Evaluation of 3-Dimensional Superimposition Techniques on Various Skeletal Structures of the Head Using Surface Models

ObjectivesTo test the applicability, accuracy, precision, and reproducibility of various 3D superimposition techniques for radiographic data, transformed to triangulated surface data.MethodsFive superimposition techniques (3P: three-point registration; AC: anterior cranial base; AC + F: anterior cranial base + foramen magnum; BZ: both zygomatic arches; 1Z: one zygomatic arch) were tested using eight pairs of pre-existing CT data (pre- and post-treatment). These were obtained from non-growing orthodontic patients treated with rapid maxillary expansion. All datasets were superimposed by three operators independently, who repeated the whole procedure one month later. Accuracy was assessed by the distance (D) between superimposed datasets on three form-stable anatomical areas, located on the anterior cranial base and the foramen magnum. Precision and reproducibility were assessed using the distances between models at four specific landmarks. Non parametric multivariate models and Bland-Altman difference plots were used for analyses.ResultsThere was no difference among operators or between time points on the accuracy of each superimposition technique (p>0.05). The AC + F technique was the most accurate (D<0.17 mm), as expected, followed by AC and BZ superimpositions that presented similar level of accuracy (D<0.5 mm). 3P and 1Z were the least accurate superimpositions (0.79<D<1.76 mm, p<0.005). Although there was no difference among operators or between time points on the precision of each superimposition technique (p>0.05), the detected structural changes differed significantly between different techniques (p<0.05). Bland-Altman difference plots showed that BZ superimposition was comparable to AC, though it presented slightly higher random error.ConclusionsSuperimposition of 3D datasets using surface models created from voxel data can provide accurate, precise, and reproducible results, offering also high efficiency and increased post-processing capabilities. In the present study population, the BZ superimposition was comparable to AC, with the added advantage of being applicable to scans with a smaller field of view.     

Comparison of the Accuracy of Voxel Based Registration and Surface Based Registration for 3D Assessment of Surgical Change following Orthognathic Surgery

Purpose

Superimposition of two dimensional preoperative and postoperative facial images, including radiographs and photographs, are used to evaluate the surgical changes after orthognathic surgery. Recently, three dimensional (3D) imaging has been introduced allowing more accurate analysis of surgical changes. Surface based registration and voxel based registration are commonly used methods for 3D superimposition. The aim of this study was to evaluate and compare the accuracy of the two methods.

Materials and methods

Pre-operative and 6 months post-operative cone beam CT scan (CBCT) images of 31 patients were randomly selected from the orthognathic patient database at the Dental Hospital and School, University of Glasgow, UK. Voxel based registration was performed on the DICOM images (Digital Imaging Communication in Medicine) using Maxilim software (Medicim-Medical Image Computing, Belgium). Surface based registration was performed on the soft and hard tissue 3D models using VRMesh (VirtualGrid, Bellevue City, WA). The accuracy of the superimposition was evaluated by measuring the mean value of the absolute distance between the two 3D image surfaces. The results were statistically analysed using a paired Student t-test, ANOVA with post-hoc Duncan test, a one sample t-test and Pearson correlation coefficient test.

Results

The results showed no significant statistical difference between the two superimposition methods (p<0.05). However surface based registration showed a high variability in the mean distances between the corresponding surfaces compared to voxel based registration, especially for soft tissue. Within each method there was a significant difference between superimposition of the soft and hard tissue models.

Conclusions

There were no significant statistical differences between the two registration methods and it was unlikely to have any clinical significance. Voxel based registration was associated with less variability. Registering on the soft tissue in isolation from the hard tissue may not be a true reflection of the surgical change.     

