The measurement of transverse relaxation time (T2) using histogram from ROI setting in muscle activity study at 1.5 Tesla MRI

Abstract

Muscle functional MRI (mfMRI) is the method to evaluate the muscle activity before and immediately after the muscle exercise by the magnetic resonance images. Exercise of skeletal muscles enhances image contrast in T2-weighted magnetic resonance (MR) images. In previous studies, the exercised physiology used multiple-spin-echo (MSE) sequences for calculating transverse relaxation time (T2) with several small regions of interest (ROI) within the target tissue. Such a method may not represent all properties of the target tissue. The purpose of this study is to measure T2 in rest and exercise muscle at 1.5 Tesla MRI using average mode values in histogram of signal intensity of whole target muscle. A PVA-gel cylindrical phantom was scanned by a 1.5 Tesla whole body MR scanner using MSE with TR 1000, 2000, 3000, 4000 ms, TE 15, 30, ……390 ms, and right thigh of eight healthy male subjects with repetition time (TR) 2000 ms. Subjects performed knee extension exercise of the right thigh 200 times and MR images were acquired at rest and after exercises. T2 was calculated by mono-exponential linear least-squares of TE 30, 45, 60, 75 ms. Results: for the phantom study, MR signal intensity increases with increasing TR. The SI curve of TR 1000 ms is lower than other TR signals, but the relaxation curve of TR 2000 ms and more show similar MR signals. In exercise study, it has been confirmed that the method using average value in histogram is the same as the conventional method. Muscle T2 in both methods, rest and after exercise, are significantly different and an increase by combining with the result of mode values from the histogram. In addition, in the muscle activity of quadriceps muscle by knee extension exercise, involvement of other than RF which has not been reported in the past, but in this study, VL, VI, and VM were also confirmed. It could be confirmed that the effect of SNR of MR images caused by the difference in the selection of RF receiver coil as well as the ROI setting is large. In conclusion, the following findings were obtained: first, the ROI setting using histogram proposed by this study can objectively obtain the characteristics within the region of interest with reduced variation. Second, the increase of T2 after the knee extension exercise is not only involved largely in RF but other muscles also slightly involved. The influence on T2 calculated by ROI setting is related to SNR because noise of MR image is affected. Therefore, T2 analysis using histogram is useful and could be applied for muscle activity, which is not confirmed by the conventional method.