HIFU Power Monitoring Using Combined Instantaneous Current and Voltage Measurement

• Published in: IEEE transactions on ultrasonics, ferroelectrics, and frequency control Vol. 67; no. 2; pp. 239 - 247
• Main Authors: Adams, Chris, McLaughlan, James R., Carpenter, Thomas M., Freear, Steven
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.02.2020; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustic measurements; Acoustic output measurement; Acoustics; Amplifiers; Animals; Chickens; Current measurement; Electric power; Electrical measurement; Electricity; Electricity generation; Equipment Design; Frequency control; high intensity focused ultrasound; High-Intensity Focused Ultrasound Ablation - instrumentation; High-Intensity Focused Ultrasound Ablation - methods; High-Intensity Focused Ultrasound Ablation - standards; Impedance; Instantaneous current; Magnetic resonance imaging; Magnetic Resonance Imaging - instrumentation; Magnetic Resonance Imaging - methods; Models, Biological; Monitoring; Muscle, Skeletal - radiation effects; power measurement; Reproducibility of Results; Switching theory; Transducers; ultrasonic transducers; Voltage measurement; Waveforms
• DOI: 10.1109/TUFFC.2019.2941185
• ISSN: 0885-3010

During high-intensity focused ultrasound (HIFU) therapy, it is important that the electrical power delivered to the transducer is monitored to avoid underexposure or overexposure, ensure patient safety, and to protect the transducer itself. Due to ease of measurement, the transducer's potential difference may be as an indicator of power delivery. However, even when a transducer's complex impedance is well characterized at small amplitudes and matching networks are used, voltage-only (VO) monitoring cannot account for the presence of drive waveform distortion, changes to the acoustic path, or damage to the transducer. In this study, combined current and voltage (CCV) is proposed as a magnetic resonance imaging (MRI)-compatible, miniature alternative to bidirectional power couplers, which is compatible with switched amplifiers. For CCV power measurement, current probe data were multiplied by the voltage waveform and integrated in the frequency domain. Transducer efficiency was taken into account to predict acoustic power. The technique was validated with a radiation force balance (RFB). When using a typical HIFU transducer and amplifier, VO predictions and acoustic power had a maximum difference of 20%. However, under the same conditions, CCV only had a maximum difference of 5%. The technique was applied to several lesioning experiments and it was shown that when VO was used as a control between two amplifiers, there was up to a 38% difference in lesion area. This greatly reduced to a maximum of 5% once CCV was used instead. These results demonstrate that CCV can accurately predict real-time electrical power delivery, leading to safer HIFU treatments.