RadiologyCone beam computed tomography

Background.—The introduction of cone beam com-puted tomography (CBCT) has shifted maxillofacial imag-ing from a two-dimensional to a volumetric approach interms of technical data acquisition, reconstruction, imagedisplay, and image interpretation. Technologic develop-ments have facilitated the development of affordableCBCT units small enough to use in the dental office. Thiscost-effective diagnostic technology has expanding applica-tions in the treatment planning and image guidance ofoperative and surgical procedures. The fundamental princi-ples of operation of maxillofacial CBCT technology, opera-tional parameters, the task-specific equipment, imageselection, and image display modes used, and the methodof sequencing the interpretation of CBCT images werereviewed.

Principles of Operation.—Image production and imagedisplay are the two main components of CBCT imaging.Image production is accomplished in three consecutivephases: acquisition configuration, image detection, and im-age reconstruction.

Theoretically, the geometric configuration and acquisi-tion mechanics for the CBCT technique are simple (Fig 1).

Fig 1.—X-ray beam projection scheme comparing acquisition ge-ometry of cone beam imaging (left) with conventional ‘fan beam’CT (right). In cone beam geometry, multiple basis projections formthe projection data from which orthogonal planar images are sec-ondarily reconstructed. In fan beam geometry, primary recon-struction of data produces axial slices from which secondaryreconstruction generates orthogonal images. The amount of scat-ter generated and recorded by cone beam image acquisition issubstantially higher, reducing image contrast and increasing im-age noise. (Courtesy of Scarfe WC, Li ZX, Aboelmaaty W, et al.:Maxillofacial cone beam computed tomography: essence,elements and steps to interpretation. Austral Dent J 57:46-60,2012.)

136 Dental Abstracts

An x-ray source and detector are fixed to a rotating plat-form (gantry), then a divergent pyramidal- or cone-shaped source of ionizing radiation is directed throughthe middle of the region of interest (ROI). The transmit-ted attenuated radiation is projected onto an area x-raydetector opposite. The x-ray source and detector rotatearound a fulcrum fixed in the middle of the ROI. Duringrotation, multiple sequential planar projection images cov-ered by the detector or the field of view (FOV) are ac-quired in an arc measuring 180 � or more. These rawprimary data are individually called the basis, frame, orraw images and resemble cephalometric radiographic im-ages except that each is offset slightly from the previousimage. From these hundreds of two-dimensional (2D) ba-sis images, the image volume is calculated and con-structed, with the complete series of images called theprojection data. CBCT exposure incorporates the entireFOV, so one rotational sequence of the gantry of 180� orgreater will acquire sufficient data for volumetric imageconstruction.

X-ray generation can be continuous, but pulsing to coin-cide with detector activation is preferred because the pa-tient is exposed to less radiation. The scan volumedepends on the detector size and shape, beam projectiongeometry, and the ability to collimate the beam. The scanvolume shape can be either cylindrical or spherical. Two ap-proaches are available to allow scanning of an ROI greaterthan the FOVof the detector. In the first, data are obtainedfrom two or more separate scans, superimposing them ap-propriately and fusing adjacent image volumes (stitching orblending) to create a larger volumetric data set. This can bedone manually or automatically using proprietary software.The drawback to this approach is the doubled radiationdose to overlapped regions. In the second method, the po-sition of the detector is offset, the beam collimated asym-metrically, and scans performed of half of the ROI in eachoffset position.

Image quality is determined somewhat by the numberof samples obtained. The number of images is determinedby the frame rate (number of images acquired per sec-ond), completeness of the trajectory arc, and speed ofrotation.

Most CBCTs use flat panel detectors (FPDs). X-rays areusually detected indirectly by means of a scintillator thatconverts them to visible light, registered in the photo diodearray. The detector records incident x-ray photons, collects

Fig 3.—CBCT volumetric data set. As CBCT data acquisition is de-pendent on the pixel size of the area detector and not on the ac-quisition of groups of rows with sequential translational motionthat is the case in conventional MSCT, the compositional voxelsare equal in all three dimensions rather than columnar. Initial dis-play images are secondarily reconstructed from the data set atthree right angle planes (orthogonal). (Courtesy of Scarfe WC, LiZX, Aboelmaaty W, et al: Maxillofacial cone beam computed to-mography: essence, elements and steps to interpretation. AustralDent J 57:46-60, 2012.)

a charge, and sends a signal to the computer. Each basisimage is acquired and sent in milliseconds, with many hun-dreds of repeated acquisitions in a single exposure rotation.FPDs have limitations in their performance and require re-calibration periodically.

The detail of CBCT imaging is determined by individualvolume elements (voxels) produced while forming the vol-umetric data set. Voxel dimensions depend mainly on thepixel size of the area detector, with smaller pixel size captur-ing fewer x-ray photons and creating more image noise.CBCT imaging using higher resolutions may be designedto use higher dosages and achieve a reasonable signal-to-noise ratio for diagnostic image quality. Factors related todose include time sequence, exposure parameters, collima-tion, and filtration.

