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Digital Detectors: Computed Radiography (CR) for All Radiologists

Author(s): Frans Dhaenens

Journal: Iranian Journal of Radiology
ISSN 1735-1065

Volume: 4;
Issue: S2;
Start page: 6;
Date: 2007;
Original page

Introduction: Digital techniques in radiology have many advantages for radiologists, radiographers, patients, clinicians and hospital-operators from many perspectives: diagnostic quality, dose reduction, access to old and new images and reports via PACS, HIS and RIS."nRadiologists concentrate more and more on organs and pathology (super) specialties rather than in modalities. Organ based radiology requires the integration of all modalities and has to cover large geographical regions to deliver expertise, as there few top-experts. This is only possible in the digital world of HICT (Health Information and Communication Technology), where digital detectors are the first step in a long chain of informatics."nToday most of the diagnostic imaging modalities (CT, MRI, US, PET,..) used in radiology are digital except plain film imaging. In order to digitize plain x-ray (also called General Radiology) there is choice between Computed Radiography (CR) based upon storage phosphor screens and DR (Digital Radiography) based on "flat or panel" detectors. A large spectrum of applications and price ranges is available on the market."nTechnology: Both CR and DR rely on conversion and storage of x-ray energy in "pixel"-like elements in the detector. These detectors are linear (film is S-curved) and have a enormous dynamic range (1 in 100.000) so no information is lost and they resist over- and under-exposure up to 4 times."n- In CR x-ray energy is stored into the phosphor-crystals and converted into visible light after stimulation by (laser) light which is in turn captured and digitized by an optical system. Pixels (in size between 50 and 150 ┬Ám) are one after one stimulated and translated into a digital image of millions of points. "n- In DR systems x-ray energy is converted in 2 or 3 steps into electrical charge that is stored in every pixel of the detector. These charges are digitized row after row to make a digital image. The conversion of the x-rays is done by a "scintillator" or phosphor, known from screens and image amplifiers. There are 2 groups of DR: the first group is amorphous selenium + TFT pixel backplane and the second is a scintillator + TFT pixel backplane. The first directly converts x-ray into charge (2 steps) and the latter first converts x-rays into light and light into charge in a photodiode in each pixel. "nOf course the DR technology is complex and difficult to manufacture in the large formats needed in radiology (14"x17") and explains the high price/unit (>200 k$)."nDR can only be justified in a limited number of situations: high throughput (like chest), high labor-cost and lack of radiographers, radiation-dose regulations."nThe technical nature of DR is very sensitive to environmental conditions like vibrations (mobile applications) and temperature (Selenium has to be air-conditioned) and the portable models are still bulky and costly to put under the bed patients. However, in the future DR prices are expected to come down"n- There are numerous combined systems: lens or fiber coupled CCD systems guiding the x-ray image into a small (and cheap) CCD with a bad image quality."nSlit/slot methods moving a small bundle of x-rays and a line-detector over the patient during several seconds exhausting the tube and the patient...."nThere is no perfect system: most have some advantages and some drawbacks and all of them are compromises between quality and price. "nConclusion: Today CR based on powder phosphors is still the best solution in a starter situation where cost and a broad range of ap-plications are important, because it replaces all types of film-screen cassettes by 1 CR cassette, without changes to the x-ray modality and the workflow. Immediately it allows lower doses and a decrease in repeat rates to
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