Investigation of BPF algorithm in cone-beam CT with 2D general trajectories

A mathematical derivation was conducted to illustrate that exact 3D image reconstruction could be achieved for z-homogeneous phantoms from data acquired with 2D general trajectories using the back projection filtration (BPF) algorithm. The conclusion was verified by computer simulation and experimental result with a circular scanning trajectory. Furthermore, the effect of the non-uniform degree along z-axis of the phantoms on the accuracy of the 3D reconstruction by BPF algorithm was investigated by numerical simulation with a gradual-phantom and a disk-phantom. The preliminary result showed that the performance of BPF algorithm improved with the z-axis homogeneity of the scanned object.     

Radiation dose saving through the use of cone-beam CT in hearing-impaired patients

Purpose

Bionic ear implants provide a solution for deafness. Patients treated with these hearing devices are often children who require close follow-up with frequent functional and radiological examinations; in particular, multislice computed tomography (MSCT). Dental volumetric cone-beam CT (CBCT) has been reported as a reliable technique for acquiring images of the temporal bone while delivering low radiation doses and containing costs. The aim of this study was to assess, in terms of radiation dose and image quality, the possibility of using CBCT as an alternative to MSCT in patients with bionic ear implants.

Materials and methods

One hundred patients (mean age 26 years, range 7–43) with Vibrant SoundBridge implants on the round window underwent follow-up: 85 with CBCT and 15 with MSCT. We measured the average tissue-absorbed doses during both MSCT and CBCT scans. Each scan was focused on the temporal bone with the smallest field of view and a low-dose protocol. In order to estimate image quality, we obtained data about slice thickness, high- and low-contrast resolution, uniformity and noise by using an AAPM CT performance phantom.

Results

Although the CBCT images were qualitatively inferior to those of MSCT, they were sufficiently diagnostic to allow evaluation of the position of the implants. The effective dose of MSCT was almost three times higher than that of CBCT.

Conclusions

Owing to low radiation dose and sufficient image quality, CBCT could be considered an adequate technique for postoperative imaging and follow-up of patients with bionic ear implants.     

Accuracy of three-dimensional measurements using cone-beam CT

OBJECTIVES: Lesions causing intraosseous defects in the head and neck region are difficult to diagnose using two-dimensional radiography, and three-dimensional (3D) data provided by CT is useful but often difficult to obtain. Recently, cone-beam CT (CBCT) was made available, with the potential to become a practical tool in dentistry. However, there is limited evidence to prove that defect volume can be determined accurately. Therefore, this in vitro validation study aimed at establishing whether linear and 3D CBCT, using volumetric measurements, is accurate for determining osseous defect sizes. METHODS: Depth and diameter of simulated bone defects in (i) an acrylic block and (ii) a human mandible were blindly measured electronically by five examiners using CBCT. Linear measurements were compared with predetermined machined dimensions. Using software, volume extraction was performed by another examiner on the acrylic phantom and compared with known dimensions. Data were analysed using paired t-tests. RESULTS: Using the acrylic block, mean width accuracy was -0.01 mm (+/- 0.02 SE) and mean height difference was -0.03 mm (+/- 0.01 SE; P > 0.05). For the human mandible, mean width accuracy was -0.07 mm (+/- 0.02 SE) and mean height accuracy was -0.27 mm (+/- 0.02 SE; P < 0.01). Volume accuracy was -6.9 mm3 (+/- 4 SE) for automated calculations and -2.3 mm3 (+/- 2.6 SE) for the manual measurements (P < 0.001). CONCLUSIONS: CBCT has the potential to be an accurate, non-invasive, practical method to reliably determine osseous lesion size and volume. Further clinical validation will lead to a vast array of applications in oral and maxillofacial diagnosis.     

Automated identification of anatomical landmarks on 3D bone models reconstructed from CT scan images

Identification of anatomical landmarks on skeletal tissue reconstructed from CT/MR images is indispensable in patient-specific preoperative planning (tumour referencing, deformity evaluation, resection planning, and implant alignment and anchoring) as well as intra-operative navigation (bone registration and instruments referencing). Interactive localisation of landmarks on patient-specific anatomical models is time-consuming and may lack in repeatability and accuracy. We present a computer graphics-based method for automatic localisation and identification (labelling) of anatomical landmarks on a 3D model of bone reconstructed from CT images of a patient. The model surface is segmented into different landmark regions (peak, ridge, pit and ravine) based on surface curvature. These regions are labelled automatically by an iterative process using a spatial adjacency relationship matrix between the landmarks. The methodology has been implemented in a software program and its results (automatically identified landmarks) are compared with those manually palpated by three experienced orthopaedic surgeons, on three 3D reconstructed bone models. The variability in location of landmarks was found to be in the range of 2.15–5.98 mm by manual method (inter surgeon) and 1.92–4.88 mm by our program. Both methods performed well in identifying sharp features. Overall, the performance of the automated methodology was better or similar to the manual method and its results were reproducible. It is expected to have a variety of applications in surgery planning and intra-operative navigation.     

