Experimental arrangement (a) and relative position of the transducer focus and the MRI field of view (FOV) (b).
Water proton resonance frequency shift calibration curve obtained in the 4.7-T magnet that was used for MR thermometry in present work. The solid line shows a third-order polynomial fit to the mean temperature as determined from 12 measurement points located near the center of the rf coil. The dotted lines show the mean ± standard deviation of the measurements. The linear conversion curve (dashed line) often used in 1.5-T magnets is also shown for comparison.
Focal waveforms simulated numerically and measured by the fiber-optic hydrophone in water with and without the cylindrical, water-filled chamber attached (a) and in the phantom (b). The cylindrical housing did not alter the waveform, and the numerical data are in excellent agreement with the measurements.
Simulation results for acoustic and temperature fields in the phantom obtained assuming nonlinear (solid) and linear (dashed) acoustic propagations. Spatial distributions of the peak positive and peak negative pressures, intensity, heat deposition rate, and temperature after 7 s of HIFU exposure are presented axially (upper row) and in the focal plane radially (lower row). Dashed vertical lines on the heat deposition and temperature plots indicate the width of the voxel.
(a) Percent of times in seven 5-s exposures that cavitation was detected versus peak negative pressure of the HIFU exposure. Cavitation was detected with a 20-MHz PCD high-pass filtered at 15 MHz. Peak negative pressures larger than 8 MPa were used in this work; therefore, cavitation was present in all experiments. (b) Peak-detected representation of time-domain trace recorded by the PCD for three HIFU exposure levels. At the lowest exposure (2-MPa negative pressure), the signal was at the noise level, which was 20 mV. An increase to 3.7-MPa negative pressure caused little change in one case and significant increase in signal amplitude in the other. The large increase in signal was attributed to broadband emissions from cavitation. Under the exposure used in the MR experiments (8.6-MPa negative pressure), the elevated signal due to cavitation was observed immediately after HIFU was turned on, and the signal further increased at 7 s when boiling occurred.
Temperature measured by a thermocouple at the focus. At the point boiling was visually observed in simultaneous high-speed camera images, was measured, and the temperature rise suddenly plateaued. However, the presence of the thermocouple accelerated heating, and therefore, boiling occurred at 0.5 s with the thermocouple present compared to 7 s without it.
Indication of boiling after 7-s exposure by fluctuation of the electrical power delivered to the transducer in the MRI experiment.
MR magnitude images of the tissue-mimicking phantom during and after HIFU exposure. The transducer was located above the top of the images, and the sample was exposed for 20 s. (a) The image taken 5.2 s after the start of the exposure shows evidence of heating in the focal region, but boiling had not yet occurred. No evidence of cavitation is observed in the image. (b) After 7.8 s, the image shows large boiling bubbles surrounded by motion artifact. (c) After 14.3 s, motion artifacts are less evident as bubble motion may have become less violent. (d) Even 2 minutes after HIFU exposure, a high-resolution image shows the residual bubble at the HIFU focus. The bubble position corresponded to the distal end of the region of thermally denatured protein (the lesion) photographed in (e). The lesion had grown and enlarged in the direction of the transducer as has been reported to be caused by the presence of boiling (Ref. 15 ).
Two-dimensional temperature distributions measured by MR thermometry: (a) 6.4 s, (b) 7.7 s, and (c) 12.8 s after HIFU was turned on and (d) 3.4 s, (e) 20.3 s, and (f) 40 s after HIFU was turned off. Before boiling (a), the region grew nearly symmetrically about the focus. No temperature field distortion was observed even though cavitation was present. After boiling occurred at 7.1 s, the heated region migrated upward toward the HIFU source and broadened.
MR-measured temperature at the focus of the transducer over the course of the treatment (a) and comparison of measurement and calculation in the pre-boiling part of the curve (b). MRI generally tracked the heating during and the cooling following HIFU. However, because of the presence of boiling bubbles during HIFU exposure from 7 to 20 s, temperature readings were erratic. Immediately before boiling (7 s), the calculated peak temperature was . The temperature measured with MRI in the focal voxel and the calculated temperature averaged over the voxel volume was only .
Temperature distribution after 7-s exposure calculated within a single voxel in the plane perpendicular to the acoustic axis at the spatial peak of temperature. The measured MRI temperature was the temperature averaged over the voxel volume and not the peak temperature.
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