3D Mapping of Safe and Danger Zones in the Maxilla and Mandible for the Placement of Intermaxillary Fixation Screws

Intermaxillary (IMF) screws feature several advantages over other devices used for intermaxillary fixation, but using cone beam computed tomography (CBCT) scans to determine the safe and danger zones to place these devices for all patients can be expensive. This study aimed to determine the optimal interradicular and buccopalatal/buccolingual spaces for IMF screw placement in the maxilla and mandible. The CBCT volumetric data of 193 patients was used to generate transaxial slices between the second molar on the right to the second molar on the left in both arches. The mean interradicular and buccopalatal/buccolingual distances and standard deviation values were obtained at heights of 2, 5, 8 and 11 mm from the alveolar bone crest. An IMF screw with a diameter of 1.0 mm and length of 7 mm can be placed distal to the canines (2 - 11 mm from the alveolar crest) and less than 8 mm between the molars in the maxilla. In the mandible, the safest position is distal to the first premolar (more than 5 mm) and distal to the second premolar (more than 2 mm). There was a significant difference (p<0.05) between the right and left quadrants. The colour coding 3D template showed the safe and danger zones based on the mesiodistal, buccopalatal and buccolingual distances in the maxilla and mandible.The safest sites for IMF screw insertion in the maxilla were between the canines and first premolars and between the first and second molars. In the mandible, the safest sites were between the first and second premolars and between the second premolar and first molar. However, the IMF screw should not exceed 1.0 mm in diameter and 7 mm in length.     

The importance of the zygomatic arch in complex midfacial fracture repair and correction of posttraumatic orbitozygomatic deformities

Collapse of the zygomatic arch following trauma results in inadequate anteroposterior projection of the zygomatic body and an increase in facial width. Accurate assessment of the position of the zygomatic arch in relation to the cranial base posteriorly and the midface anteriorly is the key to the acute repair of complex midfacial fractures and the secondary reconstruction of posttraumatic deformities of the orbitozygomaticomaxillary complex. Loss of projection of the zygomatic arch may occur with injuries confined to the orbitozygomaticomaxillary region or in association with complex midfacial fractures. A safe anatomic approach to the zygomatic arch allows exact anatomic restoration of the zygomatic arch using miniplates and screws and results in the reconstruction of an outer facial frame with a correct anteroposterior projection and facial width. The zygomatic arch injury is diagnosed using axial CT scanning. Three-hundred and seventeen arches have been exposed through a coronal incision following acute trauma and 47 arches have been exposed in patients requiring late correction of a posttraumatic orbitozygomaticomaxillary deformity. Permanent palsy to the frontal branch of the facial nerve has occurred in one patient following the exact definition of the anatomy of this region.     

Accuracy and Reliability of a Novel Method for Fusion of Digital Dental Casts and Cone Beam Computed Tomography Scans

Several methods have been proposed to integrate digital models into Cone Beam Computed Tomography scans. Since all these methods have some drawbacks such as radiation exposure, soft tissue deformation and time-consuming digital handling processes, we propose a new method to integrate digital dental casts into Cone Beam Computed Tomography scans. Plaster casts of 10 patients were randomly selected and 5 titanium markers were glued to the upper and lower plaster cast. The plaster models were scanned, impressions were taken from the plaster models and the impressions were also scanned. Linear measurements were performed on all three models, to assess accuracy and reproducibility. Besides that, matching of the scanned plaster models and scanned impressions was done, to assess the accuracy of the matching procedure. Results show that all measurement errors are smaller than 0.2 mm, and that 81% is smaller than 0.1 mm. Matching of the scanned plaster casts and scanned impressions show a mean error between the two surfaces of the upper arch of 0.14 mm and for the lower arch of 0.18 mm. The time needed for reconstructing the CBCT scans to a digital patient, where the impressions are integrated into the CBCT scan of the patient takes about 15 minutes, with little variance between patients. In conclusion, we can state that this new method is a reliable method to integrate digital dental casts into CBCT scans. As far as radiation exposure, soft tissue deformation and digital handling processes are concerned, it is a significant improvement compared to the previously published methods.     

How Accurate Are the Fusion of Cone-Beam CT and 3-D Stereophotographic Images?

Background

Cone-beam Computed Tomography (CBCT) and stereophotography are two of the latest imaging modalities available for three-dimensional (3-D) visualization of craniofacial structures. However, CBCT provides only limited information on surface texture. This can be overcome by combining the bone images derived from CBCT with 3-D photographs. The objectives of this study were 1) to evaluate the feasibility of integrating 3-D Photos and CBCT images 2) to assess degree of error that may occur during the above processes and 3) to identify facial regions that would be most appropriate for 3-D image registration.

