The influence of geometry on intervertebral disc stiffness.

Geometry plays an important role in intervertebral disc (IVD) mechanics. Previous computational studies have found a link between IVD geometry and stiffness. However, few experimental studies have investigated this link, possibly due to difficulties in non-destructively quantifying internal geometric features. Recent advances in ultra-high resolution MRI provides the opportunity to visualise IVD features in unprecedented detail. This study aimed to quantify 3D human IVD geometries using 9.4 T MRIs and to investigate correlations between geometric variations and IVD stiffness. Thirty human lumbar motion segments (fourteen non-degenerate and sixteen degenerate) were scanned using a 9.4 T MRI and geometric parameters were measured. A 1kN compressive load was applied to each motion segment and stiffness was calculated. Degeneration caused a reduction (p < 0.05) in IVD height, a decreased nucleus-annulus area ratio, and a 1.6 ± 3.0 mm inward collapse of the inner annulus. The IVD height, anteroposterior (AP) width, lateral width, cross-sectional area, nucleus-annulus boundary curvature, and nucleus-annulus area ratio had a significant (p < 0.05) influence on IVD stiffness. Linear relationships (p < 0.05, r > 0.47) were observed between these geometric features and IVD compressive stiffness and a multivariate regression model was generated to enable stiffness to be predicted from features observable on clinical imaging (stiffness, N/mm = 6062 - (61.2 × AP width, mm) - (169.2 × IVD height, mm)). This study advances our understanding of disc structure-function relationships and how these change with degeneration, which can be used to both generate and validate more realistic computational models.

Quantifying deformations and strains in human intervertebral discs using Digital Volume Correlation combined with MRI (DVC-MRI).

Physical disruptions to intervertebral discs (IVDs) can cause mechanical changes that lead to degeneration and to low back pain which affects 75% of us in our lifetimes. Quantifying the effects of these changes on internal IVD strains may lead to better preventative strategies and treatments. Digital Volume Correlation (DVC) is a non-invasive technique that divides volumetric images into subsets, and measures strains by tracking the internal patterns within them under load. Applying DVC to MRIs may allow non-invasive strain measurements. However, DVC-MRI for strain measurements in IVDs has not been used previously. The purpose of this study was to quantify the strain and deformation errors associated with DVC-MRI for measurements in human IVDs. Eight human lumbar IVDs were MRI scanned (9.4 T) for a 'zero-strain study' (multiple unloaded scans to quantify noise within the system), and a loaded study (2 mm axial compression). Three DVC methodologies: Fast-Fourier transform (FFT), direct correlation (DC), and a combination of both FFT and DC approaches were compared with subset sizes ranging from 8 to 88 voxels to establish the optimal DVC methodology and settings which were then used in the loaded study. FFT + DC was the optimal method and a subset size of 56 voxels (2520 µm) was found to be a good compromise between errors and spatial resolution. Displacement and strain errors did not exceed 28 µm and 3000 microstrain, respectively. These findings demonstrate that DVC-MRI can quantify internal strains within IVDs non-invasively and accurately. The method has unique potential for assessing IVD strains within patients.

Phased-array of microcoils allows MR microscopy of ex vivo human skin samples at 9.4 T.

PURPOSE: The aim of this study was to demonstrate the feasibility of a custom-made phased-array microcoil within a 400 MHz animal system for the morphological characterization of human skin tissue in correlation with histopathology. MATERIALS AND METHODS: A dedicated 7-channel microcoil-based MR detector arranged in a phased-array geometry was developed to combine the advantages of both a large field of view and a high signal-to-noise ratio. Standard gradient echo sequences were adapted for the characterization of skin morphology ex vivo. RESULTS: In this study, the feasibility of using this type of microdetector, combined with specially manufactured sample holders, to achieve high-resolution MR images of fresh and formalin-fixed, normal and hidradenitis suppurativa diseased skin was successfully demonstrated. The setup presented in this work allows reliable acquisitions of high-resolution images with in-plane resolution up to 25 × 25 µm², and 100 µm in the orthogonal direction, thereby allowing the differentiation of typical layers of the skin, sebaceous glands and hair follicle. CONCLUSION: This study demonstrates that MR microscopy on skin biopsies can be applied at low cost on a standard animal MR imaging system. The successful imaging of different skin structures ex vivo is a prerequisite for non-invasive, in vivo application of skin MR microscopy for accurate complementary disease diagnosis in dermatology.

Characterization of a 3D MEMS fabricated micro-solenoid at 9.4 T.

We present for the first time a complete characterization of a micro-solenoid for high resolution MR imaging of mass- and volume-limited samples based on three-dimensional B(0), B(1) per unit current (B(1)(unit)) and SNR maps. The micro-solenoids are fabricated using a fully micro-electromechanical systems (MEMS) compatible process in conjunction with an automatic wire-bonder. We present 15 µm isotropic resolution 3D B(0) maps performed using the phase difference method. The resulting B(0) variation in the range of [-0.07 ppm to -0.157 ppm] around the coil center, compares favorably with the 0.5 ppm limit accepted for MR microscopy. 3D B(1)(unit) maps of 40 µm isotropic voxel size were acquired according to the extended multi flip angle (ExMFA) method. The results demonstrate that the characterized microcoil provides a high and uniform sensitivity distribution around its center (B(1)(unit) = 3.4 mT/A ± 3.86%) which is in agreement with the corresponding 1D theoretical data computed along the coil axis. The 3D SNR maps reveal a rather uniform signal distribution around the coil center with a mean value of 53.69 ± 19%, in good agreement with the analytical 1D data along coil axis in the axial slice. Finally, we prove the microcoil capabilities for MR microscopy by imaging Eremosphaera viridis cells with 18 µm isotropic resolution.

Investigation of NMR limits of detection for implantable microcoils.

Although NMR has the ability to investigate biological systems non-destructively, its low sensitivity primarily has hampered their investigation compared to other analytical techniques. Therefore, optimi zing radio frequency (RF) coils to improve sensitivity do offer benefits in MR spectroscopy (MRS). Sensitivity may be improved for mass- and volume-limited samples if the size of the detection RF coils matches the sample size. In this paper, the mass- and concentration-limit of detection (LOD(m), LOD(c)) for an implantable microcoil will be estimated by MRS measurements and then compared with their analytical values. For a sample containing a solution of several cerebral metabolites, for the Choline case, the LODm is 5.7 . 10(-9)mol and LODc of 3.8 mM. These preliminary results enable to open largely the biomedical applications based on cerebral metabolism investigation on small animal experiments.

NMR planar micro coils for micro spectroscopy: design and characterisation.

The goal of this study is to determine the concentration sensitivity and the limit of detection of a SNMR receiver planar micro coil with ellipsoidal geometry 1000x500 microm, fabricated using an electroplating technique and used as SNMR receiver coil at 200 MHz. The maximum signal intensity on the NMR images and simulation of RF field distribution allows defining an active volume of 0.8 microL. The localised spectroscopy based on a PRESS sequence shows that the concentration sensitivity is closed to S(C)=2.33 M(-1) and the limit of detection LOD=0.8 M. This micro-system offers the possibility of new investigation techniques based on implantable micro coils used for in vivo study of local cerebral metabolites occupying a small volume (microL to nL order).