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Basic Principles of Cone Beam Computed Tomography

      Keywords

      Key points

      • The use of cone beam computed tomography (CBCT) imaging in the dental profession has blossomed since its inception 15 years ago. CBCT unit design has undergone many changes that enhance CBCT access and practical utility in dentistry. The scanners have become smaller, scan patients in an upright position, use primarily flat panel detectors, and readily convert projection data to DICOM file formats. Units themselves have various scanning options that include the size of the area to be scanned (field of view [FOV]), voxel size (spatial resolution), bit depth (contrast resolution), and scan times (frame rate).
      • CBCT manufacturers have incorporated various aspects of imaging technology in a cost-effective, efficient, and practical manner. There are now numerous CBCT applications in many software formats that are helpful in a multitude of dental disciplines including but not limited to dentoalveolar disease and anomalies, vertical root and dentin fractures, jaw tumors, prosthodontic evaluations, and advances in orthodontic/orthognathic and implant patient evaluations. The latter also include mechanisms for surgical and prosthodontic splint design and the capability of CBCT scan data to bridge with computer-aided design/manufacturing image files for the fabrication of various dental restorations.
      • Streaking and beam hardening remain as ominous imaging artifact that compromise CBCT utility in various case situations. However, because of the popularity of CBCT, computer hardware and software developers, machine manufacturers and dental researchers will continue to improve the applications of this imaging modality for the betterment of patient care.

      Introduction

      Imaging with cone beam technology has rapidly become a popular and frequently used imaging modality to assist dentists and other health care professionals in a multitude of diagnostic tasks to improve patient care.
      Cone beam imaging technology is most commonly referred to as cone beam computed tomography (CBCT). The terminology “cone beam” refers to the conical shape of the beam that scans the patient in a circular path around the vertical axis of the head, in contrast to the fan-shaped beam and more complex scanning movement of multidetector-row computed tomography (MDCT) commonly used in medical imaging.
      First introduced at the end of the millennium,
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      A new volumetric CT machine for dental imaging based on the cone-beam technique: preliminary results.
      • Arai Y.
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      • Arai Y.
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      Development of a compact computed tomographic apparatus for dental use.
      CBCT heralded a new dental technology for the twenty-first century. Its practical applications for implant dentistry and orthognathic/orthodontic patient care were the main applications at that time. Owing to the dramatic and highly positive impact that CBCT had on these disciplines, additional applications for this technology became apparent. New software programs were developed to improve the applicability and access of CBCT for the care of dental patients.
      Two factors played a big part in the rapid incorporation of CBCT technology into dentistry, the first of which was the availability of improved, rapid, and cost-effective computer technology. The second was the ability of software engineers to develop multiple dental imaging applications for CBCT with broad diagnostic capability.

      CBCT versus computed tomography

      CBCT, by virtue of the terminology, is a form of computed tomography (CT). In a single rotation, the region of interest (ROI) is scanned by a cone-shaped x-ray beam around the vertical axis of the patient’s head. Digitized information of objects in the ROI such as shape and density is acquired from multiple angles. These imaging data are then processed by specialty software that ultimately constructs tomographic images of the ROI in multiple anatomic planes, namely the standard coronal, axial, and sagittal anatomic planes (Fig. 1) and their various paraplanar derivatives, the parasagittal, paracoronal and para-axial planes.
      Figure thumbnail gr1
      Fig. 1Standard anatomic planes of imaging used for multiplanar reconstructions in cone beam computed tomography (CBCT) and multidetector-row computed tomography.
      (Modified from Washington CM, Leaver DT. Principles and practice of radiation therapy. Philadelphia: Mosby; 2004.)
      The historically standard and more sophisticated form of CT, present since the 1970s, was developed in part by British engineer and Nobel Prize winner Sir Godfrey Hounsfield. It is of interest that by the end of the decade, the technology of Hounsfield’s first scanner was followed by the development of a larger body scanner by a group of researchers in the United States headed by American dentist and physicist Robert S. Ledley.
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      The story behind the development of the first whole-body computerized tomography scanner as told by Robert S Ledley.
      This more advanced form of CT is known as MDCT, although other terms such as multislice CT and multirow CT are used. Because MDCT is more commonly used in medicine, it is often referred to as medical CT. However, this term is a misnomer, as CBCT is now also being used and further modified for patient evaluations in medicine.
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      Conebeam CT of the head and neck, part 1: physical principles.
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      Conebeam CT of the head and neck, part 2: clinical applications.
      A more appropriate term for MDCT might be “conventional CT.” Differences between CBCT and MDCT have been widely reported.
      • Miles D.A.
      Color atlas of cone beam volumetric imaging for dental applications.
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      Computed tomography for oral and maxillofacial surgeons. Part 2: cone-beam computed tomography.
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      Cone-beam computed tomography.
      • Drage N.
      • Rout J.
      Diagnostic imaging.
      However, owing to the specific advances and innovations of CBCT technology for the care of dental patients, it has become and will remain a vital and significant imaging modality in dentistry.

