Optimization of dental CBCT exposures through mAs reduction

Objectives:

To investigate the effect of tube current–exposure time (mAs) reduction on clinical and technical image quality for different CBCT scanners, and to determine preliminary minimally acceptable values for the mAs and contrast-to-noise ratio (CNR) in CBCT.

Methods:

A polymethyl methacrylate (PMMA) phantom and an anthropomorphic skull phantom, containing a human skeleton embedded in polyurethane, were scanned using four CBCT devices, including seven exposure protocols. For all protocols, the mAs was varied within the selectable range. Using the PMMA phantom, the CNR_AIR was measured and corrected for voxel size. Eight axial slices and one coronal slice showing various anatomical landmarks were selected for each CBCT scan of the skull phantom. The slices were presented to six dentomaxillofacial radiologists, providing scores for various anatomical and diagnostic parameters.

Results:

A hyperbolic relationship was seen between CNR_AIR and mAs. Similarly, a gradual reduction in clinical image quality was seen at lower mAs values; however, for several protocols, image quality remained acceptable for a moderate or large mAs reduction compared with the standard exposure setting, depending on the clinical application. The relationship between mAs, CNR_AIR and observer scores was different for each CBCT device. Minimally acceptable values for mAs were between 9 and 70, depending on the criterion and clinical application.

Conclusions:

Although noise increased at a lower mAs, clinical image quality often remained acceptable at exposure levels below the manufacturer''s recommended setting, for certain patient groups. Currently, it is not possible to determine minimally acceptable values for image quality that are applicable to multiple CBCT models.     

下颌前牙唇舌侧骨壁厚度的锥形束 CT 研究

目的：应用锥形束 CT(CBCT)测量下颌前牙唇舌侧骨壁厚度,为即刻种植提供临床指导。方法选取132例正常牙合个体的 CBCT 图像,对其下颌前牙进行研究。测量根尖处、根尖1/4、根中1/2、根颈1/4处的唇舌侧骨壁厚度,比较左、右侧及性别之间骨壁厚度的差异。结果下前牙左右侧同名牙相应骨壁厚度无差异。除根颈1/4处外,舌侧骨壁较唇侧厚。中切牙、侧切牙和尖牙在根颈1/4处,唇侧骨壁厚度>2 mm 的比例分别占0、0和1.51%。在根中1/4处,分别占0.76%、1.44%和3.79%。性别对舌侧骨壁厚度有影响。结论应用 CBCT 建立了正常牙合个体下颌前牙区唇舌侧骨壁厚度的参考值,为即刻种植提供临床指导依据。     

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.     

Influence of CBCT parameters on the output of an automatic edge-detection-based endodontic segmentation

Objectives:

To determine the optimal CBCT settings for an automatic edge-detection-based endodontic segmentation procedure by assessing the accuracy of the root canal measurements.

Methods:

12 intact teeth with closed apexes were cut perpendicular to the root axis, at pre-determined levels to the reference plane (the first section made before acquisition). Acquisitions of each specimen were performed with Kodak 9000^® 3D (76 µm, 14 bits; Kodak Carestream Health, Trophy, France) by using different combinations of milliamperes and kilovolts. Three-dimensional images were displayed and root canals were segmented with the MeVisLab software (edge-detection-based method; MeVis Research, Bremen, Germany). Histological root canal sections were then digitized with a 0.5- to 1.0-µm resolution and compared with equivalent two-dimensional cone-beam reconstructions for each pair of settings using the Pearson correlation coefficient, regression analysis and Bland–Altman method for the canal area and Feret''s diameter. After a ranking process, a Wilcoxon paired test was carried out to compare the pair of settings.

Results:

The best pair of acquisition settings was 3.2 mA/60 kV. Significant differences were found between 3.2 mA/60 kV and other settings (p < 0.05) for the root canal area and for Feret''s diameter.

Conclusions:

The quantitative analyses of the root canal system with the edge-detection-based method could depend on acquisition parameters. Improvements in segmentation still need to be carried out to ensure the quality of the reconstructions when we have to deal with closer outlines and because of the low spatial resolution.     

