Reduction of B1 Inhomogeneity Using B1 Rectifying Fin at High Fields

Introduction B1 inhomogeneity in a human body increases as the strength of static magnetic field increases and the RF wavelength becomes smaller. Recently, various RF control methods have been developed to reduce B1 inhomogeneity. For example, some methods involve devices such as dielectric pads [1,2] or coupling coils [3,4], or techniques such as B1 shimming [5,6]. However, B1 inhomogeneity still remains in some cases of abdominal imaging, and a more effective method for reducing B1 inhomogeneity is required. In this paper, we have proposed a new method to reduce B1 inhomogeneity using a “B1 rectifying fin” combined with B1 shimming. This method can reduce B1 inhomogeneity less than 2-port B1 shimming used in some commercial MRI, and moreover, the fin has a simple structure with no capacitor or inductor. The effect of the fin on B1 inhomogeneity was analyzed using both finite-difference time-domain (FDTD) simulation and experiments. Method Design: The fin consists of a thin sheet with conductive property. In an electromagnetic field, the fin can change the magnetic flux around it, that is, it can create high or low density of the flux. This is because an electrical current flows in a direction which counters the magnetic flux across the fin. The spatial distribution of the B1 field can be controlled by using this phenomenon in an appropriate manner. Figure 1 illustrates an example of the B1 rectifying fin arranged to reduce B1 inhomogeneity. The fins around the body are arranged like a windmill, taking into consideration the phenomena of RF propagation from the RF transmit coil to the abdomen, and the position of larger and smaller B1 regions in the abdomen. Specifically, the fins are positioned near the abdomen where the B1 value is larger, and the edges of the fins are positioned near the region of the smaller B1. There is no need, in principle, to use a capacitor or inductor with this method. Simulation: The effect of the B1 rectifying fin was confirmed through numerical analysis of the electromagnetic field. The spatial distribution of the B1 field in the phantom was calculated using an electromagnetic simulation tool (xFDTD). A 2-port birdcage coil was used for RF transmission, and the RF frequency was 128 MHz. The phantom size (xy plane) was 350 x 200 mm. The conductivity and relative permittivity of the phantom were 0.55 S/m and 50, respectively. Four fins were set around the phantom. In this case, the fins covered 50 % of the surface of the phantom, and the gap between the fin and the surface was 20 mm. A B1 homogeneity value (USD) was used to evaluate B1 inhomogeneity, and USD = standard deviation of B1 / average of B1. Experiment: A human abdominal imaging experiment was conducted, using a 3T MR scanner (Varian INOVA). The B1 rectifying fins were set around the abdomen, and the placement of the fins (coverage percentage and gap) was the same as that in the simulation case. Copper mesh sheets (0.1 mm thick) were used as the material for the B1 rectifying fins for this experiment, and their weight was very small (~ 0.1 kg). The mesh sheets were put on a synthetic rubber sheet (20 mm thick), and the rubber sheet was set around the lower abdomen, like a torso coil. B1 mapping was accomplished using the double-angle method in order to evaluate the effect of the fins. The sequence parameters were FOV = 450 mm, TR/TE = 5000/6.7 ms, matrix = 128 x 64, thickness = 10 mm, flip angle = 60, 120 degrees. Results and Discussion Figure 2 shows the simulation results of the spatial distribution of the B1 field in the phantom. The B1 map in case (b), with B1 shimming alone, is more homogeneous than that in case (a), with a quadrature drive (QD). The B1 map is the most homogeneous in case (c), in which both the B1 rectifying fin and B1 shimming were used. Figure 2 (d) represents the B1 homogeneity value (USD) and the average of B1. The B1 average values were normalized with the value in case (a). The values of USD for (a)(b)(c) are 0.224, 0.164, and 0.126, respectively, and USD decreases when both the B1 rectifying fin and B1 shimming are used. The average values of B1 for (a)(b)(c) are 1, 0.97, and 0.95, respectively, and the average of B1 remains static. The B1 rectifying fin doesn’t reduce the average of B1, which means that the fin has the effects of both enhancing and diminishing the magnetic flux. It was confirmed that the B1 rectifying fin can reduce B1 inhomogeneity, while maintaining the average value of B1. Figure 3 shows the experimental results of the spatial distribution of B1 in a human abdomen. The values of USD for (a)(b)(c) are 0.222, 0.168, and 0.112, and the average of B1 for (a)(b)(c) are 1, 1.00, and 0.99, respectively. The B1 rectifying fin can contribute to reducing B1 inhomogeneity as shown in the simulation results. It is suggested that the B1 rectifying fin can be useful for reducing B1 inhomogeneity at higher magnetic field, more than 3 T. Conclusion We have proposed a new method using a B1 rectifying fin combined with B1 shimming. Both FDTD simulation and experiments were conducted, and we confirmed that the B1 rectifying fin, used with B1 shimming, was more effective in reducing B1 inhomogeneity than B1 shimming alone. Reference [1] Schmitt M et al. ISMRM 2004; 11: 197. [2] Kendra MF et al. J Magn Reson Imaging 2008; 27: 1443-1447. [3] Schmitt M et al. ISMRM 2005; 13: 331. [4] Wang S et al. ISMRM 2007; 15: 3275. [5] Nistler J et al. ISMRM 2007; 15: 1063. [6] Hajnal JV et al. ISMRM 2008; 16: 496.