      Historical development of CBCT units

      During the early development of CBCT, the technology was being advanced primarily for the dental office. Subsequently, many of the earlier units were modified to include designs that more readily fit within dental offices and clinics. The integration of CBCT imaging in dentistry has in some ways paralleled the transition of panoramic imaging x-ray machines into dental offices. Early panoramic units were mainly sit-down,
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      Evaluation of an S.S. White Panorex x-ray machine.
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      Relations between tooth positions and focal troughs of panoramic machines.
      but there was also a lay-down unit.
      • Shinozima M.
      • Kohirazawa H.
      • Kubota K.
      • et al.
      Tomorex (curved rotational tomography apparatus) in experimental and clinical practice.
      Several other sit-down machines were manufactured, but eventually units were made whereby the patient could stand upright for the panoramic exposure. Upright machines became preferable, as it is more convenient and takes less time to transfer patients into and out of these stand-up panoramic units.
      The physical size and shape of CBCT units has paralleled this panoramic pathway. One of the very first commercially available cone beam machines, the NewTom 9000 (QR srl, Verona, Italy), was a large unit that scanned the patient lying in a supine position. It was followed by the NewTom 3G (Fig. 2A). These early NewTom units eventually lost favor to smaller, sit-down chair units or to stand-up units. These smaller units with better scanner quality more readily fit into dental office space and overhead budgets (see Fig. 2B–F). Despite the previous drawbacks of the NewTom prototypes, CBCT units that scan patients in a supine position have made a comeback; the NewTom 5G (QR srl) and the SkyView (MyRay, Imola, Italy) are currently available. These units, with upright patient loading and supine position for patient scanning, are presented in Fig. 2G–H. NewTom is also producing standing machines such as the VGi.
      Figure thumbnail gr2ae
      Fig. 2(A) NewTom 3G. This supine CBCT scanner was one of the first commercially available units in North America. It was replaced by units that scanned patients seated with the head in an upright position. (B) The Accuitomo 170 (J. Morita USA, Irvine, CA). (C) The Scanora 3Dx (Soredex, Milwaukee, WI). (D) The CS 9300 (Carestream Health, Rochester, NY). (E) The Orthophos XG 3D (Sirona USA, Charlotte, NC). (F) The i-CAT FLX (Imaging Sciences International, Hatfield, PA). (G) The NewTom 5G in patient entry (left) and patient scan (right) positions. This unit is currently manufactured by QR srl, Verona, Italy. (H) The SkyView CBCT scanner (MyRay, Imola, Italy) in patient entry (left) and patient scan (right) chair positions.
      Figure thumbnail gr2fh
      Fig. 2(A) NewTom 3G. This supine CBCT scanner was one of the first commercially available units in North America. It was replaced by units that scanned patients seated with the head in an upright position. (B) The Accuitomo 170 (J. Morita USA, Irvine, CA). (C) The Scanora 3Dx (Soredex, Milwaukee, WI). (D) The CS 9300 (Carestream Health, Rochester, NY). (E) The Orthophos XG 3D (Sirona USA, Charlotte, NC). (F) The i-CAT FLX (Imaging Sciences International, Hatfield, PA). (G) The NewTom 5G in patient entry (left) and patient scan (right) positions. This unit is currently manufactured by QR srl, Verona, Italy. (H) The SkyView CBCT scanner (MyRay, Imola, Italy) in patient entry (left) and patient scan (right) chair positions.

      Effect of field of view on scanner type

      The size of the scanned object volume is called the field of view, commonly abbreviated as FOV. The FOV for units with a flat-panel detector (see later description) is a cylindrical shape in the center of the scanner between the detector and the x-ray source. The CBCT scanning controls are programmed to scan an FOV of sizes and areas that are built into the scanner by the manufacturer. Other factors that affect the FOV are the size and type of the detector and the degree of beam collimation on the x-ray tubehead. Fig. 3A demonstrates how the dimensions of a flat-panel detector’s FOV cylinder are expressed by the height of the cylinder (H) and the diameter of the base (D). The FOV is a very flexible option in contemporary scanners. The range of commercially available FOVs for flat-panel detectors can be from 3.0 cm (H) × 3.0 cm (D) to 24 cm (H) × 16.5 cm (D) (Table 1).
      Figure thumbnail gr3
      Fig. 3(A) Cylindrical shape and measurement characteristics of the field of view (FOV) for CBCT. (B) The different FOV option sizes from the Vatech CBCT (Vatech America, Fort Lee, NJ). Many CBCT units now have the capability of scanning a range of FOV sizes.
      Table 1Scanners with large field of view (FOV)
      ModelManufacturerMaximum to Minimum FOV

