SOUND GENERATION IN GRADIENT COIL STRUCTURES FOR MRI

When supporting plates of plastic material are subjected to alternatin g transverse Lorentz forces while in a strong magnetic field normal to the plate surface, compressional waves within the solid produce a mod ulation of the plate surface that launches an acoustic wave in air alo ng the magnetic field axis. We have extended our previous theory descr ibing this process to include a detailed description of the formation of an acoustic interference pattern in air described by Fraunhofer dif fraction at a distance from the plate surface. The extended theory pre dicts that the observed acoustic signal midpoint and normal to the pla te surface gives a variation with frequency in approximate agreement w ith our previous measurements. The acoustic output off axis shows acou stic blazing that produces two main diffraction peaks with a splitting inversely proportional to the velocity of sound in the plate material . The new results could have important ramifications for the minimizat ion of sound output in gradient coil design for MRI. A new arrangement of coils is proposed to ameliorate the acoustic output problem centra lly and normal to the plate by extending the frequency response of the supporting plates to much higher frequencies. Also presented are esti mates of the compressional wave velocities deduced from frequency resp onse data recorded at the center-point of a number of different plates .