History, Technique, and Safety

Abstract

The scope of dental imaging has been greatly expanded with the invention of cone beam computed tomography (CBCT). The everyday functionality of dental practice, especially for dental specialties like orthodontics and oral surgery, has certainly changed with this radiographic technology for treatment planning and evaluation. The discovery of X-rays was made by Wilhelm Conrad Rontgen in 1895, who was a known physicist for his work. He won a Nobel Prize in 1901 for this discovery. Ever since this revolution, constant technologic advancements have been made in the field of dental radiology that had resulted in improving the diagnostic accuracy and reducing the radiation exposure in every day dental practice. In dentistry, conventional two-dimensional (2-D) radiographic imaging has been widely used. However, the conventional radiographic images have limitations like inherent magnification, distortion, superimposition of structures, and lack of depth for three-dimensional anatomical objects. Over the years, the technology has improved tremendously in terms of image quality and radiation dose. With CBCT, the visualization of structures is possible with much clarity and without superimposition. The technology has shown a profound impact on the dental practice with its widespread applications.

1 Introduction

Computed tomography (CT) imaging was originally called computed axial tomography (CAT) scan. Researchers started the development of medical CT scanners in the 1960s. Later around 1970–1972, Dr. Godfrey Hounsfield (electrical engineer at EMI Central Research Laboratories, England) and another physicist Allan Cormack of Tufts University (Boston, MA) introduced the CT imaging modality for clinical applications. The Nobel Prize was awarded to both of them in 1979 for the development of computerized tomography, as the technology had a profound impact in improving the diagnostic methodology. The technology was patented by Dr. Hounsfield in 1973 [1]. In the mid-1970s, a full-body scanner was developed by a dentist-physicist Dr. Robert Ledley of Georgetown University (Washington, DC).

To capture the data, the medical CT scanner uses an X-ray beam and image detectors, mounted and fixed on a rotating gantry, which rotates around the patient to capture the region of interest. During the rotational movement of the gantry, the X-ray beam passes through the patient, and the remnant X-ray photons remaining after the attenuation are captured by the image detectors. The “raw data” acquired by this process is reconstructed by a computer algorithm, and the end result is generation of cross-sectional images of the patient’s tissues. The process uses a series of radiographic images to create sequential images, and virtual slices of body tissues are produced.

Initially, the first-generation CT scanners acquired the data in the axial plane by “slice-by-slice” scanning with a narrow fan-shaped X-ray beam and a single array of detectors. Eventually, the development of spiral CT (1989) and the multislice image detector systems (1988) leads to the acquisition of volumetric data [2]. Modern CT scanners are much faster as they use array of multiple detectors with the rotating fan-shaped X-ray beam and capture multiple slices of data simultaneously, in a short period of time. This has resulted in shorter scan times and lesser radiation dose to the patient as well [3].

With this technique, information about the internal structures is obtained by reformatting of the data and production of cross-sectional images, and the structures are visualized without superimposition. For image display, the components of the gray images are pixel and voxels. A voxel defines a point in three dimensions, whereas the pixel defines a point in two dimensions. Pixel or picture element represents the smallest single module of an image in a two-dimensional (2-D) framework. The attenuation of X-ray photons or signal by the patient’s tissues determines the value and intensity of each individual pixel that is captured by the detector, and the information is displayed on the computer screen. Pixel size effects the image resolution. Voxel adds detail and third dimension (3-D) or depth to the image.

Medical multi-detector computed tomography (MDCT) units use Hounsfield units (HU) to display relative density values of various body structures according to a calibrated gray value scale.

For viewing, the reconstruction of data produces images in multiple imaging planes. During the CT scanning process, the data is captured in the axial or transverse plane. Axial plane is an imaginary plane that divides the structure or body into upper and lower portions. From this axial data set, the computer software programs can generate multiplanar reformatted images in axial, sagittal, and coronal planes by combining the information. The sagittal plane sections the structure or body into right and left, and the coronal plane divides the structure or body into anterior and posterior sections. Also, 3-D computer-generated models of the structures can be made. MDCT units have superior contrast resolution and can display soft tissues with more superior quality. Medical- or hospital-based CT units have large footprints and are supine gantry-style units with considerably high radiation exposure to the patient. Before the introduction of CBCT for dental needs, MDCT units were utilized for diagnosis and treatment planning for only limited cases in dentistry. Factors like lower radiation dose and ease of use in dental setting lead to the development of cone beam computed tomography.

