A theoretical and experimental investigation of the tagging efficiency of pseudocontinuous arterial spin labeling.

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.     

Pseudocontinuous arterial spin labeling with optimized tagging efficiency

The adiabatic inversion of blood in pseudocontinuous arterial spin labeling (PCASL) is highly sensitive to off‐resonance effects and gradient imperfections and this sensitivity can lead to tagging efficiency loss and unpredictable variations in cerebral blood flow estimates. This efficiency loss is caused by a phase tracking error between the RF pulses and the flowing spins. This article introduces a new method, referred to as Optimized PCASL (OptPCASL), that minimizes the phase tracking error by applying an additional compensation RF phase term and in‐plane gradients to the PCASL pulse train. The optimal RF phase and gradient amplitudes are determined using a prescan procedure, which consists of a series of short scans interleaved with automated postprocessing routines integrated to the scanner console. The prescan procedure is shown to minimize the phase tracking error in a robust and time efficient manner. As an example of its application, the use of OptPCASL for the improved detection of functional activation in the visual cortex is demonstrated and temporal signal‐to‐noise ratio (SNR), image SNR, and baseline cerebral blood flow measures are compared to those acquired from conventional PCASL. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Vessel‐encoded dynamic magnetic resonance angiography using arterial spin labeling

A new noninvasive MRI method for vessel‐selective angiography is presented. The technique combines vessel‐encoded pseudocontinuous arterial spin labeling with a two‐dimensional dynamic angiographic readout and was used to image the cerebral arteries in healthy volunteers. Time‐of‐flight angiograms were also acquired prior to vessel‐selective dynamic angiography acquisitions in axial, coronal, and/or sagittal planes, using a 3‐T MRI scanner. The latter consisted of a vessel‐encoded pseudocontinuous arterial spin labeling pulse train of 300 or 1000 ms followed by a two‐dimensional thick‐slab flow‐compensated fast low‐angle shot readout combined with a segmented Look‐Locker sampling strategy (temporal resolution = 55 ms). Selective labeling was performed at the level of the neck to generate individual angiograms for both right and left internal carotid and vertebral arteries. Individual vessel angiograms were reconstructed using a bayesian inference method. The vessel‐selective dynamic angiograms obtained were consistent with the time‐of‐flight images, and the longer of the two vessel‐encoded pseudocontinuous arterial spin labeling pulse train durations tested (1000 ms) was found to give better distal vessel visibility. This technique provides highly selective angiograms quickly and noninvasively that could potentially be used in place of intra‐arterial x‐ray angiography for larger vessels. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Noncontrast‐enhanced three‐dimensional (3D) intracranial MR angiography using pseudocontinuous arterial spin labeling and accelerated 3D radial acquisition

Pseudocontinuous arterial spin labeling (PCASL) can be used to generate noncontrast magnetic resonance angiograms of the cerebrovascular structures. Previously described PCASL‐based angiography techniques were limited to two‐dimensional projection images or relatively low‐resolution three‐dimensional (3D) imaging due to long acquisition time. This work proposes a new PCASL‐based 3D magnetic resonance angiography method that uses an accelerated 3D radial acquisition technique (VIPR, spoiled gradient echo) as the readout. Benefiting from the sparsity provided by PCASL and noise‐like artifacts of VIPR, this new method is able to obtain submillimeter 3D isotropic resolution and whole head coverage with a 8‐min scan. Intracranial angiography feasibility studies in healthy (N = 5) and diseased (N = 5) subjects show reduced saturation artifacts in PCASL‐VIPR compared with a standard time‐of‐flight protocol. These initial results show great promise for PCASL‐VIPR for static, dynamic, and vessel selective 3D intracranial angiography. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Superselective pseudocontinuous arterial spin labeling

A new technique for the imaging of flow territories of individual extra‐ and intracranial arteries is presented. The method is based on balanced pseudocontinuous arterial spin labeling but employs additional time‐varying gradients in between the radiofrequency pulses of the long labeling train. The direction of the additional gradient vector is perpendicular to the selected artery and its azimuthal angle is switched after every radiofrequency pulse. The phases of the radiofrequency pulses are adopted to cancel out the phase accrual of the spins at the center of the target vessel due to the extra applied gradients. This results in efficient inversion at the targeted position, whereas elsewhere time‐varying phase changes will result in marginal inversion efficiency. By changing the moment of the added gradients, the size of the labeling focus can be adjusted. Influence of the temporal order of the additional gradients on the labeling efficiency and on the selectivity was investigated by simulations and experimental measurements. In a volunteer study, the acquisition of high signal‐to‐noise ratio flow territory images of small branches of the anterior cerebral artery distal to the circle of Willis was demonstrated. This shows the method's flexibility for dealing with complicated arterial geometries and its ability to superselectively label small intracranial arteries. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast projective carotid MR angiography using arterial spin-labeled balanced SSFP

