Real‐time functional MRI using pseudo‐continuous arterial spin labeling

The first implementation of real‐time acquisition and analysis of arterial spin labeling‐based functional magnetic resonance imaging time series is presented in this article. The implementation uses a pseudo‐continuous labeling scheme followed by a spiral k‐space acquisition trajectory. Real‐time reconstruction of the images, preprocessing, and regression analysis of the functional magnetic resonance imaging data were implemented on a laptop computer interfaced with the MRI scanner. The method allows the user to track the current raw data, subtraction images, and the cumulative t‐statistic map overlaid on a cumulative subtraction image. The user is also able to track the time course of individual time courses and interactively selects a region of interest as a nuisance covariate. The pulse sequence allows the user to adjust acquisition and labeling parameters while observing their effect on the image within two successive pulse repetition times. This method is demonstrated by two functional imaging experiments: a simultaneous finger‐tapping and visual stimulation paradigm, and a bimanual finger‐tapping task. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Cerebral blood flow MRI in mice using the cardiac‐spin‐labeling technique

Continuous arterial spin labeling MRI with a separate neck labeling coil provides a highly sensitive method to image cerebral blood flow (CBF). In mice, however, this has not been possible because the proximity of the neck coil to the brain uses the neck coil to significantly saturate the brain signal. To overcome this limitation the cardiac spin labeling (CSL) technique is introduced in which the labeling coil is placed at the heart position. To demonstrate its utility, CSL CBF was applied to image quantitative basal CBF and hypercapnia‐induced CBF changes. This approach provides a practical means to image CBF with high sensitivity in small animals, compares favorably to existing mouse CBF imaging techniques, and could broaden CBF applications in mice where many brain disease and transgenic models are widely available. Magn Reson Med 60:744–748, 2008. © 2008 Wiley‐Liss, Inc.     

Pseudo‐continuous transfer insensitive labeling technique

Transfer insensitive labeling technique (TILT) was previously applied to acquire multislice cerebral blood flow maps as a pulsed arterial spin labeling (PASL) method. The magnetization transfer effect with TILT is well controlled by using concatenated radiofrequency pulses. However, use of TILT has been limited by several challenges, including slice profile errors, sensitivity to arterial transit time and intrinsic low signal‐to‐noise ratio (SNR). In this work, we propose to address these challenges by making the original TILT method into a novel pseudo‐continuous arterial spin labeling approach, named pseudo‐continuous transfer insensitive labeling technique (pTILT). pTILT improves perfusion acquisitions by (i) realizing pseudo‐continuous tagging with nonadiabatic pulses, (ii) being sensitive to slow flows in addition to fast flows, and (iii) providing flexible labeling geometries. Perfusion maps during both resting state and functional tasks are successfully demonstrated in healthy volunteers with pTILT. A comparison with typical SNR values from other perfusion techniques shows that although pTILT provides less SNR than inversion‐based pseudo‐continuous ASL techniques, the modified sequence provides similar SNR to inversion‐based PASL techniques. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Improved pseudo‐continuous arterial spin labeling for mapping brain perfusion

Purpose:

To investigate arterial spin labeling (ASL) methods for improved brain perfusion mapping. Previously, pseudo‐continuous ASL (pCASL) was developed to overcome limitations inherent with conventional continuous ASL (CASL), but the control scan (null pulse) in the original method for pCASL perturbs the equilibrium magnetization, diminishing the ASL signal. Here, a new modification of pCASL, termed mpCASL is reported, in which the perturbation caused by the null pulse is reduced and perfusion mapping improved.

Materials and Methods:

improvements with mpCASL are demonstrated using numerical simulations and experiments. ASL signal intensity as well as contrast and reproducibility of in vivo brain perfusion images were measured in four volunteers who had MRI scans at 4 Tesla and the data compared across the labeling methods.

Results:

Perfusion maps with mpCASL showed, on average, higher ASL signal intensity and higher image contrast than those from CASL or pCASL. Furthermore, mpCASL yielded better reproducibility in repeat scans than the other methods.

Conclusion:

The experimental results are consistent with the hypothesis that the new null pulse of mpCASL leads to improved brain perfusion images. J. Magn. Reson. Imaging 2010;31:1419–1427. © 2010 Wiley‐Liss, Inc.     

Recommended implementation of arterial spin‐labeled perfusion MRI for clinical applications: A consensus of the ISMRM perfusion study group and the European consortium for ASL in dementia

This review provides a summary statement of recommended implementations of arterial spin labeling (ASL) for clinical applications. It is a consensus of the ISMRM Perfusion Study Group and the European ASL in Dementia consortium, both of whom met to reach this consensus in October 2012 in Amsterdam. Although ASL continues to undergo rapid technical development, we believe that current ASL methods are robust and ready to provide useful clinical information, and that a consensus statement on recommended implementations will help the clinical community to adopt a standardized approach. In this review, we describe the major considerations and trade‐offs in implementing an ASL protocol and provide specific recommendations for a standard approach. Our conclusion is that as an optimal default implementation, we recommend pseudo‐continuous labeling, background suppression, a segmented three‐dimensional readout without vascular crushing gradients, and calculation and presentation of both label/control difference images and cerebral blood flow in absolute units using a simplified model. Magn Reson Med 73:102–116, 2015. © 2014 Wiley Periodicals, Inc.     

Pseudo‐continuous arterial spin labeling at 7 T for human brain: Estimation and correction for off‐resonance effects using a Prescan

Pseudo‐continuous arterial spin labeling (ASL) can provide best signal‐to‐noise ratio efficiency with a sufficiently long tag at high fields such as 7 T, but it is very sensitive to off‐resonance fields at the tagging location. Here, a robust Prescan procedure is demonstrated to estimate the pseudo‐continuous ASL radiofrequency phase and gradients parameters required to compensate the off‐resonance effects at each vessel location. The Prescan is completed in 1–2 min and is based on acquisition of label/control pair‐wise ASL data as a function of the radiofrequency phase increment applied to the pseudo‐continuous ASL train. It is shown that this approach can be used to acquire high quality whole‐brain pseudo‐continuous ASL perfusion data of the human brain at 7 T. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.