Time reversal principles for wave optimization in multiple driver magnetic resonance elastography

Magnetic Resonance Elastography (MRE) quantitatively maps the stiffness of tissues by imaging propagating shear waves induced by mechanical transducers. It has been shown that by using multiple drivers, certain limitations of conventional single driver MRE can be reduced, and that by suitably adjusting the waveforms applied to these drivers, any arbitrary region of interest can be optimally illuminated (wave optimization). Typically these adjustments were derived from wave response data collected for each transducer individually, which increases the total scan time. To address this issue, we investigated the use of time reversal principles to calculate the appropriate waveforms and their potential advantages in MRE exams. A phased array acoustic driver system with four independent 'daughter' transducers was used. An additional shear 'parent' transducer was used to create shear waves at the ROI, and wave propagation data was collected with MRE both in continuous and transient wave mode. From these single source wave data, the appropriate phase and time offset relationships between the daughter transducers were derived. Separate experiments were then carried out driving the daughter transducers with these calculated motions, and wave optimization was achieved in both continuous and transient wave MRE. We conclude that time reversal principles could be used for wave optimization with multiple drivers and could potentially reduce the total scan time.