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CT Dental

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  • 1. Received: 2 August 2001Revised: 17 January 2002Accepted: 28 January 2002Published online: 30 April 2002© Springer-Verlag 2002Abstract In addition to convention-al imaging methods, dental CT hasbecome an established method foranatomic imaging of the jaws priorto dental implant placement. Morerecently, this high-resolution imag-ing technique has gained importancein diagnosing dental-associated dis-eases of the mandible and maxilla.Since most radiologists have had lit-tle experience in these areas, manyof the CT findings remain unde-scribed. The objective of this reviewarticle is to present the technique ofdental CT, to illustrate the typical ap-pearance of jaw anatomy and dental-related diseases of the jaws withdental CT, and to show where it canserve as an addition to conventionalimaging methods in dental radiology.Keywords Computed tomography ·Jaw · Teeth · ImplantEur Radiol (2003) 13:366–376DOI 10.1007/s00330-002-1373-7 H E A D A N D N E C KAndré GahleitnerG. WatzekH. ImhofDental CT: imaging technique, anatomy,and pathologic conditions of the jawsIntroductionDental CT has become an established method for imag-ing of jaw anatomy prior to dental implant placement [1,2, 3, 4, 5, 6]. The clinical use of these implants has rap-idly increased over the past 20 years. It is estimated that300,000 implants are placed each year with over 50 com-panies involved in the manufacture, marketing and distri-bution within the United States [7, 8].The term “dental CT” does not represent a particularmodality, but rather a specific investigation protocol.The main features of this protocol include the acquisi-tion of axial scans of the jaw with the highest possibleresolution together with curved and orthoradial multi-planar reconstructions (Fig. 1). Dentists commonly di-agnose and work in the submillimeter scale; hence, ahighly detailed image quality is required and challengesCT to its technical limits. This article reviews the spe-cific technique of dental CT and illustrates the typicalappearance of jaw anatomy and dental-related diseasesof the jaw.TechniqueHistoryThe technique of dental CT, also called Dentascan, wasdeveloped by Schwarz et al. in 1987, when these investi-gators first used curved multiplanar reconstructions ofthe jaw [9, 10]. At that time, the number of inserted im-plants in the jaw had steadily increased and there was acritical need for accurate imaging of the jaw anatomy,especially in the bucco-lingual plane. The major disad-vantage of CT in the jaw region, the metal artifacts fromtooth fillings, was overcome by using the axial plane forscanning instead of the coronal plane, which kept theseartifacts in the occlusion plane and hence left the jawA. Gahleitner (✉) · H. ImhofDepartment of Radiology/Osteology,Medical School, University of Vienna,Währinger Strasse 25a,1090 Vienna, Austriae-mail: andre.gahleitner@univie.ac.atTel.: +43-1-427767191Fax: +43-1-427767039A. Gahleitner · G. WatzekDepartment of Oral Surgery,Dental School, University of Vienna,Währinger Strasse 25a,1090 Vienna, Austria
  • 2. 367bone undistorted. This allowed for accurate display ofthe vertical as well as the important bucco-lingual di-mensions of the jaw in actual size, which facilitated thework of the dentist [11].Prior to this development, the first useful techniquefor pre-implant imaging of jaw anatomy was convention-al orthoradial tomography, using a complex (circular,spiral, or hypocycloidal) blurring device, such as theScanora or CommCat (Soredex, Marietta, Ga.; ImagingSciences International, Roebling, N.J.) [12, 13]. Al-though this technique is still a valuable procedure, it isprone to errors, has the known disadvantages of conven-tional tomography, and does not allow imaging of thecomplete jaw within an acceptable time frame. Due tothe higher cost and lesser availability of dental CT, con-ventional orthoradial tomography is still a standard in-vestigation in many implanologic centers.During the past years various studies have been pub-lished which have validated dental CT as an excellenttool for diagnosing dental-related pathologies [3, 14, 15,16, 17, 18, 19, 20, 21]. Since occurring changes may bevery subtle, an optimal image quality with the highestpossible resolution is essential for establishing a correctdiagnosis.PatientPrior to imaging, the patient should be informed aboutthe investigation and instructed not to move or swallowduring the scan. The investigation is performed