PURPOSE: To determine whether pulsed arterial spin-labeled (pulsed ASL) balanced steady-state free precession (bSSFP) imaging allows for rapid projective depiction of the carotid arteries without electrocardiographic (ECG) gating. MATERIALS AND METHODS: The carotid arteries of six volunteers were scanned at 1.5 T using an ASL two-dimensional bSSFP sequence. Three configurations were tested, with and without ECG gating: (a) full field-of-view (FOV) acquisition (scan time=48 s), (b) full-FOV acquisition with parallel acceleration of 2 (24 s), and sequence C (half-FOV acquisition (24 s). Vessel-to-background contrast-to-noise ratios (CNRs) and vessel lengths were compared between sequence configurations. Vessel caliber measurements were compared with those obtained from three-dimensional time-of-flight (TOF) angiography. RESULTS: The carotid arteries were seen over extensive lengths with ASL two-dimensional bSSFP. Projected vessel length and vessel-to-background CNR did not differ with ECG gating (P=NS). Nonaccelerated full-FOV and half-FOV scans provided larger apparent vessel-to-background CNR and slightly longer vessel lengths than the parallel-accelerated scans (P<0.01). Carotid diameter measurements were in agreement with those obtained from three-dimensional TOF (intraclass correlation coefficient=0.810; P<0.001). CONCLUSION: pulsed ASL bSSFP is a fast technique for projective carotid angiography that may not require ECG gating. Acquisition time may be decreased with reduced FOV or parallel imaging strategies.     

Modeling the effects of dispersion and pulsatility of blood flow in pulsed arterial spin labeling

Arterial spin labeling (ASL) is a method of using MRI to image cerebral perfusion. For the measurement to be calibrated, a model is required describing the kinetics of the flow of the inverted blood from the labeling region to the imaging region. It is common to assume plug-flow, but alternatives such as a Gaussian distribution of arrival times have also been suggested. In this study a physiologically based model for dispersion is developed and compared to existing models when fit to experimental data. The model is based on the assumption of parabolic flow in the major arteries, and also allows inclusion of cardiac pulsatility. It was found that fitting using the proposed model leads to higher perfusion estimates, with the difference becoming more pronounced in regions where the dispersion is greater. This suggests that current models may underestimate perfusion in these areas. However, fitting using the proposed model also leads to high uncertainties in parameter estimates due to non-orthogonality of the parameters. Effects due to pulsatility are expected to be observable, but when no cardiac-gating is used the mean curve over several cardiac cycles is predicted to closely match the curve which assumes constant flow.     

Multiphase pseudocontinuous arterial spin labeling (MP‐PCASL) for robust quantification of cerebral blood flow

Pseudocontinuous arterial spin labeling (PCASL) has been demonstrated to provide the sensitivity of the continuous arterial spin labeling method while overcoming many of the limitations of that method. Because the specification of the phases in the radiofrequency pulse train in PCASL defines the tag and control conditions of the flowing arterial blood, its tagging efficiency is sensitive to factors, such as off‐resonance fields, that induce phase mismatches between the radiofrequency pulses and the flowing spins. As a result, the quantitative estimation of cerebral blood flow with PCASL can exhibit a significant amount of error when these factors are not taken into account. In this paper, the sources of the tagging efficiency loss are characterized and a novel PCASL method that utilizes multiple phase offsets is proposed to reduce the tagging efficiency loss in PCASL. Simulations are performed to evaluate the feasibility and the performance of the proposed method. Quantitative estimates of cerebral blood flow obtained with multiple phase offset PCASL are compared to estimates obtained with conventional PCASL and pulsed arterial spin labeling. Our results show that multiple phase offset PCASL provides robust cerebral blood flow quantification while retaining much of the sensitivity advantage of PCASL. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.