ĭuring the operation of a conventional CT device, the X-ray beam leaving the X-ray tube is fan-shaped and is detected by detector elements arranged in an arc rotating in a direction opposite to the X-ray source around the patient lying on the moving patient table (A). However, today’s MSCTs are using increasingly divergent beams due to the presence of detector rows. Collimation results in the formation of a cone or pyramidal beam, as opposed to conventional CT devices where a fan beam leaves the collimator ( Figure 1). The divergent X-rays emitted from the X-ray tube are usually collimated by lead alloy with a rectangular opening, whose size can be adjusted depending on the size of the exposed volume, i.e., the field of view (FOV). Therefore, low energy X-rays must be absorbed by an aluminum filter or a copper filter. However, the energy of X-rays generated is usually diverse hence, not only high but also low energy X-rays leave the X-ray source, which would be absorbed by the soft tissues and would not contribute to the imaging just to the increase of the patient dose. The average time of the exposure is varying from 5 to 40 s depending on the device and the exposure settings. In the latter case, the X-ray source and the detector are in a fixed position but rotate in the vertical plane. The patient is in a standing, sitting, or supine position. The X-ray source and the detector rotate in the opposite direction from 180° to 360° depending on the exposure setting of the device during the acquisition around the patient’s head. The CBCT device consists of an X-ray source and a flat-panel detector, which are connected by a C-arm in a fixed position, but their vertical position can be adjusted according to the anatomical circumstances of the patient. In this chapter, we provide a comprehensive picture of the everyday use of CBCT as a modality in the dentomaxillofacial region and its current limitations and expected improvements. Nonetheless, it should be noted that CBCT devices operate in a wide range of dose values hence, in the particular clinical situation, the proper justification and optimization are crucial. CBCT devices offer a compact size, the ability for producing high-resolution volumetric data, and lower patient dose compared to multislice CT (MSCT). Conventional-computed tomography (CT) has been widely used in the head and neck and other anatomical regions nevertheless, by the invention of CBCT device for maxillofacial imaging in the 1990s, some of the drawbacks of CT were overcome resulting in the application of CBCT as an alternative modality in these regions. The advantages of CBCT contribute to its spreading not only in the field of dentistry, but also in maxillofacial surgery, otorhinolaryngology, rheumatology, and traumatology. The amount of radiation that passes through tissue is given by the Beer-Lambert law, which says that I(E) = (E) *, where I(E) is the beam intensity (energy flux density) per unit energy interval μ is the linear attenuation coefficient and x is the distance the photon travel through the tissue.The use of cone-beam computed tomography (CBCT) has been increasing in everyday clinical practice. As the beam is attenuated, the amount of energy left is less, and the average energy is higher. Compton scattering is about constant for different energies although it slowly decreases at higher energies.Because of the different photoelectric absorption, a heterogeneous beam traversing an absorbing medium becomes proportionately richer in high-energy photons. As E gets larger, the likelihood of interaction drops rapidly. Much of this effect is related to the photoelectric effect the probability of photoelectric absorption is approximately proportional to, where Z is the atomic number of the tissue atom and E is the photon energy. Higher energy photons travel through tissue more easily than low-energy photons because the higher energy photons are less likely to interact with matter. That is, the number of photons passing through in higher photon energy is larger compared with lower photon energy and all substances attenuate low-energy X-rays more strongly than high-energy ones. In figure 1, we can see that as the photon energy is increasing the linear attenuation coefficient is decreasing. X-ray linear attenuation coefficient for bone, muscle, and fat as a function of the incident X-ray photon energy.ġ.1 Beam Hardening - The reason why the mean energy of an X-ray beam increases with it passage through tissue.
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