organs irradiated and their radiation sensitivities.

Common unit is the millisievert - mSv Background radiation in Canada is 2 mSv per year Risk of a lethal cancer is 1 in 20,000 per mSv

ICRP (2008) Report 103

Procedure Effective DosemSv

Months of Natural Background

Very Low Dose

Bone density scan 0.0002 0.001

Low Dose

Skull series 0.05 0.3

Chest PA 0.05 0.3

Extremity 0.1 0.6

Thoracic Spine AP 0.5 3

Lumbar Spine AP 1 6

Mammography 2views 1 6

Abdomen AP 1 6

Intermediate Dose

Pelvis AP 1.6 9.6

Head CT 2 12

Upper GI Series 2.8 30

IVP 3 24

Lower GI Series 5 24

Higher doses

Chest CT 7 42

Abdomen CT 9 54

Pelvis CT 9 54

Cardiac Angiogram 8 48

Natural Background 2 12

Procedure Effective DosemSv

No Dose

MRI, US ZERO

Low Dose

CXRExtremities

<1

Intermediate Dose

IVP, lumbar spine, abdomenbone scan, CT head and neck

1 - 5

Higher doses

Chest or Abdomen CTNuclear cardiogramCardiac angiogram

5 - 10

Natural Background 2

Patient Doses

Age and Cancer Mortality Risk

Life

time

mor

talit

y ris

k pe

r 10

0,00

0 pe

r m

Sv)

10 706040 503020

15

30

10

5

0

20

25

0

Age at time of exposure

female

maleICRP 60 average

1996 Re-analysis

Age and Cancer Mortality RiskAge Relative

Risk

Male

Relative Risk

Female

Newborn 3 6

10 2 4

20 1.5 2.5

30 1 2

40 0.6 1

50 0.4 0.7

60 0.2 0.5

70 0.1 0.1

ACR Recommendations 2007

Wider dissemination of Referral Guidelines Routine monitoring of patient doses and

optimization where necessary Improved education regarding radiation

dose from x-ray procedures for all staff involved in or using diagnostic imaging

American College of RadiologyJ Am Coll Radiol 2007;4:272-284

Referral Guidelines

A useful study is one in which the result will alter management or add confidence to the clinician’s diagnosis

It is thought that about 15% of studies are not useful

CAR - Referral Guidelines in Diagnostic Imaging (2005)

Referral Guidelines

The chief causes of the wasteful use of radiology are:

Repeating investigations which have already been done

Investigation when results are unlikely to affect patient management

Investigating too often Doing the wrong investigation Failing to provide appropriate clinical information and

questions

Patient Dose in BC

In the past we have not continuously monitored patient dose

‘Snapshots’ have been taken of the situation In BC we estimated doses from CT in 2004 At VGH we estimated CR and DR doses in 2004 In VCHA we estimated CR doses in 2007

Optimum Dose

The 'optimum dose' is the minimum dose that provides the required diagnostic accuracy.

However, this is difficult to determine A simpler first approach is to determine what doses

are clearly TOO HIGH Then a Reference Dose or Reference Level is

normally set at the 75th percentile.

BC CT Dose Survey 2004

Hospitals around BC were asked to provide data on routine head, chest, abdomen, and pelvis CT

Patient age, weight, slice width, number of slices, kVp, mA, CTDIvol, DLP

Data was collected from 18 hospitals

BC Patient Dose – Head CT

Effective Dose Head CT

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Hospital

Efe

ctiv

e D

ose

mS

v

BC Reference Level

BC Patient Dose - Chest

Chart Effective Doses Chest CT

0.0

5.0

10.0

15.0

20.0

25.0

30.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Hospital

mS

v

BC Reference Level

Summary - BC CT Survey 2004

Study Range mSv

Mean DosemSv

Head 2 – 5 2.8

Chest 3 – 27 9.0

Abdomen 4 – 27 10.2

Pelvis 4 – 16 9.0

Abdo-Pelvis 7 – 32 16.5

Aldrich et al CARJ 2006;57:281

Comparison of Mean Doses (mSv)

CT Study EU 1999

US 2000

Germany2002

UK 2003

BC 2004

Head 2.0 2.0 2.8 2.1 2.8

Chest 8.8 9.1 5.7 9.9 9.0

Abdomen 7.8 8.3 - 7.1 10.2

Pelvis 7.9 5.4 7.2 - 9.0

Abdomen-Pelvis

- 12.1 14.4 9.5 16.5

Diagnostic Reference Levels

CT Study EU 1999 US 2000 UK 2003 BC 2004

Head 1050 960 930 1300

Chest 650 640 580 600

Abdomen 780 530 470 920

Pelvis 570 510 - 650

Abdomen-Pelvis

- 910 560 1100

*75% DLP values mGy.cm for a 70 kg patient

Dose-length product (DLP) is the ‘dose’ reported on each CT scanner for each patient

