TERPSSC 2001Robert M. Gagne OVERVIEW OF DIGITAL IMAGING AND RADIATION PROTECTION ISSUES Robert M....

TERPSSC 2001 Robert M. Gagne

OVERVIEW OF DIGITAL IMAGING

AND RADIATION PROTECTION

ISSUES

Robert M. Gagne

MICAB/DECS/OST

[email protected]

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


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


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?


Flat Panel Imaging Arrays(indirect conversion)

Image Formation

[From: Medical Imaging, 2000]



Flat Panel Imaging Arrays(direct conversion)

Image Formation




Number of Pixel Elements (3000 x 2500)

Pixel Element Size ( 0.14 mm x 0.14 mm )

Flat Panel Imaging Arrays




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]


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]


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?


Film/Screen “Self Limitation”

• Imaging task with large dynamic range

• Be careful not to under or over expose film

• “Self limitation” of patient exposure


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


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


Imaging System Inefficiency

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

• Photofluorographic (PFG) imaging system– mobile vans


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


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


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?



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


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


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

TERPSSC 2001Robert M. Gagne OVERVIEW OF DIGITAL IMAGING AND RADIATION PROTECTION ISSUES Robert M....

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