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Zusammenfassung:
Purpose/Introduction: Adiabatic spin-lock (SL) was shown to
enable dynamic glucose enhanced (DGE) imaging at 7T [1–3]. While the proposed pulses are robust against B1 inhomogeneity the mismatch between adiabatic and locking pulses (Fig. 1a) is in principle prone to B0 artifacts. To robustly translate the adiabatic SL to clinical field strength of 3T we want to improve the stability in the presence of B0 inhomogeneity by matching the RF amplitude of tipping and locking pulse. To achieve this a lower (and shorter) adiabatic tilting pulse is proposed and tested in vivo.
Subjects and Methods: Simulation. In addition to the commonly used hyperbolic secant (HS) adiabatic pulse [4], we introduced a Blackman windowed HS amplitude modulation
in combination with an exponential frequency modulation
(HSExp pulse), defined by the maximum amplitude B1, the bandwidth Df, the exponential parameter l, the length of the Blackman window twindow and the pulse duration tadia. Using a compiled Bloch simulation as a mex file in MATLAB, trajectory computation of the adiabatic pulse was performed for more than 380000 different parameter settings: 1 B l B 80; 5 lT B B1 B 15 lT; 500 Hz B Df B 5000 Hz; 0 ms B twindow B 3 ms; 1 ms B tadia B 10 ms. Imaging Imaging of three healthy volunteers was performed on a 3 T MRI scanner (PRISMA, Siemens), using the optimized HSExp pulse, as
well as the HS pulses [1]. On resonant SL-prepared imaging consists of an adiabatic SL pulse cluster (Fig. 1a) and a subsequent single-shot centric-spiral reordered 3D GRE readout (680 k-space lines, FA = 6, TE= 1.95 ms, TR = 3.95 ms, Grappa = 2, elliptical scanning, BW = 700 Hz/px, resolution 1.6 9 1.6 9 3 mm3, FOV = 17 9 21 9 4.8 cm3).
Results: The optimized HSExp pulse (l = 51; B1 = 7 lT;
Df = 4000 Hz; twindow = 2.8 ms; tadia = 5 ms) found by Bloch
simulations has a much smaller discontinuity of 2 lT than the HS pulse with 15 lT (Fig. 1a). The simulated magnetization trajectory reveals good performance of the adiabatic SL independent of typical B0 deviations (Fig. 1b). In vivo, also flat T1q -weighted images could be measured with significantly reduced duration and amplitude of the
tip pulses (Figs. 2, 3).
