Diagnostic Requirements For X-Ray Image Intensifiers

DIAGNOSTIC REQUIREMENTS FOR X -RAY IMAGE INTENSIFIERSRobert D. Moseley, Jr., M.D.Professor of RadiologyThe University of New MexicoSchool of MedicineAlbuquerque, New Mexico 87131For almost two decades x -rayimage intensifiers have been widely- -and now are almost universally --usedfor fluoroscopy and cine- radiography;increasingly, small format (70 -105mm)static images, produced by photogra-phing the output phosphor, have beenobtained in those circumstances thatin earlier days would have been accom-plished by larger format "spot films"made to document the findings of afluoroscopic examination.For currently available intensi-fiers, from the standpoint of thequality of diagnostic information aswell as from the standpoint of patientexposure, the most important charac-teristics of x -ray image intensifiersare conversion efficiency, (Ref. 1)modulation transfer function (themeasure of the ability of the systemto resolve image details with accept-able contrast) and signal -to -noiseratio.There are a number of recommen-dations (one international and severalnational) for the definition of andthe measurement of the conversionefficiency of these systems. Basically,the conversion efficiency is the ratiobetween the output screen luminance incandella per square meter to the x -rayexposure rate at the input screen mea-sured in milliroentgens per second.Unfortunately, simultaneouslyprecise parameters for the measurementof "diagnostic image quality" do notexist. There is as yet, incompletestandardization and no agreed uponnational or international recommenda-tions for the measurements of themodulation transfer function or thesignal -to -noise ratio of these devices,and, even if such were available, itis not clear that they define thediagnostic nature of the image. Holm(Ref. 2) has proposed practical mea-surements of image contrast and detailresolution useful both in the labora-tory and on installed image intensi-fiers. He also recommends exposurerates to produce diagnostic fluoroscopy,cine- radiography and small format photo-graphy of the output phosphor withoutthe production of subjectively disturb-ing visible quantum noise; this is atleast a practical, and maybe the onlypossible, approach to the definition ofdiagnostic image quality which, afterall, definitely contains subjectiveelements.In the radiographic mode, theintensifier is in direct competitionwith the large film /screen system andtherefore, certain attributes must beemphasized. They are, good resolution,good contrast, large field size andhigh sensitivity. The system noiseshould also be minimal, although it isrecognized that the quantum noise levelwill inevitably increase as sensitivityand resolution increase.Electron -optical intensifiersoperate by conversion of the incomingradiation energy into light and theninto photo -electrons which are thenaccelerated and focused, usually intoa minified image, by an electron lens,before conversion into an output lightimage. Within the last five years, theresolution of these intensifiers hasapproximately doubled due primarily tothe introduction of phosphors withhigher quantum detection efficiency andsuperior optical properties, the mainadvantages of the rare -earth phosphorsand Cesium Iodide. The vacuum deposi-tion of Cesium Iodide directly onto aphotocathode with 100% packing density