terpssc 2001robert m. gagne overview of digital imaging and radiation protection issues robert m....
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TERPSSC 2001 Robert M. Gagne
OVERVIEW OF DIGITAL IMAGING
AND RADIATION PROTECTION
ISSUES
Robert M. Gagne
MICAB/DECS/OST
[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