Technical considerations on the validity of blood oxygenation level‐dependent‐based MR assessment of vascular deoxygenation

A blood oxygenation level‐dependent (BOLD)‐based apparent relative oxygen extraction fraction (rOEF) as a semi‐quantitative marker of vascular deoxygenation has recently been introduced in clinical studies of patients with glioma and stroke, yielding promising results. These rOEF measurements are based on independent quantification of the transverse relaxation times T₂ and T₂* and relative cerebral blood volume (rCBV). Simulations demonstrate that small errors in any of the underlying measures may result in a large deviation of the calculated rOEF. Therefore, we investigated the validity of such measurements. For this, we evaluated the quantitative measurements of T₂ and T₂* at 3 T in a gel phantom, in healthy subjects and in healthy tissue of patients with brain tumors. We calculated rOEF maps covering large portions of the brain from T₂, T₂* and rCBV [routinely measured in patients using dynamic susceptibility contrast (DSC)], and obtained rOEF values of 0.63 ± 0.16 and 0.90 ± 0.21 in healthy‐appearing gray matter (GM) and white matter (WM), respectively; values of about 0.4 are usually reported. Quantitative T₂ mapping using the fast, clinically feasible, multi‐echo gradient spin echo (GRASE) approach yields significantly higher values than much slower multiple single spin echo (SE) experiments. Although T₂* mapping is reliable in magnetically homogeneous tissues, uncorrectable macroscopic background gradients and other effects (e.g. iron deposition) shorten T₂*. Cerebral blood volume (CBV) measurement using DSC and normalization to WM yields robust estimates of rCBV in healthy‐appearing brain tissue; absolute quantification of the venous fraction of CBV, however, is difficult to achieve. Our study demonstrates that quantitative measurements of rOEF are currently biased by inherent difficulties in T₂ and CBV quantification, but also by inadequacies of the underlying model. We argue, however, that standardized, reproducible measurements of apparent T₂, T₂* and rCBV may still allow the estimation of a meaningful apparent rOEF, which requires further validation in clinical studies. Copyright © 2014 John Wiley & Sons, Ltd.     

MRI Brain T1 Relaxation Time Changes in MS Patients Increase Over Time in Both the White Matter and the Cortex

OBJECTIVE: To test the sensitivity of whole-brain T1 relaxometry to the evolution of pathological changes in multiple sclerosis (MS). BACKGROUND: T1-weighted hypointense lesion load in the brains of patients with MS is associated with axonal loss. Other work has shown that T1 measurements may provide information complementary to existing imaging techniques, such as magnetization transfer imaging. METHODS: The authors studied 14 MS patients twice over a median time interval of 19.5 months (range, 14-22 months). Structural images and whole-brain T1 maps using a novel rapid-scanning technique (3 min/study) were performed at 3 T. Analysis focused on defining changes separately in the lesional and normal-appearing white matter (NAWM) and in the cortical gray matter. RESULTS: At baseline, there was an inverse relationship between disease duration and the NAWM T1 histogram peak height (r = -0.75, P = .03). The total white matter T1 histogram peak height decreased over time (P < .001). This could be accounted for by changes in the NAWM (P < .03). There also was a decrease (6%) in the mean (11 of 14 patients, P = .004) and in the median (7%) (13 of 14 patients, P < .001) neocortical gray matter T1 over the follow-up period. CONCLUSIONS: Brain T1 maps can be generated quickly and are sensitive to pathological changes over time. T1 values in both the gray and the white matter at the baseline visit were related to disease duration, suggesting that the T1 changes are clinically relevant. Although the absolute values will be different, it is likely that similar changes will be able to be detected at 1.5 T. The role of T1 measurement as a magnetic resonance imaging outcome measure in clinical trials now should be explored.     

