3-Directional Fast Acceleration Encoding

Introduction: For the assessment of myocardial motion or blood flow, direct measurements of regional acceleration might provide valuable information regarding flow changes or tissue function and structure. However, there is only one report of the implementation of a direct acceleration encoding phase contrast (PC) technique based on an acceleration encoded scan and an acceleration compensated scan [1]. In this study we report, for the first time, a gradient optimized acceleration encoding strategy along all three spatial dimensions. A pulse sequence implementation with a minimized echo time is presented and validated in phantom studies. Moreover, initial results of three-directional left ventricular acceleration encoding in a volunteer are shown. As a reference, the obtained results were compared to acceleration data derived from standard phase contrast MRI [2]. Methods: Sequence: An acceleration encoded phase contrast sequence scheme was developed (Fig. 1). In the read and the slice direction, the read dephaser and the slice rewinder were included in the tripolar gradient waveforms used for acceleration encoding. Two-sided acceleration encoding, i.e. equal distribution of the desired 2 gradient moment M2, was used to minimize echo time [3]. For each encoding direction (slice, read, phase), 2 tripolar waveforms with +/M2/2 (up-case) and -/M2/2 (down-case) were designed. For gradient optimization, the maximum gradient amplitude hmax (23 mT/m), the ramp time r (370 μs), and the known shapes of the slice selection and readout gradients were used. For a given acceleration encoding moment M2, the up-case was calculated by solving for the flat-top times t1, t2 and t3 of the three gradients. The down-case was then calculated from r, t1, t2, t3 and the encoding moment M2 by solving for h1, h2 and h3. The read direction was a time-reversed version of the slice direction. Echo time TE was further minimized by systematically evaluating the gradient waveform durations for different distributions of M2 between the up and down waveforms. The phase encoding direction was implemented by using hmax, r and t2 = 2*t1 and the maximum phase encoding moment. Measurements: All measurements were performed on a 3 T MR-system (Trio,