Bone labelling on micro-magnetic resonance images.

The study of trabecular bone is receiving increasing interest within the medical community working in the field of skeletal diseases, such as osteoporosis. Quantification of trabecular bone structure usually requires as a starting point a correct segmentation of the trabecular network. We have developed a probabilistic relaxation labelling technique, which uses local features of the trabecular bone images to improve segmentation. Tests on synthetic images show that bone labelling performs a more accurate segmentation than conventional techniques such as thresholding, especially by preserving the connectivity of the trabecular network. Tests on acquired data show that porosity values obtained after segmentation are in good agreement with experimental values obtained by weighing the bone samples.

Automatic NMR field-frequency lock-pulsed phase locked loop approach.

A self-contained deuterium frequency-field lock scheme for a high-resolution NMR spectrometer is described. It is based on phase locked loop techniques in which the free induction decay signal behaves as a voltage-controlled oscillator. By pulsing the spins at an offset frequency of a few hundred hertz and using a digital phase-frequency discriminator this method not only eliminates the usual phase, rf power, offset adjustments needed in conventional lock systems but also possesses the automatic pull-in characteristics that dispense with the use of field sweeps to locate the NMR line prior to closure of the lock loop.

Q optimization of RF inductors for use in NMR probes.

Radiofrequency inductors are important components of many NMR probe circuits, and a high quality factor Q of these inductors is necessary to achieve good signal-to-noise ratios. We have designed high Q inductors using an optimum coil diameter to coil length ratio of the order of 2.54. The latter was calculated from Lorenz's inductance formula for a solenoid and checked experimentally using a series of inductors of various diameters and lengths. The results show good agreement between theory and practice.

Parallel-plate split-conductor surface coil: analysis and design.

A split-conductor circular parallel-plate resonator with one end shorted to ground can be used to one's advantage as a surface coil for NMR imaging experiments. It can be easily constructed by a printed-circuit method and possesses a relatively high loaded Q with negligible frequency detuning. From the proposed equivalent circuit, an equation was derived that relates the self-resonance frequency of the resonator to the split distance measured from a reference point. An example of a 101-mm-diameter surface coil operating at 26 MHz for 31P in vivo spectroscopy is given to illustrate the concordance between calculated and experimental results. Complementary equations and formulas are also included to assist researchers in designing their own antennas to meet specific requirements.

A UHF probe for NMR micro-imaging experiments.

A long solenoid is capable of producing, at least theoretically, a B1 field that is relatively more homogeneous than other known structures. This paper describes the design of a UHF probe incorporating a one-turn solenoid associated with a pair of parallel-plate transmission line feeders and presents a practical example used in a 360-MHz spectrometer for micro-imaging experiments. Numerical evaluation of the solenoid inductance using King's method for direct calculation of elliptic integrals is also included.

In vitro magnetic resonance microimaging of experimental osteoarthritis in the rat knee joint.

OBJECTIVE: To visualize articular changes during iodoacetate induced osteoarthritis (OA) of the rat knee using high resolution magnetic resonance imaging (MRI). METHODS: All images were acquired on a 8.5T microimager on ex vivo rat knees. After optimization studies, 3D spin echo sequences were used with TR 1200 ms and TE 15 ms; 40 sagittal images were obtained with a resolution of 400 x 60 x 60 microns. OA lesions were achieved by injecting 3 mg iodoacetate in the right knee joint (Day 0). Progression of OA changes was studied at Days 5, 10, 15, 20, 30, 90 and corresponding radiographs and histological sections were obtained. RESULTS: Spin echo images of the normal rat knee clearly visualized cortical bone, calcified menisci, patella, and cruciate ligaments as dark regions. Cartilage and muscles appeared as intermediate signal. In some cases a trilaminar appearance of cartilage and growth plate was depicted. This appearance persisted at Day 5, although cartilage presented a marked loss of proteoglycans. After Day 10 the menisci appeared irregular and inhomogeneous. After Day 15, MRI showed important alterations of articular cartilage, predominant on weight bearing areas of femoral condyles. Osteophytic remodeling was also seen around the patella. From Day 20 to Day 90 anatomical changes progressively affected epiphyseal bone, leading to subchondral cysts and loss of its regular trabecular structure. CONCLUSION: High field MRI provides a sensitive method for investigating ex vivo focal erosions of cartilage and established osteochondral remodeling in experimental OA in the rat. In this model, microimaging provides more information about early modifications of cartilage and soft tissue than radiographic exploration, in good correlation with histological data.

Evaluation of cartilage repair tissue after biomaterial implantation in rat patella by using T2 mapping.

