Absolute measurements of ultrasonic pressure by using high magneticfields

摘要

A hydrophone is introduced that exploits the emf signal generatednin a conductor when sonicated in the presence of a uniform staticnmagnetic field. The method uses a small metal coil or metal membrane asna hydrophone receiver. Acoustic signals at 748 kHz are introduced in 1.5nT and 4.7 T fields and recorded both through direct electrical contactnwith the hydrophone and via RF pick-up coils, allowing wirelessnplacement of the hydrophone. Linear response Is confirmed over fournorders of magnitude in the pressure amplitude. Waveforms determined fromnthe detected voltage are shown to be in excellent agreement with thosenobtained using a calibrated polyvinylidene difluoride film, and absolutenvalues correlate within 20%. The methods are conceptually suitable fornuse in the presence of the high and uniform field of commercial MRnscanners. The hydrophone may appear particularly useful as a qualitynassurance device in therapeutic and diagnostic acoustic techniques thatnuse MRI