Fast image acquisition can improve patient’s comfort, reduce motion artifact, and allow delineating important physiological information with high temporal resolution. Recently, blipped CAIPI for simultaneous multi-slice (SMS) EPI¹ was applied successfully to high temporal resolution fMRI and DTI acquisition time reduction. Furthermore, WAVE-CAIPI² has been demonstrated that 13 slices can be simultaneously acquired in Turbo Spin Echo imaging with small noise amplification penalty. However, simultaneous-multi-slice excitation requires high RF power, because one straightforward and typical approach is to achieve multi-slice excitation by applying linear combination of RF pulses designed for individual slices. Using this method, the transmitted energy and peak power is linearly proportional to the number of slices. To overcome this problem, blipped gradient and rectangular RF pulses were designed to excite aliasing slices³. However, long time gap between gradient blips is required to avoid excessive transmitted power., MultiPINS can reduce transmitted power (roughly 50%) using the same excitation duration by combing the typical multiband and PINS excitation method together to distribute RF power evenly over time⁴. Since we only concern the excitation pattern inside an imaging object, further reduction on the transmitted RF energy is possible if RF pulses are designed to achieve the desired slices within the imaging object without any restriction on the excitation pattern outside the imaging object. Accordingly, we present simultaneous-multi-slice ROI-optimized excitation method. Compared to MultiPINS, our approach used only 79% of the RF energy to the same result (slice thickness = 3mm, MB factor = 5, 4 bandwidth time product, excitation duration 6380 $$$\mu s$$$). This excitation method was experimentally demonstrated in spin-echo SMS EPI with blipped CAIPI acquisition.

Without considering the excitation profile outside the imaging object, the multi-slice excitation method can excite the desired slices within imaging object with the minimal RF power. Given predetermined slice gradient waveform G(t) and $$$k(t)=γ\int_0^tG(s)ds$$$, the RF waveform can be derived from an optimization problem: $$RF_{ROI} (k(t))=argmin_{RF_{opt}} \| \int_0^TRF_{opt} (k(t)) e^{2πik(t)z} -m(z)\|_2^2+\alpha‖RF_{opt} ((k(t))\|_2^2$$

, where $$$m(z)$$$ is the desired excitation slice profile, $$$\gamma$$$ is the gyromagnetic ratio and $$$\alpha$$$ is a regularization parameter. Figure 1 shows RF waveforms, gradient waveforms, and excited slice profile of typical MB excitation, PINS, MultiPINS, and ROI-optimized methods. RF powers were calculated using the following excitation parameters: slice thickness = 3 mm, slice separation = 30 mm, FOV = 150mm, MB factor = 5 , BWTP = 4, gap between slice gradient blips = 20, 40, 60, 80 us, and slice gradient slew rate limit = 180 T/m/s. The duration of excitation using PINS and MultiPINS methods can be calculated based on parameters listed above. The same duration was used for typical MB excitation with constant slice gradient strength. The same gradient waveform designed by PINS and MultiPINS methods were used in ROI-optimized method. To compare RF power, the RF power in typical MB excitation with 8720 $$$\mu s$$$ duration was set to 1.

Experiments on a 3T scanner (Siemens) used spin-echo EPI with blipped CAIPI acquisition.to acquire images covering the whole brain with the parameters listed above, except FOV (slice direction) = 150 mm, FOV (imaging plane) = 192 mm, duration between two slice gradient blips = 20 ms,, FOV shift between slices =1/3, TE = 60 ms, and TR = 800 ms. Both 90 degree pulse and 180 degree refocusing pulse were further optimized by the optimal control method. Fat suppression was applied..

Table 2 shows the RF power of the PINS method (2.2) was larger than that of the typical MB method (1.4) when the time gap between two gradient blips was short (20 $$$\mu s$$$).The ROI optimized method used 79%, 82%, 84%, and 86% of the RF power required by MultiPINS method at 20, 40, 60, 80 $$$\mu s$$$ excitation gaps. Figure 2 shows simultaneously excited and acquired 5 brain images of spin-echo EPI with ROI-optimized excitation method and blipped CAIPI acquisition.Both ROI-optimized method and MultiPINS method can reduce RF power even the time gap between gradient blips is short. Unlike PINS only delivering RF pulse during gaps between gradient blips, ROI-optimized and MultiPINS methods distribute RF power over the whole excitation duration. Furthermore, ROI-optimized method can further reduce RF power by designing RF pulses with a higher degree of freedom, because only region within the imaging object was considered in the pulse design. The RF power difference between these methods gradually decreased as the excitation duration increases, because blip gaps were wider to allow RF pulses between gaps.