Quantitative T2 in the occipital lobe: the role of the CPMG refocusing rate

Characterizing white matter fiber orientation effects on multi-parametric quantitative BOLD assessment of oxygen extraction fraction

Relative oxygen extraction fraction (rOEF) is a fundamental indicator of cerebral metabolic function. An easily applicable method for magnetic resonance imaging (MRI) based rOEF mapping is the multi-parametric quantitative blood oxygenation level dependent (mq-BOLD) approach with separate acquisitions of transverse relaxation times   T 2 * and T2 and dynamic susceptibility contrast (DSC) based relative cerebral blood volume (rCBV). Given that transverse relaxation and rCBV in white matter (WM) strongly depend on nerve fiber orientation, mq-BOLD derived rOEF is expected to be affected as well. To investigate fiber orientation related rOEF artefacts, we present a methodological study characterizing anisotropy effects of WM as measured by diffusion tensor imaging (DTI) on mq-BOLD in 30 healthy volunteers. Using a 3T clinical MRI-scanner, we performed a comprehensive correlation of all parameters (  T 2 * , T2, R 2 ' , rCBV, rOEF, where R 2 ' =1/  T 2 * -1/T2) with DTI-derived fiber orientation towards the main magnetic field (B0). Our results confirm strong dependencies of transverse relaxation and rCBV on the nerve fiber orientation towards B0, with anisotropy-driven variations up to 37%. Comparably weak orientation-dependent variations of mq-BOLD derived rOEF (3.8%) demonstrate partially counteracting influences of R 2 ' and rCBV effects, possibly suggesting applicability of rOEF as an oxygenation sensitive biomarker. However, unresolved issues warrant caution when applying mq-BOLD to WM.

Measuring intra-axonal T2 in white matter with direction-averaged diffusion MRI

PURPOSE

To demonstrate how the T₂ relaxation time of intra-axonal water (T_2a ) in white matter can be measured with direction-averaged diffusion MRI.

METHODS

For b-values larger than about 4000 s/mm² , the direction-averaged diffusion MRI signal from white matter is dominated by the contribution from water within axons, which enables T_2a to be estimated by acquiring data for multiple TE values and fitting a mono-exponential decay curve. If given a value of the intra-axonal diffusivity, an extension of the method allows the extra-axonal relaxation time (T_2e ) to be calculated also. This approach was applied to estimate T_2a in white matter for 3 healthy subjects at 3 T, as well as T_2e for a selected set of assumed intra-axonal diffusivities.

RESULTS

The estimated T_2a values ranged from about 50 ms to 110 ms, with considerable variation among white matter regions. For white matter tracts with primarily collinear fibers, T_2a was found to depend on the angle of the tract relative to the main magnetic field, which is consistent with T_2a being affected by magnetic field inhomogeneities arising from spatial differences in magnetic susceptibility. The T_2e values were significantly smaller than the T_2a values across white matter regions for several plausible choices of the intra-axonal diffusivity.

CONCLUSION

The relaxation time for intra-axonal water in white matter can be determined in a straightforward manner by measuring the direction-averaged diffusion MRI signal with a large b-value for multiple TEs. In healthy brain, T_2a is greater than T_2e and varies considerably with anatomical region.

Quantitative in vivo T2 mapping using fast spin echo techniques - A linear correction procedure

A method is presented for correcting the effects of stimulated and indirect echoes on quantitative T2 mapping data acquired with multiple spin echo techniques, such as turbo spin echo. In contrast to similar correction techniques proposed in the literature, the method does not require a priori knowledge of the radio frequency (RF) pulse profiles. In a first step, for the T2 mapping protocol under investigation, signal decay curves S(TE) are simulated for a range of different RF pulse profiles. The actual signal decay S(TE) is then measured on a phantom with known T2, so the approximate RF pulse profiles can be derived via comparison with the simulated decay curves. In a second step, with the RF pulses obtained from step one, signal decay curves S(TE) are simulated for different T2 values and fitted mono-exponentially, thus allowing to deduce the relationship between true T2 and the apparent T2 (T2app) values. Results show that this relationship is approximately linear, allowing for a direct correction of T2app maps. If the amplitude of the transmitted RF field (B1) does not exceed the nominal value by more than 10%, it is shown that a B1-independent correction of T2app maps yields sufficiently accurate results for T2. A B1-dependent version is also presented. The method is tested in vitro on a phantom with different T2 values and in vivo on healthy subjects.

