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Methods: We exploit the low rank property of the concatenated temporal imaging contrasts, on top of the fact that the MRF signal is sparsely represented in the generated dictionary domain. We present an iterative scheme that consists of a gradient step followed by a low rank projection using the singular value decomposition.Results: Experimental results consist of retrospective sampling, that allows comparison to a well defined reference, and prospective sampling that shows the performance of FLOR for a real-data sampling scenario. Both experiments demonstrate improved parameter accuracy compared to other compressed-sensing and low-rank based methods for MRF at 5% and 9% sampling ratios, for the retrospective and prospective experiments, respectively. Conclusions:We have shown through retrospective and prospective experiments that by exploiting the low rank nature of the MRF signal, FLOR recovers the MRF temporal undersampled images and provides more accurate parameter maps compared to previous iterative methods.

Diffuse WM glial abnormalities were larger in magnitude than the axonal abnormalities and increased over time independently of conventional clinical or imaging metrics and despite immunomodulatory treatment. In contrast, the axonal abnormalities showed partial recovery, suggesting that patients' lower WM NAA levels represented a dysfunction, which may abate with treatment. Absence of detectable diffuse changes in GM suggests that injury there is minimal, focal, or heterogeneous between cortex and deep GM nuclei.

Since the brain's gray matter (GM) and white matter (WM) metabolite concentrations differ, their partial volumes can vary the voxel's 1H MR spectroscopy (1H-MRS) signal, reducing the sensitivity to changes. While single-voxel 1H-MRS cannot differentiate the WM from GM signals, partial volume correction is feasible in MR spectroscopic imaging (MRSI), using segmentation of the MRI that is always acquired for VOI placement. To determine the magnitude of this effect on metabolic quantification, we segmented the 1 mm3 resolution MRI into GM, WM and CSF masks that were co-registered with the MRSI grid to yield their partial volumes in every ~1 cm3 spectroscopic voxel. Each voxel then provided one equation with two unknowns – its i- metabolite's GM and WM concentrations: CiGM, CiWM. With the voxels' GM and WM volumes as independent coefficients, that over-determined system of equations can be solved for the global, averaged CiGM and CiWM. Trading off local concentrations differences offers three advantages: (i) higher sensitivity due to combined data from many voxels; (ii) improved specificity to WM versus GM changes; (iii) reduced susceptibility to partial volume effects. These improvements make no additional demands of the protocol, measurement time or hardware. Applying the approach to 18 volunteers' 3D MRSI sets, 480 voxels each, yielded N-acetylaspartate, creatine, choline and myo-inositol CiGM of 8.5±0.7, 6.9±0.6, 1.2±0.2, 5.3±0.6 mM; and CiWM of 7.7±0.6, 4.9±0.5, 1.4±0.1 and 4.4±0.6 mM. We show that unaccounted voxel WM or GM partial volume can vary absolute quantification by 5–10% (more for ratios) that can often as much as double the sample size required to establish statistical significance.

Since mild traumatic brain injury (mTBI) often leads to neurological symptoms even without clinical MRI findings, our goal was to test whether diffuse axonal injury is quantifiable with multivoxel proton MR spectroscopic imaging (1H-MRSI). T1- and T2-weighted MRI and three dimensional 1H-MRSI (480 voxels over 360 cm3, ∼30% of the brain) were acquired at 3 Tesla from 26 mTBI patients (mean Glasgow Coma Scale score 14.7), 18–56 years old, 3–55 days post injury and 13 healthy matched contemporaries. The N-acetylaspartate (NAA), choline (Cho), creatine (Cr) and myo-inositol (mI) concentrations and gray-, white-matter (GM/WM) and cerebrospinal fluid fractions were obtained in each voxel. Global GM and WM absolute metabolic concentrations were estimated using linear regression, and patients were compared with controls using two-way analysis of variance. Patients' mean NAA, Cr, Cho and mI concentrations in GM (8.4±0.7, 6.9±0.6, 1.3±0.2, 5.5±0.6 mM) and Cr, Cho and mI in WM (4.8±0.5, 1.4±0.2, 4.6±0.7 mM) were not different from controls'. NAA, however, was significantly lower in patients' than controls' WM (7.2±0.8 versus 7.7±0.6 mM, p=0.0125). The Cho and Cr levels in patients' WM positively correlated with time from mTBI. This 1H-MRSI approach allowed us to ascertain that early mTBI sequelae are (i) diffuse (not merely local), (ii) neuronal (not glial) and (iii) in the global white (not gray) matter. These findings support the hypothesis that, similarly to more severe head trauma, mTBI also results in diffuse axonal injury, but that dysfunction, rather than cell death, dominates shortly after injury.

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Single-scan 2D NMR relies on a spatial axis for encoding the indirect-domain internal spin interactions. Various strategies have been demonstrated for fulfilling the needs underlying this procedure. All such schemes use gradient-echoed sequences that leave at their conclusion solely the effects of the internal interactions along the indirect domain; they also include a real-time scheme that though simple, yields in general mixed-phase line shapes. The present paper introduces two new proposals geared up for easing the spatial encoding underlying single-scan 2D NMR methodologies. One of these is capable of delivering dispersive-free peaks along the indirect domain, and thereby purely-absorptive 2D line shapes, in amplitude-encoded experiments. The other demonstrates for the first time, the possibility to obtain single-scan 2D spectra without echoing the effects of the encoding gradient-simply by applying a single-pulse frequency sweep to encode the interactions. Both of these modes are compatible with homo- and heteronuclear correlations, and exhibit a number of complementary features vis-à-vis encoding alternatives that have so far been presented. The overall principles underlying these new spatially encoding protocols are derived, and their performance demonstrated with single-scan 2D NMR TOCSY and HSQC experiments on model compounds.