People interested in the SAR suppression using RF pulse synthesis in simultaneous multislice methods. The multislice RF pulse excitation is required in some simultaneous multi-slice (SMS) methods [1-4]. The aim of this study was to investigate the synthesis method and SAR (Specific Absorption Rate) of multislice PINS (Power Independent of Number of Slices) [4] SLR RF pulses [5] for Wideband MRI [1-3] and other SMS methods.

From the sampling theorem of the DFT (discrete Fourier transform), the DFT of a continuous function multiplied by a sampling function would generate duplications of the original spectrum [4]. And the inter-spectrum distance is inverse proportional to the sampling time [4]. This concept was applied on the synthesis method of multislice PINS [4] SLR RF pulse [5] and the synthesis method is shown in Fig. 1. The slice spectrum in frequency domain could also be displayed as the slice profile in the direction of slice selection z.

Suppose that the desired scan distance of an object to be scanned ranges from -δz to δz in the direction of slice selection. In standard single-slice RF pulse excitation, in order to offset the center of a slice by the distance δz from the gradient isocenter, the RF carrier frequency is offset by an amount f_s from the Larmor frequency.

By changing the sampling time of single slice SLR RF pulse from Δt₁ to Δt₂, single slice profile, Fig. 1(f), could become multiple slice profile, Fig. 1(g). Furthermore, if the slice selection gradient G_z, Fig. 1(d), is reduced to c₁G_z, Fig. 1(e), the number of slices within ±δz can be reduced and the slice thickness will become thicker (Fig. 1(h)).

For 3-slice (W = 3 and W is the wideband multi-slice/slab factor [1].), these equations Δt₂=4·Δt₁, c₁=(δz/2)/z_c, Δz₁=c₁·Δz were used for the synthesis of multislice PINS SLR RF pulse. Where ±z_c are the two center locations of the two outer slices, Δz₁ is the specified slice thickness and Δz is the desired slice thickness (Fig. 1). This synthesis method mentioned above was extended (with a little variation) for W = 5, 7, etc.

Also the relative values of SAR generated by the standard SLR and PINS SLR RF pulses were studied. The RF pulses were all specified with 4-msec duration and 2048 sample points. FA=30 degrees. Normalized bandwidth Δf=0.01. If δz=10.24 cm, then slice thickness Δz=1 mm. If W ≥ 2, ±z_c=±0.95δz was specified for all RF pulses in this study. A constant slice selection gradient, Fig. 1(e), was adopted for a RF pulse.

The multislice PINS and standard SLR RF pulse design, profile simulation (by Bloch equation [5]) and the SAR computation were performed by using an in-house program written in MATLAB 7.14 (The Mathworks, USA).

The 3-slice (W = 3) PINS SLR RF pulse is shown in Fig. 2(b). And its simulation profile M_xy by using Bloch equation is shown in Fig. 2(c). The relative values of SAR of the standard SLR and PINS SLR RF pulses with different number of slices W are presented in Fig. 3. It can be found that the values of SAR of PINS SLR pulses were greatly reduced compared to that of the standard SLR pulses. For example, The SAR of PINS SLR with W = 5 is only 38% of that of standard SLR with W = 5. And The SAR of PINS SLR with W = 5 is only 1.9 times of that of standard SLR with W = 1. The proposed synthesis method of multi-slice PINS SLR RF pulse has been performed successfully on synthesizing some multi-slice RF pulses for wideband MRI. SAR values of multi-slice PINS SLR RF pulses were greatly reduced compared to that of standard SLR RF pulses by factor of 2.5 to 4 in our simulation with W ranging from 3 to 5.