A noninvasive tumor oxygenation imaging strategy using magnetic resonance imaging of endogenous blood and tissue water

PURPOSE

To present a novel imaging strategy for noninvasive measurement of tumor oxygenation using MR imaging of endogenous blood and tissue water.

THEORY AND METHODS

The proposed approach for oxygen partial pressure (pO2) estimation is based on intravoxel incoherent motion diffusion MRI and the dependence of the blood R2 relaxation rate on the inter-echo spacing measured using a multiple spin-echo Carr-Purcell-Meiboom-Gill sequence and weak-field diffusion model. The accuracy of the approach was validated by comparison with (19)F MRI oximetry.

RESULTS

The results in eight rats at 4.7 T showed that tumors have longer T1 (1980 ± 186 ms) and T2 (59 ± 9 ms) relaxation times, heterogeneous blood volume fraction (0.23 ± 0.1), oxygen saturation level (Y) (0.53 ± 0.12), and pO2 (36 ± 15 mmHg) distributions compared with normal muscle (T1 1480 ± 86 ms, T2 29 ± 2 ms, blood volume fraction 0.22 ± 0.03, Y 0.49 ± 0.06, and pO2 39 ± 5 mmHg). pO2 estimates based on the novel (1)H approach were essentially identical with (19)F observations.

CONCLUSION

The study indicates that noninvasive measurement of tumor pO2 using (1)H MRI derived multiparametric maps is feasible and could become a valuable tool to evaluate tumor hypoxia.

Investigating the Relationship between Transverse Relaxation Rate (R2) and Interecho Time in MagA-Expressing, Iron-Labeled Cells

Reporter gene-based labeling of cells with iron is an emerging method of providing magnetic resonance imaging contrast for long-term cell tracking and monitoring cellular activities. This report investigates 9.4 T nuclear magnetic resonance properties of mammalian cells overexpressing MagA, a putative iron transport protein from magnetotactic bacteria. MagA-expressing MDA-MB-435 cells were cultured in the presence and absence of iron supplementation and compared to the untransfected control. The relationship between the transverse relaxation rate (R2) and interecho time was investigated using the Carr-Purcell-Meiboom-Gill sequence. This relationship was analyzed using a model based on water diffusion in weak magnetic field inhomogeneities (Jensen-Chandra model) as well as a fast-exchange model (Luz-Meiboom model). Increases in R2 with increasing interecho time were larger in the iron-supplemented, MagA-expressing cells compared to other cells. The dependence of R2 on interecho time in these iron-supplemented, MagA-expressing cells was better represented by the Jensen-Chandra model compared to the Luz-Meiboom model, whereas the Luz-Meiboom model performed better for the remaining cell types. Our findings provide an estimate of the distance scale of microscopic magnetic field variations in MagA-expressing cells, which is thought to be related to the size of iron-containing vesicles.

In vivo measurement of transverse relaxation time in the mouse brain at 17.6 T

PURPOSE

To establish regional T1 and T2 values of the healthy mouse brain at ultra-high magnetic field strength of 17.6 T and to follow regional brain T1 and T2 changes with age.

METHODS

In vivo T1 and T2 values in the C57BL/6J mouse brain were followed with age using multislice-multiecho sequence and multiple spin echo saturation recovery with variable repetition time sequence, respectively, at 9.4 and 17.6 T. Gadolinium-tetra-azacyclo-dodecane-tetra-acetic acid phantoms were used to validate in vivo T2 measurements. Student's t-test was used to compare mean relaxation values.