Technical aspects of dental CBCT: state of the art

As CBCT is widely used in dental and maxillofacial imaging, it is important for users as well as referring practitioners to understand the basic concepts of this imaging modality. This review covers the technical aspects of each part of the CBCT imaging chain. First, an overview is given of the hardware of a CBCT device. The principles of cone beam image acquisition and image reconstruction are described. Optimization of imaging protocols in CBCT is briefly discussed. Finally, basic and advanced visualization methods are illustrated. Certain topics in these review are applicable to all types of radiographic imaging (e.g. the principle and properties of an X-ray tube), others are specific for dental CBCT imaging (e.g. advanced visualization techniques).     

CT和锥形束CT配准范围与精度关系研究

目的 研究锥形束CT（CBCT）图像和CT图像的配准范围对配准精度的影响。方法 对腹部、头部和胸部各5位患者分别进行CBCT和CT扫描,将扫描后各部位图像的配准范围处理成4种模式,模式1为CT配准范围大于CBCT配准范围;模式2为CBCT与CT配准范围相同;模式3是将模式2的CT平移5 cm;模式4是将模式2的CBCT和CT两边同时减少2 cm。4种模式均使用图像分割与配准工具包进行配准,比较4种模式配准后的平均方差测度值,并分析模式2与其他3种模式的关系。结果 平均方差测度值模式3最大,其次是模式1,最小的是模式2和4,且模式2和4的平均方差测度值几乎相等（P>0.05）。对于各部位的模式2与模式1比较,差异有统计学意义（t=-4.586、-4.164、-5.618,P<0.05）;模式2与模式3比较,差异有统计学意义（t=-6.423、-8.109、-19.601,P<0.05）。结论 CBCT和CT图像的配准范围对图像配准精度有一定的影响,CBCT的配准范围与CT的配准范围越接近,配准精度越高。     

Three-dimensional reconstruction of teeth and jaws based on segmentation of CT images using watershed transformation

The objective of the present study was the development and clinical testing of a three-dimensional (3D) reconstruction method of teeth and a bone tissue of the jaw on the basis of CT images of the maxillofacial region. 3D reconstruction was performed using the specially designed original software based on watershed transformation. Computed tomograms in digital imaging and communications in medicine format obtained on multispiral CT and CBCT scanners were used for creation of 3D models of teeth and the jaws. The processing algorithm is realized in the stepwise threshold image segmentation with the placement of markers in the mode of a multiplanar projection in areas relating to the teeth and a bone tissue. The developed software initially creates coarse 3D models of the entire dentition and the jaw. Then, certain procedures specify the model of the jaw and cut the dentition into separate teeth. The proper selection of the segmentation threshold is very important for CBCT images having a low contrast and high noise level. The developed semi-automatic algorithm of multispiral and cone beam computed tomogram processing allows 3D models of teeth to be created separating them from a bone tissue of the jaws. The software is easy to install in a dentist''s workplace, has an intuitive interface and takes little time in processing. The obtained 3D models can be used for solving a wide range of scientific and clinical tasks.     

Comparison of micro-CT and cone beam CT-based assessments for relative difference of grey level distribution in a human mandible

Objectives:

The purpose of this study was to examine the ability of CT to assess the relative difference of degree of bone mineralization (grey level) parameters in a human mandible.

Methods:

Ten mandibular sections from cadavers (81.5 ± 12.1 years) were scanned using micro-CT with 27.2 μm voxel size and cone beam CT (CBCT) with 200 μm, 300 μm, and 400 μm voxel sizes. In addition, 15 clinical CBCT images from young patients (mean age 18.9 ± 3.3 years) were identified. After segmentation of bone voxels, alveolar bone and basal cortical bone regions were digitally isolated. A histogram of grey level, which is equivalent to degree of bone mineralization, was obtained from each region of the CT images. Mean, standard deviation (SD), coefficient of variation (COV), fifth percentile low (Low₅) and high (High₅) of alveolar bone and basal cortical bone regions were obtained. Percentage differences of grey level parameters between alveolar and basal cortical bones were computed.

Results:

The alveolar bone region had significantly lower Mean, Low₅ and High₅ values but significantly higher SD and COV than the basal cortical bone region for all CT images (p < 0.05). All parameters were significantly lower for the old cadaver group than for the young patient group (p < 0.05).