      Height × Width (cm)
      Minimum Voxel (mm3)Two-Dimensional OptionsBit DepthScan Time (s)kVNotes
      3D Accuitomo 170J. Morita USA, Irvine, CA, USA12 × 17 to 4 × 40.08No145.4–3090
      3D eXamKaVo, Charlotte, NC, USA17 × 23 to 8 × 80.125
      Information was not available at press time.
      148.5–26120
      CS 9300Carestream Health, Rochester, NY, USA13.5 × 17 to 5 × 50.09Panoramic cephalogram1412–2090Previously Kodak (dental division)

      Available in 2 versions
      DaVinci D3DCefla Dental, Imola, Italy15 × 15 to 7 × 70.17
      Information was not available at press time.
      12
      Information was not available at press time.
      Information was not available at press time.
      Supine position for scan
      Galileos Comfort PlusSirona USA, Charlotte, NC, USASphere 15.4 cm (D)0.125No121498Plus model is upgraded Comfort

      Uses image intensifier detector
      i-CAT FLXImaging Sciences International, Hatfield, PA, USA17 × 23 to 8 × 80.125Panoramic145–26.9120QuickScan+ option allows for ultralow dose exposures
      NewTom 5GQR srl, Verona Italy16 × 18 to 6 × 60.075No1418–26110Supine position for scan
      NewTom VGiQR srl, Verona, Italy15 × 15 to 6 × 60.075No1418–26110VGi Flex version intended for mobile use
      PaX-Reve3D PlusVatech America Inc, Fort Lee, NJ, USA15 × 19 to 5 × 50.08Panoramic1415–2490
      ProMax 3D MaxPlanmeca USA Inc, Roselle, IL, USA17 × 22 to 5.5 × 50.10Panoramic1518–2696Can obtain a stitched 26 × 23 cm FOV and upgradable to ProFace 3D Photos
      ProMax 3D MidPlanmeca USA Inc, Roselle, IL, USA17 × 20 to 5 × 40.10Panoramic cephalogram1518–2690
      Quolis Alphard 3030Asahi Roentgen Ind. Co., Ltd, Kyoto, Japan17.9 × 20 to 5.1 × 5.10.10No
      Information was not available at press time.
      17110
      Scanora 3DSoredex, Milwaukee, WI, USA13 × 14.5 to 6 × 60.133Panoramic1210–2690
      Scanora 3DxSoredex, Milwaukee, WI, USA24 × 16.5 to 5 × 50.10Panoramic
      Information was not available at press time.
      18–3490
      SkyViewMyRay, Imola, ItalySphere 22.9 cm (D)

      15.3 cm (D)

      10.2 cm (D)
      0.17No1210–3090Supine position for scan

      Uses image-intensifier detector
      WhiteFoxActeon North America, Mt. Laurel, NJ, USA17 × 20 to 6 × 60.10No1618–27105
      a Information was not available at press time.
      The FOV for image-intensifier detectors is shaped differently, not as a cylinder but rather as a sphere. The dimensions are usually measured by the diameter of the circular shape in inches (eg, 6″, 9″, 12″).
      The size of the FOV significantly affected the evolution of the CBCT scanner. Early CBCT units were restricted to a single-size FOV that was either large or small, which limited the usefulness of the scanner. The general rule was the larger the FOV, the greater the cost of the scanner. The higher cost is attributed to the larger detector size and the larger kilovoltage (kV) generator needed for imaging denser parts of the skull for orthognathic and orthodontic evaluations. The FOV most typically included the jaws, midface, and skull base. Some had options that included a more extended part of the skull toward the vertex, that is, 40 cm (H). Because of the limited indications and increased cost, the larger FOVs were not as popular for limited dentoalveolar applications. Smaller FOV units large enough to image 2 to 4 teeth of a jaw (either maxilla or mandible) was another earlier scanner option. The area covered in these smaller volumes is adequate for a thorough 3-dimensional (3D) periapical evaluation of selected teeth, alveolar bone, and a limited amount of maxillary or mandibular basal bone. Contemporary scanners are now capable of a range of FOVs (see Fig. 3B) from the smaller 3.0 cm (H) × 3.0 cm (D), to the midrange FOVs for coverage of one or both jaws, to the larger FOVs that include the cervical spine, jaws, more of the paranasal sinuses, skull base, and parts of the cranium. Larger FOVs that include superior areas of the skull are not usually indicated for most dental applications.
      Because of these technological improvements and enhancements, CBCT is now readily identified as part of the imaging equipment in modern dental clinics. Table 1 lists many of the currently available CBCT units with larger FOV capabilities along with notations of some of their other options. Table 2 is a similar listing of units with medium to smaller FOV options. Because of the constant modifications in CBCT scanner technology, manufacturers, and machine trade names, the information in these tables is time sensitive and only current at the time of publication. For additional information, other listings may also be referenced.
      • Nemtoi A.
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      • et al.
      Cone beam CT: a current overview of devices.
      Table 2Scanners with medium and small FOV
      ModelManufacturerMaximum to Minimum FOV