in the su-pine position with the cervical spine slightly overextend-ed backward. The head should be strapped to the head-rest and positioned as symmetrically as possible. Thiscan be checked in the scoutview, where both rami andthe angles of the mandible should be perfectly aligned. Ifmovement of the mandible during the scan is likely tooccur (e.g., edentulous jaws with lack of occlusion), it ispossible to immobilize the jaw by having the patient biteon a cotton roll or on fast-hardening impression material.Scanner protocolDental CT investigations can be performed either on aconventional CT, spiral CT, or a multislice CT scanner.The device should be capable of performing high-resolu-tion scans with a small focal spot and acquiring thin slic-es of 1.5 mm or less. A table feed of 1 mm is necessaryto obtain high-quality images with optimal detail in thescan plane as well as in the multiplanar reconstruction. Aspiral scan technique with 1 s per rotation is sufficient inmost cases where imaging of jaw anatomy is performedprior to implant placement. If imaging of pathologic con-ditions is required, small details can be obtained by in-creasing the scan time to 2 s per rotation. The largernumber of views due to the slower rotation speed of thetube provides the more detailed information necessaryfor imaging of frequently very subtle pathologic fea-tures. The field of view should be limited to 120 mm orless to avoid unnecessary imaging of the spine, neck, orposterior cranial fossa. A well-established universal pro-tocol is provided as an example in Table 1.Scan direction is caudocranial beginning with themandible base and extends to include the alveolar crestfor the mandible, whereas for the maxilla the scan planestarts with the alveolar crest and extends upward to in-clude all root tips. If sinusitis is present, extensions ofthe scan are recommended to exclude possible dental-related causes, such as displaced root remnants or for-eign bodies.Dose reductionDental CT images are displayed with a very low contrastsetting (bone window) due to the excellent contrast be-tween bone and soft tissue. Since no contrast medium isused and displaying soft tissue detail with digital en-hanced contrast (soft tissue window) is usually notnecessary, dental CT is ideally suited for applying dose-reduced investigation protocols [4, 22, 23]. This is pri-marily accomplished by reducing the tube current, whichleads to increased quantum noise noted in the soft tissuecompartment, whereas bone is only marginally affected.In addition, using 1.5-mm slice thickness instead of 1.0and/or using a spiral technique with a pitch factor ofmore than 1.0 can further reduce dose delivery. The lim-iting factor here is that the important visualization of themandibular canal is degraded when a high pitch factor isused, although orthoradial reconstructions are displayedFig. 1 Dental CT of the maxilla. From axial slices (center) of agiven investigation volume orthoradial reconstructions are calcu-lated (right). As a single example this reconstruction demonstratesthe alveolar crest and the left maxillary sinus
  • 3. 368in correct size. In addition, cystic lesions can be harderto detect since quantum noise can mimic mineralization.Another possibility for dose reduction is to limit thescan range of the investigation. This is easily accom-plished by excluding the occlusal region (crowns of theteeth), since this area is easily investigated by clinicalmeans. Moreover, this region is frequently obscured bymetal artifacts and hence provides limited additional in-formation.ReconstructionsAfter the examination, the axial slices are transferred toa workstation to perform multiplanar reconstructions.This is usually done manually with the aid of a dentalsoftware package on the workstation, although some CTshave a dental software package included on the operatingconsole. A planning line is drawn manually by the tech-nician along the centerline of the jaw arch, which is thebase for the subsequent orthoradial and panoramic multi-planar reconstructions. These multiple, computer-gener-ated orthoradial reconstructions are calculated perpen-dicular to the planning line. Usually, the distance be-tween each of the 40–60 orthoradial cuts is chosen to ac-commodate all reconstructions on a single sheet of film(usual distance, 1.5–3.0 mm; Fig. 2). Panoramic recon-structions are calculated along the planning line. Fre-quently, the dental software package allows calculationof two to four additional panoramic cuts parallel to theplanning line, which