Principles of HIFU Ablation

During HIFU therapy for destruction of diseased tissue, ultrasound beams are focused on a small region to be treated and high amounts of energy are deposited in that area causing the local temperature to rise. Each sonication beam treats a precisely defined portion of tissue, although in practice there can be cold spots caused by features such as blood perfusion, beam distortion due to calcification, and similar impediments.  A robotic arm moves the ultrasound beam a few millimeters at a time to treat the entire therapeutic target by using multiple treatments. Anesthesia requirements are minimal within the prostate but anesthesia is required to prevent patient movement. The high intensity focused ultrasound requires location tracking to ensure safety. This is done in HIFU for prostate cancer using imaging ultrasound. The machines in use have both a treatment transducer and an imaging transducer that is simultaneously visualizing the treatment as it is occurring.

As an acoustic wave moves through the tissue, it is absorbed and converted to heat in focal zones. With focused ultrasound beams, lower temperature heat can pass through normal tissue and the destroying temperatures can be achieved in deep tissue. The beam can therefore pass through areas, such as the rectum, without damaging it and yet damage the prostate cancer within the prostate. The beam usually has a cigar shape in the focal zone, where it is longer than wider along the transducer axis. Tissue damage is a function of both the temperature to which tissue is heated and how long tissue is exposed, and this is measured as a thermal dose. By scanning the focus, a volume of tissue can be thermally treated. The biologic response depends on maximum temperature and duration of temperature [thermal dose]. Temperature elevation as it occurs melts lipid membranes, denatures proteins, produces vascular endothelial cell damage, and ultimately leads to coagulative necrosis. At high enough temperatures, microbubbles will form, causing cavitation. In clinical applications, cavitation and microbubbles are avoided because of potential for unpredictable tissue damage due to microimplosions and shockwaves.

What is HIFU?

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Tissue Change Monitoring:

To avoid areas of under treatment, areas that are seen as green or blue spots are treated again to achieve full ablation.

HIFU: Tissue Effects

Primary thermal effect- absorption of ultrasound energy by tissue and its conversion into heat inducing irreversible damage.

Extensive Fibrosis