Inflow‐based vascular‐space‐occupancy (iVASO) MRI

Vascular‐space‐occupancy (VASO) MRI, a blood nulling approach for assessing changes in cerebral blood volume (CBV), is hampered by low signal‐to‐noise ratio (SNR) because only 10–20% of tissue signal is recovered when using nonselective inversion for blood nulling. A new approach, called inflow‐VASO (iVASO), is introduced in which only blood flowing into the slice has experienced inversion, thereby keeping tissue and cerebrospinal fluid (CSF) signal in the slice maximal and reducing CSF partial volume effects. SNR increases of 198% ± 12% and 334% ± 9% (mean ± SD, n = 7) with respect to VASO were found at TR values of 5s and 2s, respectively. When using inflow approaches, data interpretation is complicated by the fact that signal changes are affected by vascular transit times. An optimal TR‐range (1.5–2.5s) was derived in which the iVASO response during activation predominantly reflects arterial/arteriolar CBV (CBV_a) changes. In this TR‐range, perfusion contributions to the signal change are negligible because arterial label has not yet undergone capillary exchange, and arterial and precapillary blood signals are nulled. For TR = 2s, the iVASO signal change upon visual stimulation corresponded to a CBV_a increase of 58% ± 7%, in agreement with arteriolar CBV changes previously reported. The onset of the hemodynamic response for iVASO occurred 1.2 ± 0.5s (n = 7) faster than for conventional VASO. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Magnetization transfer enhanced vascular‐space‐occupancy (MT‐VASO) functional MRI

Vascular‐space‐occupancy (VASO) MRI is a novel technique that uses blood signal nulling to detect blood volume alterations through changes in tissue signal. VASO has relatively low signal to noise ratio (SNR) because only 10–20% of tissue signal remain at the time of blood nulling. Here, it is shown that by adding a magnetization transfer (MT) prepulse it is possible to increase SNR either by attenuating the initial tissue magnetization when the MT pulse is placed before inversion, or, accelerating the recovery process when the pulse is applied after the inversion. To test whether the MT pulse would affect the blood nulling time in VASO, MT effects in blood were measured both ex vivo in a bovine blood phantom and in vivo in human brain. Such effects were found to be sufficiently small (< 2.5%) under a saturation power ≤ 3 μT, length = 500 ms, and frequency offset ≥40 ppm to allow use of the same nulling time. Subsequently, functional MRI experiments using MT‐VASO were performed in human visual cortex at 3 Tesla. The relative signal changes in MT‐VASO were of the same magnitude as in VASO, while the contrast to noise ratio (CNR) was enhanced by 44 ± 12% and 36 ± 11% respectively. Therefore, MT‐VASO should provide a means for increasing inherently low CNR in VASO experiments while preserving the CBV sensitivity. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

On the measurement of absolute cerebral blood volume (CBV) using vascular‐space‐occupancy (VASO) MRI

Recently, a vascular‐space‐occupancy (VASO) MRI technique was developed for quantitative assessment of cerebral blood volume (CBV). This method uses the T₁‐shortening effect of gadolinium diethylenetriamine pentaacetic acid (Gd‐DTPA) with imaging parameters chosen that null the precontrast blood magnetization but allow the postcontrast blood magnetization to recover to equilibrium. A key advantage of VASO CBV estimation is that it provides a straightforward procedure for converting MR signals to absolute physiologic values. However, as with other T₁‐based steady‐state approaches, several important factors need to be considered that influence the accuracy of CBV values obtained with VASO MRI. Here, the transverse relaxation (T₂/T) effect in VASO MRI was investigated using multiecho spin‐echo and gradient‐echo experiments, resulting in underestimation of CBV by 14.9% ± 1.1% and 16.0% ± 2.5% for spin echo (TE = 10 ms) and gradient echo (TE = 6 ms), respectively. In addition, the influence of contrast agent clearance was studied by acquiring multiple postcontrast VASO images at 2.2‐min intervals, which showed that the concentration of Gd‐DTPA in the first 14 min (single dose) was sufficient for the blood magnetization to fully recover to equilibrium. Finally, the effect of vascular Gd‐DTPA leakage was assessed for scalp tissue, and signal extrapolation as a function of postinjection time was demonstrated to be useful in minimizing the associated errors. Specific recommendations for VASO MRI acquisition and processing strategies are provided. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Implementation of vascular‐space‐occupancy MRI at 7T

