There is a need for three-dimensional imaging in orthodontics. Cone Beam Computed Tomography (CBCT) has become increasingly popular in dental imaging, but there is need for standardisation of hardware and software if this method is to progress beyond the creation of two-dimensional images.

By Dr J. M. Palomo

Since the inception of radiography in orthodontics, there has always been the feeling that the image was not complete.  The cephalogram was revolutionary in the additional diagnostic, treatment planning and outcome assessment data, but the flat image did not tell the whole story [1,2]. When introducing the cephalogram, knowing its limitations on portraying a three dimensional (3D) subject in a two dimensional (2D) format, recommendations were to take two radiographs, a lateral view and a postero-anterior view, to compensate for such limitations [2].

Orthodontists need imaging in 3D, because patients are 3D.  In traditional cephalometry, 3D craniofacial structures are projected onto a 2D radiographic film.  This process creates cephalometric structures and landmarks that do not exist in the patient. Examples of such structures are the mandibular symphysis, articulare, the pterygoid fossa and the “key ridge.” 
The need for three-dimensional cephalometry was mentioned in the 1960s by Savara, and has been confirmed by many in the literature [3]. Altobelli specifically called attention to the lack of three-dimensional standards for pediatric and adult craniofacial patients [4]. Various manual techniques for abstracting three-dimensional coordinate data from bio-orthogonal head films have been developed [5-8]. This work remained impractical because of the time-consuming nature of pencil tracing of films and computer mouse-based landmark identification from tracings. The use of computers and digital radiographs makes the creation of a 3D image from two different 2D views of the same subject less labour intensive, but still not sufficiently practical and user-friendly for the clinical environment at this point. The importance of computer-based cephalometry was long ago recognised by Ricketts, who wrote: “Cephalometrics, when computerised, becomes the most powerful tool of information yet devised for the practicing clinician” [9].
There are different technologies that can be used for 3D radiography (Magnetic Resonance Imaging, computed tomography, ultrasound, etc) but Cone Beam Computed Tomography (CBCT) has become increasingly popular in orthodontics. CBCT, also known as Cone Beam Volumetric Tomography (CBVT), is considered by many as the latest generation scanner using CT technology. CBCT differs from the other generations of computed tomography in its capture method, which is more volumetric than linear, and the ability to have a smaller footprint than traditional CTs [10, 11]. CBCT was developed to counteract some of the limitations of earlier generations of CT scanning devices and to make 3D technology practical for dental medicine.  The radiation source consists of a conventional, low-radiation X-ray tube, and the resultant beam is projected onto a panel detector, producing a more focussed beam and considerably less scatter radiation compared to the helical CT devices. The total radiation is approximately 20% of that of a helical CT and can be equivalent to the exposure dose received during a full-mouth periapical series[12]. These innovations allow the CBCT unit to be less expensive and smaller in size than a traditional CT machine. When compared to earlier generation CT scanners, CBCT is more sensitive and more accurate, requires less radiation, captures the maxilla and mandible in a single rotation of the X-ray source, and is more cost-effective for patients. Another advantage of the CBCT technology over earlier generations of CT scanners, such as helical CT, is the low level of metal artifacts in the image. An image taken with helical CT of an area close to a metallic restoration, a crown, or an implant is very difficult to analyse and diagnose because of the artifacts and distortions that the presence of the metal creates. This is a major limitation in the use of helical CT images, since many patients have metal present in their mouths. With CBCT technology, the area around the metal presence is usually of diagnostic quality.
 
There are several CBCT scanners currently commercially available. These mostly differ in image receptor type (CCD or Amorphous Flat Panel), available field of view, dedicated or 2D/3D capabilities and overall size/weight [13].

Using the CBCT, a less than 10 seconds scan around the patient’s head can provide more diagnostic information than a panoramic X-ray, full mouth periapical series, lateral and frontal cephalograms and occlusal radiographs. All the above mentioned views can be generated from a single scan, with additional views that are impossible to obtain with traditional radiography [Figure 1]. The resulting images are user-friendly and provide far more information than conventional 2D radiographs. Three-dimensional imaging is capable of capturing both skeletal and soft tissues, which can then be displayed together or separately [Figures 2 and 3]. Axial, sagittal, and coronal “slice-by- slice” images can be observed, along with reference lines that make location of these slices less complicated. For example, even when observing only the coronal view or a small segment of a complete image, lines in the sagittal slice view indicate the height and position of the slice or object being analysed [Figure 4].

CBCT is digital by nature and uses a computer program to construct a 3D volume from a series of 2D images, which, depending on the scanner used, range from 160 to over 600 images. With the 3D format, new terminology is used, for instance, voxel (volume element) is used instead of pixel (picture element). Other terms used for 3D images are “region of interest”, abbreviated as ROI, and “field of view” abbreviated as FOV. The ROI is the 3D region to be evaluated. For example, when asking for a periapical radiograph of the mandibular incisors, the ROI is that incisor area. The FOV is the area captured during the scanning session. The resolution of an image is usually related to the size of the field of view (FOV), which is the resulting size of the image [Figure 5]. For example, if the clinician wants to visualise a cyst in the mandibular incisor area, and a large FOV that includes the entire head is used, the observer must zoom to the ROI, and the image quality will not be as good as a FOV that was focussed only in that incisor area. This concept is similar to what occurs in digital photography. If the clinician wants to see a central incisor in detail, a good starting point would be an intraoral picture of the target area, not the full smile. In the latter scenario, zooming in would make the incisor appear fuzzy, indicating poor resolution. The resolution in CBCT images ranges from 0.07 to 1 mm.

CBCT is now very popular in orthodontics, and more comprehensive information can be extracted, but there is no accepted standardised method for taking images or its use, and there are several questions that remain unanswered. This is a similar situation to when cephalograms were introduced in 1926. It took 17 years until the first accepted and widely used cephalometric analysis was introduced, and almost 30 years for the superimposition technique to be developed [14]. A meeting sponsored by the American Association of Orthodontists called the “First Roentgenographic Workshop’ took place in Cleveland, USA on March of 1957, with the main objectives of defining cephalometric points and planes, standardising the technique, clarifying the interpretation and evaluating clinical application [15]. Fifty-one years later, a similar meeting occurred, also in Cleveland, called the Joint Cephalometric Experts Group (JCEG), with similar objectives, but for CBCT instead of the cephalogram. The American Association of Orthodontists is also planning on releasing a position paper by the end of 2010, to provide some guidelines as to how and when to use CBCT.

Similar to the use of CT in medicine, CBCT in dentistry should have protocols for different situations, specifying milliamperage (mA), kilovoltage (kVp), exposure time, etc. Other needs for standardisation concern software manufacturers. New diagnostic information such as airway volume, is performed differently by different software packages, and provides different results. This interferes with the ability to create norms for such values, and interferes with proper diagnostic communication [16]. 

Different institutions such as Case Western Reserve university, USA are using CBCT on a routine basis, actually making use of all three dimensions, and trying new diagnostic methods [Figure 6]. Based on our experience so far, it seems that CBCT is here to stay, but hardware and software standardisation is very important to allow this method to progress beyond the creation of 2D images. 

References
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The author
Juan Martin Palomo DDS, MSD
Diplomate, American Board of Orthodontics
Case School of Dental Medicine
Associate Professor and Program Director - Orthodontics
Craniofacial Imaging Center - Director
10900 Euclid Ave.
Cleveland
OH 44106, USA
Tel. +1 216 368 2449
e-mail: palomo@case.edu
orthodontics.case.edu
ImagingCenter.case.edu