Adrian Rengle

In vivo high-resolution MRI (7T) of femoro-tibial cartilage changes in the rat anterior cruciate ligament transection model of osteoarthritis: a cross-sectional study

OBJECTIVE To assess OA-related changes in mean compartmental femorotibial cartilage thickness in rat knees by three-dimensional (3D) MRI (7T). METHODS MRI was performed in vivo at 7T on OA and untouched contralateral knee joints. Gradient Echo Fast Imaging 3D MR images were acquired sequentially in surgically induced OA (D0) in 40 Wistar rats (anterior cruciate ligament transection). Mean femoral (trochlear, lateral and medial) and tibial (lateral and medial) cartilage thicknesses were quantified from a 2D MRI slide in weight-bearing areas and from a 3D MRI data set. At each time-point [Day (D)8, D14, D21, D40 and D60], eight animals (16 knees) were sacrificed for concomitant histomorphometry. RESULTS As body weight dramatically increased throughout the experiment (+150%, baseline vs endpoint), all compartmental mean cartilage thicknesses noticeably decreased (D8, D14) and then remained relatively stable. Femoral compartments in OA knees were thinner at the end of the experiment than in contralateral age-matched knees. Conversely, lateral and medial tibial cartilages were thicker than controls. Histological correlation was significant only in untouched healthy cartilages (3D better than 2D). CONCLUSIONS 3D MRI (7T) enables in vivo monitoring of compartmental changes in OA-related femorotibial rat cartilage thickness vs contralateral age-matched knees.

A Dedicated Two-Channel Phased-Array Receiver Coil for High-Resolution MRI of the Rat Knee Cartilage at 7 T

In the field of small animal studies, the array coil imaging has become increasingly important. In this paper, a dedicated two-channel array coil operating at 300 MHz (7 T) for high-resolution MRI (HR-MRI) of the rat knee cartilage is presented. The average gain in signal-to-noise ratio (SNR) compared to a 15-mm multipurpose surface coil was 2.2. This SNR gain was used to improve the spatial resolution of 3-D acquisitions by decreasing the voxel size from 59 times 59 times 156 mum3 to 51 times 51 times 94 mum3 without time penalty. Also, a set of two array coils was used to perform a simultaneous acquisition of both knee joints of a rat, maintaining the same scanning time without SNR or spatial resolution degradation compared to the single knee joint acquisition. This two-channel array coil is a key element to perform HR-MRI and extract cartilage morphological parameters such as thickness and volume.

In vivo rat knee cartilage volume measurement using quantitative high resolution MRI (7 T): feasibility and reproducibility.

OBJECTIVES to assess reliability and reproducibility of quantitative MRI (7 T) in assessing rat femoro-tibial cartilage volume. METHODS 5 healthy rat knees were scanned in vivo using a 7 T experimental imager. Sagittal high resolution 3D Gradient Echo with fat suppression sequences were performed with a dedicated home-made 2-elements array coil. 3D MRI sets were used to perform manual segmentation of the 3 cartilage compartments (femoral groove, medial and lateral tibial plateaus) by using a tactile screen. To evaluate inter- and intra-observer reproducibilities, the segmentation procedure was done blindly by two trained observers. One observer repeated the operation twice, with a period of 10 months between both readings. RESULTS the mean duration to manually segment all the slices covering the cartilaginous joint was 4 hours. On the one hand, the inter-observer root mean square of coefficients of variation was 9.1%, 6.2%, 9.6% for the femoral, medial and lateral tibial compartments respectively. On the other hand, the intra-observer reproducibility was 2.1%, 3.2%, 2.5% for these cartilage compartments cited above. CONCLUSION the image quality obtained at 7 Teslas with our dedicated coil allowed segmentation of the cartilage compartments with good reproducibility. This study demonstrated that MRI is a useful technology to provide a non-invasive and reliable assessment of rat knee cartilage volume.

A dedicated two-element phased array receiver coil for high resolution MRI of rat knee cartilage at 7T

Despite that on clinical systems phased array technology is now widely used, the high field MRI experimental systems with multiple receiver channels just became available few years ago. For this reason and due to the large range of magnetic field (frequencies between 200 and 500 MHz for proton resonance), commercial phased arrays implemented in narrow bore for high field applications are rare and relatively expensive. Array coil imaging is an advanced method for acquiring high resolution images with enhanced signal-to-noise ratio (SNR) and/or enlarged field of view (FOV) compared for example to single loop surface coil. The volume of interest is then covered by several coil elements and images reconstructed for every single channel are combined afterwards. The goal of this work was to develop a dedicated two-element array coil operating at 300 MHz (7 T) for high-resolution imaging of rat knee joint in order to quantify cartilage thickness and volume. A dedicated two-element array coil with two square elements encompassing knee joint was designed and built. Decoupling between elements was achieved with a capacitor inserted on the common leg of the two elements. The average gain in SNR compared to a 15 mm reference single loop coil was 2.2. This SNR gain was used to improve spatial resolution of 3D acquisition by decreasing the voxel size from 59 x 59 x 156 mum3 to 51 x 51 times 94 mum3 without time penalty. This two element array coil is a key element to perform HR-MRI and extract morphological parameters of animal cartilage. Finally, a set of two array coils were used to perform the acquisition of both rat knee joint at the same time with no additional acquisition time or resolution decrease.

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, optimizing 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 (LODm, LODc) 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.

Optical cardiac and respiratory device for synchronized MRI on small animal

Respiratory and cardiac motion must be overcome if MRI of the thorax or abdomen is to be performed satisfactorily. An optical-based device designed to synchronize MRI acquisition on small animal was developed using a pair of optical fibers. Light from a laser diode was focused into the transmit fiber and impinged upon the moving skin. The reflected light was detected by the receive fiber and then caries to a light-voltage photodiode, were the signal was amplified and filtered. The recorded optical-based signals are well correlated with both respiratory and heart motions. The signal amplitude recorded on both rats and mice were large enough to perform an easy adjustment of gating level with good differentiation between cardiac and respiratory signal. The device developed using thin fibers is simple to use even when space available around the mice is limited (narrow coils). The signal is totally unaffected by radiofrequency impulsions or magnetic field gradients used for imaging. This optical-based trigger system was used successfully for dual cardiac and respiratory synchronization of rat and mice for heart and liver examinations at 4.7 T.

High-Resolution 1H NMR Micro spectroscopy using an Implantable Micro-coil

This study presents a new concept of implantable micro coil (1000 times 500 mum2) fabricated using an electroplating technique, used as receiver coil at 200 MHz for the measurement of small volumes and concentrations samples by NMR spectroscopy. Our goal is to determine its concentration sensitivity Sc and its limit of detection LOD. The MRI and simulation of RF field distribution allows defining an active volume of 0.8 muL. The spectroscopic results show that the Sc is closed to 0.2691 mM-1 and LOD of 11.1 mM. This micro-system offers the possibility of new investigation techniques like local cerebral metabolites occupying small volumes (muL to nL).