Effect of exposure parameters and voxel size on bone structure analysis in CBCT.

OBJECTIVE: To evaluate the effect of exposure parameters and voxel size on bone structure analysis in dental CBCT. METHODS: 20 cylindrical bone samples underwent CBCT scanning (3D Accuitomo 170; J. Morita, Kyoto, Japan) using three combinations of tube voltage (kV) and tube current-exposure time product (mAs), corresponding with a CT dose index of 3.4 mGy: 90 kV and 62 mAs, 73 kV and 108.5 mAs, and 64 kV and 155 mAs. Images were reconstructed with a voxel size of 0.080 mm. In addition, the 90 kV scan was reconstructed at voxel sizes of 0.125, 0.160, 0.200, 0.250 and 0.300 mm. The following parameters were measured: bone surface (BS) and bone volume (BV) per total volume (TV), fractal dimension, connectivity density, anisotropy, trabecular thickness (Tb. Th.) and trabecular spacing (Tb. Sp.), structure model index (SMI), plateness, branches, junctions, branch length and triple points. RESULTS: For most parameters, there was no significant effect of the kV value. For BV/TV, "90 kV" differed significantly from the other kV settings; for SMI, "64 vs 73 kV" was significant. For BS/TV, fractal dimension, connectivity density, branches, junctions and triple points values incrementally decreased at larger voxel sizes, whereas an increase was seen for Tb. Th., Tb. Sp., SMI and branch length. For anisotropy and plateness, no (or little) effect of voxel size was seen; for BV/TV, the effect was inconsistent. CONCLUSIONS: Most bone structure parameters are not affected by the kV if the radiation dose is constant. Parameters dealing with the trabecular structure are heavily affected by the voxel size.

Comparison of mandibular bone microarchitecture between micro-CT and CBCT images.

OBJECTIVES: To compare microarchitecture parameters of bone samples scanned using micro-CT (µCT) to those obtained by using CBCT. METHODS: A bone biopsy trephine bur (3 × 10 mm) was used to remove 20 cylindrical bone samples from 20 dry hemimandibles. Samples were scanned using µCT (µCT 35; SCANCO Medical, Brüttisellen, Switzerland) with a voxel size of 20 µm and CBCT (3D Accuitomo 170; J. Morita, Kyoto, Japan) with a voxel size of 80 µm. All corresponding sample scans were aligned and cropped. Image analysis was carried out using BoneJ, including the following parameters: skeleton analysis, bone surface per total volume (BS/TV), bone volume per total volume (BV/TV), connectivity density, anisotropy, trabecular thickness and spacing, structure model index, plateness and fractal dimension. Pearson and Spearman correlation coefficients (R) were calculated. CBCT values were then calibrated using the slope of the linear fit with the µCT values. The mean error after calibration was calculated and normalized to the standard deviation of the µCT values. RESULTS: R-values ranged between 0.05 (plateness) and 0.83 (BS/TV). Correlation was significant for both Spearman and Pearson's R for 8 out of 16 parameters. After calibration, the smallest normalized error was found for BV/TV (0.48). For other parameters, the error range was 0.58-2.10. CONCLUSIONS: Despite the overall correlation, this study demonstrates the uncertainty associated with using bone microarchitecture parameters on CBCT images. Although clinically relevant parameter ranges are not available, the errors found in this study may be too high for some parameters to be considered for clinical application.

A pragmatic approach to determine the optimal kVp in cone beam CT: balancing contrast-to-noise ratio and radiation dose.

OBJECTIVES: To determine the optimal kVp setting for a particular cone beam CT (CBCT) device by maximizing technical image quality at a fixed radiation dose. METHODS: The 3D Accuitomo 170 (J. Morita Mfg. Corp., Kyoto, Japan) CBCT was used. The radiation dose as a function of kVp was measured in a cylindrical polymethyl methacrylate (PMMA) phantom using a small-volume ion chamber. Contrast-to-noise ratio (CNR) was measured using a PMMA phantom containing four materials (air, aluminium, polytetrafluoroethylene and low-density polyethylene), which was scanned using 180 combinations of kVp/mA, ranging from 60/1 to 90/8. The CNR was measured for each material using PMMA as background material. The pure effect of kVp and mAs on the CNR values was analysed. Using a polynomial fit for CNR as a function of mA for each kVp value, the optimal kVp was determined at five dose levels. RESULTS: Absorbed doses ranged between 0.034 mGy mAs(-1) (14 × 10 cm, 60 kVp) and 0.108 mGy mAs(-1) (14 × 10 cm, 90 kVp). The relation between kVp and dose was quasilinear (R(2) > 0.99). The effect of mA and kVp on CNR could be modelled using a second-degree polynomial. At a fixed dose, there was a tendency for higher CNR values at increasing kVp values, especially at low dose levels. A dose reduction through mA was more efficient than an equivalent reduction through kVp in terms of image quality deterioration. CONCLUSIONS: For the investigated CBCT model, the most optimal contrast at a fixed dose was found at the highest available kVp setting. There is great potential for dose reduction through mA with a minimal loss in image quality.