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0 20 40 60 80 100 120 140

Weight kg

No

ise H

U

Optimization of Abdomen CT

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

20 40 60 80 100 120 140

Weight (kg)

Tu

be

cu

rre

nt

(mA

)

Actual mA

mA for noiseof 16 HU

Predicted and Actual mA

Decrease noise

Increase noise

Decrease dose

Increase dose

Aldrich J et al CARJ 2006 57:347

Computed Radiography(CR)

x-ray image

Exposure

Latent Image

Readout

Cassette - based system using a storage phosphor instead of film-screens

Erase

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.001 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

AEC/Room

Re

lati

ve

Lig

ht

Ou

tpu

t

Company A - Mean 4.9Company B - Mean 8.6

CR Relative Light Emission AEC Centre Cell

Optimized Level

Summary – VCH CR Survey

RadiographicStudy

Relative Dose

AP Abdomen 4 – 14

PA Chest 4 – 22

Patient Dose MonitoringIt is now technically possible to estimate patient dose

from most x-ray systems

Equipment Dose Indicator Status

CT Dose-length product DLP

Available on all CTs

Radiographic/

Fluoroscopic/

Angiographic

Dose-area product DAP

Available on all new units- can be retrofitted

CR Exposure Index (EI) of detector

All CR

Both DLP and DAP values are directly related to patient dose

Dose Area Product Meter

Dose Area Product meter Dose Area Product meter intercepts all radiation intercepts all radiation producedproduced

X-ray X-ray TubeTube

CollimatorCollimator

X-ray X-ray fieldfield

Diagnostic Reference Levels

Exam Dose Area Product

Gy cm2

DAP to E Conversion Factor

mSv/Gy cm2

Skull AP 2 0.03

Chest PA 0.11 0.12

Abdomen AP 2.6 0.2

Pelvis AP 2.1 0.2

Barium Meal 14 0.2

IVP 14 0.1

Coronary Angiography

29 0.15

Practical Measurement

Dual display of skin dose Dual display of skin dose and effective dose during and effective dose during fluoroscopy/angiographyfluoroscopy/angiography

Meter records patient dose for the whole procedure

Record these values rather than fluoroscopy time Newer units record DAP and Skin Dose

Skin Doses

During the 1990s there were many reported injuries to patients following fluoroscopy

High doses to the skin of the patient can cause erythema and even hair loss

This only occurs during long interventional angiographic procedures

The threshold for these effects is about 2000 mGy to the skin

Patient Doses - Fluoroscopy

Fluoroscopy Exam

Skin Dose Rate

mGy/min

Elbow 5

Chest PA 10

Abdomen PA 20

Digital spot 3 mGy/frame

Angiography Skin DosesPublished Data

1. Bor et al BJR 2004;77:3152. McParland et al BJR 1998;71:1753. Van de Putte et al BJR 2000;73:504

Procedure Diagnostic mGy Therapeutic mGy

Cerebral Angiography 1200 1310

Carotid Angiography 215 154

Thoracic Angiography 260 -

Hepatic Angiography 360 540

Renal 620 660

Lower Extremity 68 146

Upper Extremity 73 150

Coronary catheterization 410

PTCA 760

PTCA with stenting 1800

Skin Doses

It is essential that operators are correctly trained. A course for physicians who use fluoroscopy is

mandatory in VCHA and is now being implemented on the VCHA/FH/VIHA intranet

Skin dose should be monitored with a DAP meter, so that patients can be counselled if necessary

Summary - Referring Physicians

Avoid repeating investigations which have already been done

Choose the appropriate investigation Provide appropriate clinical information

and questions

Summary - Education

CAR Referral Guidelines should be made widely available

More information needs to be provided to users of diagnostic imaging on patient dose

Training course for physicians who use fluoroscopy

Summary – Purchasers and Managers

Monitor patient dosesCT DLPRadiographic Rooms (DR, CR) DAP and EIFluoroscopy/Angiography DAP

Only purchase new equipment which incorporates patient dose estimation systems

Display dose indicators as overlays on PACS

Summary – Radiologists, Technologists

Review patient doses Use Diagnostic Reference Levels for

comparison Optimize techniques based on this information Relatively easy to reduce higher doses