      Height × Width (cm)
      Minimum Voxel (mm3)Two-Dimensional OptionsBit DepthScan Time (s)kVNotes
      AUGE ZIOAsahi Roentgen Ind. Co., Ltd, Kyoto, Japan8 × 10 to 5.5 × 50.1Panoramic cephalogram128.5–1795
      Cranex 3DSoredex, Milwaukee, WI, USA6 × 8 to 6 × 40.085Panoramic cephalogram
      Information was not available at the time of writing.
      11–1785
      CS 9000 3DCarestream Health, Rochester, NY, USA3.75 × 5

      Stitched

      7.5 × 3.75
      0.076

      Stitched 0.2
      Panoramic cephalogram1410.890Will stitch 3 scans together
      Finecube XP62Yoshida, Tokyo, Japan7.5 × 8.10.1No148.6–3490Marketed as PreXion 3D by PreXion Inc in the USA
      GXCB-500Gendex, Hatfield, PA, USA8 × 14 to 8 × 80.125No148.9–23120Powered by i-CAT
      GXCB-500 HDGendex, Hatfield, PA, USA8 × 14 to 2 × 80.125Panoramic14
      Information was not available at the time of writing.
      120Powered by i-CAT
      GXDP-700Gendex, Hatfield, PA, USA6 × 8 to 6 × 40.2Panoramic cephalogram
      Information was not available at the time of writing.
      10–2090
      i-CAT PreciseImaging Sciences International, Hatfield, PA, USA8 × 14 to 2 × 80.125Panoramic144–23120
      I-Max Touch 3DOwandy, Croissy-Beaubourg, France8.3 × 9.30.156Panoramic cephalogram8–162086
      Orthopantomograph OP300Instrumentarium, Milwaukee, WI, USA6 × 8 to 6 × 40.085Panoramic cephalogram1410–2090
      Orthophos XG-3DSirona USA, Charlotte, NC, USA8 × 8 to 5 × 50.1Panoramic cephalogram121490
      PaX-i3D GreenVatech America, Fort Lee, NJ, USA10 × 16 to 5 × 50.12Panoramic cephalogram145.9
      Information was not available at the time of writing.
      100
      PreXion 3D EclipsePreXion, Inc, San Mateo, CA, USA8 × 11 to 8 × 7.50.15Panoramic cephalogram148.7–17.490
      PreXion3D ElitePreXion, Inc., San Mateo, CA, USA8 × 7.5 to 5.6 × 5.20.11Panoramic138.6–33.590
      Promax 3D ClassicPlanmeca USA Inc, Roselle, IL, USA8 × 8 to 8 × 40.1Panoramic cephalogram151890
      Promax 3D PlusPlanmeca USA Inc, Roselle, IL, USA9 × 14 to 5 × 40.1Panoramic cephalogram151890
      Promax 3D sPlanmeca USA Inc, Roselle, IL, USA8 × 5 to 5 × 50.1Panoramic cephalogram151890
      Suni3DSuni Medical Imaging, San Jose, CA, USA5 × 5 to 5 × 80.08Panoramic cephalogram1615–2490
      VeraviewepocsJ. Morita USA, Irvine, CA, USA8 × 10 to 4 × 40.125Panoramic cephalogram139.490Various configurations available
      a Information was not available at the time of writing.

      Features of the imaging process

      Image Capture

      As in any radiographic imaging system, CBCT requires x-ray production, x-ray attenuation by an object, signal detection, image processing, and image display. These parameters are vital to all aspects of dental imaging, but they are understandably more sophisticated for CBCT.
      During a rotational scan of an object, multiple exposures are taken at fixed intervals (angles) of the rotation. Each of these exposures is referred to as a basis image. The images are standard radiographic images captured on the detector, and the signal of each projection is unique for each of the different angles in the rotational arc. Instantaneously the image data for each basis image are sent to a data-storage area so that the detector can be cleared to capture the next basis image at a position interval further along the rotational arc. Once the rotation is complete and all the basis images are made, the complete set of basis images forms the “projection data.” The total number of basis images taken depends on the radiographer’s preferences and the scanner’s capability. This total ranges from 100 to 600 basis images per scan. The greater the number of basis images, the longer the scan time, the greater the radiation dose, and the better the quality of the constructed images. Fig. 4 demonstrates a hypothetical scheme for projection-data formation with the capture of 2 basis images.
      Figure thumbnail gr4
      Fig. 4Basis-image capture for a hypothetical CBCT rotational scan of the cervical spine. Two basis-image capture sequences are depicted in this diagram as the machine rotates counterclockwise from Position 1 to Position 2. An arrow depicts the counter-clockwise rotation. CBCT scans routinely capture in the range of 100 to 600 basis images per rotational scan.
      (Modified from Zhen X, Yan H, Zhou L, et al. Deformable image registration of CT and truncated cone beam CT for adaptive radiation therapy. Phys Med Biol 2013;58(22):7979–93.)