Methodology

CBCT scans and stereophotographic images from 29 patients were used for this study. Two 3-D images corresponding to the skin and bone were extracted from the CBCT data. The 3-D photo was superimposed on the CBCT skin image using relatively immobile areas of the face as a reference. 3-D colour maps were used to assess the accuracy of superimposition were distance differences between the CBCT and 3-D photo were recorded as the signed average and the Root Mean Square (RMS) error.

Principal Findings:

The signed average and RMS of the distance differences between the registered surfaces were −0.018 (±0.129) mm and 0.739 (±0.239) mm respectively. The most errors were found in areas surrounding the lips and the eyes, while minimal errors were noted in the forehead, root of the nose and zygoma.

Conclusions

CBCT and 3-D photographic data can be successfully fused with minimal errors. When compared to RMS, the signed average was found to under-represent the registration error. The virtual 3-D composite craniofacial models permit concurrent assessment of bone and soft tissues during diagnosis and treatment planning.     

颧根与侧颅底重要结构的解剖学测量

目的：通过测量颧根与侧颅底各重要结构间的距离关系,为临床侧颅底外科手术治疗提供定位参考。方法：取成人颅骨标本50例（去颅盖标本8例,整颅42例）100侧,用游标卡尺、圆规和直尺测量颧根与侧颅底重要结构的距离。结果：实验测得左右侧颧根与外耳门前缘中点、乳突尖、茎突、翼突外侧板根部、舌下神经管外口、茎乳孔、颈静脉孔外缘、颈动脉管外口后缘、棘孔、卵圆孔、破裂孔的距离分别为22.30±2.84mm和22.02±3.27mm、40.37±3.21mm和40.56±3.54mm、32.53±2.78mm和32.92±2.68mm、35.13±3.14mm和35.19±2.74mm、49.29±2.88mm和48.98±2.87mm、32.92±2.44mm和33.05±2.61mm、35.15±2.86mm和34.68±3.13mm、33.17±2.78mm和33.17±2.72mm、28.83±2.62mm和28.68±2.63mm、31.15±2.76mm和31.49±2.73mm、43.67±3.32mm和44.15±3.02mm,左右侧数据无差异（P〉0.05）。结论：颧根可以作为侧颅底外科手术的定位标志,为直视条件下经颞下等入路的侧颅底外科手术提供解剖学依据。     

Phase correlation applied to the 3D registration of CT and CBCT image volumes

PurposeIn this study, a 3D phase correlation algorithm was investigated to test feasibility for use in determining the anatomical changes that occur throughout a patient's radiotherapy treatment. The algorithm determines the transformations between two image volumes through analysis in the Fourier domain and has not previously been used in radiotherapy for 3D registration of CT and CBCT volumes.MethodsVarious known transformations were applied to a patient's prostate CT image volume to create 12 different test cases. The mean absolute error and standard deviation were determined by evaluating the difference between the known contours and those calculated from the registration process on a point-by-point basis. Similar evaluations were performed on images with increasing levels of noise added. The improvement in structure overlap offered by the algorithm in registering clinical CBCT to CT images was evaluated using the Dice Similarity Coefficient (DSC).ResultsA mean error of 2.35 (σ = 1.54) mm was calculated for the 12 deformations applied. When increasing levels of noise were introduced to the images, the mean errors were observed to rise up to a maximum increase of 1.77 mm. For CBCT to CT registration, maximum improvements in the DSC of 0.09 and 0.46 were observed for the bladder and rectum, respectively.ConclusionsThe Fourier-based 3D phase correlation registration algorithm investigated displayed promising results in CT to CT and CT to CBCT registration, offers potential in terms of efficiency and robustness to noise, and is suitable for use in radiotherapy for monitoring patient anatomy throughout treatment.     

A New 3D Tool for Assessing the Accuracy of Bimaxillary Surgery: The OrthoGnathicAnalyser

Aim

The purpose of this study was to present and validate an innovative semi-automatic approach to quantify the accuracy of the surgical outcome in relation to 3D virtual orthognathic planning among patients who underwent bimaxillary surgery.

