By carefully considering all the challenges of high-field VASO and filling behavior of the relevant vasculature, the proposed method can detect and quantify CBV changes with high CNR in human brain at 7T.

This study examines the value of spin-echo-based fMRI for cognitive studies at the main magnetic field strength of 3 T using a spin-echo EPI (SE-EPI) sequence and a Stroop color-word matching task. SE-EPI has the potential advantage over conventional gradient-echo EPI (GE-EPI) that signal losses caused by dephasing through the slice are not present, and hence although image distortion will be the same as for an equivalent GE-EPI sequence, signal voids will be eliminated. The functional contrast in SE-EPI will be lower than for GE-EPI, as static dephasing effects do not contribute. As an auxiliary experiment interleaved diffusion-weighted and non-diffusion-weighted SE-EPI was performed in the visual cortex to further elucidate the mechanims of functional contrast. In the Stroop experiment activation was detected in all areas previously found using GE-EPI. Additional frontopolar and ventral frontomedian activations were also found, which could not be detected using GE-EPI. The experiments from visual cortex indicated that at 3 T the BOLD signal change has contributions from the extravascular space and larger blood vessels in roughly equal amounts. In comparison with GE-EPI the absence of static dephasing effects would seem to result in a superior intrinsic spatial resolution. In conclusion the sensitivity of SE-EPI at 3 T is sufficient to make it the method of choice for fMR studies that require a high degree of spatial localization or where the requirement is to detect activation in regions affected by strong susceptibility gradients. © 2002 Elsevier Science (USA)

Functional MRI (fMRI) by means of spin-echo (SE) techniques provides an interesting alternative to gradient-echo methods because the contrast is based primarily on dynamic averaging associated with the blood oxygenation level-dependent (BOLD) effect. In this article the contributions from different brain compartments to BOLD signal changes in SE echo planar imaging (EPI) are investigated. To gain a better understanding of the underlying mechanisms that cause the fMRI contrast, two experiments are presented: First, the intravascular contribution is decomposed into two fractions with different regimes of flow by means of diffusion-weighting gradient schemes which are either flow-compensated, or will maximally dephase moving spins. Second, contributions from the intra-and extravascular Key words: spin-echo fMRI; diffusion-weighting; BOLD; quantificationThe method used routinely for functional MRI (fMRI) of the brain employs blood oxygenation level-dependent (BOLD) (1) changes in paramagnetic properties of hemoglobin in the blood, closely related to oxygen delivery and consumption. Despite the success of the widely used gradient-echo (GE) techniques for fMRI, such as echo planar imaging (EPI), spin-echo (SE) sequences are a promising alternative to map the BOLD contrast because their functional contrast is dominated by intra-and extravascular diffusion-induced dynamic averaging rather than static dephasing. Thereby, extravascular effects are supposed to be more pronounced in the parenchyma, where variations in local field strength occur on a small spatial scale, leading to enhanced dynamic averaging. Due to its relative immunity to large-scale field distortions and signal voids by compensating phase dispersions, imaging is feasible even in regions with large static frequency variations (e.g., the frontal lobe).In Refs. 2 and 3, diffusion-weighted (DW) fMRI was introduced to study the origin of the GE BOLD contrast by selectively suppressing signal from the intravascular space. It was consistently found that intravascular spins account for the majority of fMRI signal change at 1.5 T. Later, the technique was extended to SE fMRI (4) to measure the apparent diffusion coefficient and changes in T 2 simultaneously pointing to changes in both parameters in the extravascular space in addition to intravascular fMRI signal changes. At 4 and 7 T (5,6), the application of DW to SE fMRI revealed extravascular contributions which exceed one-half of the signal change for TE Ϸ T 2 , and at an even higher field strength of 9.4 T (7), virtually no suppression of fMRI signal due to DW was observed at TE ϭ 40 ms. Therefore, SE fMRI at high fields offers the possibility to observe BOLD effects with dominant contributions from the extravascular space of the parenchyma. A detailed quantification of these fractions at a certain field strength would improve the understanding of the underlying mechanisms of fMRI contrast. With this information, it should be possible to tailor SE sequences for optimized sensitivity to signal from the parenc...

