Ultrasound focusing using magnetic resonance acoustic radiation force imaging: Application to ultrasound transcranial therapy
(a) Maximal displacement response to a single phased array element’s phase. MR-ARFI image (b) obtained by the line-scan spin-echo method was used to calculate one of the maximal displacement points presented in the graph. The images were acquired by 21 W of acoustic power transmitted from the 208-element phased array, while the power transmitted from the tested element was increased to 2 W.
MR-ARFI sequences diagrams. (a) Single shot gradient-echo echo-planar sequence diagram. The displacement is caused by a single FUS pulse of 20 ms duration. (b) Line-scan spin-echo sequence diagram. In both sequences, the FUS pulses were applied 2 ms earlier than the encoding gradient in order to synchronize the actual displacement to the gradients, as tissue’s static displacement response time is few milliseconds.
(a) MRI image of an in vivo experiment. Human skull was placed between the ultrasound transducer and pig’s brain to resemble transcranial treatment in humans. (b) In vivo MR-ARFI image of pig’s brain, produced by line-scan spin-echo sequence, 1.4 kW acoustic power at 220 kHz frequency.
(a) An MRI image of the apparatus. The tissue-mimicking phantom was placed inside the cranium to resemble transcranial human treatments. (b) MR-ARFI line-scan spin-echo measurements produced by 1890 W acoustic power of acoustic fields obtained after correction by hydrophone, CT-skull, and without aberration correction. The images were acquired using InSightec ExAblate 4000, equipped with 650 kHz frequency brain transducer that was focused to about 15 mm on phantom that was wrapped in a human skull, in degassed water environment. Hydrophone aberration correction uses the phase of each phased array element measured by a hydrophone at the focus to correct the aberration. CT-skull aberration correction uses the cranial thickness and density, obtained from high-resolution CT images, to estimate the phase shift and amplitude of each element caused by the cranium.
Demonstration of nearly optimal focusing using MR-ARFI. The displacement images were acquired by line-scan spin-echo sequence and 20 W of acoustic power produced by 208-element, 1 MHz ultrasound phased array. Random aberration was added to the phase map of the transducer and was corrected by acoustic phase sensitive MR-ARFI measurements. The left image was produced after distorting the focus by applying the random aberration to apparatus. The middle image was taken after improving the focus using phase sensitive MR-ARFI measurements and right image represents nearly optimal focus without the random aberration.
Nearly optimal focus obtained by MR-ARFI adaptive focusing. The three MR-ARFI line scans produced by different phase maps are described in Fig. 5.
(a) Human skull aberration correction. Single-shot echo-planar MR-ARFI displacement maps of a tissue-mimicking phantom acquired using noncorrection (left), CT-skull based correction (middle), and ARFI based correction (right) phase maps. Images were acquired using 800 W acoustic power, 710 kHz ultrasound frequency targeted on the phantom via human skull. (b) Geometrical distortions of the echo-planar MRI sequence. MRI magnitude images of GE distortion phantom acquired by fast spin-echo (left), single-shot echo-planar (middle), and two-shot echo-planar (right) MR-ARFI sequences. The phantom was placed on the ultrasound transducer, at the focal plane, 94 mm above the isocenter of the magnet.
Transcranial focusing experiments results. One-dimensional displacements produced by three different acoustic phase maps are given in (a). An illustration of the 225-element, 710 kHz central frequency, ultrasound phased array, is given in (b) where the four segments that were used for the adaptive focusing are plotted. The acoustic phase maps obtained by the segment’s adaptive focusing technique using MR-ARFI and by the CT-skull correction are given in (b) and (c), respectively.
Summary of the calculated segments’ geometrical-adjusts and phase shifts. Transducer’s segments.
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