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Digital X-Rays
Feature Focus 		Vikram Shah, VESIT,
Contributing Editor

Introduction

For decades now, unlike any major medical imaging methods such as ultrasound, nuclear medicine etc, all of which are digital, conventional x-ray imaging remains a largely analog technology.

Why digital?


Making the transition from analog to digital could bring several advantages to x-ray imaging. These would include improvement in contrast and other aspects of image quality by means of image processing: radiological images could be compared more with those obtained from other imaging modalities, electronic distribution of images within hospitals could make remote access and archiving possible, highly qualified personnel could service remote or poorly populated regions from a central facility by means of "teleradiology"; and radiologists could use computers more effectively to help with diagnosis.

The second phosphor based approach is a stimulable phosphor system, in which the phosphor contains traps for electrons excited by incident x-rays. The latent image formed by the trapped electrons is then brought out, in the form of a blue-light image, by illuminating the phosphor, point to point, with a red laser. Unlike the intensifier system, the stimulable phosphor system cannot produce instant images, for the cassette must be carried to a laser scanner for readout by a photo-multiplier, which performs the digitization. The third commercial digital system is based on using an amorphous selenium photoconductive layer to convert x-ray photons directly to charge carriers. It uses an amorphous selenium photoconductor sensitized by depositing charges on its surface by a corona discharge, as in xerography. After exposure to x-rays, the image resides as a charge distribution on the a-Se surface, which is read out electronically and then digitized. With the use of flat panel detectors in the a-Se method the x-ray image is captured and is converted directly to a digital signal for display, processing, and storage. Even a higher resolution can be achieved due to the use of these flat panel detectors in the a-Se method.

Digital X-rays


Digital radiography is currently practiced through the use of three commercial approaches, two of which also depend on phosphor screens. The first phosphor based approach is to digitize the signal from a video camera that is optically coupled to an x-ray image intensifier to provide an instant readout


A Digital X-Ray image (Courtesy:Physics today, June 1997)

Future Potential


The flat panel detector coated with an x-ray photoconductor such as a-Se provides one of the best ways to realize the benefits of digital radiography. The a-Se method has potential for use in fluoroscopy (real-time interactive x-ray imaging) In fluoroscopy a video image on the monitor enables the radiologist to see a moving x-ray picture of the inside of the human body.

Conventional X-Ray
The conventional method for obtaining x-ray images
is to use a cassette containing film held in position behind
an intensifying phosphor screen. The screen absorbs x-rays,
gives out light and exposes the film, which is subsequently
processed to form a final image.

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