2 What Is Cone Beam Computed Tomography (CBCT)?

CBCT was introduced in the early 2000s to the main market. As stated earlier, before CBCT was introduced to the main market, the conventional CT and MDCT scanners were used by the dental specialties to obtain cross-sectional views for pathology, maxillofacial trauma, and in limited number of dental implant cases. However, the utilization of MDCT was very limited due to higher radiation doses as compared to CBCT. Cost of the procedure was also very high. Many CBCT systems are available (Fig. 1.1).

Fig. 1.1

Picture of currently available ProMax 3D CBCT scanner (Courtesy of Planmeca Oy, Helsinki, Finland)

Cone beam computed tomography (CT) has the potential to reduce the size and cost of CT scanners. Because this emerging technology produces images with isotropic submillimeter spatial resolution, it is ideally suited for dedicated dentomaxillofacial CT scanning. When combined with application-specific software tools, cone beam computed tomography can provide dentomaxillofacial practitioners with a complete solution for performing specific diagnostic and surgical tasks, such as dental implant planning.

The other terms used to describe this technology include cone beam volumetric imaging (CBVI) and cone beam volumetric tomography (CBVT).

The introduction of low-dose CBCT scanning systems has changed this approach for everyday dental practice. It is specifically designed to produce three-dimensional images of the maxillofacial region. The computer software programs are designed for dental needs.

CBCT scanners are connected to a computer, and the data or the region of interest is acquired with a single full 360° or partial rotation of the cone-shaped X-ray beam and reciprocal rotating single image detector around the patient’s head. The scan times are usually less than 15–20 s. The system uses back-projection reconstruction tomographic technique. MDCT acquires image data using multiple rows of detectors, where multiple slices must be stacked to obtain a complete image [3].

The CBCT technology exposes the whole region of interest or the head of the patient with one flat-panel detector. This baseline data are then used to generate individual image slices in different planes. In CBCT image acquisition, there is no additional mechanism needed to move the patient during the scanning, and also the use of cone-shaped beam in CBCT increases the utilization of the X-ray energy by lowering the X-ray tube heat capacity required for volumetric scanning as compared to a fan-shaped beam in MDCT [4]. CBCT units have isotropic (equal in all three dimensions) voxel resolution in which images with isotropic submillimeter spatial resolution are produced [3]. Image detectors with smaller pixels tend to capture fewer X-ray photon per voxel and thus result in more noise. Higher radiation doses are required for a reasonable signal-to-noise ratio, which improves the image quality. The following factors also effect the spatial resolution and image quality: focal spot of the X-ray generator, patient-to-detector distance, X-ray source-to-patient distance, and patient movement. Smaller focal size, reduced patient-to-detector distance, and increased X-ray source-to-patient distance minimize the geometric unsharpness of the images. In practice, movement of the patient’s head is a big factor that will deteriorate the image quality [4]. Many CBCT machines have artifact reduction tools that allow minimization of the noise due to metal streaking after acquiring the images. One company offering a tool which helps reduce movement related artifact after acquisition of the data. The rotation of the cone-shaped X-ray beam and the detector around the patient’s head generates large amount of data that is rapidly transferred from the rotating scanning system to the external computers for further processing for visualization in axial, sagittal, and coronal planes, and 3-D reconstruction is done. Images are produced with isotropic submillimeter spatial resolution, and the application-specific software tools are available for use by the dentists.

With advancements, the modern CBCT systems have integrated very well in the dental practice. Smaller footprint of the machine, simplicity of operator training procedures, ease of use, short exposure time, easy patient positioning for scanning, integration into the workflow of the practice, accuracy of information, and availability of relatively simple viewing software have led to the popularity of CBCT systems. However, the CBCT cost and radiation dose are considered to be higher as compared to the conventional 2-D dental imaging procedures. As with any other radiographic technique, the aim is to achieve optimal image quality with the lowest possible radiation dose, which could be challenging. CBCT units are smaller in size and can fit in a dental office with some modification. In majority of the CBCT machines, the patient sits in the chair for the short exposure time.

CBCT digital imaging produces 3-D data of the area of interest with diagnostically acceptable spatial resolution, much lower radiation dose, and cost as compared to the MDCT. For dental practice the first CBCT machine, NewTom 9000 (Quantitative Radiology, Verona, Italy), was developed and introduced in the European market in the late 1990s.