Conclusions:

CBCT and micro-CT provide comparable results in the assessment of relative difference in grey level distribution between alveolar and basal cortical bone regions in the human mandible. The percentage difference relative to an internal reference (basal cortical bone) can be a reliable method when assessing the degree of bone mineralization using CBCT images for both cross-sectional and longitudinal comparisons.     

Comparison of linear measurements between CBCT orthogonally synthesized cephalograms and conventional cephalograms

Objectives:

The purposes of the study are to investigate the consistency of linear measurements between CBCT orthogonally synthesized cephalograms and conventional cephalograms and to evaluate the influence of different magnifications on these comparisons based on a simulation algorithm.

Methods:

Conventional cephalograms and CBCT scans were taken on 12 dry skulls with spherical metal markers. Orthogonally synthesized cephalograms were created from CBCT data. Linear parameters on both cephalograms were measured via Photoshop CS v. 5.0 (Adobe^® Systems, San Jose, CA), named measurement group (MG). Bland–Altman analysis was utilized to assess the agreement of two imaging modalities. Reproducibility was investigated using paired t-test. By a specific mathematical programme “cepha”, corresponding linear parameters [mandibular corpus length (Go-Me), mandibular ramus length (Co-Go), posterior facial height (Go-S)] on these two types of cephalograms were calculated, named simulation group (SG). Bland–Altman analysis was used to assess the agreement between MG and SG. Simulated linear measurements with varying magnifications were generated based on “cepha” as well. Bland–Altman analysis was used to assess the agreement of simulated measurements between two modalities.

Results:

Bland–Altman analysis suggested the agreement between measurements on conventional cephalograms and orthogonally synthesized cephalograms, with a mean bias of 0.47 mm. Comparison between MG and SG showed that the difference did not reach clinical significance. The consistency between simulated measurements of both modalities with four different magnifications was demonstrated.

Conclusions:

Normative data of conventional cephalograms could be used for CBCT orthogonally synthesized cephalograms during this transitional period.     

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.     

Comparison of cone beam CT scans with enhanced photostimulated phosphor plate images in the detection of root fracture of endodontically treated teeth

Objectives:

Two-dimensional intraoral radiography is the most common tool for diagnosing root fractures (RFs). Cone beam CT (CBCT) is widely used to depict RFs in endodontically treated teeth. Beam hardening and other artefacts caused by gutta percha may result in an incorrect diagnosis when using CBCT only. A comparison of two CBCT machines with photostimulated phosphor (PSP) plate images enhanced with the equalization tool was carried out to detect RFs in endodontically treated teeth.

Methods:

66 roots were collected, decoronated and treated endodontically using the same technique with gutta percha. 33 of these roots were randomly selected and fractured; the 2 root fragments were glued together with 1 layer of methyl methacrylate and placed randomly in 8 prepared beef rib fragments. Large fields of view (FOVs) were acquired with one CBCT unit and small FOVs with the second CBCT unit. Periapical radiographs (using intraoral PSP plates) were also acquired. A contrast enhancement tool was used when evaluating the PSP plate images.

Results:

Small FOV images had significantly higher accuracy (area under the receiver operating characteristic curve) and sensitivity in detecting RFs than PSP plates and large FOV images. The specificity of the enhanced PSP images was higher than, although not significantly higher than, the small FOV images and was significantly higher than the large FOV images.

Conclusions:

CBCT small FOVs should be acquired for depicting RFs of endodontically treated teeth. Images obtained using PSP plates had the lowest rate of false-positive results and their use can save the patient a radiation dose.     

