MR-Shear Wave Elasticity Imaging (SWEI) with Bipolar Motion-Encoding Gradients
Yuan Zheng1, Michael Marx1, Rachelle R. Bitton1, and Kim Butts Pauly1

1Radiology, Stanford University, Stanford, CA, United States

Synopsis

We have demonstrated a method for shear wave elasticity imaging (SWEI). A shear wave was generated by a short focused ultrasound (FUS) pulse, and was tracked by collecting images with different delays (tdelay) between the FUS pulse and bipolar motion-encoding gradients (MEG). The time-of-flight (TOF) at each pixel was determined by the zero-crossing of the image phase as a function of tdelay. Based on the TOF map, a shear wave velocity map was generated in polar coordinates.

Introduction

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 (tdelay) between the FUS pulse and MEG, and shear wave velocity was calculated as a function of both polar angle and radial distance.

Methods

An spGRE pulse sequence with bipolar MEG and spiral readout was used to track shear wave propagation in an isotropic acrylamide phantom, by changing tdelay 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 tdelay. 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 tdelay 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).

Results

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.

Discussion

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.

Acknowledgements

The authors acknowledge our funding sources: P01 CA159992, InSightec and General Electric.

References

1, Palmeri ML, Nightingale KR. Acoustic radiation force-based elasticity imaging methods. Interface Focus. 2011 Aug 6;1(4):553-64.

2, Souchon R, Salomir R, Beuf O, Milot L, Grenier D, Lyonnet D, Chapelon JY, Rouvière O. Transient MR elastography (t-MRE) using ultrasound radiation force: theory, safety, and initial experiments in vitro. Magn Reson Med. 2008 Oct;60(4):871-81.

3, Bitton R, Kaya E and Butts Pauly K. Shear Wave Tracking in Cadaveric Breast Using MR-ARFI. Proceedings of ISMRM, 2012 (557).

4, McDannold N, Maier SE. Magnetic resonance acoustic radiation force imaging. Med Phys. 2008 Aug;35(8):3748-58.

Figures

Timing of the SWEI pulse sequence (readout not shown). Images were acquired with tdelay from 0 ms to 27 ms in steps of 1 ms. tdelay was adjusted by moving the FUS pulse relative to the MEG.

ϕ acquired with different tdelay at a single pixel. The time at which the phase is zero (here, ~11 ms) is recorded to determine a time-of-flight (TOF) map.

The TOF map acquired in an acrylamide phantom is shown in a). The FOV shown here is 9 cm by 9 cm. TOF as a function of radial distance within an 8° sector is shown in b).

Shear wave velocity map in polar coordinates with Δθ=8° and Δr=2 mm. rmin/rmax=4 mm/26 mm. Some pixels at small radial distance may not have enough points in the linear fit kernel to determine shear wave speed. Velocities at these pixels were calculated by 2d gridding.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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