Vascular‐space‐occupancy (VASO) MRI exploits the difference between blood and tissue T₁ to null blood signal and measure cerebral blood volume changes using the residual tissue signal. VASO imaging is more difficult at higher field because of sensitivity loss due to the convergence of tissue and blood T₁ values and increased contamination from blood‐oxygenation‐level‐dependent (BOLD) effects. In addition, compared to 3T, 7T MRI suffers from increased geometrical distortions, e.g., when using echo‐planar‐imaging, and from increased power deposition, the latter especially problematic for the spin‐echo‐train sequences commonly used for VASO MRI. Third, non‐steady‐state blood spin effects become substantial at 7T when only a head coil is available for radiofrequency transmit. In this study, the magnetization‐transfer‐enhanced‐VASO approach was applied to maximize tissue‐blood signal difference, which boosted signal‐to‐noise ratio by 149% ± 13% (n = 7) compared to VASO. Second, a 3D fast gradient‐echo sequence with low flip‐angle (7°) and short echo‐time (1.8 ms) was used to minimize the BOLD effect and to reduce image distortion and power deposition. Finally, a magnetization‐reset technique was combined with a motion‐sensitized‐driven‐equilibrium approach to suppress three types of non‐steady‐state spins. Our initial functional MRI results in normal human brains at 7T with this optimized VASO sequence showed better signal‐to‐noise ratio than at 3T. Magn Reson Med 69:1003–1013, 2013. © 2012 Wiley Periodicals, Inc.     

T2‐weighted 3D fMRI using S2‐SSFP at 7 tesla

In this study, the sensitivity of the S₂‐steady‐state free precession (SSFP) signal for functional MRI at 7 T was investigated. In order to achieve the necessary temporal resolution, a three‐dimensional acquisition scheme with acceleration along two spatial axes was employed. Activation maps based on S₂‐steady‐state free precession data showed similar spatial localization of activation and sensitivity as spin‐echo echo‐planar imaging (SE‐EPI), but data can be acquired with substantially lower power deposition. The functional sensitivity estimated by the average z‐values was not significantly different for SE‐EPI compared to the S₂‐signal but was slightly lower for the S₂‐signal (6.74 ± 0.32 for the TR = 15 ms protocol and 7.51 ± 0.78 for the TR = 27 ms protocol) compared to SE‐EPI (7.49 ± 1.44 and 8.05 ± 1.67) using the same activated voxels, respectively. The relative signal changes in these voxels upon activation were slightly lower for SE‐EPI (2.37% ± 0.18%) compared to the TR = 15 ms S₂‐SSFP protocol (2.75% ± 0.53%) and significantly lower than the TR = 27 ms protocol (5.38% ± 1.28%), in line with simulations results. The large relative signal change for the long TR SSFP protocol can be explained by contributions from multiple coherence pathways and the low intrinsic intensity of the S₂ signal. In conclusion, whole‐brain T₂‐weighted functional MRI with negligible image distortion at 7 T is feasible using the S₂‐SSFP sequence and partially parallel imaging. Magn Reson Med 63:1015–1020, 2010. © 2010 Wiley‐Liss, Inc.     

Diffusion‐weighted spectroscopy: A novel approach to determine macromolecule resonances in short‐echo time 1H‐MRS

Quantification of short‐echo time proton magnetic resonance spectroscopy results in >18 metabolite concentrations (neurochemical profile). Their quantification accuracy depends on the assessment of the contribution of macromolecule (MM) resonances, previously experimentally achieved by exploiting the several fold difference in T₁. To minimize effects of heterogeneities in metabolites T₁, the aim of the study was to assess MM signal contributions by combining inversion recovery (IR) and diffusion‐weighted proton spectroscopy at high‐magnetic field (14.1 T) and short echo time (=8 msec) in the rat brain. IR combined with diffusion weighting experiments (with δ/Δ = 1.5/200 msec and b‐value = 11.8 msec/μm²) showed that the metabolite nulled spectrum (inversion time = 740 msec) was affected by residuals attributed to creatine, inositol, taurine, choline, N‐acetylaspartate as well as glutamine and glutamate. While the metabolite residuals were significantly attenuated by 50%, the MM signals were almost not affected (<8%). The combination of metabolite‐nulled IR spectra with diffusion weighting allows a specific characterization of MM resonances with minimal metabolite signal contributions and is expected to lead to a more precise quantification of the neurochemical profile. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Vascular space occupancy (VASO) cerebral blood volume‐weighted MRI identifies hemodynamic impairment in patients with carotid artery disease