are positioned just slightly closer tothe buccal (outer) and lingual (inner) side from the plan-ning line (usually approximately 2 mm). These cuts areseldom of major diagnostic importance but provide avery good overview of the general situation, since thepanoramic cuts resemble conventional panoramic radio-graphs, which are familiar to dentists, oral surgeons, ormaxillofacial surgeons.Hard copyIf a single jaw is scanned, all images can be displayed onthree hard copies, of which the first two are the axialscans. This should be possible if an array of four col-umns by five rows of slices per hard copy is chosen. Theremaining single hard copy accommodates all dental re-constructions, which should be precisely in 1:1 (life-size) scale. This can be easily checked using a simpleruler next to the centimeter scale that usually is dis-played with the images. This hard copy should consist ofthe axial planning slice, which includes the planning lineand the orthoradial lines. Subsequent scans are numberedto allow correlation with the accordingly numbered or-thoradial reconstructions. Panoramic reconstructions areincluded on the same hard copy and should allow corre-lation with the orthoradial reconstructions by indicatingtheir position. The use of three hard copies offers the ad-vantage of a good overview of the entire investigationfor reporting purposes, as well as for ease of handling forthe referring clinician.Jaw anatomyNormal anatomyVisible structuresTeeth. The jaws contain 32 teeth (=16 per jaw, 8 perquadrant) which, based on their configuration and func-tion, are grouped for each quadrant into molars (3), pre-molars (2), canine (1), and incisors (2). The tooth gener-ally consists mainly of dentin which has a radiologicopacity similar to cortical bone. In the region of thecrown, the dentin is surrounded by a thin layer of enam-el, which has by far highest opacity of all natural tissues(similar density as contrast media or certain metals, e.g.,Table 1 Example of standard investigation protocols for imaging of the anatomic situation (Anatomy) or pathologic conditions (Pathol-ogy) of the jaw with dental CTAnatomy PathologyScan type Spiral IncrementalSlice thickness (mm) 1.0 1.5Table feed 1.0 1.0Field of view (mm) 120 (mandible) 120 (mandible)100 (maxilla) 100 (maxilla)Scan time (s) 1 2Matrix 512 512Tube current (mA) 25–100 25–100Tube voltage (kV) 120 120Filter High-resolution edge enhanced filter High-resolution edge enhanced filterWindow setting 2000 HU with, 400 HU center (bone window) 2000 HU with, 400 HU center (bone window)Scan plane Hard palate (maxilla) Hard palate (maxilla)Mandible base (mandible) Mandible base (mandible)
  • 4. 369titanium). The root canal is a centrally located, hypo-dense structure that is large in the crown region and de-creases in size toward the direction of the root tip. Theroot dentin is surrounded by a thin layer of cementum,which cannot be differentiated radiographically. Thetooth in the region of the root is surrounded by the nar-row periodontal space, which is hardly visible on CT. Awidened, clearly visible periodontal space is typically in-dicative of pathology. The molars in the mandible havetwo roots, whereas molars in the maxilla consist of threeroots. The first premolar of the maxilla has two roots,whereas the remaining teeth have a single root (Fig. 3).Variations in the number of roots are primarily found inboth maxillary premolars and the third molars.Alveolar crest. The alveolar crest is the part of thejawbone that holds the tooth roots, periodontal space,and the lamina dura that envelops the periodontal space.The alveolar crest is the part of the jaw where mostpathologic conditions occur.Mandible base. The mandible base is located below thealveolar crest and contains the mandibular canal with theinferior alveolar nerve and vessels. The neurovascularbundle enters the canal at the mandibular foramen andexits through the mental foramen. The mandibular canalusually is well visualized on axial slices and orthoradialreconstructions, as long as it is surrounded by corticalbone. Frequently, in short segments, this cortical laminais not present on some orthoradial reconstructions andhence the mandibular canal is obscured by the surround-ing cancellous bone. Since this is hardly ever the case forthe entire extent of the canal, the exact position of themandibular canal can be located by means of interpola-tion between other orthoradial reconstructions where thecanal is visible. Injury to the mandibular canal results inFig. 2 Complete hard copywith panoramic and orthoradialreconstructions of the mandi-ble. Upper left axial slice of themandible with planning linealong the mandibular arch to-gether with multiple numberedorthoradial lines. Upper rightthree panoramic slices recon-structed along the planningline. Lower part multiple ortho-radial reconstructions corre-sponding to the orthoradiallines visible on the axial slice.Additionally, both mental fo-ramina are indicated by twosmall arrows to facilitate theorientation between the inter-foraminal region and the tworetroforaminal regions (rightand left) The mandibular canalis visible as a small corticalring in each retroforaminal re-gion
