Arterial spin labeling (ASL) is capable of noninvasively measuring blood flow by magnetically tagging the protons in arterial blood, which has been conventionally achieved using instantaneous (PASL) or continuous (CASL) RF pulses. As an intermediate method, pseudocontinuous ASL (pCASL) utilizes a train of discrete RF pulses to mimic continuous tagging that is often unavailable on imagers due to the requirement of continuous RF transmit capabilities. In the present study, we implemented two versions of pCASL (balanced and unbalanced gradient waveforms in tag and control scans) for both transmit/receive coils and array receivers. Experimental data show a 50% +/- 4% increase of signal-to-noise ratio (SNR) compared with PASL and a higher tagging efficiency than amplitude-modulated (AM) CASL (80% vs. 68%). Computer simulations predict an optimal tagging efficiency of 85% for flow velocities from 10 to 60 cm/s. It is theoretically and experimentally demonstrated that the tagging efficiency of pCASL is dependent upon the resonance offset and flip angle of the RF pulse train. We conclude that pCASL has the potential of combining the merits of PASL, including less hardware demand and higher tagging efficiency, and CASL, which includes a longer tagging bolus and thus higher SNR. These improvements provide a better balance between tagging efficiency and SNR.

In pathologies in which slow or collateral flow conditions may exist, conventional arterial spin labeling (ASL) methods that apply magnetic tags based on the location of arterial spins may not provide robust measures of cerebral blood flow (CBF), as the transit delay for the delivery of blood to target tissues may far exceed the relaxation time of the tag. Here we describe current methods for ASL with velocity-selective (VS) tags (termed VSASL) that do not require spatial selectivity and can thus provide quantitative measures of CBF under slow and collateral flow conditions. The implementation of a robust multislice VSASL technique is described in detail, and data obtained with this technique are compared with those obtained with conventional pulsed ASL (PASL). The technical considerations described here include the design of VS pulses, background suppression, anisotropy with respect to velocity-encoding directions, and CBF quantitation issues. In conventional arterial spin labeling (ASL) techniques, including both pulsed ASL (PASL) (1-5) and continuous ASL (CASL) (6 -8), arterial blood is tagged by magnetic inversion or saturation proximal to the region of interest (ROI). Tagged blood then flows into the ROI, and the inflow is detected as a modulation of the longitudinal magnetization. In these techniques there is necessarily a spatial gap between the tagging location and the ROI. This gap results in a transit delay (␦t) for the delivery of tagged blood to the ROI. The gap (and hence the delay) can be small for single-slice imaging, but is larger for multislice or volume acquisitions. The magnitude and variability of the transit delay in relation to the T 1 decay of the tag is one of the largest potential sources of errors in the quantitation of perfusion using ASL in the normal human brain (5,6,9). In stroke and other pathologies in which flow may be slow or may follow circuitous collateral routes of delivery, ␦t can be much larger than T 1 (10), which makes conventional ASL an impractical method for obtaining accurate measures of CBF.We recently introduced a new ASL method in which the tag pulse is purely velocity-selective (VS) and not spatially-selective. This allows for the tagging of all flowing spins within a specified velocity range, regardless of location, and can in principle eliminate the problem of transit delays. We refer to this technique as VSASL, and in this work we describe the implementation of VSASL, as well as some of the considerations involved in the design of VSASL pulse sequences and the quantitation of perfusion using this technique. VSASL was introduced in abstract form in Ref. 11, and some of the issues addressed herein were described in suggested the use of VS pulses in ASL, but did not present an implementation of VSASL. THEORYIn principle, the elements of a VSASL pulse sequence are similar to those of a conventional ASL experiment and include a tagging pulse that modifies the magnetization of inflowing arterial spins, followed by a delay (TI) to allow for inflow, and a rapid image...

