Experimental analysis of acoustic noise effect due to the gradient pulsing in functional MRI

Abstract

In MRI, gradient magnetic fields are used to resolve the spatial information by frequency modulation of the received signal. I.e., the magnetic field intensity is slightly different with respect to the spatial location, where the field difference in space domain is provided with gradient coils. The gradient fields for imaging are also time varying by switching of currents on the gradient coils, which generates acoustic noise due to Lorentzian force. The acoustic or sound noise has been one of the problems in handling patients, mainly due to the fast gradient pulsing in interaction with the main magnetic field. The larger the magnetic field strength is, the louder the acoustic noise is. The intensity of the acoustic noise depends on the imaging method and the pulse sequences. In particular, fast imaging methods, which are usually used for fMRI, require fast switching of the gradient pulse, thereby generating large acoustic noise. The acoustic noise induced by gradient pulsing may interfere with signal enhancement of brain areas during fMRI. The gradient pulsing effects on fMRI are analyzed for different combinations of gradients. There are two combinations of phase-encoding and readout gradients for sagittal-view images, such as Z phase-encoding / Y readout gradients and Y phase-encoding / Z readout gradients both with X selection gradient. The experimental results show that total activations by visual stimulation are slightly larger for a combination of Z readout and Y phase-encoding gradients than those for a combination of Y readout and Z phase-encoding gradients when sagittal-view fMRI is performed. Also, we analyzed the onset time constants of hemodynamic response in the visual and motor cortices, which were affected by the activation of the auditory cortex due to an acoustic noise of gradient pulsing. The onset time constants were measured from functional MR imaging without and with previous acoustic stimulation. For various time durations of the previou...