NMR in biomedicine
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In vivo MRS of the human brain at 7 tesla allows identification of a large number of metabolites at higher spatial resolutions than currently possible at lower field strengths. However, several challenges complicate in vivo localization and artifact suppression in MRS at high spatial resolution within a clinically feasible scan time at 7 tesla. Published MRS sequences at 7 tesla suffer from long echo times, inherent signal-to-noise ratio (SNR) loss, large chemical shift displacement artifacts or long repetition times because of excessive radiofrequency (RF) power deposition. ⋯ The pulse-acquire approach resulted in a very short echo time of 1.4 ms. A cost function guided shimming algorithm was developed to constrain frequency offsets in the excited slice, therefore adiabatic frequency selective suppression could be employed to minimize artifacts from high intensity lipids and water signals in the excited slice. The high sensitivity at a TR of 1 s was demonstrated both on a supraventricular slice as well as in an area very close to the skull in the frontal cortex at a nominal spatial resolution of 0.25 cc within a feasible scan time.
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We treated traumatic brain injury (TBI) with human bone marrow stromal cells (hMSCs) and evaluated the effect of treatment on white matter reorganization using MRI. We subjected male Wistar rats (n = 17) to controlled cortical impact and either withheld treatment (controls; n = 9) or inserted collagen scaffolds containing hMSCs (n = 8). Six weeks later, the rats were sacrificed and MRI revealed selective migration of grafted neural progenitor cells towards the white matter reorganized boundary of the TBI-induced lesion. ⋯ Fiber tracking (FT) maps derived from diffusion tensor imaging confirmed the immunohistological data and provided information on axonal rewiring. The apparent kurtosis coefficient (AKC) detected additional axonal remodeling regions with crossing axons, confirmed by immunohistological staining, compared with FA. Our data demonstrate that AKC, FA, FT and T(2) can be used to evaluate treatment-induced white matter recovery, which may facilitate restorative therapy in patients with TBI.
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Brain temperature is important in stroke and trauma. In birth asphyxia, hypothermia improves outcome, but local brain temperature information is needed to optimise therapy. The proton MRS water chemical shift (δ(water) ) is temperature dependent, and the in vivo brain temperature has often been estimated by measuring δ(water) relative to the N-acetylaspartate (NAA) singlet methyl resonance. ⋯ AWC with T(Cho) , T(Cr) and T(NAA) weighted by the squared Cho, Cr and NAA peak amplitudes provided the smallest intersubject standard deviations: Thal, 0.45°C; OP, 0.33°C (for T(NAA) values of 0.65°C and 1.12°C, respectively). AWC provided resilience against simulated pathological alterations in Cho, Cr and NAA peak amplitudes, with Thal and OP T(AWC) changing by less than 0.04°C. AWC improves both intersubject reproducibility of MRS temperature estimation and resilience against pathological, anatomical and developmental variation of Cho, Cr and NAA peak amplitudes.
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MR thermometry offers the possibility to precisely guide high-intensity focused ultrasound (HIFU) for the noninvasive treatment of kidney and liver tumours. The objectives of this study were to demonstrate therapy guidance by motion-compensated, rapid and volumetric MR temperature monitoring and to evaluate the feasibility of MR-guided HIFU ablation in these organs. Fourteen HIFU sonications were performed in the kidney and liver of five pigs under general anaesthesia using an MR-compatible Philips HIFU platform prototype. ⋯ These findings were in agreement with histological analysis. Volumetric thermometry allows real-time monitoring of the temperature at the target location in liver and kidney, as well as in surrounding tissues. Thermal ablation was more difficult to achieve in renal than in hepatic tissue even using higher acoustic energy, probably because of a more efficient heat evacuation in the kidney by perfusion.
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The aim of this study was to explain the temporal variations between subjects in the blood oxygenation level-dependent (BOLD) response. Somatosensory responses were elicited with the electrical forepaw stimulus at a frequency of 10 Hz in urethane-anesthetized rats, and functional magnetic resonance imaging (fMRI) with BOLD contrast and local field potential (LFP) measurements were performed simultaneously. BOLD fMRI activation was evaluated by two different models, one based on the stimulus paradigm (the block model) and the other on the simultaneously measured evoked LFP responses. ⋯ A statistical comparison of the two models revealed that the LFP-derived model was able to explain additional BOLD variation over the block model in the somatosensory cortex in nine of 12 rats. These results suggest that there is more information regarding neuronal activity in the BOLD signal than can be exploited using the block model alone. Furthermore, the hemodynamic coupling remains unchanged in the case of temporally variable BOLD signals.