In about 2000, Elekta introduces a megavolt EPID camera with a solid state image plate based on amorphous silicon (a-Si), mounted on a folding arm.
Follow this link for more information on: Amorphous Silicon. Amorphous silicon is the non-crystalline form of silicon which can be deposited in a thin layer over a large glass plate. On this plate, hundreds of thousands of electronic components and connections can be etched with electrochemical processes, such as is done on silicon computer chips. For an application such as the EPID, a thin glass plate of about 41 cm x 41 cm is given a mesh of 512 x 512 light sensors. In combination with a fluorescent screen and filters, a flat 2-dimensional image detector is made for X-rays.
Click on the link for an explanation of the principal of the a-Si image plate: a-Si image plate principle .
In the AVL the image quality of the a-Si image plate is compared with the fluid filled EPID of Marcel van Herk and colleagues. The a-Si plate appears to make improved images at the same radiation level. This is mainly because of the higher sensitivity and the greater pixel density of 512 x 512 as opposed to the 256 x 256 on the NKI EPID. The a-Si image plate can be read at a higher speed so that more images can be compiled, which increases the image quality.
Portal image of an irradiation field shaped with a multi-leaf collimator. The a-Si EPID (left) gives a better image quality than the NKI EPID (right).The four images are obtained with 9 resp. 72 monitor units (MUs) of radiation per image. 1 MU equals about 1cGy dose in the patient. From the number of detected image frames it appears that the a-Si detector records the X-ray image five times faster than the NKI EPID.
In the NKI the self-developed EPIDs continue to be used for some time. The new accelerators are all equipped with the Elekta system based on the a-Si image plate. Within a few years all machines are equipped with this device.
The Netherlands Cancer Institute collaborates with Elekta and the software group of Marcel van Herk takes up an important role in this. The know-how which has been acquired in the programming for the NKI EPID is used to develop powerful and user-friendly software for the a-Si EPID application.