Significant Artifact Reduction at 1.5T and 3T MRI by the Use of a Cochlear Implant with Removable Magnet: An Experimental Human Cadaver Study

OBJECTIVE Cochlear implants (CIs) are standard treatment for postlingually deafened individuals and prelingually deafened children. This human cadaver study evaluated diagnostic usefulness, image quality and artifacts in 1.5T and 3T magnetic resonance (MR) brain scans after CI with a removable magnet. METHODS Three criteria (diagnostic usefulness, image quality, artifacts) were assessed at 1.5T and 3T in five cadaver heads with CI. The brain magnetic resonance scans were performed with and without the magnet in situ. The criteria were analyzed by two blinded neuroradiologists, with focus on image distortion and limitation of the diagnostic value of the acquired MR images. RESULTS MR images with the magnet in situ were all compromised by artifacts caused by the CI. After removal of the magnet, MR scans showed an unequivocal artifact reduction with significant improvement of the image quality and diagnostic usefulness, both at 1.5T and 3T. Visibility of the brain stem, cerebellopontine angle, and parieto-occipital lobe ipsilateral to the CI increased significantly after magnet removal. CONCLUSIONS The results indicate the possible advantages for 1.5T and 3T MR scanning of the brain in CI carriers with removable magnets. Our findings support use of CIs with removable magnets, especially in patients with chronic intracranial pathologies.

The effect of pulsed jet lavage in vertebroplasty on injection forces of PMMA bone cement: an animal study

Percutaneous vertebroplasty, comprising of the injection of polymethylmethacrylate (PMMA) into vertebral bodies, is an efficient procedure to stabilize osteoporotic compression fractures as well as other weakening lesions. Besides fat embolism, cement leakage is considered to be one of the major and most severe complications during percutaneous vertebroplasty. The viscosity of the PMMA during injection plays a key role in this context. It was shown in vitro that the best way to lower the risk of cement leakage is to inject the cement at higher viscosity, which is requires high injection forces. Injection forces can be reduced by applying a newly developed lavage technique as it was shown in vitro using human cadaver vertebrae. The purpose of this study was to prove the in vitro results in an in vivo model. The investigation was incorporated in an animal study that was performed to evaluate the cardiovascular reaction on cement augmentation using the lavage technique. Injection forces were measured with instrumentation for 1 cc syringes, additionally acquiring plunger displacement. Averaged injection forces measured, ranged from 12 to 130 N and from 28 to 140 N for the lavage group and the control group, respectively. Normalized injection forces (by viscosity and injection speed) showed a trend to be lower for the lavage group in comparison to the control group (P = 0.073). In conclusion, the clinical relevance on the investigated lavage technique concerning lowering injection forces was only shown by trend in the performed animal study. However, it might well be that the effect is more pronounced for osteoporotic vertebral bodies.

Augmentation of mechanical properties in osteoporotic vertebral bones--a biomechanical investigation of vertebroplasty efficacy with different bone cements

Recent clinical trials have reported favorable early results for transpedicular vertebral cement reinforcement of osteoporotic vertebral insufficiencies. There is, however, a lack of basic data on the application, safety and biomechanical efficacy of materials such as polymethyl-methacrylate (PMMA) and calciumphospate (CaP) cements. The present study analyzed 33 vertebral pairs from five human cadaver spines. Thirty-nine vertebrae were osteoporotic (bone mineral density < 0.75 g/cm2), 27 showed nearly normal values. The cranial vertebra of each pair was augmented with either PMMA (Palacos E-Flow) or experimental brushite cement (EBC), with the caudal vertebra as a control. PMMA and EBC were easy to inject, and vertebral fillings of 20-50% were achieved. The maximal possible filling was inversely correlated to the bone mineral density (BMD) values. Cement extrusion into the spinal canal was observed in 12% of cases. All specimens were subjected to axial compression tests in a displacement-controlled mode. From load-displacement curves, the stiffness, S, and the maximal force before failure, Fmax, were determined. Compared with the native control vertebrae, a statistically significant increase in vertebral stiffness and Fmax was observed by the augmentation. With PMMA the stiffness increased by 174% (P = 0.018) and Fmax by 195% (P = 0.001); the corresponding augmentation with EBC was 120% (P = 0.03) and 113% (P = 0.002). The lower the initial BMD, the more pronounced was the augmentation effect. Both PMMA and EBC augmentation reliably and significantly raised the stiffness and maximal tolerable force until failure in osteoporotic vertebral bodies. In non-porotic specimens, no significant increase was achieved.