This technology was brought to the market in the United States in 2001. For scanning in the NewTom 9000, the patient had to be in supine position and the X-ray tube and the detector rotated 360° around the patient’s head to obtain a relatively larger field of view (FOV) 15 cm × 15 cm volume. The system utilized an image intensifier and a charge-coupled device. The sensor was 8-bit displaying 256 shades of gray. Later developments were made to fabricate CBCT machines with smaller, adjustable FOVs. Later Ortho-CT based on Scanora stand (Soredex Corporation, Helsinki, Finland) made it possible where patient would sit in a chair during the scan. In 2002, 3D Accuitomo unit (J. Morita Corporation, Japan) became available in the European market. In this scanner the patient sat in a chair for exposure, and the FOV size was reduced to 3 cm × 4 cm cylinder [5].

Since then, tremendous improvements have been made in image quality. Currently the CBCT systems offer different sizes of FOVs and 12-bit sensors or more, displaying 4096 shades of gray with 12-bit. CBCT machines use single flat-panel detectors with amorphous silicon. Various FOVs, image acquisition parameters, image reconstruction algorithms, and viewing software programs have become available, providing choices for the user. It has been reported that, at this time in the market, there are around 50 CBCT devices available from 20 manufacturers, which are operating in 20 different countries around the world (Table 1.1) [5]. Today’s CBCT units are equipped with head restraining/positioning devices that help better position the patient during the scan to reduce movement artifacts. Software programs have post-processing tools that can be used to minimize image noise artifacts after data acquisition. It is important that the whole dental team be knowledgeable about the availability of these tools in the software. This will potentially reduce the number of re-exposures.

Table 1.1 Selected CBCT systems available with larger fields of view (FOVs)

3 Acquisition of CBCT Volume

CBCT devices consist of X-ray source (cone-shaped divergent beam) and a 2-D image detector. The X-ray source and the image detector are connected by an arm that rotates around the patient’s head during the scan. Rotation varies from 180° to 360°. Typically, the volumetric data is captured with single rotation around the patient’s head as the transmitted beam of radiation is aimed at the image detector. Beam collimators either match the size of the detector and the beam size or can be used to further reduce or collimate the field of view.

With most CBCT units, a series of 2-D raw base images or projections are captured. The number of raw images varies from 180 to 600 or up to 1000 in some machines. Exposure times vary from 6 s to 40 s. Many machines have pulsating radiation, which helps reduce the patient dose. The ranges for tube current (mA) and peak voltage (kVp) are 1–15 mA and 85–120 mA, respectively. After processing axial, coronal, and sagittal planes, images appear on the computer monitor as an Explore screen (Fig. 1.2).

Fig. 1.2

Explore screen from a CBCT machine with large field of view is shown. This is a typical image display in coronal, sagittal, and axial planes

4 Image Detectors Used in CBCT Units

The image detector or receptor converts the incoming remnant X-ray photons from the patient into electrical signals. Later computer processing converts these signals into visible images. CBCT machines are equipped with either image intensifier tubes/charge-coupled device (II-CCD) or a flat-panel detector (FPD). II-CCD units are usually bulkier as compared to the FPD. FPD is made up of scintillation crystal screen on a matrix of photodiodes embedded in a solid-state amorphous silicon layer with thin-film transistors. The signal intensity is proportional to the stored charges. The advantages of FPD include higher radiosensitivity, lesser radiation exposure, and better image quality.

5 Field of View (FOV)

FOV is the anatomical volume that can be captured by the detector. FOV varies in size. The machines come with various detector sizes. Machines with larger detectors offer larger FOV, with ability to collimate the X-ray beam to a small area or FOV. With collimation of the X-ray beam, the FOV can be reduced to suit the needs, and this reduces the amount of exposure to the patient. Multiple FOV options, ranging from few centimeters to full head size, are available for various clinical scenarios.

Larger detectors tend to be more expensive. Due to the cost factor, some CBCT systems offer smaller detectors with limited FOV. When there is need to acquire the larger FOV, two or more adjacent scans can be made, and the volumes can be stitched together by the computer software to produce a larger FOV.

