O52. Magnetic field inhomogeneity in the MRI scanner and image quality dependence

Abstract

Introduction Different elements in the human body can be used as the source of magnetism. However, due to the abundance of hydrogen (1H) in the human body, 1H is the most commonly used element in clinical MRI. 1H nuclei in the presence of a magnetic field, B0, precess at the Larmor frequency ( ω 0 ) which is field dependent (127 MHz at 3 Tesla). To maintain a constant ω 0 for all nuclei, the B0 field must be homogeneous. Functional MRI (fMRI) generates contrast in the images based on tiny changes in the magnetic properties of 1H in haemoglobin. An fMRI acquisition requires repeatedly imaging the brain more than 150 times. Each measurement takes ∼ 2 seconds. Due to the prolonged fMRI acquisition, several factors distort the B0 field ( Δ B0) including subject motion. This results in corrupted fMRI data. In this work, we present a novel technique to measure and perform correction (shimming) in real time for Δ B0 in the presence and absence of motion during an fMRI scan. Materials and methods A navigation technique was inserted after each fMRI measurement to create a B0 field map. From the map, Δ B0 could be evaluated and corrected in real time. Three healthy subjects were scanned with the navigated fMRI sequence. In some scans subjects were instructed to move at specific times. Results The drift in the B0 field during the acquisitions exceeded 30 Hz even in the absence of head motion. Δ B0 resulted in noisy brain activation data. The navigated fMRI was able to accurately correct for Δ B0 and motion. Conclusion MRI scanners are not equipped with tools to alert us to changes in the B0 field during fMRI acquisition. The navigated fMRI sequence is able to correct in real time for Δ B0 and motion.