Getting Acoustic Energy Precisely to its Target

Focused ultrasound (FUS) and high intensity FUS (HIFU) have found valuable application in a wide range of medical treatments: tumor ablation; treatment of epilepsy, movement disorder, and chronic pain; reversible blood-brain-barrier opening (e.g., to increase uptake of Parkinson drugs); clot lysis; and neurostimulation. They offer the potential of targeting locations deep inside the human body without requiring surgical access. The mechanisms can be thermal (e.g., thermal ablation) or non-thermal. Acoustic propagation is affected by bones, air cavities, and the general anatomy within the exposed domain, which can reduce the focusing ability and produce unwanted secondary foci. Phased array transducers allow for aberration correction, provided suitable steering parameters can be identified for a specific target shape, location and patient anatomy. Modeling enables investigation and design of novel applicators and patient-specific treatment planning.

Sim4Life offers a comprehensive environment for image-based or anatomical model-based simulation of acoustic propagation, induced heating, and resulting effect quantification: The IMG and iSEG modules add support for image integration and image-based model generation. The Virtual Population (ViP) 3.0 anatomical phantoms offer the most detailed and accurate representations of the patient population for device design or in-depth analysis. The P-THERMAL module adds a solver optimized for the modelling of thermal phenomena in living, blood-perfused tissue, while T-CEM43 adds thermal dose and effect assessment models for the quantification of thermal tissue damage and treatment efficacy. For more detail see Thermal therapies.

P-ACOUSTICS seamlessly integrates with all this functionality and permits full-wave modelling of acoustic propagation in the human body or artificial structures at unprecedented resolution and problem size. This is possible thanks to the use of high performance computing (HPC) techniques that harness the power of one or multiple graphics processing units (GPU) cards to accelerate the simulations by orders of magnitude.

Sim4Life has been applied to investigate ways of improving focusing, especially in the context of transcranial focused ultrasound, where applicators with over a thousand ultrasound transducers are used to target locations deep in the brain without opening the skull. Various focusing approaches have been investigated and compared. Superior results, also in comparison with clinically applied methods, were obtained by using the virtual source approach (also known as time reversal technique), where an initial simulation is performed with a source at the target location while recording the incoming pressure wave at the transducer elements. For the real sonication, these signals are then conjugated and applied.

Sim4Life allows to compensate for the impact of skull induced aberration and focus shifting and even enables considering the inhomogeneous nature of the skull bone based on computed tomography (CT) image data. The results indicate the possibility of considerably extending the envelope of treatable regions in the head.

When modeling HIFU tumor ablation in the liver, the virtual source approach was combined with a 4D animated anatomical model, where breathing motion was extracted from MRI images and applied to warp the body model. This allows to study the importance of motion tracking. Furthermore, the virtual source approach is successful in reducing collateral damage to the ribs, as assessed using the T-CEM43 model in combination with thermal modeling.