(A) study of 3-D volume localization technique and its application to in-vivo spectroscopy

Abstract

In recent years, several volume selection methods have been used for $^1H$ and $^{31}P$ MR spectroscopy in vivo. Most volume selection methods, however, suffer from $T_2$ decay in the volume of interest (VOI) due to the relatively long selection time. In this paper, three 3-dimensional localized volume selection techniques, the spiral scanning, the radial scanning, and water suppressed volume selection with radial gradient coil, are proposed using radial gradient field for in vivo NMR spectroscopy. In spiral and radial scanning techniques, volume selection is achieved by using a single rf pulse in each sequence thereby minimizes $T_2$ decay during the selection time. These techniques utilizes an additional radial gradient coil and two-dimensional interlaced scanning in the k-space. For water suppressed spectroscopy, spin echo and CHESS sequence are combined to the volume selection scheme using radial gradient coil. Depending on the nuclei of interest, proper trajectory in 2-D k-space can be selected. In $^1H$ spectroscopy, spiral scanning was adopted to minimize the number of interlacings, while in the 31P spectroscopy that has wide spectral range and very short T2 components radial scanning was adopted to improve the volume selectivity with the given gradient strengths. The proposed methods are derived theoretically and computer simulation was performed to prove them. The techniques were implemented with the radial gradient coil of 40cm diameter in KAIS 2.0T MRI system. Experiments are performed with phantoms as well as human volunteers and promising results were obtained in both volume localized $^1H$ and $^{31}P$ spectroscopies in vivo. The proposed methods appear simple and accurae in localizing a volume which would be used as localized spectroscopy.