Image reconstruction involves processing data to createa volumetric data set composed of voxels by a sequence ofsoftware algorithms (Fig 3). Reconstruction times varybased on acquisition parameters, hardware, and software.The two stages of reconstruction are the acquisition stageand the reconstruction stage, with each taking a numberof steps to accomplish.

Operational Parameters.—Practitioners must be cog-nizant of the operational parameters of CBCT and the ef-fects of each on image quality and radiation safety. Amongthe parameters to be considered are exposure settings, im-age resolution, frame rate, trajectory arc, and FOV.

Exposure settings can be managed via a selection offixed levels or operator manual adjustment of kilovoltage(kV) and/or milliamperes (mA). When operator adjustableexposure settings are used, it must be remembered thatthese affect both image quality and patient radiation dose,so careful selection is needed to fulfill the as-low-as-reasonably-achievable (ALARA) principle. Exposure param-eters must be appropriate for the given patient as well as forthe diagnostic task. Dental periapical diagnosis based onthe ability to discern the periodontal ligament space andsubtle changes in bone trabeculation requires higher expo-sure parameters than implant planning. The tube currentcan be reduced up to 50% without substantial loss of diag-nostic quality for relatively low-resolution tasks such as pre-surgical implant planning or orthodontic diagnosis. Thiswill significantly lower patient exposure doses.

Both spatial resolution (considering the proximity ofdetails able to be recorded separately) and contrast reso-lution (allowing the distinction between tissues of differ-ing radiodensity) must be considered in most cases.However, spatial resolution is moot when the contrast isinsufficient to differentiate between tissue densities of ad-jacent structures. Maxillofacial CBCT imaging provides suf-ficient spatial and contrast resolution to demonstratedetail in osseous structures. Image artifacts resultingfrom image acquisition, patient-related factors, the scan-ner itself, or the cone/pyramidal beam projection geome-try can compromise CBCT imaging and make itunacceptable for dental caries diagnosis, especially in re-stored dentitions. It should therefore be considered a com-plementary modality and not a replacement technologyfor all other methods.

The number of projection scans comprising a singlescan can be fixed or variable. Withmore projection data, im-age reconstruction is enhanced, spatial and contrast resolu-tions are improved, the signal-to-noise ratio is increased,and metallic artifacts are reduced. However, this requiresa longer scan time, delivering a higher radiation dose tothe patient and requiring more time for primary imagereconstruction.

Reducing the scan arc from 360� to 180�, which also re-duces patient dose by 50%, can produce images of sufficientdiagnostic quality to achieve implant planning in the upperjaw. Reductions below 180� do not acquire sufficient infor-mation for diagnostic image quality.

FOV is usually managed mechanically, but can also behandled electronically. The limitation of the FOV to thesmallest ROI needed for diagnosis using mechanical colli-mation methods is the preferred approach.

Current CBCT Equipment.—Units are designed to per-mit scanning of the patient in a standing, seated, or supine

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Fig 8.—Display mode options of CBCT volumetric data. Display modes can be divided into three categories: (1) multiplanar reformatted(MPR) consisting of linear, curved oblique and serial trans-axial images; (2) ray sum comprising images of increased section thickness;and (3) volumetric images consisting of indirect volume rendering (IVR), the most common of which being maximum intensity projection(MIP) and direct volume rendering (DVR). (Courtesy of Scarfe WC, Li ZX, Aboelmaaty W, et al: Maxillofacial cone beam computed tomogra-phy: essence, elements and steps to interpretation. Austral Dent J 57:46-60, 2012.)

position. A head stabilizing mechanism is used to minimizemotion artifact. Scan volumes are generated from a singlescan or multiple adjacent limited field volumes through dig-ital stitching.

Available units can be categorized by maximum verti-cal FOV as (1) maxillofacial, covering most of the cranio-facial skeleton; (2) dentoalveolar, covering a single orinterarch distance of 5–10 cm incorporating the maxillaand/or mandible; or (3) limited, with just 5 cm or less ver-tical height and covering local regions. FOV selection isimportant to minimize patient radiation exposure. Equip-ment should be selected to perform the intended diag-nostic tasks, with some able to deliver high-resolutionimaging that permits discernment of finely detailed struc-tures and disease processes. CBCT systems can also bestand-alone or hybrid multi-modal units that combine dig-ital panoramic radiographs with small-to-medium FOVCBCT systems.

Task-Specific Equipment and Images.—Imagesshould be viewed digitally and dynamic, facilitated bythe use of appropriate software and task-specific protocolformatting. A technique involving three stages hasprovided an efficient and consistent systematic methodo-logic approach to the CBCT image display beforeinterpretation.