Purpose

To assess the role of vascular space occupancy (VASO) magnetic resonance imaging (MRI), a noninvasive cerebral blood volume (CBV)‐weighted technique, for evaluating CBV reactivity in patients with internal carotid artery (ICA) stenosis.

Materials and Methods

VASO reactivity, defined as a signal change in response to hypercapnic stimulus (4‐second exhale, 14‐second breath‐hold), was measured in the left and right ICA flow territories in patients (n = 10) with varying degrees of unilateral and bilateral ICA stenosis and in healthy volunteers (n = 10).

Results

Percent VASO reactivity was more negative (P < 0.01) bilaterally in patients (ipsilateral: ?3.6 ± 1.5%; contralateral: ?3.4 ± 1.2%) compared with age‐matched controls (left: ?1.9 ± 0.6%; right: ?1.9 ± 0.8%). Owing to the nature of the VASO contrast mechanism, this more negative VASO reactivity was attributed to autoregulatory CBV effects in patients. A postbreath‐hold overshoot, which was absent in healthy volunteers, was observed unilaterally in a subset of patients.

Conclusion

More negative VASO reactivity was observed in patients with ICA stenosis and may be a marker of autoregulatory effects. Furthermore, the postbreath‐hold overshoot observed in patients is consistent with compensatory microvascular vasoconstriction and may be a marker of hemodynamic impairment. Based on the results of this feasibility study, VASO should be useful for identifying CBV adjustments in patients with steno‐occlusive disease of the ICA. J. Magn. Reson. Imaging 2009;29:718–724. © 2009 Wiley‐Liss, Inc.

     

3D flow‐independent peripheral vessel wall imaging using T2‐prepared phase‐sensitive inversion‐recovery steady‐state free precession

Purpose:

To develop a 3D flow‐independent peripheral vessel wall imaging method using T₂‐prepared phase‐sensitive inversion‐recovery (T₂PSIR) steady‐state free precession (SSFP).

Materials and Methods:

A 3D T₂‐prepared and nonselective inversion‐recovery SSFP sequence was designed to achieve flow‐independent blood suppression for vessel wall imaging based on T₁ and T₂ properties of the vessel wall and blood. To maximize image contrast and reduce its dependence on the inversion time (TI), phase‐sensitive reconstruction was used to restore the true signal difference between vessel wall and blood. The feasibility of this technique for peripheral artery wall imaging was tested in 13 healthy subjects. Image signal‐to‐noise ratio (SNR), wall/lumen contrast‐to‐noise ratio (CNR), and scan efficiency were compared between this technique and conventional 2D double inversion recovery – turbo spin echo (DIR‐TSE) in eight subjects.

Results:

3D T₂PSIR SSFP provided more efficient data acquisition (32 slices and 64 mm in 4 minutes, 7.5 seconds per slice) than 2D DIR‐TSE (2–3 minutes per slice). SNR of the vessel wall and CNR between vessel wall and lumen were significantly increased as compared to those of DIR‐TSE (P < 0.001). Vessel wall and lumen areas of the two techniques are strongly correlated (intraclass correlation coefficients: 0.975 and 0.937, respectively; P < 0.001 for both). The lumen area of T₂PSIR SSFP is slightly larger than that of DIR‐TSE (P = 0.008). The difference in vessel wall area between the two techniques is not statistically significant.

Conclusion:

T₂PSIR SSFP is a promising technique for peripheral vessel wall imaging. It provides excellent blood signal suppression and vessel wall/lumen contrast. It can cover a 3D volume efficiently and is flow‐ and TI‐independent. J. Magn. Reson. Imaging 2010;32:399–408. © 2010 Wiley‐Liss, Inc.     