During sustained periods of a taxing cognitive workload, humans typically display time-on-task (TOT) effects, in which performance gets steadily worse over the period of task engagement. Arterial spin labeling (ASL) perfusion functional magnetic resonance imaging (fMRI) was used in this study to investigate the neural correlates of TOT effects in a group of 15 subjects as they performed a 20-minute continuous psychomotor vigilance test (PVT). Subjects displayed significant TOT effects, as seen in progressively slower reaction times and significantly increased mental fatigue ratings after the task. Perfusion data showed that the PVT activates a right lateralized fronto-parietal attentional network in addition to the basal ganglia and sensorimotor cortices. The fronto-parietal network was less active during post-task rest compared to pre-task rest, and regional CBF decrease in this network correlated with performance decline. These results demonstrate the persistent effects of cognitive fatigue in the fronto-parietal network after a period of heavy mental work and indicate the critical role of this attentional network in mediating TOT effects. Furthermore, resting regional CBF in the thalamus and right middle frontal gyrus prior to task onset was predictive of subjects' subsequent performance decline, suggesting that resting CBF quantified by ASL perfusion fMRI may be a useful indicator of performance potential and a marker of the level of fatigue in the neural attentional system.

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A time-efficient method is described for in vivo venous blood T 1 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 T 1 by taking advantage of the continuous inflow of fresh blood with longitudinal magnetization undisturbed by previous radiofrequency pulses. In vivo measurement of venous blood T 1 in the sagittal sinus was carried out in 26 healthy children and adults aged 7-39 years. The measured venous blood T 1 values decreased with age as a whole (P 5 0.006) and were higher in females than males (P 5 0.013), matching the expected developmental changes and gender differences in human hematocrit level. The estimated mean blood T 1 values were highly correlated with normal hematocrit levels across age and gender groups (Spearman's r 5 0.93, P 5 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 The T 1 (or rate R 1 ) of blood is a critical physiological parameter. In vitro studies have demonstrated the linear dependence of both arterial and venous blood R 1 on hematocrit (Hct) level, in addition to its dependence on temperature and oxygenation level (1,2). Theoretically, in vivo measurement of blood T 1 may be used as a surrogate index of Hct, if both adequate accuracy and precision can be achieved. Human Hct level varies considerably with age and gender across people's life span. It varies widely within the first month after birth (31-67%) and gradually converges to 33-40% around the age of 2, followed by a steady increase with age through childhood and adolescence. Gender difference in Hct emerges around the age of 12, and by adulthood, the normal Hct range is 41-53% for males and 36-46% for females (3). A rapid and reliable in vivo measurement of blood T 1 therefore is desirable to account for individual variations of Hct in a number of MRI applications. For instance, Hct is a scaling factor for the widely adopted blood oxygen level-dependent signal (4). Hct, through its effects on arterial blood T 1 , affects perfusion quantification using arterial spin labeling (5), as well as the appropriate null point for black blood imaging (6) and vascular space occupancy imaging (7).To date, however, the endeavor for in vivo blood T 1 mapping in individual subjects has been dampened mainly by prohibitively long scan times. For instance, conventional T 1 measurement is typically achieved by sampling the longitudinal relaxation curve with varied inversion times (TI) following a global inversion pulse, resulting in a scan time that spans at least several minutes. Other challenges for in vivo measurement of blood T 1 include partial-volume effects and flowrelated ...

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Arterial spin labeling imaging (ASL) perfusion MRI is a relatively novel technique that can allow for quantitative measurement of cerebral blood flow (CBF) by using magnetically labeled arterial blood water as an endogenous tracer. Available data on resting CBF in schizophrenia primarily comes from invasive and expensive nuclear medicine techniques that are often limited to small samples and yield mixed results. The noninvasive nature of ASL offers promise for larger-scale studies. The utility of this approach was examined in 24 healthy controls and 30 patients with schizophrenia. Differences between groups in quantitative CBF were assessed, as were relationships between CBF and psychiatric symptoms. Group comparisons demonstrated greater CBF for controls in several regions including bilateral precuneus and middle frontal gyrus. Patients showed increased CBF in left putamen/superior corona radiata and right middle temporal gyrus. For patients, greater severity of negative symptoms was associated with reduced CBF in bilateral superior temporal gyrus, cingulate gyrus, and left middle frontal gyrus. Increased severity of positive symptoms was related to both higher CBF in cingulate gyrus and superior frontal gyrus and decreased CBF in precentral gyrus/middle frontal gyrus. These findings support the feasibility and utility of implementing ASL in schizophrenia research and expand upon previous results.