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First, the data are corrected. The initial adjustment is datareorientation to realign the patient’s anatomic features as ap-propriate for the desired task. Then the data set is optimizedfor display by adjusting grayscale brightness levels, establish-ing a contrast range, and applying filters favoring cortical andtrabecular bone. Secondary algorithms, including annota-tion, measurement, and magnification, are then applied.

Second, the data are viewed dynamically by scrollingthrough the orthogonal image stack. The scrolling is per-formed craniocaudally, then in reverse, with slower speedsadopted in areas of greater complexity. The scrolling is re-peated in the coronal and sagittal planes. Soft tissue calcifi-cations are detected more readily by repeating the processusing a slab thickness of about 10 mm and a maximum in-tensity profile (MIP) setting.

Third, the data are displayed. CBCT software providesmany visualization options. Protocols that incorporateFOV scan exposure parameters and display modes areapplied selectively to highlight anatomic features or func-tional characteristics appropriate to the specific diagnostictask (Fig 8). Thin sections are used for detail and thickersections for relationships. To visualize adjacent voxelsmost effectively, the operator can use ray sum or ray castingor volume rendering, which can be indirect (IVR) or direct(DVR).

Interpretation.—The practitioner who operates a CBCTunit or requests a specific CBCT study is professionally re-sponsible for providing information on the imaging find-ings based on evaluation of the entire image data set. Thismay also be legally mandated for third-party health insur-ance reimbursement or medical liability protection. Thepractitioner bases the interpretation on a thorough knowl-edge of the CT anatomy for the entire acquired imagevolume, anatomic variations, and observation of abnormal-ities. Qualified specialist oral and maxillofacial radiologistsmay assist diagnostically.

An interpretation report is designed to convey the find-ings of CBCT studies. Although there are no specific re-quirements for these reports, the following data shouldbe included:

1. Patient information, including patient name or otheridentifier, gender, date of birth, or age.

2. Scan information, including succession number, datethe scan was done, location of the facility, equipmentused, scan parameters, referring practitioner’s name,rationale for the procedure, and images provided,as well as any problems that occurred during theprocedure.

3. Radiologic findings, including both general and spe-cific data. The patient’s dental status, such as miss-ing teeth, restorative status, root canal filled teeth,periapical lesions, general alveolar bone status,and status of edentulous areas, is included in thegeneral information. The specific findings includeprecise anatomic, pathologic, and radiologic termi-nology that accurately describes gnathic or TMJfeatures in the ROI. The data set is reported in

a head-to-toe format. Incidental findings represent-ing significant conditions in nongnathic structuresare included.

4. Radiologic impression, which is expressed as a defini-tive or differential diagnosis. Radiologic findings arecorrelated with patient presentation and with pertinentclinical issues raised in the request for the examination.If there are previous examinations, findings can becompared to those. Recommendations for follow-upor additional diagnostic or clinical studies are sug-gested to clarify, confirm, or exclude the diagnosis.

Clinical Significance.—CBCThas been used indentistry for some time, and a large selection ofunits is now available. All can provide accurate,submillimeter-resolution images in formats thatpermit the volumetric visualization of osseousstructures in the maxillofacial region. CBCTwill continue to affect the expected standardsof care, so practitioners should become edu-cated about the performance, optimal visualiza-tion, and interpretation of volumetric data sets.

Scarfe WC, Li ZX, Aboelmaaty W, et al: Maxillofacial cone beamcomputed tomography: essence, elements and steps to interpreta-tion. Austral Dent J 57:46-60, 2012

Reprints available fromWC Scarfe, The Univ. of Louisville, School ofDentistry, Radiology and Imaging Science, Dept of Surgical andHosp Dentistry, Rm 149F, 501 S. Preston St, Louisville, KY 40292,USA; email: wcscar01@louisville.edu

Restorative DentistryRepair or replace

Background.—About half of all restorations placed ina general dental practice are done to replace defective orfailed restorations. Factors that influence these replace-ment decisions include those related to the clinician, prop-erties of the restorative materials, and patient-relatedissues. Sometimes a combination of these factors is inplay. Better understanding dentist- and patient-related char-acteristics associated with the decision to repair or replaceddefect restorations should help develop guidelines to im-prove treatment. A study was undertaken to determinewhether dentists in practices in The Dental Practice�BasedResearch Network (DPBRN) are more likely to repair or

replace defective restorations; to list the specific reasonsfor repairing or replacing restorations; and to determineif certain dentist-, patient-, and restoration-related variablesare associated with the decision to either repair or replacea restoration.

Methods.—A total of 197 practitioner-investigators(P-Is) participating in The DPBRN participated. Each pa-tient who was receiving a repair or replacement of a res-toration on a permanent tooth was asked to participate.Restoration replacement was defined as the entire re-moval of the existing restoration and any adjacent

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