In many ultrasound-based therapies, cavitation (the collapse of bubbles) can play a major role in treatment:
- inertial cavitation greatly enhances heating during HIFU
- cavitation acts in various ways to aid localised drug delivery
- inertial cavitation is a key factor in tissue fractionation (or histotripsy) and also important in shock wave lithotripsy
Mapping the spatial and temporal extent of cavitation is therefore an important concern when monitoring ultrasound treatments. This is usually attempted using B-mode ultrasound imaging, where the presence of bubbles creates increased backscatter (hyperechoes) from the probing ultrasound pulses. Alternatively, we have developed array systems and algorithms to conduct passive acoustic mapping (PAM) that can provide accurate spatio-temporal maps of the bubble activity in real-time during the therapy. Since cavitation can act to promote positive therapeutic effects, this activity should spatially correspond to the treated region, and this has been confirmed through both in-vitro and in-vivo experiments.
PAM System Development:
Systems have been developed for real-time fully 3D PAM processing. These systems consist of custom arrays made of PVDF coupled to low-noise preamplifiers and rapid data collection system. The arrays have been primarily designed to not inhibit the existing CE mark of the clinical equipment. Developed software allows for PAM in the three primary planes at real-time clinical frame rates.
Figure: Custom sparse 2D array systems for performing 3D PAM for both HIFU (left) and drug delivery (right) applications where the sensor array system design is made to not remove existing CE marking from the existing ultrasound equipment. The 32 channel PAM array for HIFU has been used in an in-vivo clinical trial and the 64 channel PAM array for drug delivery has been used in in-vitro experiments.
PAM Algorithm Development:
Both sparse linear weighting and optimal beamforming algorithms have been incorporated into improving PAM. Several issues exist to reduce the resolution and image quality of PAM images including low channel count systems and interference from scatterers and multi-bubble interaction. Improvements in the algorithms can aid in improving the resulting images to correlate the cavitation activity to the location of the bioeffect allowing for improved monitoring.
Figure: Cumulative PAM maps of in-vivo experimental drug delivery for conventional (left) and optimal (right) processing collected over 20 HIFU pulses. The white ellipse shows the bounds of the 3dB HIFU focus, which corresponds best to the cavitation activity using the optimal processing (right).
For further details please see our Publications pages.