Jiming Zhang PhD, Abstract Co-Author: Nothing to Disclose

Amol Pednekar PhD, Abstract Co-Author: Employee, Koninklijke Philips NV

Pei-Herng Hor PhD, Abstract Co-Author: Nothing to Disclose

Raja Muthupillai PhD, Presenter: Research support, Koninklijke Philips NV

In MR-ARFI, tissue displacement at the high-intensity ultrasound focus sets off a shear wave, and displacement within tissue is measured using motion encoding gradients (MEG).  We propose a Radon transform (RT) based method to estimate shear wave speed within a homogenous medium.

MR-ARFI sequence was implemented on a commercial1.5 T  platform (Sonalleve™, Philips).  A phase contrast sequence was modified to include a symmetric bipolar MEG (4 ms duration @ 27mT/m) to encode tissue displacement in the plane bisecting HIFU focus (Fig.2). By progressively increasing the time delay (τ) between HIFU discharge and MEG, shear wave propagation was captured as a series of snapshots. Data Analysis: After background phase correction, two successive RT converted the displacement map I(x,y) to Radon space J(r,Θ) with two distinct peaks at θ=0 (Fig.3). Shear wave speed was calculated from τ and the distance (d) between the two peaks (d=distance travelled by the  wave from the HIFU focus) using d/τ.  

A snapshot of the propagating shear wave is shown in Fig. 3 (left). Projection onto the J(r,Θ) space converts the circularly symmetric wavefront into a parallel lines which is further condensed to two points upon subsequent RT. The SNR computed for the wavefront region using zero phase noise was 2.3 in I(x,y), which increased to 6.0 in J(r,Θ) and was boosted to 25.5 after second Radon transform. Shear wave velocity (v) was calculated as 4.01±0.06m/s from a series of images at progressively increasing τ(Fig.4). The estimated effective shear stiffness was then calculated as μ = ρv2=16.4±0.5kPa with known phantom density of 1020kg•m-3.

An unsupervised RT based method with very high displacement SNR can be used to estimate shear wave speed in homogeneous media. It should be noted that unlike single-frequency MRE, MR-ARFI methods estimate the group velocity of shear wave propagation.

We describe a gradient echo based technique suitable for magnetic resonance acoustic radiation force imaging (MR-ARfI) and the validation of the method in a gel phantom. The propagation of the displacement wave emanating from the ultrasound focus within the phantom material was clearly captured by the method. We also describe a radon transform based method to reliably reconstruct the mechanical properties of homogeneous phantom material from MR-ARFI.

Zhang, J, Pednekar, A, Hor, P, Muthupillai, R, A Radon Transform Based Approach for Estimating Shear Wave Propagation Speed in Homogenous Medium for MR Acoustic Radiation Force Imaging (MR-ARFI).  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14015069.html