Theoretical and experimental investigation of the VASO contrast mechanism.

Vascular space occupancy (VASO)-dependent functional MRI (fMRI) is a blood-nulling technique capable of generating microvascular cerebral blood volume (CBV)-weighted images. It is shown that at high magnetic field (3.0T) and high spatial resolution (1.89 x 1.89 x 3 mm(3)), the VASO signal changes are too large (6-7%) to originate from CBV effects alone. Additional contributions are investigated theoretically and experimentally as a function of MRI parameters (TR and TE), as well as the signal-to-noise ratio, (SNR) and spatial resolution. First, it is found that an arterial spin labeling (ASL) contribution causes large negative VASO signal changes at short TR. Second, even at high fMRI spatial resolution, CSF volume contributions (7-13%) cause VASO signal changes to become more negative, most noticeably at long TR and TE. Third, white matter (WM) effects reduce signal changes at lower spatial resolution. The VASO technique has been tested using different stimulus paradigms and field strengths (1-3), giving results consistent with comparable tasks investigated using BOLD and cerebral blood flow (CBF)-based techniques. Finally, simulations show that a mixture of fresh and steady-state blood may significantly alter signal changes at short TR (< or =3 s), permitting larger VASO signal changes than expected under pure steady-state conditions. Thus, many competing effects contribute to VASO contrast and care should be taken during interpretation.     

3D single‐shot VASO using a maxwell gradient compensated GRASE sequence

The vascular space occupancy (VASO) method was recently proposed as a functional MRI (fMRI) method that is capable of detecting activation‐related changes in blood volume (CBV), without the need for a blood‐pool contrast agent. In the present work we introduce a new whole‐brain VASO technique that is based on a parallel‐accelerated single‐shot 3D GRASE (gradient and spin echo) readout. The GRASE VASO sequence employs a flow‐compensated correction scheme for concomitant Maxwell gradients which is necessary to avoid smearing artifacts that may occur due to violation of the Carr–Purcell–Meiboom–Gill (CPMG) condition for off‐resonance excitation. Experiments with 6 min of visual‐motor stimulation were performed on eight subjects. At P < 0.01, average percent signal change and t‐score for visual stimulation were ?3.11% and ?8.42, respectively; activation in left and right motor cortices and supplementary motor area was detected with ?2.75% and ?6.70, respectively. Sensitivity and signal changes are comparable to those of echo‐planar imaging (EPI)‐based single‐slice VASO, as indicated by additional visual‐task experiments (?3.39% and ?6.93). The method makes it possible to perform whole‐brain cognitive activation studies based on CBV contrast. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Experimental hypoxemic hypoxia: Changes in R2* of brain parenchyma accurately reflect the combined effects of changes in arterial and cerebral venous oxygen saturation

A two-dimensional T₂*-weighted gradient-echo sequence was used to image the rat brain before and during graded hypoxemia. Changes in R₂* (δR₂*) with respect to the control state were calculated for brain parenchyma and were compared with changes in hemoglobin saturation measured from both arterial and jugular venous blood samples. δR₂* was first correlated with the changes in arterial (δYa) and venous (δYv) hemoglobin saturations individually. Although a general trend toward a linear relationship with δR₂* was observed for both δYa and δYv, neither alone was strong (correlation coefficients r = 0.71 and 0.75 for δYa and δYv, respectively, and standard errors of the regression (SER) = 0.52 and 0.48 for δYa and δYv, respectively). However, when an “effective” cerebral blood hemoglobin saturation change (δYb) was constructed that takes into account the approximate weighting of the contributions from the arterial and venous phases of the circulation (δYb = 0.75 × δYv + 0.25 × δYa), a stronger correlation with δR₂* was obtained and there was less variance (r = 0.87 and SER = 0.35). It is concluded that an appropriate weighting of the contributions of arterial and venous phases of the circulation must be taken into account in modeling the volume susceptibility effects of deoxyhemoglobin on R₂* of brain parenchyma. In this way, a more accurate relationship between δR₂* and δYb can be obtained.     