Bayesian algorithm using spatial priors for multiexponential T(2) relaxometry from multiecho spin echo MRI

Multiexponential T₂ relaxometry is a powerful research tool for detecting brain structural changes due to demyelinating diseases such as multiple sclerosis. However, because of unusually high signal‐to‐noise ratio requirement compared with other MR modalities and ill‐posedness of the underlying inverse problem, the T₂ distributions obtained with conventional approaches are frequently prone to noise effects. In this article, a novel multivoxel Bayesian algorithm using spatial prior information is proposed. This prior takes into account the expectation that volume fractions and T₂ relaxation times of tissue compartments change smoothly within coherent brain regions. Three‐dimensional multiecho spin echo MRI data were collected from five healthy volunteers at 1.5 T and myelin water fraction maps were obtained using the conventional and proposed algorithms. Compared with the conventional method, the proposed method provides myelin water fraction maps with improved depiction of brain structures and significantly lower coefficients of variance in white matter. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Carbonaceous Materials Porosity Investigation in a Wet State by Low-Field NMR Relaxometry

The porosity of differently wetted carbonaceous material with disordered mesoporosity was investigated using low-field ¹H NMR relaxometry. Spin–spin relaxation (relaxation time T₂) was measured using the CPMG pulse sequence. We present a non-linear optimization method for the conversion of relaxation curves to the distribution of relaxation times by using non-specialized software. Our procedure consists of searching for the number of components, relaxation times, and their amplitudes, related to different types of hydrogen nuclei in the sample wetted with different amounts of water (different water-to-carbon ratio). We found that a maximum of five components with different relaxation times was sufficient to describe the observed relaxation. The individual components were attributed to a tightly bounded surface water layer (T₂ up to 2 ms), water in small pores especially supermicropores (2 < T₂ < 7 ms), mesopores (7 < T₂ < 20 ms), water in large cavities between particles (20–1500 ms), and bulk water surrounding the materials (T₂ > 1500 ms). To recalculate the distribution of relaxation times to the pore size distribution, we calculated the surface relaxivity based on the results provided by additional characterization techniques, such as thermoporometry (TPM) and N₂/−196 °C physisorption.     

Selective excitation of myelin water using inversion-recovery-based preparations.

T1 and T2 relaxation of excised frog sciatic nerve water was characterized at 7 T. Based on these findings, optimal timings for multiple inversion-recovery magnetization preparations were determined to selectively excite the so-called myelin-water T2 component. Subsequent double inversion-recovery and triple inversion-recovery preparations were used in combination with CPMG acquisitions to experimentally determine optimal timings and effect of the preparation. Using double inversion-recovery, optimal timings were found to excite magnetization that is predominantly (approximately 93%) derived from the myelin-water component. Greater selectivity (approximately 96%) was found by extending the preparation to triple inversion-recovery, at the price of decreasing SNR by a factor of approximately 2.     

Measurement of short and ultrashort T2 components using progressive binomial RF saturation.

A new magnetic resonance (MR) method for measuring T2 relaxation times in tissues is proposed. The method is based on a T2 selective saturation period followed by sampling of the remaining longitudinal magnetization. Saturation of the longitudinal magnetization is accomplished by a single binomial RF pulse of zeroth order with a constant flip angle. The T2 selectivity is controlled by the RF pulse duration. A full T2 spectrum can be obtained by performing a series of measurements with varying RF pulse duration. On a conventional 1.5 T system this approach allows detection of T2 components as short as several hundred microseconds. A major limitation is the method's susceptibility to resonance offsets. At typical offsets of 0.1-0.2 ppm the sensitivity of the method is limited to a T2 range below 20 ms, which corresponds to an RF pulse duration shorter than 50 ms. The new method was used to acquire T2 spectra from the liver of pigs in vitro on a conventional 1.5 T system. We observed a short T2 component around 17 ms and an ultrashort T2 component in the range of 0.9-1.1 ms. Numerical simulations and in vitro measurements suggest that resonance offsets have effects that require further investigation.