      Imaging Software and Data File Management

      Image reconstruction software programs, usually proprietary to each machine manufacturer, then manage the projection data and construct a 3D volumetric data set. These processed data are then accessed to construct various types of images for display. The choice of images constructed depends on the power of the imaging software and the needs and preferences of the clinician. The image selection from 3D software is not limited to a single type of image display. Depending on the capability of the software, there are multiple options of image construction from the 3D volumetric data set. Most scanner programs display a primary image reconstruction of the object in the 3 anatomic planes of imaging: the axial, sagittal, and coronal planes. These primary reconstruction displays are also referred to more typically as the multiplane or multiplanar images. Primary multiplanar reconstructions from 2 different software programs are presented in Fig. 5. The same volumetric data set can be used to also construct multiple kinds of secondary reconstructions. The choice of secondary reconstruction is often task specific, and is also related to the reconstruction options within the scanner’s proprietary software.
      Figure thumbnail gr5
      Fig. 5Examples of multiplanar reconstructions. The upper example (A) is constructed by One Volume viewer software (J. Morita USA). The lower (B) reconstruction is by CS 3D Imaging Software (Carestream Health, New York).
      At present, a variety of independent third-party imaging software is commercially available for image reconstruction of CBCT volumetric data sets. Third-party imaging software is software not associated with the capture and proprietary software of the CBCT scanner. A limited selection of third-party software is listed in Table 3.
      Table 3Third-party software available for imaging CBCT data sets
      SoftwareManufacturerUses
      CS 3DCarestream Dental, Rochester, NY, USAMultiple applications
      Dolphin 3DDolphin Imaging and Management Solutions, Chatsworth, CA, USAMultiple applications
      EasyGuideKeystone Dental, Burlington, MA, USAImplant planning
      InVivoDentalAnatomage Inc, San Jose, CA, USAMultiple applications
      OnDemand3DCybermed Inc, Irvine, CA, USAMultiple applications
      OsiriXPixmeo SARL, Bernex, SwitzerlandMultiple applications
      Procera SoftwareNobel Biocare USA, LL, Yorba Linda, CA, USAImplant planning
      Ultra-Fast CBCT Reconstruction SoftwareBronnikov Algorithms, The NetherlandsMultiple applications
      If third-party software is being used, the file format of the volume set must be converted from the proprietary file format or file language to a more universal or common digital file format. This common format must be conformant with the Digital Imaging and Communications in Medicine standard (DICOM 09v11dif); that is, the current DICOM standardized file format.
      • National Electrical Manufacturers Association
      Digital Imaging and Communications in Medicine (DICOM), PS 3 2011.
      This digital format is the International Organization for Standardization (ISO) referenced standardized digital file format for medical images and related information, namely ISO 12052. To facilitate access to health care, multiple imaging modalities (x-ray, visible light, ultrasound, and so forth) used in medicine and dentistry must be compliant with ISO 12052. Digital applications in veterinary medicine also follow this standard.
      If CBCT vendors do not specifically use the DICOM file format in their proprietary scanner software, their proprietary software should have the capability of converting the volume data to the DICOM standard file format. In so doing, they make their volume data usable in other and often more specialized software applications.
      Types of task-specific reconstruction capabilities of viewing software include, but are not limited to, panoramic reconstructions, implant planning reconstructions with 2-dimensional and 3D windows, temporomandibular joint reconstructions, airway reconstructions, and so forth. Examples of these latter reconstructions are presented in Fig. 6.
      Figure thumbnail gr6ab
      Fig. 6Examples of secondary reconstructions from various CBCT software programs. (A) Two-dimensional (2D) panoramic reconstruction. Although a CBCT scan is not indicated solely for panoramic imaging, many imaging software packages can reconstruct panoramic images from the storage data. (B) Implant planning with 2D reconstructions and a tracing of the mandibular nerve. (C) Implant planning with 2D/3-dimensional (3D) reconstructions. (D) Bilateral reconstructions of the temporomandibular joints in coronal and sagittal sections. (E) Sagittal reconstruction without (top) and with (bottom) Airway Measurement tool from InVivo 5.2 imaging software (Anatomage, San Jose, CA). When the airway is traced in the airway measurement window, the program wizard computes the volume of the airway space. Threshold values for compromised airway volumes have not yet been determined for this software.
      Figure thumbnail gr6cd
      Fig. 6Examples of secondary reconstructions from various CBCT software programs. (A) Two-dimensional (2D) panoramic reconstruction. Although a CBCT scan is not indicated solely for panoramic imaging, many imaging software packages can reconstruct panoramic images from the storage data. (B) Implant planning with 2D reconstructions and a tracing of the mandibular nerve. (C) Implant planning with 2D/3-dimensional (3D) reconstructions. (D) Bilateral reconstructions of the temporomandibular joints in coronal and sagittal sections. (E) Sagittal reconstruction without (top) and with (bottom) Airway Measurement tool from InVivo 5.2 imaging software (Anatomage, San Jose, CA). When the airway is traced in the airway measurement window, the program wizard computes the volume of the airway space. Threshold values for compromised airway volumes have not yet been determined for this software.
      Figure thumbnail gr6e
      Fig. 6Examples of secondary reconstructions from various CBCT software programs. (A) Two-dimensional (2D) panoramic reconstruction. Although a CBCT scan is not indicated solely for panoramic imaging, many imaging software packages can reconstruct panoramic images from the storage data. (B) Implant planning with 2D reconstructions and a tracing of the mandibular nerve. (C) Implant planning with 2D/3-dimensional (3D) reconstructions. (D) Bilateral reconstructions of the temporomandibular joints in coronal and sagittal sections. (E) Sagittal reconstruction without (top) and with (bottom) Airway Measurement tool from InVivo 5.2 imaging software (Anatomage, San Jose, CA). When the airway is traced in the airway measurement window, the program wizard computes the volume of the airway space. Threshold values for compromised airway volumes have not yet been determined for this software.