Estimation of tool pose based on force-density correlation during robotic drilling

The application of image-guided systems with or without support by surgical robots relies on the accuracy of the navigation process, including patient-to-image registration. The surgeon must carry out the procedure based on the information provided by the navigation system, usually without being able to verify its correctness beyond visual inspection. Misleading surrogate parameters such as the fiducial registration error are often used to describe the success of the registration process, while a lack of methods describing the effects of navigation errors, such as those caused by tracking or calibration, may prevent the application of image guidance in certain accuracy-critical interventions. During minimally invasive mastoidectomy for cochlear implantation, a direct tunnel is drilled from the outside of the mastoid to a target on the cochlea based on registration using landmarks solely on the surface of the skull. Using this methodology, it is impossible to detect if the drill is advancing in the correct direction and that injury of the facial nerve will be avoided. To overcome this problem, a tool localization method based on drilling process information is proposed. The algorithm estimates the pose of a robot-guided surgical tool during a drilling task based on the correlation of the observed axial drilling force and the heterogeneous bone density in the mastoid extracted from 3-D image data. We present here one possible implementation of this method tested on ten tunnels drilled into three human cadaver specimens where an average tool localization accuracy of 0.29 mm was observed.

Mechatronic feasibility of minimally invasive, atraumatic cochleostomy

Robotic assistance in the context of lateral skull base surgery, particularly during cochlear implantation procedures, has been the subject of considerable research over the last decade. The use of robotics during these procedures has the potential to provide significant benefits to the patient by reducing invasiveness when gaining access to the cochlea, as well as reducing intracochlear trauma when performing a cochleostomy. Presented herein is preliminary work on the combination of two robotic systems for reducing invasiveness and trauma in cochlear implantation procedures. A robotic system for minimally invasive inner ear access was combined with a smart drilling tool for robust and safe cochleostomy; evaluation was completed on a single human cadaver specimen. Access to the middle ear was successfully achieved through the facial recess without damage to surrounding anatomical structures; cochleostomy was completed at the planned position with the endosteum remaining intact after drilling as confirmed by microscope evaluation.

The effect of pulsed jet lavage in vertebroplasty on injection forces of polymethylmethacrylate bone cement, material distribution, and potential fat embolism: a cadaver study

STUDY DESIGN: In vitro testing of vertebroplasty techniques including pulsed jet-lavage for fat and marrow removal in human cadaveric lumbar and thoracic vertebrae. OBJECTIVE: To develop jet-lavage techniques for vertebroplasty and investigate their effect on cement distribution, injection forces, and fat embolism. SUMMARY OF BACKGROUND DATA: The main complications of cement vertebroplasty are cement leakage and pulmonary fat embolism, which can have fatal consequences and are difficult to prevent reliably by current vertebroplasty techniques. METHODS: Twenty-four vertebrae (Th8-L04) from 5 osteoporotic cadaver spines were grouped in triplets depending on bone mineral density (BMD). Before polymethylmethacrylate (PMMA) vertebroplasty, a pulsatile jet-lavage for removal of intertrabecular fat and bone marrow was performed in 2 groups with 8 specimens each, performing radial and axial irrigation from the biopsy needles. One hundred mL of Ringer solution were injected through 1 pedicle and regained by low vacuum via the contralateral pedicle. Eight control vertebrae were not irrigated. All specimens underwent standardized PMMA cement augmentation injecting 20% of the vertebral volume. Injection forces, cement distribution, and extravasations were quantified. RESULTS: All irrigation solution could be retrieved with the vacuum applied. A Kruskal-Wallis test revealed significantly higher injection forces of the control group as compared with the irrigated groups (P = 0.021). Dilatation of the syringe at forces above 300 N occurred in 75% of the untreated compared with 12.5% of the lavaged specimens. CT distribution analysis showed more homogenous cement distribution of the cement and significantly less extravasation in the irrigated specimens. CONCLUSION: The developed lavage technique for vertebroplasty showed to be feasible and reproducible. The reduction of injection forces would allow the use of more viscous PMMA cement lowering the risk for cement embolization and results in a safer procedure. The wash-out of bone marrow and the possible reduction of pulmonary fat embolism have to be verified with in vivo models.

