Application Cases

Experiment Name: Real-Time Monitoring of Focal Region Damage Based on Continuous Self-Sensing Ultrasound

Experimental Principle:
Monitoring the formation of HIFU-induced damage is crucial for treatment precision and controllability. In clinical focused ultrasound therapy, the assessment of coagulative necrosis in the focal region often relies on B-mode ultrasound imaging during treatment intervals. However, B-mode imaging can be affected by cavitation or boiling bubbles, and dependence on the subjective experience of clinicians is unreliable. Real-time monitoring of HIFU-induced damage is a major research focus and a critical issue requiring solutions. Existing studies utilize the amplitude of voltage signals at the terminals of HIFU transducers to monitor cavitation bubble activity. Self-sensing monitoring via HIFU transducers offers advantages such as real-time capability, low cost, and ease of integration. By extracting and analyzing the frequency and time domains of the driving voltage and current signals during HIFU transducer operation, experiments were conducted to monitor damage in ex vivo bovine liver tissue using a self-sensing ultrasound monitoring system.

Test Equipment:
ATG-2021B power signal source, ultrasound transducer, MATLAB software development platform, acoustic radiation force balance, oscilloscope, bovine liver tissue.

Figure 1: Schematic Diagram of the Experimental Principle

Experimental Procedure:
The ex vivo bovine liver tissue was placed in a polymethyl methacrylate (PMMA) box with inner dimensions of 114 mm × 100 mm × 120 mm. The bottom of the box faced the HIFU transducer. To approximate clinical conditions, the encapsulated surface of the bovine liver was positioned opposite the HIFU transducer. The transducer was driven by the ATG-2021B power signal source. Under B-mode ultrasound guidance, different irradiation points were selected, and continuous waves with an acoustic power of 60 W were applied to the ex vivo bovine liver tissue. The phase difference between the driving voltage and current of the HIFU transducer was monitored via the GUI interface. When significant fluctuations in the phase difference occurred, the treatment was stopped. B-mode ultrasound images before and after irradiation were saved, and changes in grayscale were observed to verify the accuracy of the method. After irradiation, the bovine liver tissue was sectioned along the acoustic axis into thin slices. The slice with the maximum damage was identified, rinsed with saline, and photographed for documentation.

Voltage and current probes were connected to two acquisition channels of the Pico device, and the acquisition program was initiated before activating the HIFU transducer output. The standard deviation of the acquired data was calculated to observe trends during the irradiation process. The ex vivo bovine liver tissue was irradiated for an extended period until grayscale changes appeared in the B-mode ultrasound images, at which point the power source output was stopped. The voltage signal of the transducer was filtered to remove the fundamental frequency and harmonics up to the 5th order. The background noise energy (root mean square value) was calculated using the following formula:

$$\text{RMS}=\sqrt{\frac{1}{N}\sum _{i=1}^N{x}_i^2}$$

The background noise energy was compared with that of a control group without grayscale changes, revealing variations in background noise energy during cavitation.

Experimental Results:
As the number of irradiation rounds increased, the phase difference between voltage and current exhibited a transition from minor fluctuations to significant fluctuations, as shown in Figure 2(a). When irradiation was stopped at this stage, damage was observed in the focal region, with a relatively small area of damage, as shown in Figure 3(a). Figure 2(b) compares the phase diagrams of the second and final irradiation rounds, showing significant fluctuations in the latter, with a much larger standard deviation (0.33 vs. 1.81). Standard deviation analysis of the phase for each treatment round, shown in Figure 2(c), revealed an upward trend in phase fluctuation magnitude.

Figure 2: Phase Variation Trends During HIFU Irradiation (Without Cavitation)
(a) Phase variation process during HIFU irradiation
(b) Comparison of phase fluctuations before and after damage
(c) Changes in phase standard deviation per irradiation round

Figure 3: Bovine Liver Damage
(a) Without cavitation
(b) With cavitation

Figure 4: Phase Variation Trends During HIFU Irradiation (With Cavitation)

As shown in Figure 4, when cavitation occurred during irradiation, the phase exhibited larger fluctuations compared to non-cavitation conditions, with a maximum standard deviation of 3.43. Comparison of Figure 4(a) with Figure 2(a) reveals significantly greater fluctuations and an expanded fluctuation range during cavitation. Sectioning of the bovine liver tissue revealed noticeable mechanical damage, as shown in Figure 3(b).

(a) Phase variation process during HIFU irradiation
(b) Comparison of phase fluctuations before and after damage
(c) Changes in phase standard deviation per irradiation round

In ex vivo bovine liver HIFU experiments, the intensity of phase fluctuations showed distinct changes before and after necrosis in the focal region, as shown in Figure 5. Before necrosis, the phase standard deviation was around 0.5, while after necrosis, it exceeded 1.0, indicating relatively larger fluctuations. This system reliably detected damage during HIFU irradiation, whereas B-mode ultrasound showed no significant changes. Figure 6 illustrates the damage detected using this monitoring system, demonstrating its effectiveness in preventing extensive damage due to overtreatment compared to B-mode ultrasound monitoring.

Figure 5: Comparison of Phase Standard Deviation Before and After Necrosis in the Focal Region During HIFU Treatment

Figure 6: Damage Detected in Ex Vivo Bovine Liver Tissue Using This System

Figure: Specifications of the ATG-2000 Power Signal Source

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