Aberration by soft tissues in therapeutic ultrasound: Current noninvasive correction methods and their challenges

One of the challenges of transcutaneous high-intensity focused ultrasound (HIFU) therapies is the deterioration of focus due to aberration by inhomogeneous soft tissues. This effect is of particular importance in applications where the acoustic path is through inhomogenously distributed layers of fat such as kidney and breast tumor ablation. Deformation and respiratory motion of the soft tissues result in additional complexity of the problem. Using multi-element HIFU arrays, aberrations can be corrected by implementing time delays on each element of the array to compensate for differences in travel time caused by variations in the thickness and sound speed of the tissue layers in the beam path. Various solutions have been developed to determine these time delays. In particular, noninvasive methods relying on harmonic pulse-echo sensing with the HIFU array combined with CT- or 3-D ultrasound-based simulations have shown promise. This presentation will focus on these methods and their limitations, including the isoplanatic patch — the volume where the aberration correction made at one point remains applicable. A way of measuring the isoplanatic patch size of porcine body wall will be presented, and methods to reduce the number of corrections needed to treat a large volume will be discussed.