Bone conduction in Thiel-embalmed cadaver heads

INTRODUCTION Sound can reach the inner ear via at least two different pathways: air conduction and bone conduction (BC). BC hearing is used clinically for diagnostic purposes and for BC hearing aids. Research on the motion of the human middle ear in response to BC stimulation is typically conducted using cadaver models. We evaluated middle ear motion of Thiel-embalmed whole-head specimens in terms of linearity, reproducibility, and consistency with the reported middle ear motion of living subjects, fresh cadaveric temporal bones, and whole-heads embalmed with a Non-Thiel solution of salts. METHODS We used laser Doppler vibrometry to measure the displacement of the skull, the umbo, the cochlear promontory, the stapes, and the round window in seven ears from four human whole-head specimens embalmed according to Thiel's method. The ears were stimulated with a Baha(®) implanted behind the auricle. RESULTS The Thiel model shows promontory velocity similar to that reported in the literature for whole-heads embalmed with a Non-Thiel solution of salts (0- to 7-dB difference). The Thiel heads' relative velocity of the stapes with respect to the promontory was similar to that of fresh cadaver temporal bones (0- to 4-dB difference). The velocity of the umbo was comparable in Thiel-embalmed heads and living subjects (0- to 10-dB difference). The skull and all middle ear elements measured responded linearly to different stimulation levels, with an average difference less than 1 dB. The variability of repeated measurements for both short- (2 h; 4 dB) and long-term (4-16 weeks; 6 dB) repetitions in the same ear, and the difference between the two ears of the same donor (approximately 10 dB) were lower than the inter-individual difference (up to 25 dB). CONCLUSION Thiel-embalmed human whole-head specimens can be used as an alternative model for the study of human middle ear mechanics secondary to BC stimulation. At some frequencies, differences from living subjects must be considered.

Image-based biomechanical assessment of vertebral body and intervertebral disc in the human lumbar spine

Life expectancy continuously increases but our society faces age-related conditions. Among musculoskeletal diseases, osteoporosis associated with risk of vertebral fracture and degenerative intervertebral disc (IVD) are painful pathologies responsible for tremendous healthcare costs. Hence, reliable diagnostic tools are necessary to plan a treatment or follow up its efficacy. Yet, radiographic and MRI techniques, respectively clinical standards for evaluation of bone strength and IVD degeneration, are unspecific and not objective. Increasingly used in biomedical engineering, CT-based finite element (FE) models constitute the state-of-art for vertebral strength prediction. However, as non-invasive biomechanical evaluation and personalised FE models of the IVD are not available, rigid boundary conditions (BCs) are applied on the FE models to avoid uncertainties of disc degeneration that might bias the predictions. Moreover, considering the impact of low back pain, the biomechanical status of the IVD is needed as a criterion for early disc degeneration. Thus, the first FE study focuses on two rigid BCs applied on the vertebral bodies during compression test of cadaver vertebral bodies, vertebral sections and PMMA embedding. The second FE study highlights the large influence of the intervertebral disc’s compliance on the vertebral strength, damage distribution and its initiation. The third study introduces a new protocol for normalisation of the IVD stiffness in compression, torsion and bending using MRI-based data to account for its morphology. In the last study, a new criterion (Otsu threshold) for disc degeneration based on quantitative MRI data (axial T2 map) is proposed. The results show that vertebral strength and damage distribution computed with rigid BCs are identical. Yet, large discrepancies in strength and damage localisation were observed when the vertebral bodies were loaded via IVDs. The normalisation protocol attenuated the effect of geometry on the IVD stiffnesses without complete suppression. Finally, the Otsu threshold computed in the posterior part of annulus fibrosus was related to the disc biomechanics and meet objectivity and simplicity required for a clinical application. In conclusion, the stiffness normalisation protocol necessary for consistent IVD comparisons and the relation found between degeneration, mechanical response of the IVD and Otsu threshold lead the way for non-invasive evaluation biomechanical status of the IVD. As the FE prediction of vertebral strength is largely influenced by the IVD conditions, this data could also improve the future FE models of osteoporotic vertebra.