RESULTS

A field-dependent decrease in T2 is shown and validated with phantom measurements. T2 values at 17.6 T typically increased with age in multiple brain regions except in the hypothalamus and the caudate-putamen, where a slight decrease was observed. Furthermore, T1 values in various brain regions of young and old mice are presented at 17.6 T. A large gain in signal-to-noise ratio was observed at 17.6 T.

CONCLUSIONS

This study establishes for the first time the normative T1 and T2 values at 17.6 T over different mouse brain regions with age. The estimates of in vivo T1 and T2 will be useful to optimize pulse sequences for optimal image contrast at 17.6 T and will serve as baseline values against which disease-related relaxation changes can be assessed in mice.

Simultaneous estimation of T(2) and apparent diffusion coefficient in human articular cartilage in vivo with a modified three-dimensional double echo steady state (DESS) sequence at 3 T

T(2) mapping and diffusion-weighted imaging complement morphological imaging for assessing cartilage disease and injury. The double echo steady state sequence has been used for morphological imaging and generates two echoes with markedly different T(2) and diffusion weighting. Modifying the spoiler gradient area and flip angle of the double echo steady state sequence allows greater control of the diffusion weighting of both echoes. Data from two acquisitions with different spoiler gradient areas and flip angles are used to simultaneously estimate the T(2) and apparent diffusion coefficient of each voxel. This method is verified in phantoms and validated in vivo in the knee; estimates from different regions of interest in the phantoms and cartilage are compared to those obtained using standard spin-echo methods. The Pearson correlations were 0.984 for T(2) (∼2% relative difference between spin-echo and double echo steady state estimates) and 0.997 for apparent diffusion coefficient (˜1% relative difference between spin-echo and double echo steady state estimates) for the phantom study and 0.989 for T(2) and 0.987 for apparent diffusion coefficient in regions of interest in the human knee in vivo. High accuracy for simultaneous three-dimensional T(2) and apparent diffusion coefficient measurements are demonstrated, while also providing morphologic three-dimensional images without blurring or distortion in reasonable scan times.

In vivo quantification of T₂ anisotropy in white matter fibers in marmoset monkeys

T₂-weighted MRI at high field is a promising approach for studying noninvasively the tissue structure and composition of the brain. However, the biophysical origin of T₂ contrast, especially in white matter, remains poorly understood. Recent work has shown that R₂ (=1/T₂) may depend on the tissue's orientation relative to the static magnetic field (B(0)) and suggested that this dependence could be attributed to local anisotropy in the magnetic properties of brain tissue. In the present work, we analyzed high-resolution, multi-gradient-echo images of in vivo marmoset brains at 7T, and compared them with ex vivo diffusion tensor images, to show that R₂ relaxation in white matter is highly sensitive to the fiber orientation relative to the main field. We directly demonstrate this orientation dependence by performing in vivo multi-gradient-echo experiments in two orthogonal brain positions, uncovering a nearly 50% change in the R₂ relaxation rate constant of the optic radiations. We attribute this substantial R₂ anisotropy to local subvoxel susceptibility effects arising from the highly ordered and anisotropic structure of the myelin sheath.

Algebraic T2 estimation improves detection of right temporal lobe epilepsy by MR T2 relaxometry

