Portal imaging is the acquisition of images to conform the treatment field setup before the execution of the concern treatment. Portal imaging will be acquired with the help megavoltage beam. But, now a day this trend has been modified by setting the x – ray tube in the gantry or changing the energy of linear accelerator to ordinary KV imaging energy. In all the latest linear accelerator are coming with Electronic Portal Imaging Device (EPID). In addition to that, on-board imager (OBI) is attached with the machine to verify the treatment field. But still EPID is in practice in all the linear accelerators.
Using portal imaging we can verify the treatment field is correct or not. But, it is not possible to get the good quality image to see the treatment field clearly. Because in diagnostic imaging photoelectric effect is predominant but, in therapy Compton Effect is more predominant than other effects. In Compton Effect the scattering of the photon beam would be there because of its high energy. Not only that but also Compton Effect doesn’t depend on atomic number of the material.
Figure 2: shows the EPID of Elekta machine.
Different types of portal imaging are,
1. Camera based Portal Imaging
2. Film based Portal Imaging
Camera based Portal Imaging:
In camera based portal imaging X-ray to light converter, mirror and camera would be used for portal imaging. The photon beam is allowed to fall on metal plate attached with phosphor and then fall on the slanted glass mirror, after that it will be catch by using the video camera.
Film based Portal Imaging:
Film based portal imaging is traditional medium of portal imaging. In Film based portal imaging the ready pack films or autoradiograph quality films are used for verification.
Advantages of Portal Imaging:
1. Treatment field verification during the treatment gives the quality treatment.
2. Images will be available immediately for reference.
3. While x-ray imaging patient have to be shift from treatment room to x-ray room it leads to unnecessary error in the setup.
Disadvantages of Portal Imaging:
1. Bulky attachment is one of the major disadvantages.
2. Poor image quality.
3. Not cost effective.
1. Why image quality in portal imaging are very poor?
a) Compton is predominant in therapy
b) Because of high energy
c) Photoelectric effect is dominant
d) A and B is correct
e) A and C is correct
d) A and B is correct
1. The Physics of Radiation Therapy 4th Edition by Faiz. M. Khan
4. Portal imaging, K A LANGMACK, DPhil, Medical Physics Department, Lincoln County Hospital, Greetwell Road, Lincoln, UK