Simultaneous Determination of the Macroscopic and Microscopic Static Magnetic Field Inhomogeneity (MAGSUS)

Abstract

A simple nuclear magnetic resonance imaging method for simultaneous mapping of the macroscopic static field B and the microscopic Larmor frequency distribution Ψ within the picture elements (pixels) is presented using multiecho acquisition. The sequence (45°)-τ 1, 1 -(−45°)-τ D-(180°)-τ D + τ 1, 1 /2-echo 1-τ-echo 2-τ-echo 3 . is based on the 1- 1 hard-pulse excitation which provides a transverse magnetization with narrow minima as a function of offset from the transmitter frequency. This excitation results in images with dark lines of constant Larmor frequency for B field mapping. The slice-selective spin-echo rephasing mode with additional gradient echoes allows one to take advantage of the phases of the transverse magnetization components with different Larmor frequencies. The interference of the magnetization of the spin ensembles within the pixels depends strongly on the time interval between the slice-selective 180° pulse and the data acquisition. For "echo 1," high contrast C max/min = S max/ S min > 5 between the signals of pixels at excitation maximum ( S max) and excitation minimum ( S min) is obtained even in cases with broad Larmor frequency distribution in each pixel. Images from "echo 2" and subsequent echoes show clearly reduced contrast C max/min < 2 of the line pattern in regions of pronounced microscopic inhomogeneity. For example, trabecular bone marrow shows a broadened width of the field distribution with a range of more than 20 Hz within each picture element. The contrast C max/min in the second or later echo images is not clearly reduced in muscle tissue or subcutaneous fat with better microscopic field homogeneity. The sequence is used to find volumes of interest with good macroscopic and microscopic homogeneity for localized spectroscopy in vivo and to determine the Larmor frequency distribution within bone marrow for correlations with the trabecular bone density. The MAGSUS method does not require further data processing.