Effect of physiological breathing motion simulator on displacement estimation.

<p>(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).</p

Overview of the experiments.

<p>(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.</p

Shear wave visualization.

<p>(A) Effect of the second Motion Encoding Gradient (MEG) on the phase images. TE = 23.8 ms, δ = 7 ms, G_e = 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.</p

MR-ARFI displacement as a function of gelatin phantom properties.

<p>(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.</p

Localization of the acoustic beam by MR-ARFI.

<p>(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.</p

Example of <i>in vivo</i> MR-ARFI imaging.

<p>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.</p

FUS beam profile measurement and ARFI validation.

<p>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.</p

<i>In vitro</i> controlled heating results in a tofu phantom.

<p>(a) The temperature increase as measured by the MR (blue) versus the requested temperature increase (red) for a tofu phantom heated under PID control in a spiral pattern. (b) A single MR magnitude frame 15 minutes into the heating experiment illustrating the temperature increase (color overlay) on the magnitude image.</p

Magnetic Resonance Imaging Assessment of Effective Ablated Volume following High Intensity Focused Ultrasound

<div><p>Under magnetic resonance (MR) guidance, high intensity focused ultrasound (HIFU) is capable of precise and accurate delivery of thermal dose to tissues. Given the excellent soft tissue imaging capabilities of MRI, but the lack of data on the correlation of MRI findings to histology following HIFU, we sought to examine tumor response to HIFU ablation to determine whether there was a correlation between histological findings and common MR imaging protocols in the assessment of the extent of thermal damage. Female FVB mice (n = 34), bearing bilateral <i>neu</i> deletion tumors, were unilaterally insonated under MR guidance, with the contralateral tumor as a control. Between one and five spots (focal size 0.5 × 0.5 × 2.5 mm³) were insonated per tumor with each spot receiving approximately 74.2 J of acoustic energy over a period of 7 seconds. Animals were then imaged on a 7T MR scanner with several protocols. T1 weighted images (with and without gadolinium contrast) were collected in addition to a series of T2 weighted and diffusion weighted images (for later reconstruction into T2 and apparent diffusion coefficient maps), immediately following ablation and at 6, 24, and 48 hours post treatment. Animals were sacrificed at each time point and both insonated/treated and contralateral tumors removed and stained for NADH-diaphorase, caspase 3, or with hematoxylin and eosin (H&E). We found the area of non-enhancement on contrast enhanced T1 weighted imaging immediately post ablation correlated with the region of tissue receiving a thermal dose CEM43 ≥ 240 min. Moreover, while both tumor T2 and apparent diffusion coefficient values changed from pre-ablation values, contrast enhanced T1 weighted images appeared to be more senstive to changes in tissue viability following HIFU ablation.</p></div

Comparison of GRE and segEPI sequences with and without GRAPPA.

<p>TA = acquisition time; TE = echo time; TR = repetition time; FA = flip angle; SNR = signal-to-noise ratio.</p