To evaluate the ability of MR T2 mapping (8.5 T) to characterize ex vivo longitudinally, morphologically and quantitatively, alginate-based tissue engineering in a rat model of patellar cartilage chondral focal defect. Calibrated rat patellar cartilage defects (1.3 mm) were created at day 0 (D0) and alginate sponge with (Sp/C+) or without (Sp/C-) autologous chondrocytes were implanted. Animals were sacrificed sequentially at D20, D40 and D60 after surgery and dissected patellae underwent MRI exploration (8.5 T). T2 values were calculated from eight SE images by using nonlinear least-squares curve fitting on a pixel-by-pixel basis (constant repetition time of 1.5 s, eight different echo times: 5.5, 7.5, 10.5, 12.5, 15.0, 20.0, 25.0 and 30.0 ms). On the T2 map, acquired in a transversal plane through the repair zone, global T2 values and zonal variation of T2 values of repair tissue were evaluated versus control group and compared with macroscopic score and histological studies (toluidine blue, sirius red and hematoxylin-eosin). "Partial", "total" and "hypertrophic" repair patterns were identified. At D40 and D60, Sp/C+ group was characterized by a higher proportion of "total" repair in comparison to Sp/C- group. At D60, the proportion of "hypertrophic" repair was two fold in Sp/C- group versus Sp/C+ group. As confirmed morphologically and histologically, the T2 map also permitted the distinction of three types of repair tissue: "total", "partial" and "hypertrophic". "Total" repair tissue was characterized by high T2 values versus normal cartilage (p<0.05). Zonal variation, reflecting the collagen network organization, appeared only at D60 for Sp/C+ group (p<0.05). "Hypertrophic" tissue, mainly observed at D60, presented high T2 global values without zonal variation with cartilage depth. These results confirm the potency of the MR T2 map (8.5 T) to characterize macroscopically and microscopically the patterns of the scaffold guided-tissue repair of a focal chondral lesion in the rat patella ("total", "partial" and "hypertrophic"). On T2 map, three parameters (i.e. MRI macroscopic pattern, T2 global values and zonal variation of T2 values) permit to characterize chondral repair tissue, as a virtual biopsy.

T2 mapping: an efficient MR quantitative technique to evaluate spontaneous cartilage repair in rat patella.

OBJECTIVE: To evaluate the ability of T2 mapping on an 8.5 T imager to characterize morphologically and quantitatively spontaneous repair of rat patellar cartilage following full thickness defect. METHODS: Patellar cartilage defects were created in 24 rats knees on D0. Eight rats per time-point were killed on D20, D40 and D60 after surgery. T2 maps of repair tissue in patellar defects were obtained from eight different axial spin echo images on an 8.5 T imager. Global, superficial and deep T2 values were evaluated in spontaneous repair tissues (3x8 right patellae) vs the opposite patellae (3x8 left patellae) of the same animals. MR data were compared with macroscopic and histological studies. RESULTS: T2 map was able to identify morphologically three types of repair tissue observed macroscopically and histologically: 'total', 'partial' and 'hypertrophic' repair tissue. 'Total' and 'partial' repair tissues were characterized by global T2 values almost similar to controls, whereas 'hypertrophic' repair tissues were characterized by T2 global values higher than controls. Zonal variation between superficial and deep T2 values observed in controls was not depicted in repair tissue before D60. CONCLUSION: T2 map is able to characterize quantitatively and qualitatively rat patellar cartilage repair, and thus can be promoted, as a non invasive technique, in clinical longitudinal studies of articular cartilage repair.

T2 mapping of rat patellar cartilage.

PURPOSE: To investigate the usefulness of magnetic resonance (MR) T2 mapping in characterizing the evolution of cartilage matrix content and thickness during the maturation and aging process. MATERIALS AND METHODS: Patellae from four groups of rats aged 4 weeks, 8 weeks, 4 months, and more than 6 months ("old rats") were studied ex vivo with an 8.5-T microimager. T2 values were calculated on transverse rat patellar sections and displayed with a color scale (the T2 map) on a pixel-by-pixel basis. Biochemical and histologic studies were performed to evaluate the influence of proteoglycans and collagen contents on T2 values of the patellar cartilage. RESULTS: On the T2 map, the maturation process until 10 weeks was characterized by a decrease in T2 values and in cartilage thickness. The biochemical data revealed a global decrease in proteoglycans and a progressive global increase in collagen content, whereas the histologic study revealed subtle zonal variation in matrix constituents with depth. As aging progressed, the T2 values were low, without important variations, whereas the global cartilage thickness decreased. The cartilage matrix became globally more fibrotic, especially in the deepest zone. Biochemical analysis revealed that collagen content was more determinant of MR signal intensity than was proteoglycans content during maturation and aging. CONCLUSION: T2 mapping allows characterization of variations in cartilage matrix constituents and thickness.