Underground variations in BEarth: Implications for near‐surface MRI

Abstract

AbstractMagnetic resonance imaging (MRI) has been proposed as a possible near‐surface imaging technique for locating objects buried to a depth of a few meters in fully hydrated soils. The viability of this proposed technique will depend on the choice of the static magnetic field, which could be the Earth's magnetic field (BEarth) or some conveniently applied higher value. This study addresses the practicality of using various magnitude static magnetic fields to perform near‐surface imaging by focusing on the restrictions the magnetic field places on major aspects of the MRI procedure. Measurements of the magnitude and direction of the Earth's magnetic field were made as a function of depth down to 6 m at a test site. These values were used to simulate in situ magnetic resonance images arising from the water in the hydrated soil. Images show that the greatest limitation is spatial susceptibility variations that affect the static magnetic field used by the hypothetical system. Higher applied magnetic fields alone only improve the signal‐to‐noise ratio. Systems with a low Q resonant coil are needed when using either the Earth's magnetic field or a higher applied magnetic field. These conditions produce a spatial distortion of less than 9 cm. © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 35B: 153–167, 2009