For electronic portal imaging, describe the principles of acquisition, assessment, and consequences of a positioning error being found.
Electronic portal imaging refers to an electronic detector replacing the radiographic film for portal films. It is faster to perform as the film does not need processing after the image is taken.
The principle of EPI devices are that ionising radiation gives rise to some sort of signal that can then be detected electronically. This can include:
- Fluorescing metal which is imaged by a video camera
- An array of small radiation dosimeters (eg. liquid ion chambers, silicon diodes or scintillation detectors)
The advantage of EPI is that images can be acquired and viewed prior to treatment delivery. If fiducial markers are used (eg. prostate cancer), the therapy staff can ensure that the target organ has not shifted significantly before delivering radiation. If significant shift has occurred, then the patient can be repositioned before treatment takes place.
EPI still suffers from the megavoltage imaging limitations seen in normal portal imaging (poor contrast between tissues due to Compton Interactions).