Introduction (Cone-Beam CT)

 
While the invers Radon transformation, the mathematical background of the computed tomography was elaborated in 1917, the first tomographic imaging system developed only in the second half of the XX century following the dramatic development in computer science. In the 1980’s among all the theoretically possible structures the third generation of the CT systems became dominant.
The first version of these devices had a single detector row along an arc opposite to the x-ray tube and both were mounted on a common framework. Since the examined part of the object is limited by the thickness of the single detector row the x-ray beam should be also collimated according to the active surface of the detector in order to avoid the unnecessary dose to the patient. The shape of the x-ray beam becomes similar to a thin fan which is opened up in the plane of the rotation; therefore these systems are also called to fan-beam CT systems. The data acquisition time can be shortened by adding more detector rows and consequently increasing the axial field of view. These are the multi detector row CT (MDRCT) systems that are often referred as multislice CT (MSCT), because they can acquire data for more slices during one rotation. Recent devices can have 256 or 320 detector rows in an array where the original thin fan shape is getting thicker and becomes similar to a cone. This is the reason, why these systems sometimes also called to cone-beam CT systems. Nevertheless in the daily practice the cone-beam CT denomination refers to a system having a planar detector with large flat active area mounted on a simple, slow rotating framework.
Due to their simple construction the cone-beam CT systems are used in a wide range of applications. In human diagnostic imaging dental radiology, imaging arteries with contrast agents (angiography), planning radiotherapy of the head and neck region are some of the examples. There are CBCT systems designed for imaging laboratory animals and ex-vivo samples, and industrial systems to perform non-destructive testing of electronic components.
The main advantage of the cone-beam CT technology is the very high and isotropic spatial resolution due to the small pixel size of the detector and the relatively high geometric magnification factor. These systems are equipped with a low power x-ray source, therefore the dose to the patient can be kept low. The low power also allows having a closed box design especially for those systems are designed to image small samples. Last but not least the simple structure and small footprint makes them very cost effective alternative to the third generation CT systems.
The limitations of the cone-beam CTs are related to the relatively small dynamic range and high image noise resulting in low soft tissue contrast and therefore restricted diagnostic value. In some applications, especially in dynamic studies using contrast agents, the acquisition time is too long due to the slow rotation. However even with these conditions ECG and respiratory gated imaging can be performed applying strategies are routinely used in nuclear medicine imaging techniques.