|University of Illinois at Urbana-Champaign | Department of Electrical and Computer Engineering | Department of Bioengineering
Department of Statistics | Coordinated Science Laboratory | Beckman Institute | Food Science and Human Nutrition | Division of Nutritional Sciences | College of Engineering
|Tuesday, June 22nd, 2021|
Visualizing an FUS Beam In Situ for Non-Thermal FUS Therapies
Focused ultrasound (FUS) can provide a means of noninvasive treatment of tissues. However, currently the gold standard for monitoring FUS is MRI temperature mapping. Because of the expense, lack of portability and slow update of temperature maps from MRI, ultrasonic solutions to monitoring of FUS remain an important clinical goal. In some applications of FUS, the goal is not to heat tissues, but to cause a biological effect such as opening of the blood brain barrier (BBB) or by collapsing microbubbles in a tumor. In such cases, what is needed is not temperature mapping but a visualization of the beam of the FUS source in the context of surrounding tissues.
Real-time visualization of the field distribution of the FUS source during treatment would allow the localization of the intersection of the FUS beam with the tissue. Furthermore, real-time visualization of the FUS beam in the context of the tissue would allow proper positioning of the FUS beam for therapy especially during tissue motion. In this work, in order to visualize the FUS beam in situ, a straightforward reconstruction technique was employed on data collected by a linear array system co-aligned with the FUS source. The technique would work by pulsing the FUS source during the off portion of the duty cycle and recording the scattered field on an aligned array. Figure 1 shows the pulsing sequence utilized by the scheme.
Figure 1. Depiction of the excitation sequence for both the FUS source and the imaging array transducer
The reconstruction technique used the scattered signal from the medium to reconstruct the intensity field pattern of the FUS beam in situ and superimpose the intensity field image on a B-mode of the scattering medium. Figure 2 shows an experimental configuration. A 6-MHz single-element transducer (f/2) was used as the FUS source and aligned perpendicular to the field of a linear array (L9-4) operated by a SonixOne system equipped with a SonixDAQ. The array had 128 elements and a measured center frequency of 5 MHz. The 6- MHz FUS source was pulse excited and the fields scattered from the sample were received by each element of the linear array.
Figure 2. Experimental Setup
Bistatic beamforming was applied to the channel data to focus the receiving array at each point in the field and reconstruct the intensity field pattern from the FUS source. The intensity field pattern was then superimposed on a registered B-mode image of the sample acquired using the same linear array. The superimposed image can be used to provide anatomical context of the FUS beam in the sample being treated. To quantify the quality of the FUS beam reconstruction, the field pattern was mapped using a wire target. Specifically, the wire target was placed in the field and the intensity field pattern was reconstructed by moving the wire throughout the focal region of the FUS source while recording on the listening array.
The intensity field pattern reconstructed from a homogeneous scattering phantom was compared to the field characteristics of the FUS source characterized by the wire technique. Figure 3 shows an image of the reconsutrcted FUS beam superimposed on a B-mode of a homogenous phantom. The beamwidth estimates at the FUS focus using the in situ reconstruction technique and the wire technique were 1.5 mm and 1.34 mm, respectively. The depth of field estimates for the in situ reconstruction technique and the wire technique were 11.8 mm and 11.75 mm, respectively.
Figure 3. (a) Single shot of the passive beam of the homogeneous phantom. (b) Beamformed image of the bistatic passive beam in the homogeneous phantom. (c) Overlaid beam on the B-mode image of the homogeneous phantom. (d) Bistatic beam estimated using the wire target. The -6-dB bistatic beamwidth and depth of field were estimated to be 1.5 mm and 11.8 mm, respectively.
Next, an inhomogeneous phantom was constructed by increasing the number density of glass beads on one side of the phantom by a factor of 10. This resulted in a 10 dB increase in the scattered intensity. The change in brightness of the B-mode in the bright section was used to correct for beam reconstruction effects. The resulting images are shown in Figure 4.
Figure 4. Two layered phantom of different glass bead density. (a). Reconstructed passive beam. (b). Overlaid of the beam onto the B-mode image. (c) FUS beam reconstructed using an intensity mask created from the B-mode image.
Finally, the FUS beam was reconstructed in a chicken breast sample. The reconstructed beam superimposed on a chicken breast is shown in Figure 5.Therefore, we conclude that the novel reconstruction technique was able to accurately visualize the field of an FUS source in the context of the interrogated medium.
Figure 5. (a) Reconstructed passive beam in the chicken breast. b) Overlay of the beam onto the B-mode image of the chicken breast.
This work was supported by a grant from the NIH (NIH R21EB023403)
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