Association of previous injury and speed with running style and stride‐to‐stride fluctuations

Running‐related injuries remain problematic among recreational runners. We evaluated the association between having sustained a recent running‐related injury and speed, and the strike index (a measure of footstrike pattern, SI) and spatiotemporal parameters of running. Forty‐four previously injured and 46 previously uninjured runners underwent treadmill running at 80%, 90%, 100%, 110%, and 120% of their preferred running speed. Participants wore a pressure insole device to measure SI, temporal parameters, and stride length (S_length) and stride frequency (S_frequency) over 2‐min intervals. Coefficient of variation and detrended fluctuation analysis provided information on stride‐to‐stride variability and correlative patterns. Linear mixed models were used to compare differences between groups and changes with speed. Previously injured runners displayed significantly higher stride‐to‐stride correlations of SI than controls (P = 0.046). As speed increased, SI, contact time (T_contact), stride time (T_stride), and duty factor (DF) decreased (P < 0.001), whereas flight time (T_flight), S_length, and S_frequency increased (P < 0.001). Stride‐to‐stride variability decreased significantly for SI, T_contact, T_flight, and DF (P ≤ 0.005), as did correlative patterns for T_contact, T_stride, DF, S_length, and S_frequency (P ≤ 0.044). Previous running‐related injury was associated with less stride‐to‐stride randomness of footstrike pattern. Overall, runners became more pronounced rearfoot strikers as running speed increased.     

Improved carotid intraplaque hemorrhage imaging using a slab‐selective phase‐sensitive inversion‐recovery (SPI) sequence

Intraplaque hemorrhage in atherosclerotic plaques has been associated with accelerated plaque growth as well as exacerbation of clinical symptoms. The identification of intraplaque hemorrhage using magnetic resonance imaging primarily relies on the detection of methemoglobin on T₁ weighted images. Current techniques are limited by insufficient intraplaque hemorrhage‐wall contrast and poor blood suppression. In this study, a slab‐selective phase‐sensitive inversion‐recovery (SPI) technique is proposed by combining a phase‐sensitive reconstruction with a T₁ weighted sequence specifically designed to achieve improved intraplaque hemorrhage imaging. The SPI sequence was optimized and then used on ex vivo plaque specimens for histology based validation and intraplaque hemorrhage‐wall contrast‐to‐noise ratio comparison with magnetization‐prepared 3D rapid acquisition gradient echo MP‐RAGE. SPI and MP‐RAGE were also tested on a group of atherosclerosis patients to compare in vivo intraplaque hemorrhage‐wall contrast‐to‐noise ratio and blood suppression effectiveness. On ex vivo specimens SPI had better intraplaque hemorrhage identification accuracy and a significantly higher intraplaque hemorrhage‐wall contrast‐to‐noise ratio (P = 0.01) than MP‐RAGE. Similar results were found in the in vivo test: Slab‐selective phase‐sensitive inversion‐recovery provided a significantly improved intraplaque hemorrhage‐wall contrast‐to‐noise ratio (P < 0.01) and blood suppression efficiency (P < 0.01). In conclusion, SPI is a novel technique optimized for intraplaque hemorrhage detection and validated against histology. It has demonstrated its capability for improved in vivo intraplaque hemorrhage identification and blood suppression in atherosclerosis patients. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

In vivo venous blood T1 measurement using inversion recovery true‐FISP in children and adults

A time‐efficient method is described for in vivo venous blood T₁ measurement using multiphase inversion‐recovery‐prepared balanced steady‐state free precession imaging. Computer simulations and validation experiments using a flow phantom were carried out to demonstrate the accuracy of the proposed method for measuring blood T₁ by taking advantage of the continuous inflow of fresh blood with longitudinal magnetization undisturbed by previous radiofrequency pulses. In vivo measurement of venous blood T₁ in the sagittal sinus was carried out in 26 healthy children and adults aged 7–39 years. The measured venous blood T₁ values decreased with age as a whole (P = 0.006) and were higher in females than males (P = 0.013), matching the expected developmental changes and gender differences in human hematocrit level. The estimated mean blood T₁ values were highly correlated with normal hematocrit levels across age and gender groups (Spearman's r = 0.93, P = 0.008). The longitudinal repeatability of this technique was 4.0% as measured by the within‐subject coefficient of variation. The proposed multiphase inversion recovery‐prepared balanced steady‐state free precession imaging method is a feasible technique for fast (<1 min) and reliable in vivo venous blood T₁ measurement and may serve as an index of hematocrit level in individual subjects. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Effect of blood flow on double inversion recovery vessel wall MRI of the peripheral arteries: Quantitation with T2 mapping and comparison with flow‐insensitive T2‐prepared inversion recovery imaging

