TERPSSC 2001 Robert M. Gagne

OVERVIEW OF DIGITAL IMAGING

AND RADIATION PROTECTION

ISSUES

Robert M. Gagne

MICAB/DECS/OST

rmg@cdrh.fda.gov

[From: Handbook of Medical Imaging, Volume I., Chapter 4, J. Beutel et al, eds, SPIE Press 2000]

TERPSSC 2001 Robert M. Gagne

What is the purpose of this presentation?

• Here are a couple reasons:– interest in digital imaging!

– some concerns related to radiation safety and effectiveness

– potential for exposure increase (and/or reduction??)

• Forces a re-visit of some actions in the far radiological health past– imaging system inefficiency

• Review of options (regulatory or otherwise) for dealing with actual and/or perceived concerns

TERPSSC 2001 Robert M. Gagne

What kind of equipment?• Digital radiography (DR)

– not digital fluoroscopy using CCD cameras

– not film digitizers

• Three different types of DR systems – flat panel imaging arrays

– computed radiography systems

– CCD based - optically coupled systems

What are the concerns?• No equivalence to "speed" or “self

limitation” as in screen/film systems• ”Inefficient" systems possible?

TERPSSC 2001 Robert M. Gagne

Flat Panel Imaging Arrays(indirect conversion)

Image Formation

[From: Medical Imaging, 2000]

[From: Handbook of Medical Imaging, Volume I., Chapter 4, J. Beutel et al, eds, SPIE Press 2000]

TERPSSC 2001 Robert M. Gagne

Flat Panel Imaging Arrays(direct conversion)

Image Formation

[From: Medical Imaging, 2000]

[From: Handbook of Medical Imaging, Volume I., Chapter 4, J. Beutel et al, eds, SPIE Press 2000]

TERPSSC 2001 Robert M. Gagne

Number of Pixel Elements (3000 x 2500)

Pixel Element Size ( 0.14 mm x 0.14 mm )

Flat Panel Imaging Arrays

[From: Handbook of Medical Imaging, Volume I., Chapter 4, J. Beutel et al, eds, SPIE Press 2000]

[From: Handbook of Medical Imaging, Volume I., Chapter 4, J. Beutel et al, eds, SPIE Press 2000]

TERPSSC 2001 Robert M. Gagne

Computed Radiography

Image Formation

Pixel Elements ( 0.2 mm x 0.2 mm )

Pixel Elements (2160 x 1800)

[From: Handbook of Medical Imaging, Volume I., Chapter 5, M. Yaffee, eds, SPIE Press 2000]

TERPSSC 2001 Robert M. Gagne

CCD based - lens coupled DR system

Pixel Element ( 0.14 mm x 0.14 mm )

Pixel Elements (3000 x 2500)

CCD based - fiber optic coupled DR system[From: Handbook of Medical Imaging, Volume I., Chapter 4, J. Beutel et al, eds, SPIE Press 2000]

[From: P. Sund et al, Proc. SPIE 3977: 437; 2000]

TERPSSC 2001 Robert M. Gagne

What are radiation protection and safety issues?

• Unique characteristics of screen/film imaging systems– “self limitation” of patient

exposure

– concept of "speed" defined and understood

• New considerations for digital radiography– no “self limitation” as in

screen/film systems

– no consensus on “speed”

– "inefficient" systems possible?

TERPSSC 2001 Robert M. Gagne

Film/Screen “Self Limitation”

• Imaging task with large dynamic range

• Be careful not to under or over expose film

• “Self limitation” of patient exposure

TERPSSC 2001 Robert M. Gagne

Film/Screen “Speed”

• Film/screen “speed”– speed = 100/E where E is exposure in

mR to produce an optical density of 1.0

– position on exposure axis dependent on “speed”

– higher “speed” number translates to lower patient exposure

Difference in speed of about 2

TERPSSC 2001 Robert M. Gagne

DR “Speed”

• DR operating point – equivalence to film/screen “speed” set

at installation?

– no “self limitation” except at extreme ends of the gray-scale transfer curve

– patient exposure increase / decrease / equivalence compared to film/screen?

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 20 40 60 80 100 120

Relative Exposure

Pix

el V

alu

e

Gain 1

Gain 2

Gain 3

TERPSSC 2001 Robert M. Gagne

Imaging System Inefficiency

• Chest radiography - screening program (60s-70s) for cardiopulmonary disease– need for rapid, economical imaging system

• Photofluorographic (PFG) imaging system– mobile vans

TERPSSC 2001 Robert M. Gagne

Public Health Concerns

• Low detection rate for tuberculosis, heart disease, and other respiratory diseases

• High patient radiation exposure vs conventional screen/film radiography– BRH develops standard technique for

estimating patient exposure

TERPSSC 2001 Robert M. Gagne

Digital Radiography (CCD based - lens coupled)

X-rays

Visible light

Phosphor

(object)

Lens

- Not all but focus on those with large object and small image

- Less than 1 % of light photons

make it to film!!

CCD Camera

(image)

Digital Radiography System

• Careful system design to overcome inefficiencies

TERPSSC 2001 Robert M. Gagne

Level of Public Health Concern

• Installation base is small (even smaller for CCD based - lens coupled systems)– computed radiography not included

• Basic questions: Is there evidence of higher patient radiation exposure with these imaging systems screen/film radiography?

[From: Medical Imaging, 2000]

TERPSSC 2001 Robert M. Gagne

Options• Tracking exposure levels

– NEXT 2001 chest radiography

– “diagnostic reference levels” (ICRP, AAPM, many others)

– practical tools for managing radiation dose levels to patients

• Quality assurance programs– “Performance evaluation of

Computed radiography system,” Med. Phys. 28(3), March 2001

• Diagnostic x-ray Performance Standard– performance requirements on levels

of imaging performance such as detective quantum efficiency (DQE)

– dose display at operator’s console

TERPSSC 2001 Robert M. Gagne

Dose Display• Previous slide describes several viable

options– TEPRSSC radiation standards and

safety committee

• Diagnostic x-ray Performance Standard– dose display at operator’s console for all

radiographic equipment

• Practical considerations yet to be explored or evaluated– cost

– effectiveness

– alternatives?

– dose descriptor and definition?

– tie to “diagnostic reference levels”

– effective resource allocation

TERPSSC 2001 Robert M. Gagne

Summary• Different types of DR

– flat panel imaging arrays

– computed radiography systems

– CCD based - optically coupled systems

• Radiation safety and effectiveness issues– no equivalence to "speed" as in

screen/film systems

– "inefficient" systems

– present revisits the past

– potential for exposure reduction and/or increase??

• Options for dealing with perceived and/or actual concerns– one suggested regulatory approach