Decreasing the size of FOV or beam collimation improves image quality by decreasing scatter artifacts in the image. The extent of anatomic coverage should be based on clinical evaluation by the treating clinician. Over collimation or too narrow collimation to achieve smaller FOV may result in excluding essential anatomic structures needed for evaluation, and thus a “not needed” retake of CBCT may be needed. Scarfe and Farman [6] published a FOV categorization of the different CBCT systems according to the CBCT volume height and provided examples of coverage as follows:

• Craniofacial region: Height > 15 cm (extending from the head vertex to the inferior mandibular border).

• Maxillofacial region: 10–15 cm in height (nasion to inferior mandibular border).

• Interarch region: 7–10 cm (extending from the inferior nasal concha to the mandible).

• Single arch/jaw: 5–7 cm (maxillary or mandibular arch only).

• Localized to region of interest: 5 cm or less in height (1–2 teeth and surrounding bone, temporomandibular joints).

FOV can also be classified large for craniofacial coverage (>10 cm in height) and small to medium for dentoalveolar coverage (variable depending on the region of interest <10 cm in height). Smaller volume or FOV should be considered if it addresses the diagnostic needs [7].

6 Reconstruction Process and Display of CBCT Images

With a single rotational movement of the CBCT machine used for exposure of 20 s or less, approximately 100 to more than 600 individual frames may be captured by the acquisition computer. A volumetric data set is created with the individual basic frames by a series of algorithms or reconstruction process at the processing computer or the workstation. Both computers are connected via an Ethernet connection for transfer of the acquired individual frames from acquisition computer to the workstation for processing.

For image display, the CBCT units also use HU units. However, in CBCT, the measured density numbers correspond to the grayscale values and do not directly represent HU units. Smaller field of view scans have more discrepancies related to the density values as the contributing structure may be located outside the area of interest. After exposure, the raw data set is reconstructed with the computer software to produce cross-sectional images in axial, coronal, and sagittal planes (orthogonal planes). Sectional or reconstructed panoramic image can also be obtained with multiplanar reconstruction from the same data set. Furthermore, cross-sectional images of a region of interest can also be obtained perpendicular to the curve of the dental arch, which are widely used for implant treatment planning.

The operator can change the cross section or slice thickness. With this data, lateral cephalometric images can also be generated. Other advantages of CBCT images include no image magnification or distortion, and the software designed for dentistry is equipped with tools such as ruler for accurate 1:1 linear and angular measurements.

The CBCT data can be displayed in various formats such as volume rendering and maximum intensity projection (MIP). Spatial relationship between structures can be visualized by volume rendering, which gives a three-dimensional impression of the volume with different colors and transparency levels, based on attenuation or gray values. In a MIP, only the highest voxel value is displayed within the selected thickness in area of interest.

The contrast and brightness can be easily adjusted (changing the window width and window level) to improve the display. Bit depth of the system determines the number of shades of gray available to display the attenuation. Display of the images depends on the ability of the system to display the variations in attenuation and the capability of the image detector to show the subtle contrast variances.

Most newer machines offer 14-bit or 16-bit image detectors, which translates to 2¹⁴ (16,384) and 2¹⁶ (65,536) shades of gray or contrast display, respectively. Higher bit-depth systems require more processing time and considerably larger data set files, which may require greater storage capability.

Segmentation of an area of interest is a very useful tool when separation of certain structures is desired from the volume for in-depth analysis.

CBCT machines from the most major manufacturers have the capability to export data in standard DICOM (digital imaging and communications in medicine) format. With DICOM data, the user can use a third-party software to view and analyze the data set.

Some CBCT machines offer “volume stitching,” where a larger field of view can be obtained by stitching the data sets obtained by a smaller detector.

7 Factors Influencing Image Quality in CBCT

In order to understand the image quality, the noise, scatter, spatial, and contrast resolution of an image must be discussed. Image noise and scatter are factors that are often encountered. Image noise occurs due to inconsistent distribution of signal and inconsistent attenuation or gray values and is visualized as “grainy appearance” of an image. Image noise can potentially degrade the image display and obscure the structure of interest. In order to decrease noise, the exposure time has to be increased.

Scatter in the images is produced by diffraction of X-ray photons from the original pathways, and upon interaction with the image detector, these photons end up producing nonuniformed increased intensities of structures. This interaction results in inferior contrast resolution on the resultant image. Changes of scatter are more with larger image detector or field of view. As CBCT machines use a single 2-D image detector with a cone-shaped beam, scatter is more seen with CBCT as compared to medical CT scanners [5]. Scatter is also produced by the patient from anatomical structures or existing restorations. Current CBCT systems have software tools to minimize the scatter after running the artifact reduction algorithm.