      X-Ray Tube and Generator Systems

      Because CBCT is a radiographic imaging system, scanners have x-ray tubes with kV and milliamperage exposure controls. Although the time of exposure is usually an exposure control for an x-ray system, in CBCT the time of the exposure is actually dependent on the number of basis images and the degree of spatial resolution requested in the voxel size. The smaller the voxel size and the greater the number of basis images, the longer the exposure. The major difference in a CBCT exposure compared with the exposure of intraoral and panoramic imaging is that the CBCT exposure consists of capturing the series of multiple basis images. Because of the process of basis-image projection, the x-rays are not generated during the entire rotational path. In most units, the exposure is pulsed at intervals so that there is time between basis-image acquisition for the signal to be transmitted from the detector area to the data-storage area and the detector to rotate to the next site or angle of exposure. Hence, the x-ray tube does not generate x rays for the entire rotational cycle. These intervals may inherently reduce patient exposure during the time interval that the detector is not ready to receive x rays. These intervals are also beneficial for the x-ray duty cycle, reducing heat buildup during an exposure cycle.
      In general, the longer the exposure and the more basis images produced, the longer it takes to complete the rotational arc. This time for the acquisition of basis images is known as the frame rate. For a shorter exposure, the rotational arc remains the same but the frame rate is reduced. In this scenario where less basis images are taken, the radiation exposure is less, the rotational arc takes less time, and the scanner parts rotate faster. The clinician can actually observe the slower or longer scan times necessary for longer exposures with higher frame rates.
      A data set with fewer basis images may be undersampled. Undersampled data sets have a lower signal-to-noise ratio, and thus lack the range of image contrast capable with a more complete volumetric data set. However, depending on the diagnostic task, the degree of image degradation from these smaller data sets with shorter exposures may still be adequate for certain diagnostic tasks. This shorter feature reduces patient exposure and is particularly helpful in reducing motion artifact in scans of younger patients, geriatric patients, or those with disabilities, during which motion artifact is more difficult for the patient to control. These smaller data sets also have less computational and construction time. With fewer data, they require less storage space. An example of undersampling is illustrated in Fig. 7.
      Figure thumbnail gr7
      Fig. 7(A) Sagittal temporomandibular joint reconstruction from projection data processed from a full quota of basis images in the projection data set. (B) Sagittal temporomandibular joint reconstruction from a shorter exposure scan that has fewer basis images in the projection data and resulting volumetric data set. There is less detail and contrast resolution in the resulting image display than with projection data from a full quota of basis images used for construction of the volumetric data set.
      Exposure factors for a CBCT scan can be preset from an exposure selection guide, or can be determined by automated features in the image-acquisition software from a scout exposure. Some units may have a direct automated exposure feedback feature in the detector that determines the exposure factor for more optimal signal detection. The automated exposure control predates CBCT technology and has been available since the introduction of charge-coupled device sensors for digital panoramic radiography. CBCT units that scan larger object areas (larger FOV) generally need higher kV potentials. The higher kV is necessary for adequate penetration of denser and larger anatomic structures in the maxilla, midface, and skull base. Consequently, higher kV is often necessary for adequate diagnostic quality of the larger FOV data sets.

