Читать книгу Orthodontic Treatment of Impacted Teeth - Adrian Becker - Страница 77

Hounsfield conceived the idea of CT in 1967 and, together with Cormack, invented the first commercially viable CT scanner in 1972. He and Cormack were later awarded the Nobel Prize for their contributions to physiology and medicine. However, the use of CT in dentistry only lasted 24 years until, in 1996, the QRsrl Company from Verona, Italy introduced CBCT with the ‘NewTom 9000’. Radiation was 90% less than for a routine medical CT. This new technology was referred to as digital volume tomography (DVT) and it has revolutionized the world of dental and maxillofacial imaging. In comparison with CT, CBCT has made imaging simpler, more accessible and cheaper. The NewTom 9000 and its successor the NewTom 3G, which was launched in 2004, employed an image intensifier connected to a charge‐coupled device (CCD) camera‐type detector. This was not new technology and other manufacturers, such as Morita (3D Accuitomo), Hitachi (CB MercuRay) and Sirona (Galileos), chose similar technology. All machines employed an image intensifier‐type detector, reconstructed to a sphere‐shaped volume.

In the first years of the new millennium, the newer and superior flat panel detector (FPD) technology was introduced to the dental market by Imaging Sciences International, with its first CBCT machine, the iCAT, employing an amorphous silicon FPD. Within a short time thereafter, all new CBCT machines were fitted with an FPD and today there are over 20 CBCT manufacturers worldwide.

The FPD is superior to its predecessors in all characteristics including its size and weight, but also because it prevents information loss due to the peripheral truncation from which image intensifiers suffer, with their spherically shaped volume. The FPD used in CBCT machines employs indirect conversion, in which the X‐ray energy is converted first into light energy and from there into a signal. The amplitude of the signal from each pixel in the detector is dependent on the amount of illumination indirectly converted. Indirect conversion FPDs have become standard detectors in all CBCT machines. Direct conversion technology from X‐ray energy straight to a signal is (at the time of writing this chapter) the latest expected CBCT developmental stage, which has so far only reached panoramic radiography. It produces high‐resolution quality images with better signal‐to‐noise ratios and dose efficiency.

The detailed workings of a CBCT machine are beyond the scope of this book and will only be discussed here insofar as they relate directly to the context of impacted teeth. For a comprehensive description of the manner in which CBCT works, the reader is referred to a supplement article that appeared in the Australian Dental Journal in 2017 [27].