Portal Imaging Technology: Past, Present, and Future.

Many different electronic portal imaging devices (EPIDs) have been developed to improve geometric accuracy in radiation therapy. This article describes the two types of EPIDs that have become available commercially-the television camera-based EPID and the matrix ion chamber EPID-as well as describing the amorphous silicon array, a device that may become available in the future for portal imaging. In addition, the various image registration techniques that identify geometric errors from the portal images are described. These include interactive techniques, landmark-based techniques, contrast-based techniques and hybrid techniques. Although great improvements in portal imaging technology have been made, more development needs to be directed towards making portal imaging convenient and reliable. Image quality must be improved further, to improve the robustness of image registration techniques and more thought must be given to integrating and automating the various steps in the image registration process. Otherwise, too much time will have to be devoted to these tasks. Finally, and most importantly, users will have to decide what is the best way of using EPIDs clinically. Much development is required before the full potential of this exciting technology can be realized.