CBCT use in modern endodontics

14 January, 2014
 

Cone beam computer tomography (CBCT) is a modern, three-dimensional imaging system, which produces high-quality images using relatively low doses of radiation. Initially introduced to dentistry for implant planning and maxillofacial uses, CBCT is increasingly being used in endodontics. These systems are changing the way we diagnose, treatment plan and assess outcomes in endodontics.

This article will address the following three questions:

1 What is a CBCT scan?

2 How do CBCT scans compare with intra-oral radiographs?

3 When is a scan beneficial?

What is a CBCT scan?

CBCT scanners use a pulsed beam of X-ray radiation to produce highly accurate three-dimensional images. A cone-shaped X-ray beam is directed through the area of the patient who is to be imaged, and a digital detector collects the attenuated beam. During the scan, the X-ray tube and the detector revolve around the patient’s head in a manner similar to an OPT (Fig 1).

Scans can be performed over 180 or 360 degrees of rotation and typically take 10-20 seconds. The pulsed nature of the X-ray beam means that the actual exposure time of the scan is significantly lower than the time taken to perform the scan – for example, a 20-second scan can result in less than 3.5 seconds of exposure time.

Cone beam CT scans deliver a significantly lower dose to the patient than conventional CT scanning. Conventional CT scanners emit a narrow, fan-shaped beam of X-rays, which means that multiple rotations of the scanner must occur to image a volume of tissue. The cone-shaped beam enables CBCT scanners to gather all the required data in a single sweep. In addition, conventional scans are performed using a constant beam of X-rays, rather than the pulsed beam used in CBCT. This means that patients are exposed to radiation for less time with CBCT scanning and hence receive a lower radiation dose. Advances in detector technology have further lowered the dose required for CBCT scanning.

There are many different CBCT scanners on the market. Some scanners, such as the iCat, scan large volumes of tissue (e.g. both jaws) and are useful for maxillofacial surgery and implant planning. Other machines, such as the Accutomo, scan much smaller volumes of tissue (e.g. a single tooth) and are very useful in endodontics. Scanners capable of imaging variable volumes of tissue are becoming increasingly popular in dental practices because they are highly flexible and, therefore, have a wide range of applications.

In general, radiation doses are lowest when exposure parameters are lowest and field of view is small1. However, different types of scanner will emit differing doses even when the same exposure parameters and field of view are used2 (Fig 2). How do CBCT scans compare with intra-oral radiographs? Intra-oral views, such as periapical radiographs, suffer from several limitations that can hinder diagnosis and treatment planning. For a geometrically accurate image of a tooth to be generated using a periapical radiograph, several principles must be adhered to: the film or detector must be parallel and as close as possible to the long axis of the tooth, the X-ray beam must be parallel, and it must meet the long axis of the tooth and the detector at 90 degrees.

While long cones and beam-aiming devices can help achieve this, in reality it is virtually impossible to produce an accurate image with intra-oral radiography; even the best periapical radiographs have approximately 5 per cent magnification. This can be complicated further by anatomical limitations, such as a shallow palate, which could bend the film or detector, rendering the image useless.

CBCT scans do not rely on this geometry to produce images and, therefore, these scans overcome distortion. Such scans have been shown to be geometrically accurate in three dimensions 3, enabling reliable measurements to be taken in any plane.

The second limitation of intra-oral radiography is anatomical noise. Anatomical noise is generated when unwanted structures are superimposed on the image. Such structures (e.g. the maxillary buttress or the cortical plate) can limit the information that is obtained from the image. CBCT scans do not have this limitation, because the anatomy before and beyond the tomographic slice being viewed is not visible, enabling a clear view of the desired structure to be obtained.

The final limitation of intra-oral radiography is a lack of reproducibility. It is virtually impossible to take two periapical radiographs from the same angle, even with customised film holders.

This can make comparing two images, taken at different times, quite difficult. CBCT scans overcome this because they image a volume of tissue which can be viewed in any plane, allowing the comparison of exactly the same tomographic slice in two separate scans. It has been shown that measurements from CBCT scans are reproducible over time, regardless of variations in patient positioning, so different scans taken at different times can be reliably compared4.

Despite these advantages, it is important to note that CBCT scans also have limitations. Even the best-quality CBCT scans have a lower resolution than digital intra-oral radiographs: CBCT scans have a typical resolution of two line pairs per millimetre, whereas digital radiographs can have a resolution of up to 20 line pairs per millimetre.

