X-ray sensors for megavolt radiation are used, for example, in radiation therapy equipment for the treatment of cancer. The radiation is used on the one hand for the treatment, so that the tumor shrinks, on the other hand, however, X-ray images can also be taken with the same radiation. Based on these images, the patient can be properly positioned and ensured that the tumor and non-healthy tissue is irradiated. In addition, the treatment quality can be checked.
In the image detector, a scintillator converts the incoming radiation into optical light. The molecules of the scintillator are excited by the high-energy photons and the excitation energy is released again in the form of light. This emitted light is then captured by an image sensor and the charges generated in the photodiodes are read out by a high-precision analogue circuit and converted into a digital image.
The development of an image detector for megavolt radiation poses various challenges to the design of the detector. For example, radiation in the megavolt range used in radiotherapy has much higher energy than conventional x-ray radiation used in diagnostic equipment. Appropriate precautions are needed to capture the high energy particles in the sensor and to produce a good image. Furthermore, the geometric arrangement of radiation source, patient and image detector requires a large-area image sensor whose diagonal can be 30 cm or more. The reading of low-noise images from such large sensors therefore places high demands on the analog circuit design in order to minimize line noise and other sources of interference. Also, the ionizing radiation of the source can damage electronic components over time. To ensure reliable and long-lasting operation of the image detector, radiation-proof components must be used and the architecture must be designed accordingly.
In order to generate meaningful images, the radiation source and the read out process of the image data must be synchronized. Depending on the organ or application to be recorded, this synchronization can make different demands on image quality, readout rate (frame rate) or maximum tolerated dose. In order to protect the end user of the machine against such complex settings, different recording modes are normally defined, which the user can select according to his application.
For the development and improvement of existing and new image detectors, the characterization of detectors and their image quality is essential and requires a clearly defined procedure and stable evaluation method. While optimizing a single imaging parameter, this is the only way to ensure that all other characteristics are preserved. In order to check the product quality and to meet the product specifications, the developed methods can also be used during production.