Effect of electric potential and current on mandibular linear measurements in cone beam CT.

OBJECTIVES: The purpose of this study was to compare mandibular linear distances measured from cone beam CT (CBCT) images produced by different radiographic parameter settings (peak kilovoltage and milliampere value). METHODS: 20 cadaver hemimandibles with edentulous ridges posterior to the mental foramen were embedded in clear resin blocks and scanned by a CBCT machine (CB MercuRay(TM); Hitachi Medico Technology Corp., Chiba-ken, Japan). The radiographic parameters comprised four peak kilovoltage settings (60 kVp, 80 kVp, 100 kVp and 120 kVp) and two milliampere settings (10 mA and 15 mA). A 102.4 mm field of view was chosen. Each hemimandible was scanned 8 times with 8 different parameter combinations resulting in 160 CBCT data sets. On the cross-sectional images, six linear distances were measured. To assess the intraobserver variation, the 160 data sets were remeasured after 2 weeks. The measurement precision was calculated using Dahlberg's formula. With the same peak kilovoltage, the measurements yielded by different milliampere values were compared using the paired t-test. With the same milliampere value, the measurements yielded by different peak kilovoltage were compared using analysis of variance. A significant difference was considered when p < 0.05. RESULTS: Measurement precision varied from 0.03 mm to 0.28 mm. No significant differences in the distances were found among the different radiographic parameter combinations. CONCLUSIONS: Based upon the specific machine in the present study, low peak kilovoltage and milliampere value might be used for linear measurements in the posterior mandible.

Evidence-based evaluation of three imaging methods for the temporomandibular disc.

OBJECTIVES: To review the literature and evaluate the evidence for the diagnostic outcome of arthrography, computed tomography (CT) and magnetic resonance imaging (MRI) in the assessment of temporomandibular joint (TMJ) disc position. METHODS: A literature search from 1978 to 1994 was conducted. Data on sensitivity, specificity, predictive values and likelihood ratios were collected and, if not available, calculated for each imaging method. Measures of observer performance were also recorded. RESULTS: In total, more than 400 publications were retrieved on imaging of TMJ disc position, 219 on arthrography, 99 on CT and 147 on MRI, with a marked decrease in recent years. The majority of the publications (54%) presented series of patients. The diagnostic outcome cold be obtained from only 7% of the studies and the observer performance from only two of these. Arthrography had the highest diagnostic outcome for the diagnosis of anterior disc position; sensitivity 0.90, specificity 0.80, positive predictive value 0.88 and negative predictive value 0.82. Corresponding figures for CT were 0.66, 0.68, 0.66 and 0.74 and for MRI 0.86, 0.63, 0.67 and 0.83. The diagnostic outcomes expressed as the likelihood ratios for positive test outcome were 4.5 for arthrography, 2.3 for MRI and 2.1 for CT. The outcomes in diagnosing sideways and rotational displacements were higher for MRI (sensitivity 0.81, specificity 0.87, positive predictive value 0.82, negative predictive value 0.88) than for arthrography (0.64, 0.83, 0.70, 0.79). The likelihood ratios were 6.2 for MRI and 3.8 for arthrography. The interobserver performance ranged between moderate to substantial for arthrography and was almost perfect for MRI. CONCLUSIONS: Based on current evidence on the diagnostic outcome, MRI seems to be the method of choice for diagnosing TMJ disc position. Arthrography has a higher diagnostic outcome for anterior disc position but the disadvantage of being an invasive method. We suggest that the quality of the evidence should be improved and that an analysis of the impact of the imaging methods on patient treatment should be performed.

Temporomandibular joint disc position in arthrography. A descriptive analysis of the observer performance.

OBJECTIVES: To explore the relationship between the use of diagnostic cues and observer performance in the diagnosis of the temporomandibular joint (TMJ) disc position. METHODS: Thirty arthrographic examinations using fluoroscopy alone and combined with either transcranial radiography or tomography were selected. Seven observers assessed the examinations with the aid of reference images and written diagnostic cues for three disc diagnoses. After one month three of the observers repeated the assessments and noted how they applied the diagnostic cues. Later, they made a consensus report on how the cues were applied to each joint. RESULTS: The seven observers agreed unanimously with the reference diagnosis by fluoroscopy alone in 11 joints and in 14 when supplemented with transcranial radiography or tomography. Seven joints were given three different diagnoses by the seven observers when fluoroscopy was used alone and five when supplemented with transcranial radiography or tomography. The cues were applied with confidence in those joints where the observers were unanimous but in those with three different diagnoses, the anatomical features described in the cues were difficult to identify or the findings were ambiguous as they did not match the cues. CONCLUSIONS: It appears difficult to define radiographic cues applicable to all TMJ disc positions.