Comparison of 3D bone models of the knee joint derived from CT and 3T MR imaging

PurposeTo examine whether magnetic resonance (MR) imaging can offer a viable alternative to computed tomography (CT) based 3D bone modeling.MethodsCT and MR (SPACE, TrueFISP, VIBE) images were acquired from the left knee joint of a fresh-frozen cadaver. The distal femur, proximal tibia, proximal fibula and patella were manually segmented from the MR and CT examinations. The MR bone models obtained from manual segmentations of all three sequences were compared to CT models using a similarity measure based on absolute mesh differences.ResultsThe average absolute distance between the CT and the various MR-based bone models were all below 1 mm across all bones. The VIBE sequence provided the best agreement with the CT model, followed by the SPACE, then the TrueFISP data. The most notable difference was for the proximal tibia (VIBE 0.45 mm, SPACE 0.82 mm, TrueFISP 0.83 mm).ConclusionsThe study indicates that 3D MR bone models may offer a feasible alternative to traditional CT-based modeling. A single radiological examination using the MR imaging would allow simultaneous assessment of both bones and soft-tissues, providing anatomically comprehensive joint models for clinical evaluation, without the ionizing radiation of CT imaging.     

Spiral CT angiography and 3D reconstruction in patients with aortic coarctation

The objective of this study was to assess the reliability of spiral CT angiography (CTA) and 3D reconstruction in patients with aortic coarctation (CoA). Eighteen patients with suspected or surgically proven coarctation were examined by spiral CT. In addition to the axial slices, 3D reconstructions, such as shaded surface display (SSD) and maximum intensity projection (MIP), were used to determine the diameters of the CoA and the pre- and poststenotic aorta and to visualise the collateral vessels. Diameters derived from cardiac catheterization were compared with those from CTA in 8 patients. The degree of aortic stenosis was correlated with blood pressure gradients (BPG) in 12 patients. The difference between the diameters of the CoA and the pre- and poststenotic aorta derived from MIP and angiography was not statistically significant (p = 0.69). With SSD the internal thoracic artery was detected in 16 and the posterior intercostal artery in 13 cases. The degree of aortic stenosis correlated poorly with the BPG (r = 0.51, r ² = 0.26). CTA with 3D reconstruction represents a reliable noninvasive technique for the assessment of the degree of CoA and the visualisation of collateral vessels. It may serve as a follow-up investigation after intervention or surgical treatment. Received 22 November 1996; Revision received 5 February 1997; Accepted 10 March 1997     

Fusion of computed tomography data and optical 3D images of the dentition for streak artefact correction in the simulation of orthognathic surgery

OBJECTIVE: To determine the limits of accuracy of fusion of optical three-dimensional (3D) imaging and computed tomography (CT) with and without metal artefacts in an experimental setting and to show the application of this hybrid system in 3D orthognathic surgery simulation. METHODS: Ten plaster casts of dental arches were subjected to a CT scan and optical 3D surface imaging. Subsequently, the first molars in the plaster casts were supplied with metal restorations, bilaterally, and new CT scans and optical surface images were assessed. The registration of the surface data of the two imaging modalities of the study models without and with metal restorations was carried out. The mean distance between the two data sets was calculated. From a patient a CT scan of the skull as well as optical 3D images of plaster casts of the dental arches were acquired. Again the two imaging modalities were registered and virtual orthognathic surgery simulation was carried out. RESULTS: The mean distance between the corresponding data points of CT and optical 3D surface images was 0.1262+/-0.0301 mm and 0.2671+/-0.0580 mm, respectively, for the plaster casts without and with metal restorations. The differences between these data were statistically significant (P<0.0005). For the patient case a mean difference of 0.66+/-0.49 mm and 0.56+/-0.48 mm for mandible and maxilla, respectively, was calculated between CT and optical surface data. CONCLUSION: The accuracy of the fusion of 3D CT surface data and optical 3D imaging is significantly reduced by metal artefacts. However, it seems appropriate for virtual orthognathic surgery simulation, as post-operative orthodontics are performed frequently.     

Positioning accuracy for lung stereotactic body radiotherapy patients determined by on-treatment cone-beam CT imaging

Stereotactic body radiotherapy for early stage non-small cell lung cancer is an emerging treatment option in the UK. Since relatively few high-dose ablative fractions are delivered to a small target volume, the consequences of a geometric miss are potentially severe. This paper presents the results of treatment delivery set-up data collected using Elekta Synergy (Elekta, Crawley, UK) cone-beam CT imaging for 17 patients immobilised using the Bodyfix system (Medical Intelligence, Schwabmuenchen, Germany). Images were acquired on the linear accelerator at initial patient treatment set-up, following any position correction adjustments, and post-treatment. These were matched to the localisation CT scan using the Elekta XVI software. In total, 71 fractions were analysed for patient set-up errors. The mean vector error at initial set-up was calculated as 5.3 ± 2.7 mm, which was significantly reduced to 1.4 ± 0.7 mm following image guided correction. Post-treatment the corresponding value was 2.1 ± 1.2 mm. The use of the Bodyfix abdominal compression plate on 5 patients to reduce the range of tumour excursion during respiration produced mean longitudinal set-up corrections of -4.4 ± 4.5 mm compared with -0.7 ± 2.6 mm without compression for the remaining 12 patients. The use of abdominal compression led to a greater variation in set-up errors and a shift in the mean value.     

