This section is a collection of useful information on cone beam computed tomography primarily directed towards residents.
A traditional CT scanner acquires images by rotating around the patient multiple times and collecting images using a 1 D detector array (the craniocaudal dimension is neglible w.r.t the transverse dimension). The image produced in each rotation is displayed as an axial slice. Stacking multiple axial slices on one another gives the image for the whole volume.
A Cone beam CT scanner utilizes a 2 D flat panel detector. As a result it can acquire the image of the whole volume in a single rotation around the patient. The beam that emerges from such a scanner produces a 3D cone shape on the detector (unlike a traditional CT beam that has a shape like a pizza slice). Hence the name cone beam CT which refers to the method of acquisition of a volumetric image of the patient in a single rotation using a xray source and a 2D detector (most commonly a flat panel detector).
Note that cone beam CTs have been in common use in dental and interventional radiological practices world wide almost a decade prior to their use in commercial RT systems.
Principle and Theory
Jaffrey et al identified several key characteristics that an imaging system should possess in order to be used as an aid to IGRT
- Contrast sensitivity sufficient to discern soft tissue
- High spatial resolution and low geometric distortion for precise localization of soft-tissue boundaries
- Operation within the environment of a radiation treatment machine
- Large field-of-view capable of imaging patients up to 40 cm in diameter
- Short image acquisition time ~within a few minutes
- Negligible harm to the patient from the imaging procedure i.e. dose much less than the treatment dose
- Compatibility with integration into an external beam radiotherapy treatment machine
Basically consists of a xray source that is coupled to a flat panel imaging device. The Flat panel detector converts ionization events into electrical signals which is then processed and reconstructed to give the image.
Inital detectors were based on CCDs optically coupled to a phospor screen. However this system was replaced by an amorphous silicon based detector system as the "coupling effeciency" (a fancy term for the percent of signal received by the CCD that is transferred from the phospor) was low. This had the disadvantage that in order to get a good contrast to visualize soft tissues the doses needed to be higher.
The amorphous silicon based imaging system consists of flat panel of hydrogenated amorphous silicon (http://en.wikipedia.org/wiki/Amorphous_silicon) fabricated as a thin film transistor (TFT). Photodiodes are embedded in this TFT matrix. A phosphor screen converts xrays into visible light which is then absorbed by the photodiodes and an electrical signal is generated proportional to the xray radiation received. For those interested aSi flat panels are the same tech that is there in the LCD monitor you are probably viewing this page on.