Abstract

Therapeutic Effect of Second Harmonic Focal Spots in High Intensity Focused Ultrasound Treatment of Tissues with Nonlinear Acoustic Properties

C.W.Connor (Massachusetts Institute of Technology, Boston, MA, USA); K.Hynynen (Brigham & Women's Hospital, Harvard Medical School, Boston, MA, USA)

e-mail: cwc@mit.edu

It has long been known that the ultrasound intensities used in high intensity focused ultrasound are sufficient to cause non-linear acoustic effects. These non-linear effects have mostly been disregarded in treatment planning since it has generally been considered that the hyperthermia induced by the focus of the fundamental ultrasound wave substantially outweighs the heating from any co-incident second harmonic waves. However, we have recently shown that certain non-axisymmetric excitation patterns of sector vortex arrays may create regions of intense heating due to the induced second harmonic field; these regions are co-incident with areas of destructive interference of the fundamental wave. Consequently, the hyperthermia induced in these regions is due solely to the second harmonic field induced by the non-linear acoustic behavior of the tissue. As a corollary, we note that treatment prediction algorithms that neglect non-linear acoustic behavior would produce markedly erroneous results for these acoustic configurations. These acoustic fields are produced with tissues having absorption and non-linear characteristics equivalent to human liver, and at pressures suitable for high intensity focused ultrasound therapy. The potential therapeutic effects of these second harmonic focal spots are therefore of interest in the development of abdominal HIFU procedures.

Section : 5