Seizure related abnormalities may be detected with T2 relaxometry, which involves quantitative estimation of T2 values. Accounting for the partial-volume effect of cerebrospinal fluid (CSF) is important, especially for voxel-based relaxometry, VBR. With a mono-exponential decay model, this can be accomplished by including a baseline constant. An algebraic calculation, which accommodates this constant, offers improved T2 estimation speed over the commonly used non-linear fitting approach. Our objective was to compare the algebraic approach against three fitting approaches for the detection of seizure related abnormalities. We tested the performance of the four methods in the presence of noise using simulated data as well as real data acquired at 3 T with a Carr-Purcell-Meiboom-Gill sequence from 45 healthy subjects and 24 patients with confirmed right temporal lobe epilepsy. A quantitative analysis was performed on spatially normalized data by measuring T2 in various regions and with a whole brain tissue segmentation analysis. The detection rate of hippocampal T2 changes in patients was assessed by comparing the regional T2 measurements from each patient against the control data with a z-score threshold of 2.33. The algebraic method yielded high sensitivity for detection of hippocampal abnormalities in the epileptic patients in regional assessment and in follow-up single-subject VBR. This can be attributed to the relatively small variance across healthy subjects and improved precision in the presence of CSF and noise in simulation. In conclusion, the algebraic method is better than fitting based on faster calculation speed and better sensitivity for detecting seizure-related T2 changes.

Fast spin echo sequences for BOLD functional MRI

At higher field strengths, spin echo (SE) functional MRI (fMRI) is an attractive alternative to gradient echo (GE) as the increased weighting towards the microvasculature results in intrinsically better localization of the BOLD signal. Images are free of signal voids but the commonly used echo planar imaging (EPI) sampling scheme causes geometric distortions, and T₂* effects often contribute considerably to the signal changes measured upon brain activation. Multiply refocused SE sequences such as fast spin echo (FSE) are essentially artifact free but their application to fast fMRI is usually hindered due to high energy deposition, and long sampling times. In the work presented here, a combination of parallel imaging and partial Fourier acquisition is used to shorten FSE acquisition times to near those of conventional SE-EPI, permitting sampling of eight slices (matrix 64  ×  64) per second. Signal acquisition is preceded by a preparation experiment that aims at increasing the relative contribution of extravascular dynamic averaging to the BOLD signal. Comparisons are made with conventional SE-EPI using a visual stimulation paradigm. While the observed signal changes are approximately 30% lower, most likely due to the absence of T₂* contamination, activation size and t-scores are comparable for both methods, suggesting that HASTE fMRI is a viable alternative, particularly if distortion free images are required. Our data also indicate that the BOLD post-stimulus undershoot is most probably attributable to persistent elevated oxygen metabolism rather than to delayed vascular compliance.

Functional contrast based on intermolecular double-quantum coherences: Influence of the correlation distance

A functional MRI (fMRI) study with visual stimulation of healthy subjects was performed at 3T exploiting intermolecular double-quantum coherences. The correlation distance, d(c), was varied between 60 and 300 microm for different evolution times, tau. Robust activation was obtained in all experiments with average signal changes (DeltaS(2)) = 8.4 +/- 0.7% and 9.4 +/- 0.8% for tau = 15 and 20 ms, respectively) exceeding those normally associated with conventional blood oxygen level-dependent (BOLD) fMRI. Relaxation-rate changes (DeltaR(2) = 0.33 +/- 0.36 s(-1) and DeltaR(2)* = 0.77 +/- 0.54 s(-1)) were similar to those commonly obtained for the extravascular BOLD effect. The number of activated voxels increased with increasing d(c) until a plateau was reached at approximately equal 120 microm. Similar trends were observed for the activation-induced percent signal change and for the maximal Z-scores. These effects were quantitatively explained by a reduced sensitivity at short d(c) due to increasing signal attenuation related to diffusion and an increasing amount of signal fluctuations in the fMRI time series due to imperfect suppression of unwanted coherence pathways. Consistent indications of a preferential selection of susceptibility changes in blood vessels of a particular size were not obtained.