The spatial resolution is measured in line pairs per millimeter (lp/mm). It is the ability to distinguish fine detail or structures that are located very close together. Higher spatial resolution means clear or sharp distinction between the shades of gray or structures on the image. Higher spatial resolution can be attained with smaller voxel size. However, this requires higher exposure time. Scans with smaller field of view also have superior spatial resolution.

Ability of an image to display subtle differences between tissues of different radiodensities is the contrast resolution. In other words, clear distinction in various shades of gray is seen on the image. Following factors tend to decrease the contrast resolution: image noise, scatter, larger fields of view, reduced milliamperes, and kilovoltage settings of the X-ray generator.

8 Imaging Protocols for CBCT and Indications

Utilization of CBCT imaging has certainly increased over the last decade, and many research studies have been published that document that its use enhances the diagnosis in a significant number of clinical cases and thus has shown to improve treatment outcomes [7]. The clinician may choose to this technology if it is believed that there will be benefit in the patient care. However, the factors like higher radiation exposure, greater cost, and cost of interpretation must be considered.

Only after detailed clinical examination, the need for any radiographic imaging must be determined. Following main parameters influence the imaging protocol and image quality: exposure settings, voxel size, scan time, and field of view. The operator must understand that change in the exposure setting will effect the image quality and radiation dose to the patient. Spatial resolution is the ability of an image to display detail. Different voxel sizes are offered with CBCT machines. Voxel size should be specified for acquisition or reconstruction stage.

Longer scan times acquired more basic frames. The advantage is fewer artifacts and better image quality. One must remember that using the longer scan time protocol will result in longer reconstruction times and more radiation dose to the patient [8].

Some CBCT machines are equipped with low-dose or ultralow-dose exposure modes.

CBCT may be advocated for patients in situations where plain conventional radiographic images are not adequate for addressing the diagnostic issues [9].

General common reasons for obtaining CBCT scan in orthodontics would include localization of unerupted teeth, resorption of root, bone grafting, and assessment of cleft palate [10,11,12].

Depending on the indication or the region of interest, the CBCT field of view (small, medium, or large) may be selected. The fields of view and possible indications are listed in Table 1.3. Once the data set is acquired, the clinician should screen through the full volume systematically in all three dimensions. The region of interest should be evaluated in axial, sagittal, and coronal planes. It must be kept in mind that the CBCT data must be evaluated in entirety or the complete volume by the clinician prescribing the scan. The scan can be read by an oral radiologist to get an additional report on the region of interest and incidental findings of significance and to rule out pathologic conditions.

After gathering this information, the practitioner can refine or change the initial diagnosis and the proposed treatment plan, as needed.

The CBCT volumetric data is usually backed up in the proprietary format. The data export is usually done as DICOM v3 (digital imaging and communications in medicine standard version v3) format, so that the data can be imported and viewed in third-party software applications as needed.

The following guidelines for the use of CBCT imaging can be recommended for the orthodontic clinical practice:

• Anomalies of the teeth, especially marked oligodontia and supernumerary teeth, impactions (esp. permanent incisors and canines), transpositions.

• Anomalies of the craniofacial complex, especially craniofacial syndromes, including cleft lip and palate. Class II division 2 malocclusions.

• Orthodontic treatment involving functional orthopedics (fixed or removable functions), maxillary orthopedics, self-ligating non-extraction protocols, extractions of permanent teeth, multiple TADs (temporary anchorage devices) and orthognathic surgery.

• Airway obstruction, including constant mouth breathing, snoring, obstructive sleep apnea.

• Class II division 1 malocclusions with mandibular retrognathia Class III malocclusions with mandibular prognathia and/or maxillary hypoplasia.

• Early treatment cases involving facebow headgear, facemask, lip bumpers.

• Cases prone to root resorption, especially those involving trauma, previous history of root resorption, marked intrusion/extrusion cases.

• Advanced periodontitis.

A task force was formed by the American Association of Oral and Maxillofacial Surgeons to study the possible indications for CBCT, clinical use, radiation exposure, safety, and legal issues in oral and maxillofacial surgery practices. The published applications and usage of CBCT at the clinical practitioner level were reviewed. They identified the current position of academic leaders in the field. A nationwide survey was done to determine how CBCT was being used and adopted by institutions and private practices. According to this published paper, the best practices supported evaluation of the entire CBCT volume with a written report with the findings, patient exposure, and FOV. The use of ALARA (“as low as reasonably achievable”) principle was emphasized. They reported that the third-party patterns for reimbursements varied widely and seem to lack consistency [13].