颞骨高分辨率CT各向同性的研究

目的　研究颞骨高分辨率CT是否能够达到各向同性并筛选出最佳扫描参数组合。方法　采用Philips公司Mx 80 0 0 4层螺旋CT机进行扫描 ,MxVIEW工作站进行图像后处理。矩阵采用5 12× 5 12 ,视野 (FOV)为 2 5 0mm× 2 5 0mm ,选用头颅标本 4具 ,8耳 ,分别用准直 0 5mm、重建间隔5 0 % (0 3mm)、不同螺距 (0 6 2 5、0 875、1 2 5 0、1 75 0 )得到直接扫描轴、冠状面图像各 4组。通过图像重建技术 ,得到准直 0 5mm、重建间隔 83 3% (0 5mm) ,准直 1 0mm、重建间隔 5 0 % (0 5mm)的重建图像各 4组。轴面选用镫骨、弓形下窝、前庭上神经管、鼓膜张肌、前庭导水管等 9个解剖结构 ,冠状面选用鼓膜张肌、锤骨外侧韧带、前庭下神经管、蜗窗等 9个解剖结构。利用多平面重建 (MPR)技术 ,采用上述解剖结构 ,分别将 12组轴、冠状扫描图像与相应的重建图像的显示质量进行统计评分处理 ,验证能否达到各向同性并筛选出达到各向同性的最佳参数组合。利用这个最佳参数组合 ,采用患者组 15耳进一步验证其正确性。结果　螺距 0 6 2 5、0 875 ,准直 0 5mm、重建间隔 0 3mm 2组扫描参数的MPR图像与直接图像能够达到各向同性。 [前组t0 0 5( 2 ) =- 0 78,P >0 0 5 ;后组t0 0 5( 3 ) =- 1 93,P>0 0 5 ]。采用准直 0     

MSCT多平面和曲面重组技术在结直肠癌诊断中的应用价值

目的:探讨MSCT多平面重组(MPR)和曲面重组(CPR)技术在结直肠癌诊断中的临床应用价值。方法:回顾性分析36例经手术证实的结直肠癌病例,术前均行MSCT平扫、增强扫描、MPR和沿病变长径的CPR。将CT显示的影像特征与手术证实的结果进行对比分析。结果:MSCT对浆膜层浸润的显示率为93.1%(27/29),淋巴结转移显示率为83.3%(5/6),远处脏器转移显示率为60.0%(3/5),肿块显示率为100.0%(36/36),肿瘤供血显示率为100.0%(23/23)。结论:MSCTMPR和CPR可以更好地显示病灶及其与周围脏器的关系,为外科手术治疗提供独特的信息。     

320层螺旋CT 4D动态重建在髌骨脱位半脱位诊断中的初步应用

目的:探讨320层螺旋CT 4D动态重建在髌骨脱位半脱位诊断中的价值。方法:将24例髌骨脱位半脱位可疑病例,行动态扫描重建,测量诊断指标,然后统计分析数据。结果:在4个指标中,诊断髌骨脱位半脱位最有价值的是髌股吻合角与髌骨倾斜角,最有价值的角度是30°。结论:320层螺旋CT 4D动态重建在髌骨脱位半脱位病变诊断中具有独特的价值,可以多角度对髌骨的相对位置进行评价,消除某些角度产生的假阳性与假阴性,减少误诊率与漏诊率。     

Influence of size of field of view (FOV), position within the FOV,and scanning mode on the detection of root fracture and observer’s perception of artifacts in CBCT images

Objective:To assess the influence of field of view (FOV) size, scanning position within the FOV and scanning mode on the detection of root fracture and artifact perception.Methods:Forty single-rooted premolars restored with NiCr and AgPd posts were divided into two groups: fractured and sound. All teeth were scanned using four CBCT scanning protocols varying FOV sizes (80 × 80 mm and 50 × 55 mm) and scanning modes (Standard and High Definition). The sample was positioned within the FOV in two pre-set positions (central and lateral) and in four positions established by the operator (quadrants). Detection of root fracture and artifact perception were assessed by two observers using 5-point and 4-point scales. Sensitivity, specificity, accuracy, and AUC values were calculated and compared by ANOVA two-way and Tukey’s test. Chi-square and Fisher’s exact test were used to assess artifact perception. The level of significance was set at p < 0.05.Results :The central position within the FOV presented higher sensitivity, specificity, accuracy, and AUC values and differed from the lateral position within the FOV for the studied metal posts (p<0.05). Quadrant 2 presented the best sensitivity, accuracy, and AUC values (p<0.05). The lateral position within the FOV, AgPd posts, quadrants 1 and 3 and protocols 1 (SM, 80 × 80) and 2 (HD, 80× 80) presented higher frequency of artifacts classified as “severe”.Conclusion:Positioning the object in the center or closer to the anterior periphery of the FOV while using a small FOV improved the detection of root fracture and decreased artifact perception.     