The effect of global cerebral vasodilation on focal activation hemodynamics

In view of the potential of global resting blood flow level to confound the interpretation of blood oxygenation level-dependent (BOLD) fMRI studies, we investigated the effect of pronounced elevation in baseline cerebral blood flow (CBF) on BOLD and CBF responses to functional activation. Twelve healthy volunteers performed bilateral finger apposition while attending to a radial yellow/blue checkerboard. Three levels of global CBF increase were achieved by inhaling 5, 7.5 or 10% CO2. CBF and BOLD signals were simultaneously quantified using interleaved multi-slice pulsed arterial spin labeling (PASL) and T2*-weighted gradient echo sequences. Increasing basal CBF produced a significant decrease in the activation-induced BOLD response, with the slope of the optimal linear fit of activation versus basal BOLD signal changes of -0.32 +/- 0.01%/% for motor and visual cortex regions of interest (ROIs). While the modulation in basal flow level also produced a statistically significant effect on the activation-induced CBF change, the degree of relative attenuation of the flow response was slight, with a slope of -0.18 +/- 0.02%/% in the motor and -0.13 +/- 0.01%/% in the visual cortex ROI. The current findings describe a strong attenuation of the BOLD response at significantly elevated basal flow levels and call for independent quantification of resting CBF in BOLD fMRI studies that involve subjects and/or conditions with markedly elevated global perfusion.

Venous refocusing for volume estimation: VERVE functional magnetic resonance imaging

A novel, noninvasive magnetic resonance imaging-based method for measuring changes in venous cerebral blood volume (CBV(v)) is presented. Venous refocusing for volume estimation (VERVE) exploits the dependency of the spin-spin relaxation rate of deoxygenated blood on the refocusing interval. Interleaved CPMG EPI acquisitions following a train of either tightly or sparsely spaced hard refocusing pulses (every 3.7 or 30 msec, respectively) at matched echo time were used to isolate the blood signal while minimizing the intravascular blood oxygenation level dependent (BOLD) signal contribution. The technique was employed to determine the steady-state increase in the CBV(v) in the visual cortex (VC) in seven healthy adult volunteers during flickering checkerboard photic stimulation. A functional activation model and a set of previously collected in vitro human whole blood relaxometry data were used to evaluate the intravascular BOLD effect on the VERVE signal. The average VC venous blood volume change was estimated to be 16 +/- 2%. This method has the potential to provide efficient and continuous monitoring of venous cerebral blood volume, thereby enabling further exploration of the mechanism underlying BOLD signal changes upon physiologic, pathophysiologic, and pharmacologic perturbations.

Quantitative analysis of temporal lobe white matter T2 relaxation time in temporal lobe epilepsy

The objective of this study was to assess temporal lobe white matter (WM) quantitatively using T2 relaxometry in patients with pharmacologically intractable temporal lobe epilepsy (TLE). T2 relaxometry was performed using a dual-echo sequence with 23 contiguous oblique coronal slices in 56 consecutive TLE patients and in 30 healthy subjects. Averages of six slices were chosen to calculate T2 relaxation time in the temporal lobe WM (WM-T2) and the hippocampus (Hippo-T2). Twenty-seven patients had unilateral hippocampal atrophy (HA), and twenty-nine patients had normal hippocampal volumes (NV) on volumetric MRI. Mean WM-T2 was increased ipsilateral to the seizure focus in TLE patients with HA and those with NV (P < 0.001). Contralateral mean WM-T2 was increased in left and right TLE with HA (P < 0.001) and in right TLE with NV (P = 0.001). There was a positive correlation between WM-T2 and Hippo-T2. Individual analysis showed a prolongation of WM-T2 in about 70% of TLE patients with HA and NV. In half of the patients, WM-T2 increase was bilateral and symmetric. However, in 33% of patients with NV and bilateral symmetric increase in Hippo-T2, WM-T2 provided a correct lateralization of the seizure focus. Regardless of the pattern of T2 abnormalities, that is, bilateral symmetric or ipsilateral, the majority of patients with HA became seizure-free after surgery, while those with NV did not have a favorable outcome. In patients with NV, WM-T2 measurement may provide additional lateralizing information compared to Hippo-T2.