Material and Method

For the validation of this new semi-automatic approach, CBCT scans of ten patients who underwent bimaxillary surgery were acquired pre-operatively. Individualized 3D virtual operation plans were made for all patients prior to surgery. During surgery, the maxillary and mandibular segments were positioned as planned by using 3D milled interocclusal wafers. Consequently, post-operative CBCT scan were acquired. The 3D rendered pre- and postoperative virtual head models were aligned by voxel-based registration upon the anterior cranial base. To calculate the discrepancies between the 3D planning and the actual surgical outcome, the 3D planned maxillary and mandibular segments were segmented and superimposed upon the postoperative maxillary and mandibular segments. The translation matrices obtained from this registration process were translated into translational and rotational discrepancies between the 3D planning and the surgical outcome, by using the newly developed tool, the OrthoGnathicAnalyser. To evaluate the reproducibility of this method, the process was performed by two independent observers multiple times.

Results

Low intra-observer and inter-observer variations in measurement error (mean error < 0.25 mm) and high intraclass correlation coefficients (> 0.97) were found, supportive of the observer independent character of the OrthoGnathicAnalyser. The pitch of the maxilla and mandible showed the highest discrepancy between the 3D planning and the postoperative results, 2.72° and 2.75° respectively.

Conclusion

This novel method provides a reproducible tool for the evaluation of bimaxillary surgery, making it possible to compare larger patient groups in an objective and time-efficient manner in order to optimize the current workflow in orthognathic surgery.     

Ultrasound versus Cone-beam CT image-guided radiotherapy for prostate and post-prostatectomy pretreatment localization

PurposeTo evaluate the accuracy of an intra-modality trans-abdominal ultrasound (TA-US) device against soft-tissue based Cone-Beam Computed tomography (CBCT) registration for prostate and post-prostatectomy pre-treatment positioning.MethodsThe differences between CBCT and US shifts were calculated on 25 prostate cancer patients (cohort A) and 11 post-prostatectomy patients (cohort B), resulting in 284 and 106 paired shifts for cohorts A and B, respectively. As a second step, a corrective method was applied to the US registration results to decrease the systematic shifts observed between TA-US and CBCT results. This method consisted of subtracting the mean difference obtained between US and CBCT registration results during the first 3 sessions from the US registration results of the subsequent sessions. Inter-operator registration variability (IOV) was also investigated for both modalities.ResultsAfter initial review, about 20% of the US images were excluded because of insufficient quality. The average differences between US and CBCT were: 2.8 ± 4.1 mm, −0.9 ± 4.2 mm, 0.4 ± 3.4 mm for cohort A and 1.3 ± 5.0 mm, −2.3 ± 4.6 mm, 0.5 ± 2.9 mm for cohort B, in the anterior-posterior (AP), superior-inferior (SI) and lateral (LR) directions, respectively. After applying the corrective method, only the differences in the AP direction remained significant (p < 0.05). The IOV values were between 0.6–2.0 mm and 2.1–3.5 mm for the CBCT and TA-US modalities, respectively.ConclusionsBased on the obtained results and on the image quality, the TA-US imaging modality is not safely interchangeable with CBCT for pre-treatment repositioning. Treatment margins adaptation based on the correction of the systematic shifts should be considered.     

The role of the zygomatic arch in the statics of the skull and its adaptive shape

The zygomatic arch of mammals is usually considered a phylogenetic relic of the fenestrations of the skull roof which may be observed in morphological sequences of primitive vertebrate skulls. If this concept is correct, the element is comparable (though not homologous) to the jugal arches of diapsid reptiles. Two major questions then remain unanswered: why different elements are maintained in reptiles and mammals during evolution, and why the arches are maintained as relics of ancestral forms. It is tempting to respond to the latter question with a very simple answer, namely that the elements function in order to sustain mechanical stresses. In this paper, we raise the questions which quality of stresses occurs in a primate skull within the zygomatic arches and what relationship these stresses hold to the morphology of these bony elements. An answer has been sought by means of finite element stress analysis. We found that the zygomatic arch in primate skulls represents a structure which carries, under all biologically relevant conditions, either compressive or tensile stresses. In a very simple model of the human skull under bite forces, a strip of stresses occurs lateral to the orbits, which seems roughly comparable to the zygomatic arch. Once such a structure exists and is used as an insertion of adductor muscles, it will be exposed to bending stress in side view and in frontal view. Morphological details of the zygomatic arch (curvature, profile, suture) are well suited to sustain the evoked stresses by a minimum of material.     

