Compact CT devices show clinical promise

Conebeam CT (CBCT) evolved from microCT technology. The first devices, developed for nonhuman studies, allowed 3D imaging with high spatial resolution to be applied to studies of bone architecture and mineralization, adipose tissue quantification, visualization of heart valves' structural failures, quantification of radionuclide tomographic images, vasculature, and experimental endodontology.¹ All of these techniques required spatial resolution in the range of 1 to 60 micron, warranting exposure times of 20 minutes to several hours.

Increasingly compact scanners are now being developed for dental and maxillofacial imaging. These include CBCT devices, also called digital volume tomography scanners, volumetric CT, or modified micro CT devices.^2-5 Such systems are cheaper and smaller than conventional CT scanners, and they emit less ionizing radiation.^2,3,6 The size of each device depends on whether the patient will be sitting or supine. Some devices are available for intraoperative use.⁷

CBCT systems use a cone-shaped x-ray beam rather than a collimated fanbeam. Height and diameter of the beam's cylindrical field-of-view can vary from 3 to 4 cm up to nearly 20 cm. More modern systems are extending this limit still further.^2-5 Some machines allow the FOV to be selected to suit the particular examination. FOV mode options may include facial, panoramic, implant, and dental.⁵ Software is sometimes available to change the FOV in older systems.

Image data are recorded in a single 360 degrees or 180 degrees gantry rotation. A 2D sensor uses either image intensifier or digital detector technology. The C-arm moves through an angle of 190 degrees in one device designed specifically for intraoperative 3D imaging (Siremobil ISO-C-3D, Siemens Medical Solutions).⁴ Exposure time is 9.6 seconds or more, and voxel size can be as small as 0.1 mm.⁵ Exposure times of six seconds or even less are becoming possible as devices continue to evolve.

Gantry rotation produces raw data for primary reconstruction, and options for slice thickness depend on the individual machine. Primary images can be used for secondary reconstructions in all planes and for producing 3D views. Creation of 2D images perpendicular to the dental arch or in panorama may be available on some systems. These are similar to images produced by reformatting software used with conventional CT scanners such as DentaScan (GE Healthcare).^2,5

The radiation dose from CBCT is much lower than that associated with conventional CT, ranging from two or three times lower to as much as six times lower,^2,8,9 although low-dose protocols can reduce the dose associated with a conventional CT examination to nearly that of a CBCT scan.⁸ CBCT's effective radiation dose is still higher than that for a plain-film x-ray, however. Studies have reported the CBCT dose to be three to 10 times higher than the dose for a panoramic radiograph.^8,10 Varying the CBCT FOV will have an impact on the effective radiation dose.

CBCT machines are unable to discriminate soft tissue because exposure settings and image detection techniques (for instance, use of an image intensifier) make contrast resolution much lower than that of conventional CT. Further, CBCT can provide only partial projection data, unlike conventional CT. Because it is impossible to estimate CT values accurately with partial projection data, CBCT image reconstructions are based on calculations of relative values.³ This is not usually a problem, given that the main objects of interest are generally bones and/or teeth. Metal causes artifacts as in conventional CT.

CBCT has excellent high-contrast resolution as a result of the small size and geometry of its isotropic voxels.⁴ Voxels used in conventional CT are usually 0.3 mm or larger, whereas CBCT voxels can be as small as 0.1 mm.3,5 Conventional CT also has relatively low longitudinal resolution compared with its axial resolution because longitudinal CT images are produced by summing axial ones.⁵

Submillimeter isotropic resolution has become possible with 64-slice CT scanners, and comparative studies of radiation exposure and resolution between this advanced technology and CBCT are now warranted.

Opinions differ on the accuracy of CBCT systems. Two studies rated geometric accuracy for the New Tom 9000 CBCT system (Quantitative Radiology) to be good.^2,11 A separate report concluded that the scanner underestimated distances between skull sites.¹² This should not affect the system's reliability for linear evaluation measurements of structures more closely associated with dentomaxillofacial imaging, however. Another group of authors compared the accuracy of limited CBCT and spiral CT when measuring the vertical distance from a reference point to the alveolar ridge. Results from five cadaver mandibles showed a statistically significantly smaller measurement error for limited CBCT than for spiral CT (1.4% and 2.2%, respectively; p < .0001).¹³

Accurate examination of delicate structures in the oral and maxillofacial regions is often difficult on conventional x-rays, and images may be insufficient for diagnostic use. Conventional radiographs provide a 2D view of 3D structures, whereas CBCT enables all structures to be evaluated in any plane desired. The use of CBCT in many dentomaxillofacial fields appears promising. Evidence at present generally consists of preliminary results or case reports. Clear recommendations on the clinical use of CBCT are not yet available.

ORAL AND MAXILLOFACIAL SURGERY

Conventional CT is recommended for imaging associated with implant surgery only if cases are anatomically difficult or treatment is particularly extensive.¹⁴ CBCT, with its lower radiation burden, is an option for 3D imaging of routine cases. It can evaluate preoperative bone volume and structure as well as complications involving implants (Figure 1).¹⁵ A radiopaque template is especially useful for preoperative implant planning if the FOV is small, the area of interest is wide, and occluding teeth are not visible on the images.¹⁶

CBCT can be useful for clinical assessment of alveolar bone grafting.¹⁷ This conclusion follows evaluation of 17 bone bridges after alveolar bone grafting in patients with cleft lip and palate. A separate study concluded that CBCT with multiplanar visualization can assist with pretreatment evaluation and decision making for complex impacted mandibular third molars. The authors used CBCT before surgical removal of 92 third molars to visualize the location of the teeth and neighboring structures.¹⁸ CBCT images were made for a total of 81 lower and 11 upper wisdom teeth, where existing orthopantomograms had not been sufficiently clear to allow a definitive diagnosis. Detailed visualization of the impacted third molar was possible in each case in all planes. Exact reconstruction of the course of the inferior alveolar nerve was successful in 93% of cases.

