Portal films are the most straightforward of the treatment verification films. A radiographic film is placed on the distant side of the patient to the beam. Two small amounts of radiation (typically 2 – 3 MU) are given off by the machine, one with collimation ‘on’ and one with an open beam. This double exposure allows the position of the collimation relative to the patient’s anatomy to be checked.
Electronic Portal Imaging
Electronic portal imaging refers to an electronic detector replacing the radiographic film for portal imaging. 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)
Modern linear accelerators have a kilovoltage on board imaging (OBI) device. This is mounted so its axis is perpendicular to that of the megavoltage beam. Kilovoltage imaging gives much better discrimination between bone and soft tissue and assists with patient positioning. A digital x-ray detecting device is mounted opposite the kilovoltage source.
Kilovoltage images can perform fluoroscopic imaging to detect organ motion.
Cone Beam CT
Cone beam CT is performed with either the EPI (megavoltage cone beam CT) or the OBI (kilovoltage cone beam CT).
- Kilovoltage CBCT has the benefits of increased tissue contrast and resolution (to sub-millimetre).
- Megavoltage CBCT has poorer contrast but has less problems with high-Z artefact seen with kilovoltage imaging.
Unlike diagnostic CT (which typically uses a number of fan beams) the cone beam CT is performed using a single beam which rotates 180 degrees around the patient.