Blood suppression in the lower extremities using flow‐reliant methods such as double inversion recovery may be problematic due to slow blood flow. T₂ mapping using fast spin echo (FSE) acquisition was utilized to quantitate the effectiveness of double inversion recovery blood suppression in 13 subjects and showed that 25 ± 12% of perceived vessel wall pixels in the popliteal arteries contained artifactual blood signal. To overcome this problem, a flow‐insensitive T₂‐prepared inversion recovery sequence was implemented and optimal timing parameters were calculated for FSE acquisition. Black blood vessel wall imaging of the popliteal and femoral arteries was performed using two‐dimensional T₂‐prepared inversion recovery‐FSE in the same 13 subjects. Comparison with two‐dimensional double inversion recovery‐FSE showed that T₂‐prepared inversion recovery‐FSE reduced wall‐mimicking blood artifacts that inflated double inversion recovery‐FSE vessel wall area measurements in the popliteal artery. Magn Reson Med 63:736–744, 2010. © 2010 Wiley‐Liss, Inc.     

Non‐contrast‐enhanced pulmonary vein MRI with a spatially selective slab inversion preparation sequence

We propose a non‐contrast‐enhanced, three‐dimensional, free‐breathing, electrocardiogram‐gated, gradient recalled echo sequence with a slab‐selective inversion for pulmonary vein (PV) MRI. A sagittal inversion slab was applied prior to data acquisition to suppress structures adjacent to the left atrium (LA) and PVs, thereby improving the conspicuity of the PV and LA. Compared with other MR angiography methods using an inversion pulse, the proposed method does not require signal subtraction and the inversion slab is not parallel to the imaging slab. The feasibility of the proposed method was demonstrated in healthy subjects. The inversion slab thickness and inversion time were optimized to be 60 mm and 500 ms, respectively. Compared to conventional gradient recalled echo imaging without inversion, the proposed technique significantly increased the contrast‐to‐noise ratios between the LA and the right atrium by 20‐fold (P < 0.01), increased the contrast‐to‐noise ratios between the PVs and right atrium by 10‐fold (P < 0.03), and increased the contrast‐to‐noise ratios between the PVs, LA and pulmonary artery by 4‐fold (P < 0.01 for both). The signal‐to‐noise ratios of the PVs and the LA were similar with and without the inversion slab (P > 0.3). The proposed technique greatly enhances the conspicuity of the PVs and LA without significant loss of signal‐to‐noise ratio. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast diffusion‐weighted steady state free precession imaging of in vivo knee cartilage

Quantification of molecular diffusion with steady state free precession (SSFP) is complicated by the fact that diffusion effects accumulate over several repetition times (TR) leading to complex signal dependencies on transverse and longitudinal magnetization paths. This issue is commonly addressed by setting TR > T₂, yielding strong attenuation of all higher modes, except of the shortest ones. As a result, signal attenuation from diffusion becomes T₂ independent but signal‐to‐noise ratio (SNR) and sequence efficiency are remarkably poor. In this work, we present a new approach for fast in vivo steady state free precession diffusion‐weighted imaging of cartilage with TR << T₂ offering a considerable increase in signal‐to‐noise ratio and sequence efficiency. At a first glance, prominent coupling between magnetization paths seems to complicate quantification issues in this limit, however, it is observed that diffusion effects become rather T₂(ΔD ～ 1/10 ΔT₂) but not T₁ independent (ΔD ～ 1/2 ΔT₁) for low flip angles α ～ 10 ? 15°. As a result, fast high‐resolution (0.35 × 0.35 ? 0.50 × 0.50 mm² in‐plane resolution) quantitative diffusion‐weighted imaging of human articular cartilage is demonstrated at 3.0 T in a clinical setup using estimated T₁ and T₂ or a combination of measured T₁ and estimated T₂ values. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Estimating cerebral blood volume with expanded vascular space occupancy slice coverage

