Comparison of high-resolution magnetic resonance imaging and micro-computed tomography arthrography for in-vivo assessment of cartilage in non-human primate models.

BACKGROUND: Non-human primate (NHP) could be an interesting model for osteoarthritis (OA) longitudinal studies but standard medical imaging protocols are not able to acquire sufficiently high-resolution images to depict the thinner cartilage (compared to human) in an in vivo context. The aim of this study was thus to develop and validate the acquisition protocols for knee joint examination of NHP using magnetic resonance imaging (MRI) at 1.5 T and X-ray micro-computed tomography arthrography (µCTA). METHODS: The first phase of the study focused on developing dedicated in vivo HR-MRI and µCTA protocols for simultaneous acquisitions of both knee joints on NHP. For MR, a dedicated two-channel receiver array coil and acquisition sequence were developed on a 1.5 T Siemens Sonata system and tuned to respect safety issues and reasonable examination time. For µCTA, an experimental setup was devised so as to fulfill similar requirements. The two imaging protocols were used during a longitudinal study so as to confirm that repeated injections of loxaglic acid (contrast agent used for µCTA) didn't induce any bias in cartilage assessment and to compare segmentation results from the two modalities. Lateral and medial cartilage tibial plateaus were assessed using a common image processing protocol leading to a 3D estimation of the cartilage thickness. RESULTS: From HR-MRI and µCTA images, thickness distributions were extracted allowing for proper evaluation of knee cartilage thickness of the primates. Results obtained in vivo indicated that the µCTA protocol did not induce any bias in the measured cartilage parameters and moreover, segmentation results obtained from the two imaging modalities were consistent. CONCLUSIONS: MR and µCTA are valuable imaging tools for the morphological evaluation of cartilage in NHP models which in turn can be used for OA studies.

Electro-optic probe for real-time assessments of RF electric field produced in an MRI scanner: Feasibility tests at 3 and 4.7 T.

During magnetic resonance imaging (MRI) examinations, the average specific absorption rate (SAR) of the whole body is calculated as an index of global energy deposition in biological tissue without taking into account the presence of metallic implants or conductive materials. However, this global SAR calculation is not sufficient to ensure patient safety and a local SAR measurement should be carried out. Several measurement techniques have already been used to evaluate the local SAR, in particular electric field (E-field) probes, but the accuracy of the measurements and the resolutions (spatial and temporal) depend strongly on the measurement method/probe. This work presents an MR-compatible, subcentimeter probe based on an electro-optic (EO) principle enabling a real-time measurement of the local E-field during MRI scans. The experiments using these probes were performed on two different MR systems (preclinical and clinical) having different static magnetic field strengths and with different volume coil geometries. The E-field was measured with unloaded (in air) and loaded volume coils in order to assess the sensing characteristics of the optical probe. The results show an excellent linearity between the measured E-field and the radiofrequency (RF) magnetic field in both experimental conditions. Moreover, the distribution of the E-field throughout the volume coil was experimentally determined and was in good agreement with numerical simulations. Finally, we demonstrate through our measurements that the E-field depends strongly on the dielectric properties of the medium.

New trends in MRI of cartilage: Advances and limitations in small animal studies.

Due to the actual interest for bioengineering in the osteoarthritis (OA) healing context, researchers need accurate qualitative and quantitative methodologies to evaluate in vivo the integration and functionality of their cartilage-like biomaterials. As in clinical diagnostic strategies, advances in Magnetic Resonance Imaging (MRI) seem promising for non-vulnerant assessments of articular cartilage bio-architecture and morphology in small animal models. These experimental models are commonly used to monitor the physiopathology of OA and to evaluate therapeutic responses mediated by chondroprotective drugs or tissue engineering. Nowadays, the application of MR protocols to in vivo small animal cartilage imaging is achievable with the development of high magnetic fields and the adaptation of methodologies to reach the required spatial resolution and contrast. The purpose of this article is to summarize these current MRI strategies used for in vivo small animal articular cartilage assessments.