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Abstract:
Introduction: Outer volume suppression (OVS) techniques were developed to overcome the geometrical limitations of PRESS MRSI. Conventional OVS pulses based on amplitude modulation are not particularly selective. In 2000, very selective suppression (VSS) pulses developed by Le Roux [1] were successfully applied in brain and prostate MRSI [2]. VSS pulses are quadratic-phase pulses designed based on a weighted least square FIR filter. A novel straight forward design method of quadratic-phase pulses based on the complex Remez algorithm was proposed by Schulte [3]. The resulting pulses have a very sharp excitation profile and a equi-ripple error function in the pass and stop band. In this work, the successful use of equi-ripple designed quadratic-phase saturation (QPS) pulses for OVS in PRESS and slice selective MRSI is demonstrated. The problems of progressing T1 relaxation during the application of multiple OVS bands as well as of significantly different T1
relaxation times are solved by flip angle adjustment.
Methods: All MRI and MRSI experiments were performed on a
Philips Achieva 3T scanner. The novel quadratic-phase pulses were successfully implemented and applied prior to excitation. Suppression bands can be repeated up to four times. Pulse duration and flip angle of each suppression band in each cycle are separately adjustable.
Results and Discussion: Equi-ripple QPS pulses have a very sharp excitation profile (Figure 1). Due to the high bandwidth chemical shift displacement is significantly reduced. Excellent OVS in combination with PRESS CSI can be achieved using QPS pulses. Repetition of the QPS bands leads to improved suppression. The optimal flip angle depends on the T1 relaxation time and increases with decreasing T1. Therefore best simultaneous suppression of
metabolites with different T1 relaxation times, like fat and water, can be achieved combining 2 to 4 cycles of QPS suppression bands with flip angles adjusted alternating to fat and water. Gradually decreasing suppression in multiple suppression bands due to progressing T1 relaxation can be avoided by gradual flip angle adjustment. Therefore, also slice selective CSI with up to 10 suppression bands becomes feasible (Figure 2), which is preferable in comparison to PRESS CSI to avoid chemical shift displacement.