Impact of orthognathic surgery on the body posture

BackgroundPostural control is classically described as being based on the visual, vestibular, and proprioceptive musculo-articular sensory systems. The influence of mandibular proprioception on postural stabilization remains controversial. Most previous studies analyzed how postural stability is influenced by partial changes in mandibular proprioception (dental occlusion and jaw position).Research QuestionIn the present experiment, we asked whether drastic mandibular changes, resulting from orthognathic surgery (including dental, joint and muscular efferents), modify postural control.MethodsThe analyzes were performed in 22 patients tested before, and 2.5 months, after orthognathic surgery for treatment of dysmorphic jaws. Experiments were performed under 4 experimental conditions: 2 visual conditions: Eyes Open (EO) and Eyes Closed (EC), and 2 occlusal conditions: Occlusion (OC: mandible positioned by the contact of the teeth), and Rest Position (RP: mandible positioned by the muscles without tooth contact). The analyses focused on head orientation in the frontal plane and on postural stabilization in a static task, consisting of standing upright.ResultsThe results show that, 2.5 months after orthognathic surgery, head orientation in the frontal plane was improved, since patient’s external intercanthal lines became closer to the true horizontal line when they were tested EC and in OC condition. Postural responses, based on the wavelet transformation data, highlight an improvement in maintaining an upright stance for all the tested sensory conditions. However, such improvement was greater in the EC and RP conditions.SignificanceThese results show, for the first time, that after drastic mandibular changes, the weight of proprioceptive cues linked to the mandibular system may be so enhanced that it may constitute a new reference frame to orient the head in space, in darkness, and improve static postural stabilization, even in the presence of visual cues.     

A review of dental cone-beam CT dose conversion coefficients

Objective:The purpose of this study was to review the literature to examine the usage and magnitude of effective dose conversion factors (DC_E) for dental cone beam CT (CBCT) scanners.Methods:A PubMed literature search for publications relating to radiation dosimetry in dental radiography was performed. Papers were included if they reported DC_E, or reported ICRP 103 effective dose and dose-area product. 71 papers relating to dental CBCT dosimetry were found, of which eight reported effective dose conversion factors or provided enough information to calculate dose conversion factors. Scanner model, effective dose, dose-area product, tube voltage, field of view size and DC_E were extracted from the papers for analysis.Results:DC_E values ranged from 0.035 to 0.31 µSv/mGy-cm² with a mean of 0.129 µSv/mGy-cm² (SD = 0.056). When categorized into small (<100 cm²), medium (100–225 cm²) and large (>225 cm²) fields of view (FOV), linear fits to the effective dose and dose-area product yielded slopes of 0.129, 0.111 and 0.074 µSv/mGy-cm² for small, medium and large FOVs respectively.Conclusion:The range of reported DC_E values and spread with respect to field of view category suggests that DC_E values that depend on FOV would provide more accurate effective dose estimates. Tube voltage was found to be a smaller factor in determining DC_E. Reasonable values for DC_E taking into account FOV size were obtained. There is considerable room for more work to be done to examine the behaviour of DC_E with changes to patient age and dental CBCT imaging parameters.     

Stereoscopic 3D CT vs standard 3D CT in the classification of acetabular fractures: an experimental study

The accuracy of stereoscopic and standard three-dimensional (3D) CT in the classification of acetabular fractures was compared. A receiver operating characteristic (ROC) analysis was performed by two radiologists and two surgeons blinded to the presence of acetabular fractures in an animal model (a total of 62 porcine hips, 40 with artificial acetabular fractures). Classification of acetabular fractures was adopted from the literature. Interpretation was performed on a workstation using two specific volume rendering algorithms; unshaded and shaded bone. The ROC analysis did not demonstrate any benefit in stereoscopic 3D CT compared with standard 3D CT.     

3D displays for craniofacial surgery

3D visualisation from tomographic image sequences has turned out to be a useful addition to diagnosis and surgical planning in craniofacial surgery. However, its clinical use still suffers from the very large variety of different methods and parameters from which the surgeon may choose. This is true not only of the data acquisition but also for of documentation of the results. Furthermore, there is no standardisation of procedures according to classes of malformations. This paper presents a systematic investigation of these problems. It proposes a standardisation of craniofacial malformations and describes an optimisation of the procedure of 3D visualisation. The procedure described has become a standard tool for craniofacial surgery in our hospital.