of digital receptors (Box 2-1). In addition to reduced exposure, a huge advantage is the ability to enhance images once they are captured. Electronic image storage and image transmission are also advantages of digital receptors compared with film-based systems. While automated analysis can be performed on a film-based image that is converted into a digital image through a process called analog to digital conversion, data is lost in the process, whereas with digital images automated analysis can be performed without any lost data. Staff efficiency is also increased with the use of digital receptors: There is no downtime spent waiting for the image to be processed.
Box 2-1 Advantages and disadvantages of digital cephalometric imaging
Advantages
•Exposure reduction
•Image enhancement
•Digital image storage
•Automated analysis
•Image transmission
•Increased staff efficiency
Disadvantages
•High initial cost
•Differences in projection geometry
There are two potential disadvantages to the use of digital receptors: (1) cost and (2) differences in projection geometry (see Box 2-1). There is no doubt that the initial cost of a digital cephalometric unit is higher than the cost of a film-based system. However, when one factors in the costs of film, processing chemistry, and lost staff efficiency, the difference in initial cost is recouped rather quickly. The issue of differences in projection geometry is covered in the section entitled “Digital Versus Conventional Cephalometry.”
PSP plates
PSP plates are image receptors that convert x-radiation into an electrical charge contained within the imaging plate. PSP plates come in all sizes (from 0 to an 8 × 10-inch plate) for cephalometry. The PSP plate is placed in the 8 × 10-inch cassette with the intensifying screens removed. The imaging plate is coated with europium-activated barium fluorohalide. The electronic information is converted into a visible image by subjecting the phosphor plate to a helium-neon laser. The PSP plate in turn emits a blue-violet light at 400 nm that is captured by the scanner and converted into a digital image. As a final step, the plate must be exposed to white light to remove the latent image; this step is performed in most scanners automatically. PSP plates are considered to be indirect digital images because the x-ray data is captured as analog or continuous data and converted into digital data in the scanner. This is the same reason that film-based images that are scanned as digital images are considered to be indirect digital images.
CCD/CMOS receptors
Direct digital cephalometric x-ray machines use either a CCD or CMOS receptor for image capture. While these two types of digital receptor differ with regard to image capture and data transfer, both generate comparable images. Some panoramic cephalometric combination machines use only one sensor that has to be moved depending on the type of image captured. Other combination machines use two sensors, which is much more efficient and decreases the risk of damaging the sensor by dropping it. The majority of these units capture an image in a scanning motion either horizontally or vertically (Figs 2-5 and 2-6). This type of image capture differs from film-based and PSP imaging, which capture the image in a single exposure. Image capture with the scanning motion requires the patient to remain motionless for up to 10 seconds. The possibility for motion artifact increases as the exposure (or in this case scanning) time increases. At least two companies (Carestream and Vatech) have produced a “one-shot” image capture system that potentially can create the same projection geometry as conventional cephalometry while significantly decreasing the time the patient must remain motionless.
Fig 2-5 Graphic representation of scanning motion for direct digital cephalometric units.
Fig 2-6 Examples of direct digital (scanned) lateral (a) and PA (b) cephalograms.
Digital Versus Conventional Cephalometry
Not all digital cephalometric images are the same. Cephalometric images captured on a PSP plate have the same projection geometry used to capture a film-based image. The majority of digital receptors, however, capture the image with a scanning motion and therefore have different magnification factors than in film-based cephalometry. Chadwick et al reported differences among several different systems that appear to be system dependent and recommended that the magnification factor be experimentally determined prior to any cephalometric analysis.8 McClure et al compared digital cephalometry with film-based cephalometry and found no differences in linear measurements; however, in their study, pretreatment cephalograms were compared with posttreatment cephalograms.9 The time frame between pre- and posttreatment images may introduce the confounder of active growth during the orthodontic treatment.
CBCT
CBCT began to appear in the late 1990s. CBCT machines consist of a radiation source shaped like a cone and a solid-state detector that rotates around the patient’s head and captures all of the scan data in a single rotation.10 This raw data is then reconstructed in the coronal, axial, and sagittal planes (also known as multiplanar reformation) (Fig 2-7). The data can be further reconstructed to produce either 2D images such as panoramic or cephalometric images (Fig 2-8) or 3D data sets11 (Fig 2-9). The images produced with CBCT are not magnified, so standard cephalometric analysis must be altered to address this difference in projection geometry.
Fig 2-7 Multiplanar reformation.
Fig 2-8 Examples of CBCT-derived lateral (a) and PA (b) cephalograms.
Fig 2-9 Examples of 3D reconstructions. (a) Lateral cephalometric rendering. (b) PA cephalometric rendering. (c) Submentovertex rendering.
While the name implies similarity with conventional CT, the two technologies differ in a number of ways. The most important difference for the patient is the difference in dose. Conventional CT produces a four- to tenfold higher dose than CBCT when imaging the maxillofacial region.6 There are several reasons for this difference in dose, but the fundamental difference is that CBCT captures the entire data set in one rotation, whereas conventional CT requires multiple rotations to capture the data. This single rotation decreases the dose but also is more susceptible to patient motion. If the patient moves during conventional CT imaging, only that slice of data is impacted. However, patient movement affects every voxel during CBCT image capture. Another difference has to do with