CBCT-based bone quality assessment: are Hounsfield units applicable?

CBCT is a widely applied imaging modality in dentistry. It enables the visualization of high-contrast structures of the oral region (bone, teeth, air cavities) at a high resolution. CBCT is now commonly used for the assessment of bone quality, primarily for pre-operative implant planning. Traditionally, bone quality parameters and classifications were primarily based on bone density, which could be estimated through the use of Hounsfield units derived from multidetector CT (MDCT) data sets. However, there are crucial differences between MDCT and CBCT, which complicates the use of quantitative gray values (GVs) for the latter. From experimental as well as clinical research, it can be seen that great variability of GVs can exist on CBCT images owing to various reasons that are inherently associated with this technique (i.e. the limited field size, relatively high amount of scattered radiation and limitations of currently applied reconstruction algorithms). Although attempts have been made to correct for GV variability, it can be postulated that the quantitative use of GVs in CBCT should be generally avoided at this time. In addition, recent research and clinical findings have shifted the paradigm of bone quality from a density-based analysis to a structural evaluation of the bone. The ever-improving image quality of CBCT allows it to display trabecular bone patterns, indicating that it may be possible to apply structural analysis methods that are commonly used in micro-CT and histology.     

Trabecular bone histomorphometric measurements and contrast-to-noise ratio in CBCT

Objectives:

The aim of this study was to evaluate how imaging parameters at clinical dental CBCT affect the accuracy in quantifying trabecular bone structures, contrast-to-noise ratio (CNR) and radiation dose.

Methods:

15 radius samples were examined using CBCT (Accuitomo FPD; J. Morita Mfg., Kyoto, Japan). Nine imaging protocols were used, differing in current, voltage, rotation degree, voxel size, imaging area and rotation time. Radiation doses were measured using a kerma area product-meter. After segmentation, six bone structure parameters and CNRs were quantified. Micro-CT (μCT) images with an isotropic resolution of 20 μm were used as a gold standard.

Results:

Structure parameters obtained by CBCT were strongly correlated to those by μCT, with correlation coefficients >0.90 for all studied parameters. Bone volume and trabecular thickness were not affected by changes in imaging parameters. Increased tube current from 5 to 8 mA, decreased isotropic voxel size from 125 to 80 μm and decreased rotation angle from 360° to 180° affected correlations for trabecular termini negatively. Decreasing rotation degree also weakened correlations for trabecular separation and trabecular number at 80 μm voxel size. Changes in the rotation degree and tube current affected CNR significantly. The radiation dose varied between 269 and 1153 mGy cm².

Conclusions:

Trabecular bone structure can be accurately quantified by clinical dental CBCT in vitro, and the obtained structure parameters are strongly related to those obtained by μCT. A fair CNR and strong correlations can be obtained with a low radiation dose, indicating the possibility for monitoring trabecular bone structure also in vivo.     

MSCT及MPR技术在肠梗阻诊断中的应用价值

目的:探讨MSCT及多平面重组技术(MPR)在肠梗阻诊断中的应用价值。方法:对拟诊肠梗阻的189例患者行MSCT动态增强扫描,将薄层容积扫描数据,在工作站行后处理,分别对肠梗阻的病因、部位及程度作出诊断,然后与术后病理结果逐一对照。比较单独观察横断面图像与横断面结合MPR图像诊断肠梗阻的诊断符合率,2种方法的诊断一致性及肠梗阻诊断的信心指数评分。结果:在单独观察横断面图像与横断面结合MPR图像诊断肠梗阻时,诊断符合率分别为91.1%和95.6%,两者间差异无统计学意义(χ~2=2.14,P>0.05),但诊断一致性进一步提高(K值分别为0.61、0.76),2种方法间信心指数评分差异有统计学意义(P>0.05)。结论:单独观察横断面图像与横断面结合MPR诊断肠梗阻符合率间差异无统计学意义,但利用MPR技术能够增加肠梗阻的诊断信心,并可使不同阅片者间诊断一致性增加。MPR技术是肠梗阻诊断的重要补充方法。