Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

scintillator(CSI)~TFT detector elements -glass substrate ~-electrode- selenium layer- TFT detector elements- glass substrateFIGURE 11-10. A: A cross-section of an indirect detection flat panelsystem is shown. X-rays interact with a scintillator, where visible light isemitted and diffuses toward the photosensitive detector elements ofthe panel. B: Direct flat panel detectors often use a layer of amorphousselenium photoconductor coupled to a thin-film transistor (TFT) array.In a direct detection flat panel device, x-ray interaction in the seleniumlayer releases electrons (or electron holes), which are used to form thesignal directly. An electric field applied across the selenium layer minimizesthe lateral spread of the electrons, preserving spatial resolution.Selenium (Z = 34) has a higher atomic number than silicon (Z = 14), but it isstill quite low compared with conventional x-ray intensifying screen phosphors thatcontain elements such as gadolinium (Z = 64) or cesium (Z = 55), and therefore theattenuation coefficient is relatively low at diagnostic x-ray energies (40 to 130 keY).Intensifying screens need to be kept thin enough to reduce light spreading andthereby preserve spatial resolution. However, because electrons do not spread laterallyin selenium direct detectors due to the electric field, selenium detectors aremade much thicker than indirect detection systems, to improve x-ray detection efficiency.Making the selenium detectors thicker compensates for the relatively low x-ray absorption of selenium. In addition to selenium, other materials such as mercuriciodide (Hgh), cadmium telluride (CdTe), and lead iodide (Pbh) are beingstudied for use in direct detection flat panel systems.Mammography presents some of the greatest challenges for digital detectors becauseof the high spatial resolution that are required to detect and characterize microcalcifications.Small field-of-view digital mammography has been used for several years