Corrected sagittal tomography of the temporomandibular joint. Influence of errors in film and patient positioning on linear and angular measurements.

The aim of this study was to assess the variation in film and patient positioning in horizontally corrected sagittal tomography of the temporomandibular joint (TMJ). The influence of this variation on linear and angular measurements of some anatomic structures of the TMJ was also studied. There was no significant variation in film positioning in the tomograph, using a multi-film cassette. The variation in positioning the patient in the tomograph was significant when four dental assistants were asked to place the Frankfort plane parallel to the horizontal plane. The measurements with the film in a straight position were compared with the measurements with the film angulated, to simulate the mean variation in film and patient positioning. Linear and angular measurements of anatomic structures were performed in arthrotomograms of 58 joints, representing joints with superior disk position and joints with anterior disk position with and without reduction. There was a difference between the two measurements for four different distances and one angle independent of diagnosis. No differences could be found between patients belonging to the different diagnostic groups. The results indicate that variation in patient positioning influences linear and angular measurements of anatomic structures in TMJ arthrotomograms.

Observer performance and diagnostic outcome in diagnosis of the disk position of the temporomandibular joint using arthrography.

Observer performance and diagnostic outcome in the diagnosis of the disk position of the temporomandibular joint by arthrography were investigated. Three different radiographic techniques were studied: fluoroscopy, fluoroscopy in combination with lateral transcranial radiography, and fluoroscopy in combination with corrected sagittal tomography. Seven observers were asked to assess 30 joints representing three diagnoses, namely superior disk position and anterior disk position with and without reduction. The intraobserver agreement was high for all three techniques. The interobserver agreement of the seven observers when the disk position was diagnosed by fluoroscopy in combination with lateral transcranial radiography (Kappa index 0.68) was higher than that with fluoroscopy alone (Kappa index 0.54) and that with fluoroscopy in combination with corrected sagittal tomography (Kappa index 0.56). The sensitivity, specificity, and predictive values of fluoroscopy in combination with lateral transcranial radiography were also higher than those of the other radiographic techniques. Judged from this study, the observer agreement and the diagnostic outcome for arthrography with the use of the three examined techniques are high when well-defined criteria and reference images are applied.

Clinical follow-up with different disc positions.

A clinical follow-up of 45 previously symptomatic patients with arthrographically diagnosed TMJ disc positions was performed at least 1 year (average, 4 years 4 months) after treatment. The patients were divided into three groups according to disc position: (1) superior disc position, n = 5; (2) anterior disc displacement with reduction, n = 17; and (3) anterior disc displacement without reduction, n = 23. Ninety-two percent of the patient were treated conservatively. Seventy percent of the patients reported being symptom-free or improved. The frequency of joint clicking did not change considerably, but the amount of crepitating sounds increased markedly. No difference was found regarding treatment outcome between the anterior disc displacement groups with and without reduction.

Some anatomical factors of the upper compartment of the temporomandibular joint related to the disc position.

Temporomandibular joint (TMJ) arthrograms of 20 joints with superior disc position (SDP), 20 joints with anterior disc displacement with reduction (ADDWR), and 20 joints with anterior disc displacement without reduction (ADDWOR) were compared. The distance from the insertion of the posterior disc attachment of the upper joint compartment to the top of the mandibular fossa in joints with ADDWR was significantly longer than in joints with SDP and ADDWOR (p = 0.003). The distance from the top of the mandibular fossa to the top of the articular eminence in joints with ADDWR was significantly shorter than in joints with ADDWOR (p = 0.006). The entire distance of the upper joint compartment in joints with ADDWR was significantly longer than in joints with SDP and ADDWOR (p = 0.02).

Angulation and prominence of the posterior slope of the eminence of the temporomandibular joint in relation to disc position.

A steep articular eminence has been proposed as an aetiological factor in internal derangement of the temporomandibular joint. Arthrograms of 20 joints each of three groups, superior disc position (SDP), anterior disc displacement with reduction (ADDWR) and anterior disc displacement without reduction (ADDWOR) were compared. No correlation was found between a steep articular eminence and anterior disc displacement. The posterior slope of the articular eminence of the joints with ADDWOR was significantly less prominent than in those with SDP and ADDWR.