Optimal Variable Refocus Flip Angle Control Method and Echo Train Length for Suppressing Exposure to Radio Frequency

The popularization of 3-Tesla magnetic resonance imaging (MRI) has improved the quality of images and shortened typical examination times. However, a side effect of this is increased exposure to radio frequency (RF) radiation. The amount of RF exposure can be controlled using a technique called variable refocus flip angle (vRFA). Controlling vRFA is also an important for improving the signal to noise ratio (SNR) and for reducing blurring on MRI images. In this study, we examined the influence of controlling vRFA and echo train length (ETL) on SNR. To do this, we used a device that can arbitrarily control three angles—the fifth RFA, the RFA centered in k-space, and the final RFA. Using a phantom, T1 and T2 values were made equal to gray- and white-matter, respectively. The repetition time was 5000 ms and echo time 90 ms. By setting the fifth RFA to 40° and using an ETL of 11–15, the signal shifted smoothly to a pseudo steady-state (PSS), and a stable signal was obtained. Further, we were able to suppress blurring by gently changing the k-space-centered RFA. In the final echo, we were able to maintain PSS by increasing the final RFA up to 180°, resulting in a high SNR. Results of this study showed the changes reduced RF exposure. Using an ETL of 30, blurring was reduced, though RFA control was similar to that used with ETLs of 11–15; although slightly higher RF exposure was required to obtain a high SNR, the fifth RFA was required to be 60°–90°.