Bone and dental fractures can be evaluated by CBCT as well (Figure 1). It is often advantageous to scan trauma patients in a supine position, and the FOV is crucial in facial fractures. Because of CBCT's small FOV and its lack of soft-tissue visualization, preoperative assessment of orbital fractures remains the domain of conventional CT in order to correctly image concomitant fractures and assess the state of orbital muscles and retrobulbar hematoma.

Linsenmaier et al have successfully used an intraoperative CBCT device with a C-arm for postsurgical imaging of the facial skeleton.⁴ CBCT has also been used with success in diagnoses of acute dental trauma.¹⁹ The dental fracture line is easier to visualize in nonrestored teeth than in teeth with metal restorations, which can produce artifacts.

Foreign bodies occur as a sequela of trauma or therapeutic interventions, and their removal is always preferred if the risks are not too high. CBCT is an appropriate tool for the detection of radiopaque foreign bodies. Items made of wood and resin can be missed if located in muscular tissue. Detectability decreases further when the object is adjacent to highly radiopaque tissue such as bone.²⁰

Clinical cases of radicular cyst and recurring ameloblastoma have been described in the literature.^15,19 CBCT offers a good means of assessing the lesion's size and location, assuming its dimensions are within the imaging FOV and soft-tissue evaluation is not mandatory. Conventional CT or MRI should be used if these criteria are not met, and CBCT can be used as a complementary imaging method in these cases. Tumor invasion by squamous cell carcinoma of the bone structures, for example, is an indication for complementary CBCT examination.¹⁵

More experience and studies are needed to evaluate CBCT's possible benefits. Soft-tissue tumor cannot be visualized on CBCT images, and contrast-enhanced studies are not possible. Combining conventional CT and/or MRI with CBCT is one option that could be explored in further research.

DENTAL INFECTION, ORTHODONTICS

CBCT has been evaluated for the diagnosis of periapical lesions in a study of 31 teeth from 24 patients.²¹ Periapical lesions showed up on 71% of radiographs and 97% of CBCT studies. Apical-marginal communication was shown on 7% of the periapical x-rays and 19% of the CBCT studies. Artifacts caused by root canal posts, however, prevented visibility of the marginal bone level in 23% of teeth examined with CBCT.

Periapical lesions of maxillary molars were seen expanding into the maxillary sinus on 32% of x-rays and 68% of CBCT studies. CBCT could thus become a valuable tool for diagnosing odontogenic sinusitis and complicated endodontic cases.

CBCT is a reliable tool in evaluating periodontal defects as well. Both CBCT and conventional CT displayed only a slight deviation in the extent of periodontal defects when compared with histological specimens. Both techniques permitted imaging of anatomic osseous structures in three planes, true to scale and without overlay or distortion. CBCT scans showed better image quality.²²

We have found that CT can often detect secondary changes of a tooth root fracture, that is, a vertical intra-alveolar bone pocket to the level of the fracture line, often combined with root canal post ends at the level of the fracture line. The modality can also visualize suspected root fracture (Figure 2).

CBCT is indicated in orthodontics for the detection of impacted and ectopic teeth and the demonstration of the amount of bone available for orthodontic tooth movement (Figure 3). Patients with cleft lip and palate may receive CBCT so practitioners can visualize the size of the alveolar cleft and also evaluate the position and movement of any disrupted teeth.²³ We have found that possible resorption caused by impacted teeth is often difficult to visualize with panoramic x-rays, intra-oral x-rays, or tomographic images. CBCT permits detection or exclusion of resorption relatively easily.

CBCT can play a valuable role in investigating temporomandibular joints (Figure 4), but MRI can also be used in this manner. MRI involves no ionizing radiation and permits the joint's soft tissues to be visualized. CBCT can be recommended if the goal of imaging is to assess the temporomandibular joint's bone structures. Pathological changes such as formation of osteophytes, erosion, fractures, ankylosis, and developmental abnormalities, as well as the position of the condyle in the fossa

in open and closed mouth conditions, can be monitored. Images obtained in planes parallel or perpendicular to the long axis of the condyle should be reconstructed.²⁴

Various groups have considered CBCT for a number of otorhinological questions. These include evaluations of the osteomeatal unit or the middle ear. Dalchow and coworkers reported the use of CBCT in examinations of the temporal bone and its value in patients with conductive hearing loss.^25,26 They concluded that CBCT is an excellent technique to examine the middle ear cleft and inner ear, and it expands the number of potential diagnostic applications in the lateral skull base.

Despite its smaller FOV and lack of soft-tissue delineation, CBCT offers an interesting alternative to multislice CT if a distinct area has to be examined or followed up. CBCT imaging is cheaper than MSCT, involves a relatively low radiation dose, and provides high-resolution images.

DR. SUOMALAINEN is senior consultant in the radiology department at Helsinki University Central Hospital in Finland. DR. ROBINSON is a radiologist at the Urania Diagnostic Center in Vienna.

References

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