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Annual Scientific Meeting of the Canadian Organization of Medical Physicists — July 9–12, 2014, The Banff Centre, Banff, Alberta, Canada

Acoustic Focusing and Tissue Interaction in Image-Guided HIFU

High-intensity focused ultrasound has matured from a laboratory curiosity into a clinically deployed modality for ablating tissue deep within the body without an incision. When paired with real-time diagnostic ultrasound for guidance, the physics of both the treatment beam and the imaging beam must be understood in parallel. Canadian and Australian centres that have introduced HIFU for uterine fibroids, prostate disease, and bone metastases rely on the same fundamental acoustic principles, even when hardware platforms differ. The 2014 COMP meeting in Banff brought together medical physicists working on these systems to compare dosimetry, quality assurance, and image registration.

Ultrasound-guided HIFU distinguishes itself from MR-guided systems by offering real-time feedback at lower cost, using portable consoles rather than whole-room installations. The trade-off is that B-mode imaging, elastography, and harmonic detection must be tuned to track tissue changes and bubble activity as they occur. Australian clinicians in Melbourne and Brisbane have adopted these platforms in private practice, complementing the public hospital system funded through Medicare. For delegates arranging accomodations in Banff, the scientific sessions provided a rare opportunity to examine how acoustic engineering translates into clinical throughput.

Wave Propagation in Focused Beams

Therapeutic transducers operate at frequencies between 0.5 and 4 MHz, chosen as a compromise between penetration and focal sharpness. Lower frequencies reach deeper targets but produce wider focal zones, while higher frequencies concentrate energy into smaller volumes for delicate structures. Phased array transducers allow electronic steering by applying time delays to individual elements, critical when the target lies behind ribs or air-filled bowel. The wavelength in soft tissue is roughly 1.5 mm at 1 MHz, setting a practical lower limit on focal dimensions.

Nonlinear propagation becomes significant at HIFU pressures, where the waveform distorts during travel and energy shifts into harmonics, increasing heating at the focus. Modelling tools such as the Westervelt and KZK equations predict energy deposition, particularly when beams converge or cross tissue interfaces with different impedance. Researchers at the University of Sydney have validated these simulations against hydrophone measurements in tissue-mimicking phantoms.

Tissue Heating and Thermal Dosimetry

The primary mechanism of cell death in most HIFU procedures is thermal coagulation, where temperatures above 55°C maintained for at least one second denature proteins irreversibly. Lower temperatures applied for longer durations can also produce ablation through sustained hyperthermia, particularly when combined with chemotherapy or radiation. The thermal dose concept, expressed in cumulative equivalent minutes at 43°C, provides a framework for predicting biological effect across different exposure patterns.

Heat diffusion during and after sonication determines final lesion size, as thermal conduction spreads energy beyond the acoustic focus. Perfusion cools tissue by carrying heat away in the bloodstream, complicating dose calculations in well-vascularised organs such as the liver. The Arrhenius integral formalises the relationship between time, temperature, and cell death, giving physicists a quantitative tool for planning. Calibration uses radiation force balances and hydrophones before clinical use, with the Therapeutic Goods Administration requiring annual verification for devices on the Australian Register of Therapeutic Goods.

Cavitation and Mechanical Bioeffects

When acoustic pressure exceeds a threshold related to ambient pressure and frequency, dissolved gas comes out of solution to form bubbles that oscillate violently. Stable cavitation produces microstreaming that enhances heating and disrupts cell membranes, while inertial cavitation collapses bubbles violently and can cause mechanical tissue damage far beyond the focal zone. Inertial cavitation is generally undesirable during thermal ablation but forms the basis of histotripsy, where mechanical fractionation liquefies tissue for non-thermal applications.

Monitoring for cavitation is essential because it changes the acoustic environment unpredictably. Passive cavitation detectors, single-element transducers listening for broadband emissions, can be integrated into the therapy probe for real-time feedback. Active approaches use interleaved imaging pulses to detect echogenicity changes that correlate with bubble activity. Sonographers in Perth and Adelaide have noted that accurate detection requires careful setting of mechanical and thermal indices, values mandated under ARPANSA guidelines for diagnostic ultrasound.

Ultrasound Imaging for Treatment Guidance

The same probe that delivers therapy can also produce B-mode images of the target, allowing clinicians to visualise anatomy and align the focus before firing. Real-time monitoring during sonication reveals hyperechoic regions at the focus as tissue coagulates, providing immediate confirmation that energy is reaching the intended site. Colour Doppler can be overlaid to confirm vascular disruption, while elastography maps stiffness changes that accompany coagulation necrosis.

Image registration is challenging because the therapy transducer sits on the skin while the target lies several centimetres deep. Calibration phantoms verify geometric accuracy before each session, as required by accreditation bodies in both Australia and Canada. Harmonic imaging and microbubble contrast agents help delineate vascular tumours and track perfusion after ablation. The Australian Medicare Benefits Schedule covers contrast-enhanced ultrasound for specific indications. For physicists comparing imaging modalities across platforms, sessions covered related topics such as the role of the medical physicist in proton therapy centers, where image guidance is equally central.

Clinical Workflow and Treatment Planning

A typical HIFU procedure begins with planning ultrasound or MRI to map the target volume and identify critical structures to avoid, such as nerves, bowel, and bone interfaces. The treatment plan divides the volume into individual sonications positioned to overlap with neighbours, ensuring complete coverage while sparing intervening tissue. Treatment margins of a few millimetres account for physiological motion, including respiration and patient movement, which can shift soft tissue during the procedure.

Patient preparation includes fasting, bladder filling for pelvic treatments, and sometimes sedation or general anaesthesia. Real-time monitoring allows the operator to pause or adjust power if the patient reports discomfort or imaging reveals off-target heating. Post-treatment imaging confirms ablation extent, often with contrast-enhanced studies showing non-perfused regions. Quality assurance programmes track device performance over time, including output power, focal position, and imaging calibration, with incident reporting feeding back into protocol refinements.

Practical Guidance for Clinical Physicists

The following points summarise practical considerations raised during the Banff sessions for physicists establishing or maintaining an ultrasound-guided HIFU programme.

  • Verify acoustic output annually against a traceable standard, and after any transducer repair or replacement.
  • Map the acoustic path in planning software to identify potential standing wave sites before approving the treatment plan.
  • Calibrate geometric accuracy of the imaging overlay using a tissue-mimicking phantom with embedded targets before each clinical session.
  • Monitor mechanical and thermal indices on the imaging console, recognising that HIFU operation will exceed diagnostic limits under controlled conditions.
  • Document cavitation detection thresholds and integrate passive cavitation detectors into the treatment console if available.
  • Establish a reporting pathway for unexpected thermal or mechanical effects, linking to national incident systems where applicable.

Participation in multi-centre working groups remains an effective way to benchmark practice and share phantoms, protocols, and incident learning. Medical physicists from Brisbane, Melbourne, and Hobart found that comparing notes with Canadian colleagues clarified several ambiguities in their own procedures. The COMP community welcomes correspondence from Australian counterparts interested in future consensus statements on focused ultrasound dosimetry. Delegates can revisit the programme details and abstracts to identify sessions most relevant to their clinical setting.