Beam characterization of a custom, handheld focused ultrasound system using thermochromic liquid crystal films

Focused Ultrasound (FUS) continues to gain acceptance in the clinical realm for its ability to provide effective, non-invasive therapeutic treatments to almost any region of the body for a host of ablative and non-ablative applications. The development of FUS devices and their implementation requires reliable characterization and quality assurance methods to verify acoustic pressure fields and focal region characteristics. The use of hydrophones is a conventional technique for mapping the acoustic field in 3 dimensions to provide focal dimensions and location. Hydrophones, however, are expensive and may be damaged even at relatively low acoustic amplitudes. Data collection with these devices can also be labor intensive and difficult to accurately reproduce. We present preliminary findings for the development of an alternative characterization process for FUS transducers that is relatively inexpensive and time efficient. Thermochromic liquid crystal (TLC) film sensors exploit the thermochromic effect, in which exposure to specific temperature changes cause a visible change in colour. The method was tested on a portable FUS system developed in-house for FUS-based therapeutic applications, comprised of a 3.57 MHz FUS transducer, and a custom-fabricated coupling cone. The results demonstrated that this method using TLC films was able to accurately provide dimensions of the focal zone and its position relative to the transducer hardware. Numerical simulations were performed along with acoustic hydrophone measurements to corroborate this data, which were found to be in general agreement. With future refinements, this cost-effective method could be practical as an expedient and cost-effective characterization technique for in-house FUS transducer development.