      Image Sensor Systems

      Two types of image detectors are used as the sensors in contemporary CBCT units. A scanner will have either (1) a charge-coupled device with a fiber-optic image intensifier, or (2) an amorphous silicon flat-panel detector. Examples are presented in Fig. 8. During the initial introduction of CBCT, most units were constructed with the large, bulky image-intensifier detectors. In the latter half of the first decade of commercial CBCT development, CBCT scanners have nearly all transitioned to the smaller, flat-panel linear array detectors. However, as noted in Tables 1 and 2 that list representative CBCT imaging systems, Sirona and MyRay still manufacture scanner units with this type of detector.
      Figure thumbnail gr8
      Fig. 8Two CBCT scanners from Sirona USA. The Galileos (top) has a charge-coupled image-intensifier detector. The Orthophos XG 3D unit (bottom) has the smaller flat-panel detector. The detectors are demarcated with dotted outlines. Differences between the two are described in the text.
      The image-intensifier detectors are larger and make the scanners’ overall dimensions larger, which may be critical for certain office designs. In addition to being more sensitive and susceptible to distortion from magnetic fields, image displays from these detectors also demonstrate greater distortion of the grid dimensions when moving away from the center of the detector (Fig. 9A), which ultimately reduces measurement accuracy of the reconstructed images.
      • Scarfe W.C.
      • Farman A.G.
      What is cone-beam CT and how does it work?.
      Because of their sensitivity to magnetic fields, the image-intensifier detectors require more frequent calibration. In addition, the phosphors in image intensifiers lose their sensitivity over time and use, and the entire image-intensification unit may need to be replaced to maintain image quality. This process is very expensive.
      • Miles D.A.
      CBCT: facilitating comprehensive, high-tech diagnostics.
      Despite the drawbacks, in certain cases the data sets from these detectors are more compatible with “bridging” to some of the data sets used in computer-aided design and manufacturing (CAD/CAM) technology, and thus remain useful.
      Figure thumbnail gr9
      Fig. 9Distortion patterns produced by image detectors. (A) Grid type X is the type of grid-distortion pattern produced by the image-intensifier detector that affects the image construction and is subsequently noted in the image display. There is distortion of the image grid when moving away from the center. (B) With flat-panel detectors (ie, grid type Y) the image receptor area receiving the signal from the flat-panel detector’s scintillator is flat. Therefore, even at more distant areas from the center of the grid, there is minimal to no distortion of the grid pattern.
      The flat-panel detectors are thin, amorphous silicon transistor panels with a cesium iodide scintillator. The scintillator is the part of the detector used to amplify the electrical signal from the x-ray attenuation. Besides being smaller and less bulky, the flat panels have minimal distortion of the image dimensions at the periphery of an image display (see Fig. 9B); hence, these units are considered to generate better data sets. Because these detectors are smaller than their image-intensifier predecessors, CBCT units with the flat-panel detectors have smaller footprints. This feature alone had made the flat-panel detector more popular. Differences in image quality between these detectors are shown in Fig. 10.
      Figure thumbnail gr10
      Fig. 10Comparative reconstructions of two different scans of the same posterior left maxillary quadrant from a scanner with a flat-panel detector (left) and one from a charge-coupled device image intensifier (right). The improved image quality and the higher signal-to-noise ratio are noted in the left image.
      (Courtesy of Dr Bruno Azevedo, Western University, Pomona, CA.)
      Another property of the image detector is the bit depth, an exponential binary property expressing the total number of gray shades the detector is able to discriminate. A 14-bit detector (ie, 214) can display 16,384 shades of gray. The range of bit depth of commercial CBCT units ranges between 12 and 16 bits (see Tables 1 and 2), indicating the wide range of contrast discrimination capability. Although the detector is capable of this degree of gray-scale discrimination, limiting features to the contrast resolution include the lower bit depth of the imaging software and the monitor display, and the visual perception of the viewing clinician. Even though bit depth is important for contrast resolution, the American College of Radiology has concluded that there is no added benefit to diagnostic interpretations by the use of higher than 8-bit depth in the workstation’s operating system.
      • Andriole K.P.
      • Ruckdeschel T.G.
      • Flynn M.J.
      • et al.
      ACR-AAPM-SIIM practice guideline for digital radiography.

