Fig 1.
(Ai) Schematic diagram of MR-guided focused ultrasound system (MRgFUS). The transducer is a 16-element annular array with focal spot size of 0.5 x 0.5 x 2 mm3. (Aii) MRI axial magnitude image of the experimental setup, demonstrating: the location of the transducer; water bath linking the motorized transducer to a water-tight membrane; acoustic gel above the membrane; and tumor and surrounding tissue. (B) The motion encoding sequence used to acquire both the acoustic radiation force displacement and the shear wave propagation. The illustrated sequence was acquired with both positive and negative bipolar motion encoding gradients (MEGs). The sonication duration (δ/2) coincided with the first half of each bipolar gradient period. The images were acquired in an 8% gelatin phantom, where subtracting the image with negative MEG polarity (middle) from the positive image (left) results in the final phase image (right). The scale bar in the images represents 8 mm.
Fig 2.
FUS beam profile measurement and ARFI validation.
The normalized FUS beam profile in the lateral (A) and axial (B) directions. (C) The displacement versus ultrasound pressure in a bovine gelatin phantom where the peak negative pressure was varied from 3.2 to 12.2 MPa.
Fig 3.
Localization of the acoustic beam by MR-ARFI.
(A) Seven coronal slices were acquired at -3, -2, -1, 0, 1, 2 and 3 mm away from the US focal plane along the depth axis with -3 dB contour lines. (B) The diameter of the area enclosed within a -3 dB contour line was calculated at different depths. (C) The peak displacement amplitude at each depth.
Fig 4.
Effect of physiological breathing motion simulator on displacement estimation.
(A) Schematic diagram of the MR-compatible respiratory motion simulator. A 1d linear motor is used to periodically depress a water bottle, which generates sinusoidal motion at a membrane connected to the bottle via plastic tubing. The phantom resting on the membrane is thus raised and lowered by the water pressure. The system generates 0.3 Hz sinusoid motion with an amplitude of 4 mm. (B) Comparison of the displacement with and without artificial sinusoidal motion. (C) Comparison of image quality of bovine gelatin phantom with and without motion, sonication and trigger. Magnitude and phase images are shown on the top and bottom row. Column (i) as a control, indicates the image quality without artificial motion. After motion was induced, images were acquired without (column (ii, iv)) and with the trigger (iii, v).
Fig 5.
MR-ARFI displacement as a function of gelatin phantom properties.
(A) MR-ARFI was tested in gelatin phantoms with 3 different concentrations, 5% (first column), 10% (second column) and 15% (third column). The images acquired with negative MEG polarity (second row) were subtracted from images acquired with positive MEG polarity (first row) to yield the final phase images (third row). (B) The calculated displacement was 0.96 ± 0.03, 0.81 ± 0.05 and 0.48 ± 0.03 μm in 5, 10 and 15% gelatin phantoms, respectively.
Fig 6.
Example of in vivo MR-ARFI imaging.
Phase images without (A) and with (B) FUS-induced displacement, where the red arrow indicates the location of focus. (C) Magnitude image, where the red circle indicates the tumor region. Corresponding H&E of the treated (D) and untreated (E) tumors at 48-hour post ablation time point. F provides the calculated displacement: 6.0 ± 0.3 μm before and 2.9 ± 0.7 μm after ablation.
Fig 7.
(A) Effect of the second Motion Encoding Gradient (MEG) on the phase images. TE = 23.8 ms, δ = 7 ms, Ge = 140 mT/m. The images are 85 × 85 mm2 in size. MR sequences (i,ii) and (iii,iv) the corresponding phase images of the shear wave. (B) Shear wave velocity estimation. (i). MR sequence used to estimate shear wave speed by varying delay t. (ii). Shear profiles analysis. Lines are extended from the focus outwards in all directions (360 degree sweep, at θ = 1 degree increment) to determine the distance of the peak minima from the focus. Propagation speed of the shear wave was calculated by measuring the radii of the peak and minima as a function of the delay time. (C) Shear wave propagation in 8% gelatin phantom. Phase images at shear wave propagation times of 10, 12, 14 and 18 ms corresponding to time delays t = 0, 1, 2 and 4 ms. (D) Distance of the peak minima from the focus versus delay time t in 8% gelatin and tofu phantoms. (E) Shear wave velocity in 8, 10% gelatin and tofu phantoms. (F) Shear wave velocity map.