The researchers concluded that the ability to assess the density or quality of bone is limited and, in regions where it is clearly soft tissue, the HU vary greatly and provide little or no meaningful data. In a study by Katsumata et al, 10 the authors found that calculated density (HU) on a CBCT scan varied widely from a range of −1500 to over +3000 for different types of bone. A review of the literature has found that while there is an acknowledgment that a deficiency exists with CBCT systems, they do not correctly display HU, there has been little research conducted to correct this deficiency. In the absence of such a system, it is difficult to interpret the grey levels or to compare the values resulting from different machines. To date, the manufacturers of dental CBCT systems have not used a standard system for scaling the grey levels representing the reconstructed values. HU provide a standard scheme for scaling the reconstructed attenuation coefficients in medical CT systems. The displayed grey levels in CBCT systems are arbitrary and do not allow for the assessment of bone quality, which can be performed with HU in medical CT. 21 HU provide a quantitative assessment of bone density as measured by its ability to attenuate an X-ray beam. 10– 14įor successful implant planning, it is important to be able to assess the bone quality in addition to the height and width of bone and the distance to other anatomical structures such as the mandibular canal or sinus region. 1– 9 The disadvantages associated with CBCT scanners are: increased scatter radiation, the limited dynamic range of X-ray area detectors, beam hardening artefacts and the inability to display actual Hounsfield units (HU) as in medical CT. Cone beam CT (CBCT) systems offer many advantages over medical CT for dental treatment and planning, including a lower radiation dose to the patient in most instances, shorter acquisition times for the resolution desired in dentistry, an affordable cost alternative to medical CT, better resolution and greater detail.
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