  • 5. 370paralysis or numbness of the chin and edge of the mouth.Since the neurovascular bundle within the mandibularcanal also supplies the teeth, sudden loss of tooth vitalitywithin a whole quadrant can occur. Damage to the man-dibular canal during placement of implants (or extractionof third molars) therefore represents one of the majorissues for legal steps taken against dentists.Maxillary sinus. The maxillary sinus can reach far mesi-ally and between the roots of the molars and premolars.Due to the close relationship with these other structures,the maxillary sinus can be easily affected by inflammato-ry conditions and cystic lesions of the adjacent teeth. Asa frequent variant, a bony septum may be visible in thesinus floor (Underwood septum), which can complicatepre-implant augmentative procedures such as the “sinuslift” (Fig. 4) [24].Bone volume/resorption/atrophyAfter the loss of teeth, atrophy of the alveolar crest is atypical occurrence. This is due to the loss of chewingforces in the jaw and can lead to a complete loss of thealveolar crest in edentulous patients. Since the height ofthe alveolar crest in both jaws can be up to 4 cm, a thirdof the patient’s face is lost and a typical surplus of softtissue is present. Cawood and Howell have describedand classified the bone volume loss in anatomic studiesthat can be applied to dental CT due to the perfect visu-alization of the alveolar crest in the orthoradial plane[25]. These six resorption classes represent typical ap-pearances of jaw atrophy after tooth loss (Fig. 5). In gen-eral, after extraction of a tooth (class 2), a continuous re-duction of bone occurs until the alveolar crest demon-strates a “knife-edge” appearance (class 4). If atrophycontinues, further bone height is lost until only the jawbase remains as a thin bone layer (class 6). Although at-rophy is a continuous process, single classes can beskipped. For example, class 2 can directly transform intoclass 4 by loss of the buccal cortical bone (juga alveol-aria), which results in a knife-edge alveolar crest. The re-sorption classes are an important consideration when im-plantation is planned, and have an influence on the selec-tion of implant dimensions and type as well as for thechoice of a possible augmentation procedure such as si-nus lift, onlay graft, or lateral augmentation. These oper-ative procedures are used to artificially enhance theavailable bone volume by deposition of autologous boneor artificial bone replacement material.Bone qualityBone quality, as described by Lekholm and Zarb, is ofmajor importance for the success of an implant place-ment [26]. For preoperative planning bone quality hasbeen categorized into four classes that basically describethe relation of cortical and cancellous bone in a specifiedregion of the jaw (Fig. 6). The amount of cortical bone isFig. 3 Axial slice through the alveolar crest of the maxilla. Thenumbers of each tooth are given according to international nomen-clature (the first digit representing the quadrant and the seconddigit the tooth counted from the midline). In addition, the incisivecanal (double arrowhead) and a small part of the maxillary sinus(single arrowhead) is visibleFig. 4 Axial slice through the maxillary sinus. Prominent Under-wood septum within both sinuses (arrows). Two root tips of tooth27 are visible within the left septum (arrowheads)
