# Presentation: 940583

## 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]

- Robert M. Gagne

- TERPSSC 2001

## 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

- Robert M. Gagne

- TERPSSC 2001

**Notes:**
What is the purpose of this presentation?  I would like to give you some of my reasons for the presentation.  Keep in mind that I’m assuming the reasons are appropriate for presentations to the Radiological Health Council.  First, I am going to talk about an issue which touches both radiation safety and effectiveness.  As such, does it fall under the auspices of the Rad Health and/or Medical Device Act?  This question may be a bit moot but is interesting nonetheless.  Second, it is unclear to em where these sort of discussions belong within the Center?  At this point, only two of the Offices (OST, ODE) know some of the details.  Lastly, somewhat similar to the last point, we thought it time to get some intra-CDRH publicity on this issue.

In fact, the radiological health issue forces a re-visit to the past for a similar sort of problem and possible solutions.  I am speaking, specifically about photofluorographic imaging systems.  (Details to follow.)

So, we wanted to use this time to acquaint the Council with the issue, action items already accomplished and the radiation safety and effectiveness concerns.  At the same time, we solicit their counsel on the matter.   

## 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?

- Robert M. Gagne

- TERPSSC 2001

## 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]

- Robert M. Gagne

- TERPSSC 2001

## 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]

- Robert M. Gagne

- TERPSSC 2001

## Flat Panel Imaging Arrays

**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]

- Robert M. Gagne

- TERPSSC 2001

## 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]

- Robert M. Gagne

- TERPSSC 2001

## 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]

- Robert M. Gagne

- TERPSSC 2001

## 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?

- Robert M. Gagne

- TERPSSC 2001

## Film/Screen “Self Limitation”

**Film/Screen “Self Limitation”**

- Imaging task with large dynamic range
- Be careful not to under or over expose film
- “Self limitation” of patient exposure

- Robert M. Gagne

- TERPSSC 2001

## 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

- Robert M. Gagne

- TERPSSC 2001

## 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?

- Robert M. Gagne

- TERPSSC 2001

## Imaging System Inefficiency

- Chest radiography - screening program (60s-70s) for cardiopulmonary disease
- need for rapid, economical imaging system
- Photofluorographic (PFG) imaging system
- mobile vans

- Robert M. Gagne

- TERPSSC 2001

**Notes:**
In the 60s and 70s, there were several screening programs in the US for cardiopulmonary disease.  I’m pretty old but I actually wasn’t a member of the government at the time.  I do, however, remember getting a screening radiograph with the device that I am going to describe next.  

Since the screening program was intended to apply to a large population of people, there was a need for a rapid, economical imaging system.  The photofluorographic (PFG) x-ray machine possessed some of these characteristics.  These x-ray systems were commonly used in mobile vans and transported from place to place.  The PFG system, basically,  consists of a phosphor screen to convert absorbed x-rays to visible light, an optical lens system to focus the light and a film magazine to record the image.

The figure which comes from a 1975 FDA report shows the basic components of the PFG system. 

## Public Health Concerns

**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

- Robert M. Gagne

- TERPSSC 2001

**Notes:**
As the years went by, there was an increasing public health concern with this screening program for two important reasons:
- a low detection rate for tuberculosis, heart disease and respiratory diseases exactly the diseases to be detected under the screening program; and
- a high patient exposure for the PFG systems versus conventional screen/film radiography as the means of imaging the chest.

As can be seen from the sample of references of the period, this concern was manifested in a series of recommendations, policy statements and articles in professional journals.  The Bureau of Radiological health was involved in developing standard techniques for estimating patient exposures when using PFG systems (DHEW Publication No. (FDA) 75-8007).  Yes, I was here in this period of time.  I’m that old!

## Digital Radiography (CCD based - lens coupled)

- Careful system design to overcome inefficiencies

- Robert M. Gagne

- TERPSSC 2001

**Notes:**
Notice the similarity between this depiction of the Imaging Dynamics digital radiography (DR) system and the old PFG system.  They appear similar because they are similar.  The only difference is that the DR system uses a CCD camera instead of film to record the image.  Since they are similar, the new DR system suffers from the same source of inefficiency as the PFG system from the 60s.  I should point out that I am trying to equate the public health concern with the PFG systems to that associated with the newer DR systems.  The newer DR systems represent a much smaller public health concern.  The intent is only to try to “go down the right path” from the beginning for the newer devices. 

## 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]

- Robert M. Gagne

- TERPSSC 2001

## 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

- Robert M. Gagne

- TERPSSC 2001

**Notes:**
In the past, with respect to radiological health activities, our approach to problem solution was focused on the risk side of the risk/benefit equation, e.g., recommend a limit on exposure for making an image with a PFG system.   This  approach was taken for the PFG systems.  This sort of recommendation can have the effect of eliminating the device since worthwhile images are not possible at normal exposures.  Currently, we haven’t abandoned the risk side but we are also focusing on the benefit side of the risk/benefit equation.  For example, guidance documents for 510(k) submittal for devices providing digital images whether mammography and/or non-mammography requests some fairly sophisticated imaging performance information.  In a nutshell, this information allows you to make some value judgments on the imaging performance per unit dose, i.e., a very important parameter.  On the other hand, the current Diagnostic X-ray System Performance Standard addresses mainly the risk side.  Quite frankly, the Performance Standard has not kept with the sophistication level of the community whether user, government or physics.  

## 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

- Robert M. Gagne

- TERPSSC 2001

## 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

- Robert M. Gagne

- TERPSSC 2001