Quantitative magnetic resonance imaging of ultrasound fields

A Magnetic Resonance Imaging (MRI) technique is proposed that can be used to directly image an ultrasound (US) field in tissue in a non-invasive manner. Through the use of an oscillating magnetic field gradient, spin phase is modulated in proportion to the displacement amplitude arising from the ultrasound field. Based on MR measurements of spin displacement, pressure and intensity can also be obtained. A 5 cm diameter, focused US transducer (515 kHz) was used to direct an US field into a small MR imaging cavity surrounded by a resonant field gradient coil (0.4 Tesla/meter) while imaging with a spin-echo sequence. This allowed the clear visualization of US induced motions of less than 10 nm. Based on MR measures of these motions, measurements of US pressure, intensity and speed-of-sound can also be obtained in tissue.