Research Interest

Acoustic and thermal modelling

GR ter Haar, S Woodford, I Rivens, in collaboration with N Saffari University College, London
Source of funding: EPSRC

HIFU treatment involves the creation of multiple adjacent ablated regions (lesions) to ensure complete ablation of the targeted region. Each lesion is roughly ellipsoidal, with a length of 6-30 mm and a diameter of 0.1-3.0 mm, depending on the transducer geometry. Effective treatment requires the ability to plan the necessary positions and sizes of lesions, and to place the lesions sufficiently accurately to produce confluent damage. 

Broadly speaking, there are two types of difficulties encountered during treatment: delivery of ultrasound to the desired location and compensating for inhomogeneities in the heating profile. Ultrasound delivery is complicated by many effects. Tissue inhomogeneity and tissue motion due to the cardiac and respiratory cycles make aiming the ultrasound difficult. The ribcage reflects ultrasound strongly, so it is difficult to provide adequate power to organs such as the liver or pancreas without causing localized “hot spots” at the rib/tissue interface or in regions of high absorption, such as the skin. During treatment, the properties of the tissue will change — small changes in temperature can affect acoustic properties such as the sound speed, causing thermal defocusing while treated regions will have very different acoustic properties.  A 100% probability of necrosis can only be assured with substantial overtreatment, which is associated with long treatment times and the raised probability of side effects. In addition, overtreatment of a region can lead to the formation of a cavitation cloud, which acoustically shields other regions, limiting the ability to treat the whole target. Finally, long treatment times will provide time for the body’s regulatory responses such as vasodilation, complicating treatment planning.  Thus the aim is to provide each region of tissue with a “thermal dose” that is associated with, e.g., 99% necrosis. 

Most cells surviving this process are expected to be sufficiently damaged to become apoptotic, and any remaining cells will die from ischaemia, since they are isolated from sources of nutrition and oxygen.  Large volumes of tissue exposed to ultrasound are unlikely to be heated uniformly. Blood flow cools the tissue and reduces damage, particularly protecting cells within the vessel wall. Thus highly-perfused regions and tissue near large blood vessels may be spared thermal necrosis and apoptosis. Clearly a treatment protocol that ignores this cooling effect has a reduced probability of success.

We have therefore constructed a tissue heating model that includes the effect of various sized vessels and different blood flow rates on the temperature achieved by an ultrasound exposure.  This will become a component of our treatment planning software.

A comparison of the critical blood flow velocity for which increasing the treatment duration reduces the impact of perfusion and typical flow velocities.