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Abstract:
Purpose/Introduction: Ultra high field MRI benefits 1H MRS
applications by offering increased chemical shift dispersion, improved SNR and reduced J-coupling, but also raises technical challenges such as shortened T2 and T2* relaxation times, severe chemical shift displacement artifacts and signal disuniformity. In this work, a parallel transmission based pulse design method that combines LSQR [1] and optimal control (OC) [2] approaches is proposed to achieve spectral-spatial pulses (SPSP) that tackle aforementioned technical challenges in ultra-high field 1H MRS.
Subjects and Methods: The B1 + B0 field maps [3] were acquired with a home-built 8-channel TxRx array head coil [4] and a spherical spectroscopy phantom on a 9.4 T Magnetom SIEMENS scanner (SIEMENS Healthcare, Erlangen, Germany), as shown in Fig 1a. The desired excitation pattern is a uniform rectangle (5 cm 9 7.5 cm) in a
slice of 22 cm FOV, as shown in Fig 1b, and a uniform band along the spectral axis, with 2 kHz (or 4 ppm) bandwidth that is sufficient to cover the spectral range of interest at 9.4 Tesla (1.6 kHz). Based on a spiral trajectory, a spatially selective excitation pulse is designed first using the LSQR method [1], and is afterwards used as
an initial guess for the OC method [2] for further optimization into an SPSP pulse. Spinor-domain based Bloch simulations were implemented to produce the saturation profiles of the designed pulses.
Results: By comparison of the spectral-spatial saturation profiles of the pulses (duration 4.10 ms, maximum B1
+ amplitude 34.5 uT) designed using the OC method (Fig. 2) and the combined method (Fig. 3), it can be seen that the initial guess calculated by the LSQR method helps to direct the OC method to an optimized global minimum instead of sticking in the local minimum with bad performance.
Discussion/Conclusion: The simulation results show promising performance of the proposed method in parallel transmit SPSP pulse design. Similar excitation and saturation pulses may be used in future for FID MRSI at 9.4 T [5, 6], which could lead to a homogeneous spatial-spectral excitation profile and better lipid suppression at
reduced SAR deposition in comparison to spatially selective outer volume suppression.
