the axial (horizontal) and cross‐sectional (vertical) ‘slices’ as selected provided information in the bucco‐lingual plane, which had been generally impossible to discern with routine plane radiography. These views contributed materially to the evaluation of the prognosis of the intended treatment. Thus, the bucco‐lingual proximity of teeth and the existence and extent of oblique root resorption all become assessable, and these were and are important factors in determining choice of teeth for extraction or indeed whether to undertake treatment at all.
In a study performed in 1988 [19], the prevalence of resorption of the roots of incisor teeth, as associated with an impacted canine, was investigated by plain 2D radiography and found to affect 12% of the individuals in the sample. When the same investigators repeated their study 12 years later using spiral CT scanning [23], the number of affected individuals increased to 48%! There can be little doubt that this was due to this vastly improved diagnostic tool and to the fact that resorption of the buccal or palatal aspects of the roots of the incisor teeth cannot be seen on regular radiograph. It is only when the buccal or palatal resorption has become sufficiently extensive to cause a change in the shape of the mesio‐distal profile of the root that it may be identified by plain 2D radiography and this type of resorption would go undiagnosed.
CT offers advantages in assessing the proximity of the impacted tooth to an adjacent pathological entity. It also provides valuable assistance in evaluating aberration in the shape and appearance of the crowns and roots of teeth that were suspected of having been damaged or have suffered from abnormal development due to past trauma [24].
Conventional spiral CT machines, as used in routine hospital practice for imaging various parts of the body, expose the body to an X‐ray beam in the form of a progressive spiral, encircling the body over a specific, defined area, with continuous radiation during the whole scanning time. This submits the patient to a high dose of ionizing radiation and has been a subject of concern when considering its use in the dental context. The dosage was evaluated by Dula et al. [25, 26] using what they called a ‘hypothetical mortality risk’. In this assessment, the mortality risk associated with routine dental radiographs ranged between 0.05 and 0.3 × 10−6 units, depending on the type and number of radiographs performed, while a CT scan of the dental area alone was assessed at 28.2 × 10−6 for the maxilla and 18.2 × 10−6 for the mandible.
Cone beam computerized tomography
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].
Cone beam computed tomography technology
The X‐ray source, emitting a pyramidal‐shaped beam, is mounted on a gantry facing a detector. Unlike the rotation centre in a panoramic machine, which slides during its rotation, the CBCT gantry axis is fixed. The patient is positioned with the centre of the region of interest (ROI) in close proximity to the gantry's rotation axis. The gantry performs 180–360° rotation around the patient’s head, during which, in most machines, multiple exposures (150 to about 1000) are taken. The number of exposures taken depends on the protocol chosen in the specific machine, but certain machines radiate continuously during the exposure. The 2D images taken in this single rotation of the patient during the scan are collectively known as raw data, which is then reconstructed into a 3D volume. This procedure is called a primary reconstruction and is carried out by a software algorithm. The algorithm will calculate and create the volume according to the protocol chosen and is depicted in the form of a cylindrical stack of axial slices, reminiscent of thin slices of salami, one on top of the other.
Processing the scanned information
When the orthodontist’s CBCT machine is ‘in‐house’, the imaging process is initiated with the software supplied by the machine manufacturer, or with third‐party software. When the patient is referred to an imaging centre, the imaging technician will produce a DICOM (Digital Imaging and Communications in Medicine) set, representing the international standard for image format and file structure for communication, handling, storing, and printing of medical and dental imaging and image‐related information. It will be handed over together with a viewer program supplied by the CBCT unit manufacturer, or with a third‐party software viewer preferred by the imaging centre. Often, however, more digitally savvy orthodontists may decide to use their own third‐party software to achieve the same ends. In this case only, the DICOM set is handed over and the orthodontists employ their own software.
Many orthodontists will prefer to patronize a professional imaging centre, who may offer a service to perform a work‐up of the case. The technicians at the centre are experts in specifically providing an accurate positional diagnosis of the tooth/teeth in the three planes of space and in relation to the adjacent teeth and other anatomical structures. The work‐up should be adequate to the task of analysing and discovering the existence of pathological entities, leading to the diagnosis of the cause of the impaction, a task for which this modality is second to none.
Secondary and online reconstructions can alter the patient position by tilting in any desired axis in space and slicing it in any direction and in any chosen slice thickness. The slices are not limited to straight cuts, with the panoramic view being the most popular curved cut. A thick panoramic view looks similar to the traditionally irradiated 2D panoramic view and it has the advantage that its form is without horizontal or vertical magnification and, therefore, without distortion. It is possible to avoid overlapping of individual