  • 6. 371responsible for the primary stability of the implant,whereas cancellous bone is responsible for long-termstability. Although class 1 indicates optimum stability ofthe implant, studies have revealed classes 2 and 3 tohave the best long-term results, with class 4 having themost frequent premature implant loss [5, 27].Measurements/implantsEvaluation of bone quantity is performed by measuringthe height and width of the alveolar crest for a specifiedregion. These values serve as an overview of the avail-able bone quantity and do not serve as a suggested im-plant size. This is because the oral surgeon has multiplechoices of implant sites and implantation directions us-ing different angulations and different diameters. Thechoice is not solely based on the available amount ofbone (bone-demanded implantation), but must take pro-sthodontic and cosmetic factors into account. Moreover,immediately prior to implant placement a canal isdrilled, which usually is 1–2 mm longer than the final in-serted implant. Thus, injury of anatomic structures canoccur even if the final implant does not reach thesestructures; hence, the radiology report concerning mea-surements cannot serve as the sole factor for implantchoice.Jaw pathologyIn addition to imaging of jaw anatomy and its variants,dental CT has proved to be an excellent tool for diagnos-ing pathologic conditions of the jaw. Most lesions inthis region are visible only in the millimeter or even sub-millimeter scale and therefore visualization of these al-terations was not possible with typical CT protocols.This changed with the advent of dental CT where it waspossible to visualize objects on the submillimeter scale.Still, conventional imaging (dental film, panoramic radi-ography) is the basic screening investigation for diagnos-ing pathologic conditions, but CT can aid in revealingadditional features and in localization of a lesion or evenhelp to exclude the presence of a pathologic conditionmore easily. In the following section, the most frequentpathologies found in the jaw are summarized.Chronic apical periodontitisChronic apical periodontitis (CAP) is a frequent findingin patients with pulpitis and in patients after dental treat-ment by root canal filling. It is characterized by an en-largement of the periodontal space at the periapical re-gion of the tooth. Dental CT can demonstrate the root tipwithin a small osteolytic region (the enlarged periodontalspace) [16]. If bacteria spreads into the surrounding can-cellous bone in the chronic stage, a reactive enlargementof trabeculae occurs. This entity is described as “scleros-ing ostitis,” a form of chronic osteomyelitis. When theCAP reaches cortical bone, a periosteal reaction (Fig. 7)Fig. 5 Classification of boneatrophy according to Cawoodand Howell [25]. Schematicrepresentation of atrophicchanges in the anterior midlineof the maxilla (upper part) andmandible (lower part)Fig. 6 Classification of bone quality according to Lekholm andZarb [26]
  • 7. 372or reactive sinusitis can be visible (Fig. 8). After perfora-tion of the apical periodontitis through the cortical boneinto the surrounding tissue, an infiltrate with soft tissueedema can be seen in this compartment.Radicular cystsWhen a CAP remains untreated, it tends to grow untilthe granulation tissue around the root apex transformsand becomes a cystic, epithelial-lined lesion. These ra-dicular cysts are the most common type of cyst in thejaw and can reach a substantial size with the risk of frac-ture [17, 28, 29]. They are characterized by a large, well-defined radiolucency (>1 cm) with the apex of a non-vital root in the epicenter of the lesion (Fig. 9). The sizeat which the transformation from CAP occurs is approxi-mately 1 cm with a large overlap. Although a clear dif-ferentiation between CAP and radicular cyst based oncriteria other than size is impossible, the choice of treat-ment is often different (operative vs endodontic treat-ment).Dentigerous cystsThe second most common cyst in the jaw, the dentiger-ous cyst, also called follicular cyst, is located aroundthe crown of an impacted tooth and attaches to the ce-mento-enamel junction, which helps in the differentia-tion from radicular cysts [30, 31]. Dentigerous cysts aresharply delineated and frequently demonstrate a localFig. 7 Axial slices demonstrat-ing Chronic apical periodontitis(CAP) of tooth 35 (arrow) withsurrounding sclerosing ostitisand lingual-sided perforationand periosteal reaction (arrow-heads)Fig. 8 Panoramic slices of themaxilla. A CAP of tooth 27with perforation into the leftmaxillary sinus (arrows) andreactive dentogen sinusitis(arrowheads)
  • 8. 373Fig. 9 Radicular cyst within the left maxillary sinus arising fromtooth 26. A root tip (arrow) is visible in the center of the lesion.Reactive mucosal swelling within the left sinusFig. 10 Dental CT of the mandible demonstrating a dentigerouscyst (arrow) arising from tooth 35. Adjacent roots are reached butnot surrounded by the cystFig. 11 Dental CT of a frac-tured tooth 11 and 21 (arrow-head) after trauma, demonstrat-ing a mesio-distal fracture line.Orthoradial reconstructionsdemonstrate oblique fracture ofboth teeth (arrows)
