Timing is everything: How subtle timing changes in MRI echo planar imaging can significantly alter mechanical vibrations and sound level.

Modern MRI relies on the well-established Echo-Planar-Imaging (EPI) method for fast acquisition. EPI is the workhorse of diffusion and functional MRI in neuroscience as well as of many dynamic applications for clinical body imaging. Its speed stems from rapidly switching currents through the 'gradient coils' responsible for spatial encoding. These quick changes, within a strong static magnetic field induce Lorentz forces that generate mechanical vibrations, leading to the loud, characteristic MRI noise. This acoustic noise is already a significant concern in standard clinical scanners, requiring hearing protection and risking image degradation or even hardware strain. In ultra-high-field systems (≥7T), the issue is exacerbated due to stronger Lorentz forces. In this study, we introduce a novel model that characterizes the acoustic spectrum for a given EPI scan. The spectrum results from interference between multiple identical periods of alternating currents, making the relative timing between them a key factor. We show that even subtle timing changes significantly alter the sound level. Scans of either high spatial resolution or high temporal resolution, on clinical 7T and investigational 10.5T human scanners, corroborated the model and its predicted acoustics spectra. Acoustic energy changes of up to 47-fold were reached in close to mechanical resonances and up to 5-fold in other regions. Intriguingly, under certain conditions, doubling the acquisitions per unit time actually reduced the minimal acoustic energy two-fold. Not less important, 'ghosting-artifacts' exhibit strong dependence on the scan's acoustic characteristics and the benefit of subtle time delays of internal correction-acquisitions ('navigators') is also demonstrated.