      Scatter and Beam-Hardening Artifact

      Scatter and beam-hardening artifact occurs in CT imaging where image reconstructions of a data set are necessary for review of the data volume. Dense metal structures frequently in the FOV for dental applications present metal artifact on CBCT reconstructions. Silver amalgam, precious and semiprecious metal alloys used in coronal restorations, dental implants, silver-point endodontic fillings, and, to a lesser extent, gutta percha endodontic fillings, all create these artifacts in image reconstructions. The artifact presents as light or dark streaks, or as a dark periphery adjacent to metallic borders. Scatter artifact is seen as radiopaque lines and patterns of metallic density that “scatter” on image reconstructions. The main types of beam hardening are the dark streaks or dark bands that show up in the image reconstructions. The latter often simulate disease such as recurrent caries or fractures in endodontically treated teeth. The light streaks often superimpose regular anatomy, and may also significantly degrade image quality.
      These artifacts are prominent problems for dental applications with CBCT, as metallic restorations are often within the FOV of most CBCT scans of dental patients. The metallic restorations then cause the resultant beam hardening and streak artifact, which then compromises the image quality with the various areas of dark and light artifact. Fig. 11 illustrates examples of how these artifacts degrade image quality and make image assessments difficult.
      Figure thumbnail gr11
      Fig. 11Beam hardening and streak artifact in CBCT image reconstructions. (A) Axial section with dental implant in #18 region highlighted by black arrow. (B) Beam-hardening artifact is indicated by red arrows. The green arrows depict streak artifact. (C) The locations of cross-sectional and parasagittal reconstructions are shown. (D) The effect of beam hardening simulating peri-implantitis and alveolar bone defects in the cross-sectional and parasagittal reconstructions. (E) The effect of streak artifact creating the outline of a “ghost” implant (as well as other radiopaque streak outlines) in the cross-sectional reconstruction. The streak artifact makes it more difficult to discern the validity of the cortical bone outlines.
      (Courtesy of Dr. Gerald Marlin, Washington, DC.)
      Recent attempts via software correction algorithms have been reported that have the potential to control these visible artifacts on image reconstruction.
      • Bechara B.
      • McMahan C.A.
      • Geha H.
      • et al.
      Evaluation of a cone beam CT artefact reduction algorithm.
      • Bechara B.
      • Moore W.S.
      • McMahan C.A.
      • et al.
      Metal artefact reduction with cone beam CT: an in vitro study.
      However, the application of software correction modes to reduce these artifacts have been inferior to noncorrected software viewing programs when evaluating peri-implant and periodontal disease
      • Kamburoglu K.
      • Kolsuz E.
      • Murat S.
      • et al.
      Assessment of buccal marginal alveolar peri-implant and periodontal defects using a cone beam CT system with and without the application of metal artefact reduction mode.
      as well as root fractures.
      • Bechara B.
      • McMahan C.A.
      • Moore W.S.
      • et al.
      Cone beam CT scans with and without artefact reduction in root fracture detection of endodontically treated teeth.
      Consequently, there are no immediate methods to correct or minimize these prominent artifacts. The best way to avoid streaking and beam hardening is to try to keep the FOV as small as possible in an attempt to minimize or keep these metals outside the FOV. In so doing, one may be able to minimize their impact on image reconstructions.

      Summary

      CBCT is now a well-accepted diagnostic tool for the care of dental patients. Design changes in the evolution of contemporary CBCT scanners include making the units smaller, and making changes whereby instead of needing to be scanned in a supine position, the patient either sits or stands upright during the scan. Along with these design changes, better stabilization devices for the patient’s head and chin were produced. Mechanical changes included the switch to smaller, flat-panel silicon detectors with better image quality compared with the bulkier, cumbersome, and eventually more costly image-intensifier detectors.
      Variable kV and multiple options for voxel size, FOV dimensions, scan times, and so forth, then followed, alongside more powerful software applications for the care of dental patients. The ability of CBCT manufacturers to use various aspects of imaging technology in a cost-effective, efficient, and practical manner means that there are now numerous CBCT applications that are helpful in a multitude of dental disciplines. These applications include, but are not limited to, dentoalveolar abnormality, vertical root fractures, jaw tumors, prosthodontic evaluations, and advances in orthodontic/orthognathic and implant patient evaluations. The latter also include mechanisms for surgical and prosthodontic splint design and the capability of CBCT scan data to bridge with CAD/CAM image files for fabrication of various dental restorations. This approach facilitates implant and prosthodontic rehabilitation by synchronously planning and subsequently milling coronal restorations for teeth and rootform implants. As the demand for CBCT technology continues to increase, so will the number of new applications for improved diagnostic techniques.

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