As the strength of a static magnetic field increases, the B₁⁺ inhomogeneity in a human body increases. Various kinds of multi-channel RF transmission techniques for reducing B₁⁺ inhomogeneity have recently been developed. One of these methods, RF shimming^1,2, is currently used in commercial MRI systems. The effect of the number of RF transmit channels (ch) has been investigated numerically and experimentally^3-6. However, the effect for lumbar spine (L-spine) imaging, especially for the partial region in a sagittal plane, has not yet been investigated. In this study, the effect of the number of RF transmit channels between 2ch and 4ch regional RF shimming for L-spine imaging was investigated, using numerical simulation. It was shown that 4ch RF shimming can contribute to improving B₁⁺ homogeneity and reducing the transmit RF power more than 2ch RF shimming.

The effect of the number of RF transmit channels was confirmed using numerical simulation of an electromagnetic field. The spatial distributions of B₁⁺ in the human body were calculated using an electromagnetic simulation tool (CST STUDIO SUITE^TM). Figure 1 shows the simulation model. A four-channel coil was used for RF transmission⁷. The inner bore size (x-y plane) was 74 x 65 cm. Hugo (height: 180 cm, weight: 90.3 kg) was used as a human model. Figure 2 shows the schematic of the setting of the Region of Interest (ROI) used for RF shimming. The whole ROI covered the entire abdominal region, and the partial ROI covered the back side where the L-spine exists. B₁⁺ homogeneity was optimized, using B₁⁺ map data in each region. The value of B₁⁺ inhomogeneity (U_SD) was defined as below:

$$U_{SD}=\frac{\sigma}{\overline{B_{1}^{+}}},$$

where $$$\sigma$$$ is the standard deviation of B₁⁺, and $$$\overline{B_{1}^{+}}$$$ is the average of B₁⁺ in a sagittal slice at x = 0 mm. The normalized value of the total RF transmit power (P_SUM) was defined as below:

$$P_{SUM}=\frac{\sum_i^N|x_{i}|^{2}}{P_{SUM_{QD}}},$$

where $$$ x_{i}$$$ represents the RF parameter, $$$i$$$ represents the channel number, $$$N$$$ represents the number of the channels (2 or 4), and $$$P_{SUM_{QD}}$$$ represents the total power in the case of quadrature (QD) drive. The RF transmission mode was QD / 2ch RF shimming / 4ch RF shimming. 2ch RF shimming was conducted by combining B₁⁺ maps of two channels (ch1 and ch3 / ch2 and ch4). 4ch RF shimming was conducted by using a B₁⁺ map of each channel. The U_SD or P_SUM was minimized in the RF shimming algorithm. The maximum local SAR was defined as the maximum value of 10g SAR in the body, and the values were calculated for the cases of RF shimming.

Figure 3 shows the U_SD and B₁⁺ maps in the case of RF shimming (minimization of U_SD). The $$$\overline{B_{1}^{+}}$$$ in each ROI was normalized to 1 μT. The U_SD in the case of 4ch RF shimming is smaller than that in the case of 2ch RF shimming. Figure 4 shows the P_SUM and B₁⁺ maps in the case of RF shimming (minimization of P_SUM, maintaining the U_SD and $$$\overline{B_{1}^{+}}$$$ in the case of QD). The P_SUM in the case of 2ch RF shimming is almost the same as that in the case of QD. On the other hand, the P_SUM in the case of 4ch RF shimming is less than that in the case of QD or 2ch RF shimming. Figure 5 shows the maximum local SAR in the case of RF shimming for the partial ROI. The maximum local SAR in the case of 4ch RF shimming is almost the same or less than those in the cases of QD and 2ch RF shimming.In the case of 4ch RF shimming in Fig. 4, the higher the reduction ratio of the P_SUM is, the narrower the setting of the ROI is. The spatially asymmetric B₁⁺ field can be created by using a four-channel transmit coil in Fig 4 (d). This is considered to contribute to the reduction of transmit RF power. In Fig. 5, it is indicated that the regional RF shimming method shown in this study can reduce U_SD and P_SUM, not exceeding the value of the maximum local SAR in QD. The effect of the number of RF transmit channels for RF shimming for the partial region in L-spine imaging has been investigated using numerical simulation. It is shown that 4ch RF shimming can contribute to reducing the B₁⁺ inhomogeneity and the transmit RF power more than 2ch RF shimming.