CBCT scans are vulnerable to beam hardening and scatter, which can reduce the image quality even further. Scatter occurs when X-ray photons are diverted from their path by the tissues they are passing through; some of these scattered photons will contact the detector and be incorporated into the data set, lowering the quality of the image.

Beam hardening occurs when an X-ray beam passes through a dense object, for example, a metal filling. The beam is said to be ‘hardened’ when the lower energy photons are removed by the dense object, leaving only the higher-energy photons. This phenomenon can produce dark bands or streaks in the image.

Computer algorithms are usually applied to CBCT scans to help reduce the effects of scatter and beam hardening.

CBCT scans and periapical radiographs should be viewed as being complementary. Good-quality periapical radiographs are an excellent diagnostic tool, but it is important to be aware of the limitations of these images and how CBCT can be used to overcome them.

When is a scan beneficial?

Difficult diagnosis

Periapical radiolucencies are not visible on periapical radiographs until the apical granuloma has eroded into the cortical plate5. This is an example of the anatomical noise of the cortical plate masking the presence of the periapical lesion. This phenomenon can be frustrating for the clinician, when a patient reports a history of symptoms of apical periodontitis and no radiographic evidence can be detected. CBCT scans can be of use in such cases because of their ability to overcome anatomical noise. This has been demonstrated by ex vivo6 and in vivo studies 7,8.

Difficult root canal treatment/retreatment

The high-quality images obtained from low-volume CBCT scans are very useful to plan the endodontic treatment of cases that are technically challenging. In particular, they are useful in the location and identification of additional/unfilled roots and canals. By scrolling through the tomographic slices in the coronal, sagittal and axial planes, it is possible to follow the path of individual roots and canals in order to determine their location, number, anatomy and patency (Fig 3).

Unusual anatomy

CBCT scans are also invaluable in the management of complex anatomical cases, such as dens invaginatus and dilaceration. In such cases, careful treatment planning is essential in order to ensure successful management. In unusual anatomical situations, often the canals are located in atypical sites and access can be extremely difficult, making the risk of perforation high (Fig 4). Apical surgery Planning apical surgery must always be done with consideration of local anatomical structures, as well as the location and size of the periapical lesion. CBCT scans allow the identification of structures such as the inferior alveolar nerve and the maxillary sinus, as well as identifying periapical lesions on other roots of the tooth which may not be visible on a periapical radiograph. Information such as this will help inform prognosis, risk of surgical complications and flap design (Fig 5).

Root resorption

Root resorption can broadly be categorised as internal or external. Internal resorption can be inflammatory or replacement resorption, whereas external resorption can be surface, inflammatory, replacement or cervical resorption. It can be difficult to distinguish between internal resorption and external cervical resorption. Traditionally, parallax radiographs have been used to determine the location of the resorptive lesion; however, it has been shown that CBCT is more accurate than parallax in distinguishing between internal resorption and external cervical resorption9,10.

Accurate diagnosis of the type of lesion is essential for correct management. CBCT scanning will help determine the type, location and extent of the resorptive lesion, as well as aiding decision-making regarding restorability (Fig 6).

Trauma

Dental trauma can be one of the most challenging and stressful things to deal with in dentistry. Until recently, conventional wisdom to determine the nature and extent of a traumatic dental injury has been to take a standard periapical radiograph, as well as horizontal and vertical parallax views. Using this technique, three periapicals and one upper occlusal would be required to image the upper anterior teeth. Even with these radiographs, determination of the position of a root fracture following trauma is highly dependent on the angle of the X-ray beam11, so fractures may still be missed. CBCT is more comfortable than intraoral radiography for recently traumatised teeth and there is a significant difference in the localisation and diagnosis of root fractures between conventional radiographs and CBCT12.

The IADT guidelines have recently been amended to reflect the superior diagnostic ability of CBCT, however, scans are still not routinely recommended.

Conclusions

Obviously, whenever ionising radiation is used in dentistry, we must endeavour to keep the dose to the patient as low as possible and exposures must be justified. With this in mind, selection criteria for the use of CBCT in endodontics have been produced and these guidelines can be obtained from http://www.sedentexct.eu

Exposures should be made in line with published selection criteria and the principles of ALARA. As with any radiographic exposure, CBCT scans must be fully reported by an appropriately trained individual.

Cone-beam computer tomography has overcome the limitations of intra-oral radiographs and has improved our ability to diagnose and treatment plan. CBCT scanning is now being used in research to help us understand how teeth heal following root canal treatment and this, in turn, will drive improvements in how we perform these procedures.

Ultimately, these advancements allow us to improve the quality of care we provide to our patients, and allow us to be more confident in the treatment we provide.

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