Original contributionThree-dimensional arterial spin labeling imaging with a DANTE preparation pulse
Introduction
Arterial spin-labeled (ASL) [1,2] is a non-invasive magnetic resonance imaging (MRI) technique that can be used to obtain cerebral blood flow (CBF) images without contrast agent administration. The continuous ASL (CASL) method uses a spatially selective radiofrequency pulse (RF) and a gradient pulse to invert the magnetization of inflowing arterial blood [3]. This labeled blood reaches the microcirculation vasculature in brain tissues after a period of time known as arterial transit time (ATT). The signal intensity of labeled images is composed from both blood and tissue signal, and their proportions can be calculated from a two-compartment exchange model [4]. When the time period between labeling and imaging, known as the post labeling delay (PLD), is shorter than the ATT [5], bright intravascular signal artifacts caused by labeled arterial spins will appear on the ASL image. To reduce these artifacts the PLD should be set to be longer than the longest expected ATT value for each individual subject [6].
In cases of cerebrovascular disease ATT can be longer than in normal subjects. Detecting the presence of delayed spins due to collateral or slow flow with stenotic proximal vessels may be clinically useful [7], but quantitative CBF values distal to these regions will be incorrect [6]. Vascular suppression (VS) techniques with large bipolar crusher gradients have been proposed to reduce the vasculature signal [5,8], but these lead to T2 contrast increase and a reduction of SNR [6]. Some of these methods, such as Motion-Sensitized Driven-Equilibrium (MSDE), introduce non-uniformities on the signal intensity distribution by eddy currents and B1+ inhomogeneity on ASL images [9,10]. Recent work has shown that the Delays Alternating with Nutation for Tailored Excitation (DANTE) preparation pulse has flow signal suppression effects [11], whilst preserving uniform signal intensity distributions.
The purpose of this study was to investigate the feasibility of using DANTE preparation pulses for ASL VS by comparing it to MSDE. First, signal suppression effects were compared using a flow phantom, and then the homogeneity of the perfusion signal was assessed from in vivo images.
Section snippets
Materials and methods
This study consisted of signal simulations, phantom scans, and in vivo scans of healthy volunteers. All scans were performed on a 3.0 Tesla Discovery750 MRI system (GE Healthcare, Waukesha, WI, USA), with a 32-channel receive-only head coil system. Three-dimensional (3D) ASL was performed with VS by either DANTE or MSDE preparation pulses, see Fig. 1.
Phantom study
The normalized signal attenuation values for static and moving water by both MSDE and DANTE ASL can be seen in Fig. 4. The signal decay patterns of moving water were similar for both methods. For static water, a gradual signal decrease was observed as FA and b-value increased with DANTE and MSDE ASL respectively. However, rapid signal reduction was observed around b = 7.5 s/mm2 on MSDE ASL. For moving water, the normalized DANTE ASL signal with 5 degrees FA was similar to MSDE ASL with a of
Discussion
The proposed DANTE ASL acquisition was successfully performed for both phantom and human imaging.
On the phantom study, the static signal of MSDE ASL reduced rapidly around b = 7.5 s/mm2 (Fig. 4). This seems to agree with previous studies that have reported that eddy currents and local B1 inhomogeneity will cause signal loss [10]. Although the flow signal using DANTE ASL was lower than the theoretical value, but using a correction factor of k = 1.51, the trend of signal changes as a function of
Conclusion
The flow signal suppression effect of DANTE ASL method was confirmed by the phantom experiment. Preliminary in vivo examination showed that the DANTE ASL technique had more homogenous vascular suppression effects than MSDE. For vascular signal suppression in ASL, DANTE may be an alternative technique to MSDE.
Acknowledgments
We would like to express our appreciation to the volunteers and to our colleagues. This study is supported in part by Grants-in-Aid for Scientific Research (C) (15K09916) from the Japan Society for the Promotion of Science (H.K.)
Conflict of interest
Hiroyuki Kabasawa is an employee of GE Healthcare Japan Corporation.
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