Magnetic resonance imaging of ultrasound fields: Gradient characteristics

Phase-contrast magnetic resonance imaging (MRI) is used to image particle d isplacements arising from a 0.515-MHZ focused ultrasound (US) field. The te chnique used a phase-locked, self-resonant gradient matched to the US frequ ency in conjunction with a spin-echo sequence to generate phase images of U S-induced displacement parallel to the US propagation direction. The gradie nt design was numerically optimized to provide maximum linearity and magnit ude while minimizing gradient inductance, The windings were fabricated of L itz wire to minimize resistive losses and mounted in an oil-cooled imaging chamber. When driven by a resonance power supply, a peak magnetic held grad ient of 0.40 T/m was attained with a peak current of 20 amp in a volume of 53 cm(3). achieving stable oscillation at the required US frequency. Clear detection of the nanometer scale particle motions of the US field was achie ved and allowed quantitative, noninvasive visualization of the entire US he ld. While the required gradient slew rate for US detection is beyond that r ecommended for in vivo application, this imaging method opens new possibili ties for in vitro or ex vivo research in the study of the interaction of US with tissue. (C) 2000 Wiley-Liss, Inc.