In vivo monitoring of focused ultrasound surgery using local harmonic motion

Abstract

The present study established the feasibility of a technique for monitoring focused ultrasound (FUS) lesion formation in vivo using localized harmonic motion (LHM) measurements. Oscillatory motion (frequencies between 50 and 300 Hz) was generated within tissues by induction of a periodic radiation force with a FUS transducer. The harmonic motion was estimated using cross correlation of RF ultrasonic signals acquired at different instances during the motion by using a confocal diagnostic ultrasound transducer. The technique was evaluated in vivo in rabbit muscle (14 locations) in an magnetic resonance (MR) imager for simultaneous ultrasound harmonic motion tracking and MR thermometry. The measured maximum amplitude of the induced harmonic motion before and after the lesion formation was significantly different for all the tested motion frequencies, and decreased between 17 and 81% depending on the frequency and location. During the FUS exposure a drop in the maximum amplitude value was observed and a threshold value could be associated to the formation of a thermal lesion. A series of controlled sonications was performed by stopping the exposure when the threshold value in LHM amplitude was reached and the presence of a thermal lesion was confirmed by MR imaging. LHM measurements were also used to perform a spatial scan of the tissues across the exposure region and the thermal lesions could be detected as a reduction in the maximum motion amplitude value at the sonication region.

Figure 1

Beam profiles: (a) lateral and (b) acoustical axis for transducer with 1.689 MHz central frequency (f_c), external diameter (D_ext) of 100 mm, internal diameter (D_int) of 70 mm and geometrical focus (R_c) of 80 mm; (c) lateral and (d) acoustical axis for transducer with 1.536 MHz central frequency (f_c), external diameter (D_ext) of 100 mm, internal diameter (D_int) of 14 mm and geometrical focus (R_c) of 80 mm.

Figure 2

Experimental set-up used for the LHM measurements. FUS and diagnostic transducer assembly were placed in a water tank facing up, with the target tissues on top of the tank. The tank was introduced into the MR Imager to allow visualization of tissues and temperature maps during the sonication. Materials inside the tank were MR-compatible; the excitation system was outside the MR room and the signals were introduced into the MR room through a penetration panel using filtering when necessary.

Figure 3

Harmonic motion amplitude for each of the fourteen locations studied before and after sonication at three different modulation frequencies (50, 100 and 150 Hz). The values shown are the average of 5 measures at each location; error bars show the standard deviation.

Figure 4

Example of tissue displacement (in mm) at location No. 6 as a function of time (a) before and (b) after lesion formation. The results for a 50, 100 and 150 Hz burst are presented. The amplitude decreases after lesion formation for all the frequencies.

Figure 5

Average of normalized LHM amplitude as a function of frequency for all the experiments. The motion amplitude was normalized by the maximum value obtained at each location (50 Hz burst before lesion formation). The difference between before and after lesion formation was significant for all the frequency bursts (50 Hz, p<0.0001; 75 Hz, p<0.0001; 100 Hz, p<0.0001; 125 Hz, p<0.0001; 150 Hz, p<0.0001; 175 Hz, p<0.0001; 200 Hz, p=0.0001; 225 Hz, p=0.0005; 250 Hz, p=0.008; 275 Hz, p=0.004; 300 Hz, p=0.04).

Figure 6

Temperature and LHM amplitude during the sonication for the location No. 13. (a) MRI temperature map (coronal) at the end of the sonication (43.8 s) and (b) corresponding T2-MR image after the sonication. (c) Plot of the temperature rise at the focus (left axis, round markers with continuous line) and of the normalized LHM amplitude (right axis, cross markers with dashed line) as a function of time for the same location.

Figure 7

Temperature (a) and dose (b) for all the locations at the instant when the harmonic motion dropped below the threshold.

Figure 8

Time to reach the threshold during the controlled exposures experiments as a function of the acoustic intensity in situ of the exposure.

Figure 9

MR sagittal image where controlled exposures No. 22 to 27 (a) before any sonication was performed, (b) after exposures 22 to 26 were made, and (c) when exposure 27 was made (at the same location as exposure 26.

Figure 10

Spatial LHM scan after two sonications (at locations Nos. 13 and 14) and the corresponding sagittal and coronal T2-weigthed MR images. The focus of the FUS transducer was located at the same vertical position for both sonications (at 50 mm in the sagittal image). The sonications were made 10-mm apart following the scan direction (superior/inferior). (a) Value of the LHM amplitude following the scanning direction. Corresponding T2-weighted sagittal (b) and coronal (c) MR images of the lesions.