The use of storage phosphors for portal imaging in radiation therapy: therapists' perception of image quality.

Two drawbacks in quality of portal radiographs in radiation therapy are their low contrast and low spatial resolution. These are due to the low differential absorption of body tissues at therapeutic energies and to a relatively large radiation source. We used an experimental, high-contrast sensitivity storage phosphor imaging system (Eastman Kodak Co.) to produce portal images. The system consists of a storage phosphor detector, a high-contrast sensitivity laser scanner (12 bit), an image processing module, and a laser printer (12 bit). Patients undergoing radiation therapy treatments had both a conventional portal image and a storage phosphor image taken. Both were displayed side-by-side and were evaluated independently by three radiotherapists according to quality of information to verify the treatment field. Each of the three radiotherapists rated the storage phosphor images to be better (p less than 0.001) than the conventional images. However, rated improvements of low-contrast storage phosphor images of the pelvis and abdomen (40) were significantly lower than those of high-contrast (head, neck, and chest) images (53).