Treatment verification using a digital photostimulable phosphor system

Good quality megavoltage portal images can be gained using a digital photostimulable phosphor imaging system. One such computed radiography (CR) system, designed for use in diagnostic radiology departments as a replacement for film-based systems, has been used at the Norfolk and Norwich Hospital, UK to produce localisation portal images. The CR system provides a fast imaging medium, which is useful for single or double exposure techniques. It has certain advantages over fast localisation film in that there are no optimum film dose and latitude considerations to take into account as the system adjusts its readout parameters to compensate for a range of exposures and required latitudes. With respect to electronic portal imaging devices (EPIDs), CR portal imaging has the advantage that it is not machine specific, however it cannot be used for on-line assessment of patient position or on-line adjustment. The practical problems associated with this method of portal imaging are discussed. There is also the possibility of passing CR images in digital format directly to a portal image assessment terminal, thus eliminating the need for hard copies and without the loss in quality which would be associated with the transfer of film images into the system using a digitiser