As there has been a marked increase in the use of CBCT imaging in dentistry and especially in specialties like orthodontics, the American Academy of Oral and Maxillofacial Radiology published a paper [14] supporting the safe use of CBCT in practice. The paper summarized the potential benefits and risks of this technology in orthodontic diagnosis, treatment planning, and outcomes to aid the clinicians. The recommendation was to use the principle of justification based on clinical exam for each individual patient. The benefits must overweigh the potential risks associated with radiation exposure. The position paper provides the following guidelines for using CBCT in orthodontics: (a) image selection criteria and recommendation should be used, (b) radiation risks and dose to the patient must be taken into account, and (c) the clinician must maintain professional competency in acquisition and interpretation.

9 Benefits and Risks of CBCT

CBCT imaging offers many obvious advantages over the conventional 2-D extraoral imaging modalities like lateral cephalometric imaging, which is widely used. It must be emphasized that CBCT utilizes ionizing radiation and thus involves known potential risks that are associated with the use of radiation. The use of radiographic imaging should not be considered routine for practice. Like with any other radiographic imaging modality, the clinician must weigh in potential benefits that will be gained by the CBCT exposure versus risks to the patient.

According to the known principle of ALARA (as low as reasonable achievable), the radiation exposure should be kept to the minimum to protect the patient from any unnecessary radiation exposure. Based on guidelines for CBCT in Orthodontics¹⁴, it has been recommended that following viewpoints must be taken into consideration in determining the need and potential benefits of a radiographic modality such as CBCT for an orthodontic patient:  

• 1. Is the operator obtaining the images trained adequately and is using the best exposure parameters under the circumstances?

• 2. Will this procedure provide additional useful information that will further aid in clinical diagnosis and treatment planning? 

• 3. Is the practitioner competent in the interpretation of CBCT images?

Before prescribing any radiographic images, it should be kept in mind that ionizing radiation can potentially lead to genetic mutations and carcinogenesis. The risk of potential side effect is more with greater exposure. Unnecessary radiographic imaging such as CBCT may contribute to an increased risk in the patients. The three guiding principles of justification, optimization, and dose limitation must be considered when ordering radiographic images [8].

Clinician’s professional judgment is essential to justify every radiographic exposure. The decision to expose a patient must be made after a detailed clinical examination and only if the clinician thinks the findings from the acquired images will provide additional information that will benefit the patient. It is important for the clinician to understand the functioning of the chosen imaging modality including the advantages and the limitation. This will result in gaining the adequate and best quality images with minimum exposure to the patient, using the optimal exposure parameters. After the images are attained, the suitable formatting should be done.

The interpretation should not be limited to the region of interest only. The entire CBCT volume must be evaluated.

CBCT imaging is an excellent tool for the orthodontists and oral surgeons and helps to improve diagnosis, treatment planning, and outcome assessment in appropriate cases. Literature shows that CBCT is a powerful imaging modality and provides orthodontists with 3-D images of the craniofacial osseous structures, dentition, and soft tissue (Figs. 1.3 and 1.4). A virtual cephalometric image can also be made (Fig. 1.5). This information is important for diagnosing malocclusion. “While orthodontists await the American Association of Orthodontists’ position paper on identifying appropriate cases for CBCT imaging, case selection using current evidence-based criteria suggest that complex craniofacial and surgical cases and cases of missing or impacted teeth may be the most suitable candidates for CBCT imaging (Fig. 1.6) although the absolute need for CBCT imaging must be determined on a case-by-case basis.” [15]

Fig. 1.3

(a) Hard tissue renderings from CBCT data are shown. (b) Example of another type of rendering with soft tissue is shown

Fig. 1.4

(a) Enhanced depth 3-D model. (b) Enhanced depth with soft tissue

Fig. 1.5

Virtual cephalometric image from a CBCT unit

Fig. 1.6

Reconstructed panoramic image (a) and cross-sectional images (b) from CBCT data showing the relationships of the forming tooth with the surrounding structures

Other common uses for CBCT evaluation of anatomical structures such as maxillary sinuses (Fig. 1.7), inferior alveolar canal and foramen (Fig. 1.8), and evaluation of periapical pathology (Fig. 1.9). Advantages of CBCT technology include lower cost, faster acquisition, and lower radiation dose to the patient as compared to MDCT. A personal computer is utilized for data reconstruction and viewing with an interactive software designed for dental applications. As compared to MDCT, the limiting factors for CBCT technology may include greater image noise and noticeably poor soft tissue contrast.

