Strategies in therapeutic ultrasound for predicting in situ fields and quantifying the impact of tissue inhomogeneities - PROJECT SUMMARY / ABSTRACT
Focused ultrasound surgery (FUS) describes a treatment modality in which ultrasound energy is delivered to a
targeted tissue site to achieve a therapeutic effect. FUS has already gained regulatory approval in the U.S. for
treating uterine fibroids, pain palliation for bone metastases, and prostate ablation. Other applications including
liver cancer and neurosurgery remain active topics for clinical trials and research. The appeal of FUS is clear:
Nonionizing energy can be delivered to a treatment site noninvasively. Ultrasound can be used as a tool to
induce a range of therapeutic effects, including thermal ablation, enhanced drug delivery, and mechanical
disintegration of soft tissues with precise boundaries. However, in order to fully realize the clinical potential of
FUS, capabilities should be developed for patient-specific treatment planning that is comparable to what is
performed in radiotherapy. At present, typical treatment planning for abdominal targets remains
unsophisticated; rather than utilizing detailed imaging information from each patient in conjunction with full-
wave simulations to predict in situ pressure and temperature fields, treatments generally rely on real-time
feedback from MR thermometry to customize the therapy. This approach does not depend explicitly on
calibration of the ultrasound source and can be successful; however, accurate control of thermal treatments
can be difficult while non-thermal treatments that utilize nonlinear fields and cavitation require an alternate
approach. To improve the safety and effectiveness of FUS for abdominal targets, this project seeks to develop
and test strategies for predicting in situ pressure and temperature fields. To address the underlying challenges,
acoustic holography is a tool that has unique capabilities: It provides an efficient way to capture ultrasound field
information in 3D from measurements in 2D. In addition, backprojection from a measured hologram can be
used to reconstruct the vibrations at the transducer surface, thereby defining an accurate boundary condition
for modeling wave propagation in any known medium to predict the in situ field at any given output level. The
first aim is to reduce the time required for recording an accurate hologram from many hours to tens of minutes
or less, using continuous scanning and specially designed array hydrophones. The second aim is to optimize
strategies for determining the boundary conditions needed to characterize the full range of operating conditions
of phased-array transducers, including beam steering, high output levels, and challenging geometries of
clinical interest. The third aim seeks to use measurement and simulation tools to quantify how tissue
inhomogeneities distort an incident ultrasound beam, including the development of indices that capture the
impacts of readily identified geometrical features on 3D ultrasound fields – e.g., orientation and curvature of the
skin surface, uniformity of transcutaneous fat, and rib location. Overall, this effort will impact public health by
providing a basis for improved treatment planning capabilities in FUS applications.