  • 9. 374expansion of the buccal or lingual cortical plate. Adja-cent roots of neighboring teeth can be easily reached,but usually are not surrounded by the cyst completely(Fig. 10).Root fracturesHorizontal root fractures, which usually occur after trau-ma, are easily diagnosed by clinical examination andconventional radiographic techniques, whereas verticalroot fractures are visualized with dental film only whenthe fracture line is oriented at least partially within thedirection of the X-ray beam. Studies have demonstratedthat dental CT is superior to dental film in diagnosingvertical root fractures because CT is not sensitive tobeam orientation [19]. These fractures usually occur as aresult of conservative restorations of a tooth with a postor in endodontically treated teeth.The limitations of dental CT in the diagnosis of den-tal fractures, resulting in false-negative readings, in-clude small fissures below the resolution capability ofCT and superimposed metal artifacts from root posts.In addition to obscuring a root, these artifacts can alsomimic fracture lines, but these limitations can be over-come if the fracture extends below the root post and isvisible there. Although horizontally oriented fractureslying in the scanning plane are difficult to visualizewith CT, oblique fractures remain easily detectable(Fig. 11).Fig. 12 Orthoradial reconstruc-tion of region 17: extractionsocket (arrowheads) demon-strating an oro-antral fistulafrom the trifurcation (arrow) tothe right maxillary sinus withreactive sinusitisFig. 13 Foreign body in the lingual-sided soft tissue of the mandi-ble (arrows). Panoramic radiography revealed the impression ofthe foreign body located within the visible extraction socket (arrow-heads)
  • 10. 375Sinus fistulaSinus fistula frequently occurs as a complication aftertooth extraction or after root resection in the maxillarymolar region and must be treated to prevent maxillary si-nus inflammation. Dental CT can clearly localize thecorresponding osseous defect in the alveolar ridge andfrequently orthoradial reconstructions offer optimal visu-alization for preoperative planning (Fig. 12) [14].Foreign bodiesDental CT can help in localizing foreign bodies that canbe found after or during dental treatment, which other-wise would be hard to detect with conventional radiolog-ic methods. Most dental instruments and materials areradio-opaque, which helps in identifying the source andexact location. Materials usually found include root orcrown fillings, gutta-percha, endodontic instruments, androot posts. These materials are typically located in themaxillary sinus, the alveolar ridge, or in the adjacent softtissue and can be a source of chronic infection and pain(Fig. 13).Implant placement “periimplantitis”Correct implant placement is crucial to prevent early im-plant loss or clinical complications, which is especiallyimportant if implant perforation into the maxillary sinusor nasal cavity occurs [32, 33, 34, 35, 36]. Another ma-jor complication is perforation of the implant into themandibular canal, which can lead to paresthesia of themental region and loss of vitality of the more mesiallylocated teeth. The term “periimplantitis” describes an os-teolytic layer around implants due to chronic infectionand/or malocclusion and it is the major radiologic indi-cator for imminent implant loss (Fig. 14).Impacted teethDental CT offers superb visualization of impacted teethand can help the clinician to plan his treatment preoper-atively or prior to orthodontic therapy [37, 38, 39]. Theposition of the tooth within the alveolar crest as well asthe relation to surrounding structures is clearly dis-closed. Resorption of adjacent roots and hooks, in par-ticular, are easily detected and quantified by dental CT(Fig. 15).ConclusionThis review summarizes the capabilities of dental CT asan imaging method for dentistry. Anatomic features aswell as the appearance of frequent dental pathologies aredescribed with their typical findings, which the radiolo-gist should communicate to the referring clinician.Fig. 14 Axial slice demonstrating extensive “periimplantitis” inregion 26 (arrow) and misplacement of implants in the anteriormaxilla (arrowheads)Fig. 15 Axial slice of the maxilla. Horizontally positioned andimpacted tooth 23 with a hook (arrow) at the apex within the leftmaxillary sinus complicating a planned extraction
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