In this study, we provided the experimental results of DT-MREIT to evaluate the electromagnetic field distribution of muscle when applying the therapeutic currents into the biological tissue.

Imaging experiments were performed using chunks of bovine muscle. Two silver wire electrodes were positioned 3 cm apart inside the muscle as shown in Fig 1. Using the 3T MRI scanner (Achieva TX, Philips, Netherlands) and single-shot spin-echo EPI (SS-SE-EPI) sequence, we collected DTI data with b-values of 500 sec/mm² and TR/TE = 4000/72 msec. One reference MR data was also obtained without diffusion sensitized gradient to measure the diffusion tensor. Using a voltage stimulator (BSLSTMB, BIOPAC Systems, Inc., USA), electric pulses were applied through the electrode pair with 100 V amplitude and 100 μs width for MREIT data collection. We used the multi-gradient echo pulse sequence to obtain the induced magnetic flux density data called Bz data. The imaging parameters were as follows: TR/TE = 200/1.6 msec, ES = 2.3 msec, NE = 32, FOV = 200*200 mm², imaging matrix = 128*128, imaging time = 51.2 sec (Fig. 2).

For the current density estimation, we adopt the projected current density method¹ which approximately recovers the best three-dimensional current density using the measured data of B_z. Based on the model about the relation between the water diffusion tensor and the conductivity tensor, we set the conductivity tensor as a scalar multiple of the diffusion tensor. Since diffusion tensor information is available from the acquired DTI images, we computed the position-dependent scale factor using the measured B_z data. Then, the conductivity tensor is estimated as the multiplication of the water diffusion tensor and the scale factor. Once we obtained both the current density as a 3*1 vector and the conductivity tensor as a 3*3 matrix, we computed the electric field by multiplying the inverse of the conductivity tensor to the current density vector.

Figure 3(a) and (b) show the acquired MR magnitude and B_z images of the bovine muscle, respectively. Figure 3(c) plots the computed projected current density image when the therapeutic currents are injected. The current density image indicates the internal current flows which exist not only the electrodes but also surrounding regions. Figure 4(a) is the recovered electric filed map using our anisotropic conductivity tensor with the projected current density whereas (b) shows the electric field map produced by using the method of Kranjc et al.² Comparing two methods, projected current density method shows enhanced signals in both the electrodes and surrounding tissues by considering the anisotropy of muscle.Electric field map obtained by using the DT-MREIT method is significantly different from the one obtained by using the previous simpler method. Since the accurate electric field mapping is important to correctly estimate the coverage of the electrical treatment, future studies should include more rigorous validations of the new method through in vivo and in situ experiments.