Tissue elasticity is often affected by pathological changes, and could provide diagnostic value. Tissue elasticity can be probed using shear waves, which can be generated by applying a short pulse of focused ultrasound (FUS)¹. The speed at which a shear wave propagates in the plane perpendicular to the FUS beam direction is directly related to the shear modulus, and can be measured by tracking the propagation of the wave. Shear wave tracking has been done mostly using ultrasound imaging techniques, but can also be achieved with MRI. The advantage of using MRI instead for shear wave tracking is that the tissue elasticity map can be directly registered to other MR images, such as temperature maps acquired during MR-guided focused ultrasound (MRgFUS) treatment. Previous work by Souchon et al. used an spGRE pulse sequence with bipolar motion-encoding gradients (MEG) to visualize shear waves². Bitton et al. tracked shear wave propagation using a spin-echo pulse sequence with unipolar MEG³, and calculated an average shear wave speed for each radial spoke covering a few degrees, based on a time-of-flight (TOF) map constructed by peak detection. In this work, shear wave tracking was achieved using an spGRE pulse sequence with bipolar MEG. A TOF map was generated by finding the zero-crossing of the motion-encoded phase at each pixel as a function of the delay time (t_delay) between the FUS pulse and MEG, and shear wave velocity was calculated as a function of both polar angle and radial distance.An spGRE pulse sequence with bipolar MEG and spiral readout was used to track shear wave propagation in an isotropic acrylamide phantom, by changing t_delay between the FUS pulse and the second lobe of MEG from 0 ms to 27 ms in steps of 1 ms (Fig. 1). MR images were acquired on a 3T GE MR750 scanner (GE Healthcare, Milwaukee, WI) with a single-channel local coil. Pulse sequences were developed and implemented on the scanner using RTHawk (HeartVista, Inc., Menlo Park). 2D images were collected with FA/TR/TE=30°/500 ms/13.5 ms, slice thickness=3 mm. The spiral readout had 24 interleaves with BW=62.5 kHz, FOV=28 cm and resolution=1.5 mm. Each lobe of the MEG was 6 ms long, with plateau magnitude of 4 G/cm. FUS was generated by a multi-element phased-array transducer (Exablate 2000, Insightec, Haifa, Israel). The pulse duration was 6 ms and the acoustic power was 21.6 W. The HIFU beam was parallel to the MEG and perpendicular to the imaging slice. A full image was collected at each t_delay. After subtracting the phase of a baseline image that was acquired without FUS, phase images characterizing the shear wave propagation were generated. The phase of each pixel is the integral of tissue displacement with the amplitude of MEG⁴: $$$\phi=\int x(t)G(t)dt$$$. Therefore ϕ is sensitive to t_delay since it determines how x(t) and G(t) overlap. ϕ=0 when the displacement partly overlaps with both gradient lobes and phases accrued during the two lobes exactly cancel each other. To calculate TOF, we estimated the time when ϕ is 0 by performing 1d gridding between the maximum and minimum measured phases (Fig 2). A shear wave velocity map was generated by dividing the 2d TOF map into 8° sectors³, and calculating the shear wave speed at radial increments of 2 mm by linear fitting (using a sliding Kaiser-Bessel kernel with w=10 mm, b=6, FWHM≈5 mm). Fig. 3a shows the TOF map, and Fig. 3b shows the points that fall within an 8° sector. The TOF increases roughly linearly with radial distance, confirming that the phantom is largely isotropic in the radial direction. The shear wave speed map is shown in Fig 4 in polar coordinates from r=4 mm to r=26 mm. The arithmetic mean of wave speed was 1.51 mm/ms, with standard deviation of 0.04 mm/ms. Note the standard deviation is affected by the width and shape of the linear fit kernel.We have demonstrated the feasibility of a method for SWEI with MRI using bipolar MEG. A high quality TOF map was generated using the zero-crossing time of the motion-encoded phase, which is relatively simple and robust. Shear wave speed as a function of the radial distance and polar angle was calculated, which might be used to detect pathological changes in tissue. Although we implicitly assumed the wavefront is always perpendicular to radial spokes in calculating the speed map, this assumption is not fundamentally necessary and more advanced algorithms can be used to determine shear wave velocity from the TOF map.