Human dental arch shape evaluated by Euclidean-distance matrix analysis

Form differences between biological structures can be evaluated using several approaches. When landmark data are available, a recently proposed method (euclidean-distance matrix analysis) seems to be able to differentiate between size and shape differences. This method also localizes those areas which differ most between the two structures. We have applied it to analyze the sexual dimorphism in dental arch form in a sample of 50 men and 45 women. Subjects ranged in age between 20 and 27 years, and had sound dentitions. Fourteen landmarks, corresponding to the centers of gravity (centroids) of the occlusal surfaces of all permanent teeth (right second molar to left second molar), were individualized on the dental casts of subjects. All the possible linear distances between pairs of teeth were computed, thus creating four mean form matrices (one for each arch within sex). Gender differences were tested by using euclidean-distance matrix analysis. No significant differences were demonstrated in the shape of arches, while male arches proved to be slightly bigger than female arches. © 1993 Wiley-Liss, Inc.     

以乳突切迹和翼钩为基点的侧颅底分区方法

目的：探讨以乳突切迹和翼钩为基点的侧颅底分区新方法。方法：在乳突切迹后缘、翼钩、枕骨大孔前缘中点和颧根四个结构间相互连线,区分侧颅底并测量连线的长度。结果：乳突切迹与侧颅底重要结构的关系密切,切迹后缘与翼钩连线和正中线将侧颅底分成内、外侧两个大的三角区,每个区再分成前后两个三角区共四个三角区,即腭和颞下三角、咽三角、关节和听三角、血管神经三角,其中血管神经三角的三边长度左右侧分别为（74.52±5.47）mm和（74.66±5.41）mm、（59.77±3.84）mm和（59.67±3.56）mm、（42.23±3.11）mm和（42.48±2.60）mm。结论：本研究提供了新的侧颅底分区方法,且血管神经三角的区域划分更为科学,为临床侧颅底手术入路和定位提供了解剖学参考。     

The functional significance of the squamosal suture in Australopithecus boisei.

A juvenile Australopithecus boisei specimen from the Omo basin, southern Ethiopia, is found to exhibit and extraordinarily large overlap of the temporal squama on the parietal, a phenomenon shared with at least two adult specimens of A. boisei. An attempt is made to interpret the overlap as a structural (bony/ligamentous) adaptation necessitated by the unique combination of certain components of the masticatory system of A. boisei. These are: (1) the massiveness and strength of the temporalis muscle, (2) its relatively anterior location, and (3) the lateral position of the masseter muscle due to the flaring of the zygomatic arches. The effect of the temporalis muscle is to create excessive pressure on the portion of the squamosal suture along the parietal, while the lateral placement of the masseter and the resultant increase of pressure on the temporal squama via the zygomatic arch tend to "loosen" the contact between the temporal and parietal bones.     

Validation of a deformable image registration produced by a commercial treatment planning system in head and neck

In recent years one of the areas of interest in radiotherapy has been adaptive radiation therapy (ART), with the most efficient way of performing ART being the use of deformable image registration (DIR). In this paper we use the distances between points of interest (POIs) in the computed tomography (CT) and the cone beam computed tomography (CBCT) acquisition images and the inverse consistence (IC) property to validate the RayStation treatment planning system (TPS) DIR algorithm. This study was divided into two parts: Firstly the distance-accuracy of the TPS DIR algorithm was ascertained by placing POIs on anatomical features in the CT and CBCT images from five head and neck cancer patients. Secondly, a method was developed for studying the implication of these distances on the dose by using the IC. This method compared the dose received by the structures in the CT, and the structures that were quadruply-deformed. The accuracy of the TPS was 1.7 ± 0.8 mm, and the distance obtained with the quadruply-deformed IC method was 1.7 ± 0.9 mm, i.e. the difference between the IC method multiplied by two, and that of the TPS validation method, was negligible. Moreover, the IC method shows very little variation in the dose-volume histograms when comparing the original and quadruply-deformed structures. This indicates that this algorithm is useful for planning adaptive radiation treatments using CBCT in head and neck cancer patients, although these variations must be taken into account when making a clinical decision to adapt a treatment plan.     

