Absolute measurements of ultrasonic pressure by using high magnetic fields

A hydrophone is introduced that exploits the emf signal generated in a conductor when sonicated in the presence of a uniform static magnetic field. The method uses a small metal coil or metal membrane as a hydrophone receiver. Acoustic signals at 748 kHz are introduced in 1.5 T and 4.7 T fields and recorded both through direct electrical contact with the hydrophone and via RF pick-up coils, allowing wireless placement of the hydrophone. Linear response Is confirmed over four orders of magnitude in the pressure amplitude. Waveforms determined from the detected voltage are shown to be in excellent agreement with those obtained using a calibrated polyvinylidene difluoride film, and absolute values correlate within 20%. The methods are conceptually suitable for use in the presence of the high and uniform field of commercial MR scanners. The hydrophone may appear particularly useful as a quality assurance device in therapeutic and diagnostic acoustic techniques that use MRI.