Reverberant Magnetic Resonance Elastographic Using a single Mechanical Driver

Reverberant elastography provides fast and robust estimates of shear modulus. However, reverberant elastography uses multiple mechanical drivers, hampering clinical utility.  In this work, we hypothesize that a single mechanical driver can generate reverberant shear fields in constrained organs such as the brain. To corroborate this hypothesis, we imaged the brain of a healthy volunteer; and two constrained phantoms containing spherical inclusions with diameters ranging from 4-18 mm. As a secondary goal, we assessed the feasibility of recovering shear modulus from a single component of the reverberant wave field. Viable reverberant and subzone elastograms were produced only when obtained at 50 and 100 Hz in phantoms. Different levels of reverberance were exhibited in different displacement components (70-82% for phantoms and 87-93% for the clinical case); however, wavefields obtained when imaging at 50 Hz and 100 Hz were not significantly different (p>0.05). Errors incurred in reverberant elastograms varied from 5% to 65% when imaging at 50 Hz and 2% to 55% when imaging at 100 Hz. Errors incurred in subzone elastograms ranged from 4% to 18% at 50 Hz and 5% and 50% at 100 Hz. The contrast-to-noise ratio of reverberant elastograms ranged from 20 dB to 44 dB compared to 25 dB to 31 dB in subzone elastograms. The accuracy of the elastograms acquired from the phantom containing internal shear wave reflectors did not differ noticeably. The global brain stiffness estimated from reverberant and subzone elastograms was 2.36 ± 0.95 kPa and 2.5 ± 1.1 kPa, respectively, when imaging at 50 Hz, and 2.56 ± 0.828 kPa and 2.89 ± 1.3 kPa respectively, when imaging at 70 Hz. The phantom study revealed that performance varied depending on the component of displacement used to compute reverberant elastograms; however, the clinical study demonstrated similar performance of reverberant and subzone elastograms