Fig. 1.7

CBCT maxillary images produced for dental implant treatment planning with an opaque radiographic marker in the left posterior maxilla. (a) Reconstructed panoramic image. (b) Cross-sectional images. Pneumatization of the maxillary sinus is also visible in the region of interest (red arrow)

Fig. 1.8

CBCT images produced for dental implant treatment planning. (a) reconstructed panoramic and (b) cross-sectional views. Inferior alveolar nerve marking is also shown by red line. (c and d) Coronal CBCT views show corticated inferior alveolar canal (green arrows) and the mental foramen (red arrows)

Fig. 1.9

Cross-sectional views show root resorption with periapical low-density lesion with corticated borders (red arrow)

10 CBCT Radiation Doses

A variety of CBCT machines are available in the market (Table 1.1). The radiation doses with each machine are going to be different due to variability in device type, imaging protocols including the field of view, and exposure parameters (mA, Kv). Due to this, the effective dose, which is used to estimate the risk in humans, from CBCT may be similar or greater as compared to the conventional intraoral full-mouth radiographic survey or panoramic image. The effective dose from medical CT is more substantial to the patient as compared to CBCT. Variability in doses has been found in the previous printed literature. Therefore, effective doses for the various modalities are listed in (Tables 1.2 and 1.3) from two different sources.

Table 1.2 Effective doses from CBCT systems [5]

Table 1.3 Effective doses from different imaging modalities [4]

Ludlow et al. [16] published with meta-analysis of the published data analyzing the effective doses of nine dental CBCT units. Dentists prefer high-resolution images and high signal-to-noise ratios. This technique is associated with higher radiation dose. In many clinical situations, such absolute high-quality image may not be needed as lower dose data may provide sufficient information for diagnostic tasks (Fig. 1.10).

Fig. 1.10

Ultralow-dose CBCT scan: Images appear slightly grainy. CBCT images also show cleft palate (red arrows)

11 Artifacts on CBCT Images

An artifact can be described as any distortion in the image which is unrelated to the patient. These are volumetric data set errors and do not represent the corresponding region of the patient’s tissues. It has been reported that CBCT images inherently have more artifacts due to the use of lower energy spectrum and beam geometry as compared to the medical CT units [8]. Many artifacts result from discrepancies between the physical imaging process and mathematical assumptions in the data reconstruction algorithms. One has to be careful during the interpretation process. The noise level is found to be more in CBCT. Artifacts or errors on CBCT images are often seen as dark or white areas or streaks. The following artifacts may be encountered in the CBCT images:

Beam hardening artifact occurs when there is more attenuation in the center of the structure as compared to the edges. Structures located adjacent to high-density metallic objects like amalgam restorations, dental implants, and orthodontic brackets may appear missing or burnt-out. Streak artifacts are commonly seen around the amalgam restorations and cast metal restorations, as these metals have a higher atomic number. Radiographically, they appear as dark or white streaks (Fig. 1.11).

Fig. 1.11

Scatter artifacts (blue arrows) due to metal are seen on large field of view CBCT image. Other findings include unilateral concha bullosa (red arrow) and mucus retention pseudocyst (yellow arrow) on the floor of the left maxillary sinus

Scatter may also cause streak artifacts. Scatter occurs when X-ray photons are diffracted from the original path after interaction with the matter or patient’s tissues (Table 1.4).

Table 1.4 Indication for different CBCT fields of view (FOVs)

Partial volume averaging artifacts are seen when an image voxel contains more than one type of tissue. Thus, after attenuation, the gray value is not representative of any specific tissue. It appears as hazy or blurred tissue outlines. This artifact is more associated with larger-sized voxels, where the object being imaged is smaller and the voxel is larger in size. If both hard and soft tissues are included in one voxel, the end result will be an average of brightness values of different tissues. The region may have a “step appearance,” and the displayed pixel may both be representative of either tissue.

Ring artifacts are seen on the displayed images due to a faculty pixel in the detector.