A model for quantifying cerebral blood volume (CBV) based on the vascular space occupancy (VASO) technique and varying the extent of blood nulling yielding task‐related signal changes with various amounts of blood oxygenation level‐dependent (BOLD) and VASO weightings was previously described. Challenges associated with VASO include limited slice coverage and the confounding inflow of fresh blood. In this work, an approach that extends the previous model to multiple slices and accounts for the inflow effect is described and applied to data from a multiecho sequence simultaneously acquiring VASO, cerebral blood flow (CBF), and BOLD images. This method led to CBV values (7.9 ± 0.3 and 5.6 ± 0.3 ml blood/100 ml brain during activation [CBV_ACT] and rest [CBV_REST], respectively) consistent with previous studies using similar visual stimuli. Furthermore, an increase in effective blood relaxation (0.65 ± 0.01) compared to the published value (0.62) was detected, likely reflecting inflow of fresh blood. Finally, cerebral metabolic rate of oxygen (CMRO₂) estimates using a multiple compartment model without assumption of CBV_REST led to estimates (18.7 ± 17.0%) that were within published ranges. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Respiratory self‐gating for free‐breathing abdominal phase‐contrast blood flow measurements

Purpose

To demonstrate the feasibility of using a free‐breathing (FB) respiratory self‐gated (RSG) approach for abdominal phase‐contrast (PC) blood flow measurements.

Materials and Methods

PC‐magnetic resonance imaging (MRI) flow measurements were performed within the right renal artery, common hepatic artery, and main portal vein during breath‐hold (BH) and FB with both signal averaging and RSG in eight healthy volunteers. The resultant images were qualitatively scored by two independent reviewers blinded to acquisition techniques. Blood flow volume and cross‐sectional vessel size measurements were compared for three techniques.

Results

The overall efficiency for the RSG‐PC sequence was 38.9% ± 4.7%. Images acquired with RSG effectively mitigated respiratory motion artifacts, which were clearly evident within FB signal‐averaged images. RSG produced similar image quality to that of BH techniques (P > 0.146) and resulted in similar vessel size measurements (P = 0.694). Flow results for both FB RSG and signal‐averaged reconstructions correlated well with BH flow measurements (r = 0.97 and 0.92, P < 0.001). However, only the RSG methods demonstrated excellent absolute agreement with BH‐PC flow measurements (P = 0.600), with signal‐averaged methods resulting in significant overestimations.

Conclusion

RSG methods can limit respiratory motion artifacts to reduce flow measurement inaccuracies during free‐breathing PC measurements in the abdomen. J. Magn. Reson. Imaging 2009;29:860–868. © 2009 Wiley‐Liss, Inc.     

Unambiguous assignment of the H3S and H3R deuterations of cerebral (2‐13C) glutamate by 13C NMR at 18.8 tesla

We used high‐field ¹³C NMR (18.8 T) to assign unambiguously the isotopic shifts induced by the deuterium substitutions of the H3_proR and H3_proS hydrogens of (2‐¹³C) glutamate in extracts of the brain from deuterated animals. Monodeuterated H3R or H3S glutamate diastereoisomers were produced stereospecifically either by chemical synthesis or by coupling the reactions of isocitrate dehydrogenase and aspartate aminotransferase in deuterated medium, respectively. We show that the (3S‐²H) or (3R‐²H) deuterations induce characteristic small (Δ₂ = ?0.058 parts per million (ppm)) or large (Δ₂ = ?0.071 ppm) vicinal isotopic shifts upfield of the perprotonated (2‐¹³C) glutamate resonance (at 55.5 ppm). Isotopically shifted (2‐¹³C, 3S‐²H) or (2‐¹³C, 3R‐²H) glutamate singlets are conveniently observed by high‐field ¹³C NMR in brain extracts from deuterated rats. Since the (3S‐²H) or (3R‐²H) glutamate diastereoisomers are produced stereospecifically by the cytosolic or mitochondrial isoforms of aconitase and isocitrate dehydrogenase, our results will facilitate the ¹³C NMR investigation of these enzymatic activities and their role in subcellular glutamate trafficking. Magn Reson Med 63:1088–1091, 2010. © 2010 Wiley‐Liss, Inc.