MR techniques are sensitive to the early stages of osteoarthritis, characterized by disruption of collagen and loss of proteoglycan (PG), but are of limited specificity. Here, water compartments in normal and trypsin-degraded bovine nasal cartilage were identified using a nonnegative least squares multiexponential analysis of T 2 relaxation. Three components were detected: T 2,1 ؍ 2.3 ms, T 2,2 ؍ 25.2 ms, and T 2,3 ؍ 96.3 ms, with fractions w 1 ؍ 6.2%, w 2 ؍ 14.5%, and w 3 ؍ 79.3%, respectively. Trypsinization resulted in increased (P < 0.01) values of T 2,2 ؍ 64.2 ms and T 2,3 ؍ 149.4 ms, supporting their assignment to water compartments that are bound and loosely associated with PG, respectively. The T 2 of the rapidly relaxing component was not altered by digestion, supporting assignment to relatively immobile collagen-bound water. Relaxation data were simulated for a range of TE, number of echoes, and SNR to guide selection of acquisition parameters and assess the accuracy and precision of experimental results. Based on this, the expected experimental accuracy of measured T 2 s and associated weights was within 2% and 4% respectively, with precision within 1% and 3%. These results demonstrate the potential of multiexponential T 2 analysis to increase the specificity of MR characterization of cartilage. Magn Reson Med 61:803-809, 2009.
The balance between excitatory glutamatergic projection neurons and inhibitory GABAergic interneurons determines the function of cortical microcircuits. How these neurotransmitters relate to the functional status of an entire macro-scale network remains unknown. The posteromedial cortex (PMC) is the default mode network (DMN) node with the greatest functional connectivity; therefore, we hypothesized that PMC glutamate and GABA predict intrinsic functional connectivity (iFC) across the DMN. In 20 healthy men, we combined J-resolved magnetic resonance spectroscopy to measure glutamate and GABA in the PMC and resting fMRI followed by group Independent Components Analysis to extract the DMN. We showed that, controlling for age and GM volume in the MRS voxel, PMC glutamate and GABA explained about half of the variance in DMN iFC (represented by the network’s beta coefficient for rest). Glutamate correlated positively and GABA correlated negatively with DMN iFC; in an alternative statistical model which included the glutamate/GABA ratio, the ratio correlated positively with DMN iFC. Age had no independent effect on DMN iFC. No other network was associated with PMC glutamate or GABA. We conclude that regional neurotransmitter concentrations in a network node strongly predict network but not global brain iFC.
Muscle bioenergetics assessed using P-MRS is highly correlated with walking speed and partially explains age-related poorer performance in fast and long walking tasks.
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