## 3 Results and Discussion
### Imaging of the Neck  
The simultaneous application of US imaging and MRI to the neck was performed to determine the feasibility of the proposed method of compensation. Examples of T1-weighted images that were obtained with the simultaneous imaging system are shown in Figure 8. The MR-visible fiducial markers that were attached to the ultrasound probe are indicated by the arrows in Figure 8. The cross section of an MR image corresponding to the cross section of an ultrasound image is shown in Figure 9. First, the sound velocity in the acoustic gel pad was estimated as 1360 m/s by simultaneous US imaging and MRI. The difference between the estimated value and the reference value was 50 m/s. Beamforming was corrected using an estimated sound velocity of 1360 m/s for the pad and a sound velocity of 1540 m/s for biological tissues in the neck region. The resulting image was compared to a conventional B-scan image as shown in Figure 10. Image degradation due to variations in the sound velocity was reduced by compensating for the sound velocity, as shown in Figure 10a. Without compensation, the conventional B-scan image was blurred as a result of fluctuations in the sound velocity, as shown in Figure 10b. To determine the improvement in the spatial resolution, the half-width of the autocorrelation function in the horizontal direction was calculated for the region of the red square shown in Figure 10. The ratio of the half-width of the autocorrelation in the conventional image with that in the compensated image was calculated to be 0.60. The improvement in the SNR was 3 dB. The results indicated that simultaneous US imaging and MRI is a feasible way to reduce image degradation due to layers of subcutaneous fat by compensating for the sound velocity to correct beamforming.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**ROIs**} & \multicolumn{2}{c}{**The Half-Width of the**} & \multicolumn{2}{c}{**The Half-Width of the**} & \multirow{2}{*}{**Ratio of the Half-Width**} \\  & **Autocorrelation Function for** & & **Autocorrelation Function for** \\  & **a Compensated Image** & & **a Conventional Image** & \\ \hline Left square & 1.35 mm & 2.55 mm & 0.53 \\ Center square & 0.85 mm & 2.60 mm & 0.30 \\ Right square & 1.33 mm & 3.02 mm & 0.44 \\ Average & 1.18 mm & 2.72 mm & 0.43 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Comparison of the half-widths of the autocorrelation functions in the lateral direction.  
Figure 7: Comparison of B-mode images of the phantom obtained using the proposed method of compensation (**a**) and a conventional approach (**b**). The three red squares indicate ROIs for calculation of the autocorrelation function for a B-mode image.  
Figure 8: Examples of MR images obtained using the simultaneous multimodality imaging system. Each image corresponds to a cross section, and rows of fiducial marker arrays are indicated by arrows. The probe was equipped with two rows of marker arrays, as shown in Figure 1.  
Figure 10: B-mode images of the neck obtained using the proposed method of compensation (**a**) and a conventional approach (**b**). A red square indicates a ROI in part of the thyroid for the autocorrelation function in each B-mode image.  
Figure 9: The estimated cross section of the MR image corresponding to the cross section of the ultrasound image according to coordinates of MR-visible fiducial markers.