Motion artifact is usually caused by the movement of the patient during the scanning process. Movement of the patient can be easily recognized by double margins or blurry cortical outlines (Fig. 1.12). In case of severe movement, the resultant scan is of no diagnostic value and must be acquired again. To reduce the chance of patient movement during the exposure, the operator should educate the patient about the procedure, use head-restraining devices available with the CBCT unit, and use shorter exposure time.

Fig. 1.12

Bony outlines appear double or blurry due to movement artifacts

Scanner-related artifacts have been described as round or ring artifacts, which result from problems associated with improper scanner calibration. Example of another type of machine calibration-related artifact is shown as black line in the middle of the scan (Fig. 1.13). After discussion with the manufacturer, it was concluded that either the detector or beam was off-centered. Realignment and calibration was needed as the corrective action. Some of the machines with smaller detector operate by acquiring two scans to cover the full skull, and thus the data sets have to be overlapped correctly and “stitched” by the computer program. Imperfections in overlap before stitching can also appear as stitching artifacts that show a step formation (Fig. 1.14).

Fig. 1.13

Images show a solid black line in the midline (red arrows). It was reported as calibration error of the machine

Fig. 1.14

Coronal image shows irregularity of the bony margins at the mid-ramus level. This error was attributed to problems in alignment of the two volumes before stitching

12 Training for Using CBCT

The practicing dentist should be aware of the state laws and requirements before purchasing the unit. It is best to check with the governing agency directly to rule out any restrictions that may be in place for purchasing the CBCT unit. In the United States, certain states do require a “certificate of need.” Legal ramifications should also be understood. Radiographic images may be acquired in a dental office or independent dental radiology imaging centers. In the United States, any licensed dentist may own and operate a CBCT unit. Non-dentists, who own an imaging center, can also own and operate these units. In both cases, adequate training is required, which is typically provided by the manufacturer [17].

Before operating the unit for the patient care, the training is critical for the whole dental office. After the installation of the unit, the manufacturer is responsible for hands-on training on how to use the machines, proper patient positioning, and the various modes of image capture. This will enable the operator and the referring dentist to acquire the best quality images to fulfill the diagnostic aim and also will keep the radiation dose as low as possible for the patient.

Referring Dentist: The referring dentist has to clearly indicate why the CBCT images are needed and justify the use. Other conventional imaging alternates may be considered before prescribing CBCT. It is important to select the appropriate cases and understand the associated risks with the use of radiation. The region of interest or field of view must be identified before exposure. The dentist must be trained in the proper use of the viewing software program for manipulating the CBCT data. Different software tools are provided to help the clinician in the process of visualization of the region of interest in different enhancement modes.

He or she should have sufficient knowledge of anatomy, variation of anatomy, incidental findings, and other pathologic conditions, in order to correctly interpret the CBCT data. The referring dentist is responsible for evaluation of complete CBCT data set to rule out abnormalities. Finding of the significance should be reported. Many practitioners choose to send CBCT for interpretation by an oral and maxillofacial radiologist. This is similar to other procedures, where a procedure may be referred to a specialist if the general dentist or the specialist does not feel competent to perform the task or to seek a second opinion.

Machine Operator: It is the responsibility of the referring dentist to convey the region of interest and the diagnostic aim to the person exposing the patient or the operator of the CBCT unit. A written prescription from the referring dentist is advised for clear directions to the operator and documentation. This will result in dose optimization and will reduce or eliminate the need for retaking the images.

Quality assurance is very important. If any errors are noted on the images, corrective actions must be taken. It is recommended that the machines should be calibrated annually by the factor certified maintenance staff to maintain the proper functioning of the machine and the image quality. Therefore, training should be done for the whole dental team. Dawood et al. [1] stated that the comprehensive training should include training from the manufacturer by a training specialist on how to operate the particular CBCT machine, an update on radiation risks and imaging pitfalls, and the selection criteria. Training should also include the use of the viewing software tools, to help in the interpretation process of cross-sectional and three-dimensional CBCT images.

13 Summary

CBCT technology has come a long way. Technically, the CBCT technology has evolved momentously over the past decade and thus is considered indispensable for many diagnostic scenarios. The image quality has improved tremendously; the cost of the CBCT machines and the scanning time has decreased. Various companies have developed task-specific software tools which are aimed for orthodontists and oral surgeons. As the technology is being integrated in the dental practice more and more, it is important that the principles of patient selection for imaging and radiation protection guidelines must be followed.