Measuring the hemodynamic response with functional magnetic resonance imaging (fMRI) together with functional near-infrared spectroscopy (fNIRS) may overcome limitations of single-method approaches. Accordingly, we measured the event-related hemodynamic response with both imaging methods simultaneously in young subjects during visual stimulation. An intertrial interval of 60 s was chosen to include the prolonged post-stimulus undershoot of the blood oxygenation level dependent (BOLD) signal. During visual stimulation, the BOLD signal, oxy-, and total hemoglobin (Hb) increased, whereas deoxy-Hb decreased. The post-stimulus period was characterized by an undershoot of the BOLD signal, oxy-Hb, and an overshoot of deoxy-Hb. Total Hb as measured by fNIRS returned to baseline immediately after the end of stimulation. Results suggest that the post-stimulus events as measured by fNIRS are dominated by a prolonged high-level oxygen consumption in the microvasculature. The contribution of a delayed return of blood volume to the BOLD post-stimulus undershoot in post-capillary veins as suggested by the Balloon and Windkessel models remains ambiguous. Temporal changes in the BOLD signal were highly correlated with deoxy-Hb, with lower correlation values for oxy-and total Hb. Furthermore, data show that fNIRS covers the outer 1 cm of the brain cortex. These results were confirmed by simultaneous fMRI/fNIRS measurements during rest. In conclusion, multimodal imaging approaches may contribute to the understanding of neurovascular coupling. D

Functional perfusion imaging with a separate labeling coil located above the common carotid artery was demonstrated in human volunteers at 3 T. A helmet resonator and a spin-echo echo-planar imaging (EPI) sequence were used for imaging, and a circular surface coil of 6 cm i.d. was employed for labeling. The subjects performed a finger-tapping task. Signal differences between the condition of finger tapping and the resting state were between -0.5% and -1.1 % among the subjects. Functional perfusion imaging using magnetically labeled water as an endogenous tracer has proven to be a valuable tool for investigating task-related brain activity (1,2). The advantages of perfusion-based functional imaging in comparison to the widely used blood oxygenation level-dependent (BOLD) technique include a potentially better localized area of activation (3) and the feasibility of quantification (4 -6). This study shows that functional perfusion imaging with continuous arterial spin labeling (CASL) can be performed with a separate labeling coil located above the common carotid artery in humans. For functional imaging, the temporal resolution of CASL is poor because it requires labeling periods of several seconds prior to image acquisition. A quantification of the cerebral blood flow (CBF) during task activation requires the acquisition of images with and without CASL, and would further increase the effective sampling interval. Therefore, in this study, labeling was applied for all repetitions of the functional run, and as a result the temporal resolution and the sensitivity of the functional study were increased by factors of 2 and ͌2 (7), respectively, while the ability to quantify blood flow changes was maintained.CASL approaches (8,9) for measuring the CBF are in principle more sensitive than methods based on pulsed labeling, but their use in humans is confronted with two major problems (10). First, the application of long offresonance radiofrequency (RF) pulses causes magnetization-transfer (MT) effects. Second, the transit time from the labeling plane to the imaging slice results in a loss of sensitivity due to the relaxation of spins in the arterial blood. Finally, at 3 T RF power deposition may also be an issue of concern. The first problem can be addressed by keeping the MT influence constant for the labeling and control experiments (11,12). Complete elimination of MT effects is achieved by using separate labeling and imaging coils, which additionally removes the need for RF pulsing during the control acquisition (13,14), and in general reduces the total RF-power requirement. Multislice perfusion imaging can easily be implemented by this method. However, influences from transit-time differences in the brain are increased if labeling is performed at the neck, and quantitative maps of CBF cannot easily be obtained. Alsop and Detre (15) showed that the introduction of a post-label delay (PLD) markedly reduces transit-time effects, provided that the longitudinal relaxation times of arterial blood and brain tissue are nearly ...

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Continuous arterial spin labeling (ASL) using a locally induced magnetic field gradient for adiabatic inversion of spins in the common carotid artery of human volunteers is demonstrated. The experimental setup consisted of a helmet resonator for imaging, a circular RF surface coil for labeling, and gradient loops to produce a magnetic field gradient. A spin-echo (SE) echo-planar imaging (EPI) sequence was used for imaging. The approach is independent of the gradients of the MR scanner. This technology may be used if the imaging gradient system does not produce an appropriate magnetic field gradient at the location of the carotid artery-for example, in a head-only scanner-and is a prerequisite for the development of a system that allows continuous labeling during the imaging experiment. Magn Reson Med 48:543-546, 2002.

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