儿童下牙弓发育的测量

本研究测量了１９３６名６－１２岁男女儿童的下颌牙弓的宽度和弦长。儿童前牙弓在７－９岁时增长最快,为此时恒尖牙萌出所致,１０岁以后各年龄组均数差异检验没有显著性意义。在整个替牙期,儿童前牙弓宽度约增加２ｍｍ,弦长增加１．８ｍｍ,儿童后牙弓的宽度和弦长约增加１ｍｍ左右。男孩前牙弓宽度比女孩大１ｍｍ左右。后牙弓的宽度和弦箕各年龄组均为男大于女,约１ｍｍ在右。可以把小学年龄阶段儿童的牙列分为乳牙弓、第１恒     

Birotundal diameter of the human sphenoid bone from age six years to early adulthood

Longitudinal data for a twenty-year period are presented for a dimension of the human sphenoid bone as seen on postero-anterior roentgenograms of 22 males and 18 females (healthy, American-born Caucasoids). Birotundal diameter measured as the distance between the centers of the right and left foramen rotundum averaged near 32.0 mm for each sex at age 6 years. Individual increase in size to age 26 years ranged between 3.2 mm and 8.3 mm. There is no correlation between size at age 6 years and change between this age and early adulthood. For 54 males and 50 females at age 9 years, intercorrelations were obtained among six variables: birotundal diameter, head breadth, bizygomatic diameter, bigonial diameter, upper and loweer dental arch widths at the permanent first molars. The r's were nonsignificant or low positive (with the exception of upper-lower dental arch, r = 0.76). The results of this study are aligned with three previous studies of human cranial base width.     

The clinical feasibility and performance of an orthogonal X-ray imaging system for image-guided radiotherapy in nasopharyngeal cancer patients: Comparison with cone-beam CT

The demand for greater accuracy of intensity-modulated radiotherapy (IMRT) has driven the development of more advanced verification systems for image-guided radiotherapy (IGRT). The purpose of this study is to investigate setup discrepancies measured between an orthogonal X-ray guidance system (XGS-10) and cone-beam computed tomography (CBCT) of Varian in the IMRT of patients with nasopharyngeal cancer (NPC). The setup errors measured by XGS-10 and CBCT at the treatment unit with respect to the planning CTs were recorded for 30 patients with NPC. The differences in residual setup errors between XGS-10 system and CBCT were computed and quantitatively analyzed. The time of image acquisition and image registration was recorded. The radiation doses delivered by CBCT and XGS-10 were measured using PTW0.6CC ionization chambers and a water phantom. The differences between setup errors measured by the XGS-10 system and CBCT were generally <1.5 mm for translations, indicating a reasonably good agreement between the two systems for patients with NPC in the translation directions of A-P (P = 0.856), L-R (P = 0.856) and S-I (P = 0.765). Moreover, compared with CBCT, XGS-10 took much shorter image acquisition and registration time (P <0.001) and delivered only a small fraction of extra radiation dose to the patients (P <0.001). These results indicate that XGS-10 offers high localization accuracy similar to CBCT and additional benefits including prompt imaging process, low imaging radiation exposure, real time monitoring, which therefore represents a potential attractive alternative to CBCT for clinical use.     

Growth changes in measurements of upper facial positioning

Growth changes in the position of the midline upper face are examined for samples of Pan troglodytes, Gorilla gorilla, and modern humans. Horizontal and vertical distances between nasion and the anterior end of the cribriform plate are plotted against stage of dental development. Kendall's nonparametric correlations between facial positioning and stage of dental development are tested for significance. In African apes, the upper face becomes more projecting and positioned higher relative to the anterior cranial base. The extent of this horizontal and vertical separation reflects primarily facial size. In modern humans, the upper face becomes more projecting but is relatively stable in its vertical position. Comparison of Pan and modern human crania in the youngest dental age category indicates that the upper face of modern humans is positioned lower early in postnatal life. The position of the upper face (glabella) relative to the anterior and posterior cranial base is presented for several fossil hominid crania. The fossil crania are similar to Pan and modern humans in facial projection relative to the anterior cranial base. However, glabella is positioned low in the fossil crania. Total facial projection (relative to hormion) for Sts 5 is similar to the mean for Gorilla. Fossil Homo and robust australopithecine crania display very projecting upper faces. We suggest that the upper face of Homo is projecting due to the length of the anterior cranial fossa, while robust australopithecines possess a thick frontal bone.