They are placed on the gantry at a 90 o offset to the primary beam. OBI devices are standard in modern linear accelerators. prostate cancer), the therapy staff can ensure that the target organ has not shifted significantly before delivering radiation.ĮPI still suffers from the megavoltage imaging limitations seen in normal portal imaging (poor contrast between tissues due to Compton Interactions). The advantage of EPI is that images can be acquired and viewed prior to treatment delivery. liquid ion chambers, silicon diodes or scintillation detectors) An array of small radiation dosimeters (eg.Fluorescing metal which is imaged by a video camera.The principle of EPI devices are that ionising radiation gives rise to some sort of signal that can then be detected electronically. It is faster to perform as the film does not need processing after the image is taken. Electronic Portal ImagingĮlectronic portal imaging refers to an electronic detector replacing the radiographic film for portal imaging. Portal images suffer from reduced contrast due to reliance of Compton interactions on electron density rather than atomic number. This double exposure allows the position of the collimation relative to the patient’s anatomy to be checked. 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. A radiographic film is placed on the distant side of the patient to the beam. Portal films are the most straightforward of the treatment verification films. It is important for IGRT systems to be rapid and have minimal impact on patient throughput. If significant deviation has occurred, the radiotherapy delivery may be mildly adjusted (requiring minimal replanning) or completely replanned. The target volume is identified and compared with the volume in planning. More advanced IGRT techniques involve image acquisition prior to every treatment, usually with an on board imaging device, electronic portal imaging or cone beam CT. Portal imaging, on board imaging and cone beam CT are all classified under IGRT.īasic IGRT involves portal films, which are checked on a daily or weekly basis to ensure bony anatomy is aligned with the intended treatment plan. Image guided radiotherapy refers to the use of imaging modalities during treatment to monitor and adjust radiotherapy delivery.
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