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Passive cavitation mapping, which generates images using bubble acoustic emission signals, has been increasingly used for monitoring and guiding focused ultrasound surgery. Passive cavitation mapping can be used as an adjunct to magnetic resonance imaging to provide crucial information on the safety and efficacy of focused ultrasound surgery. The most widely used algorithm for passive cavitation mapping is delay-and-sum. One of the major limitations of delay-and-sum is its suboptimal computational efficiency. Although frequency-domain delay-and-sum can partially resolve this issue, such an algorithm is not suitable for imaging the evolution of bubble activity in real time and for cases in which cavitation events occur asynchronously. This study investigates a transient angular spectrum approach for passive cavitation mapping. The working principle of this approach is to back propagate the received signal to the domain of interest and reconstruct the spatial-temporal wave field encoded with the bubble location and collapse time. The transient angular spectrum approach is validated using an in silico model and water bath experiments. It is found that the transient angular spectrum approach yields similar results to delay-and-sum, but it is one order of magnitude faster. The results obtained by this study suggest that the transient angular spectrum approach is promising for fast and accurate passive cavitation mapping.

The penetration depth of photoacoustic imaging in biological tissues has been fundamentally limited by the strong optical attenuation when light is delivered externally through the tissue surface. To address this issue, we previously reported internal-illumination photoacoustic imaging using a customized radial-emission optical fiber diffuser, which, however, has complex fabrication, high cost, and non-uniform light emission. To overcome these shortcomings, we have developed a new type of low-cost fiber diffusers based on a graded-scattering method in which the optical scattering of the fiber diffuser is gradually increased as the light travels. The graded scattering can compensate for the optical attenuation and provide relatively uniform light emission along the diffuser. We performed Monte Carlo numerical simulations to optimize several key design parameters, including the number of scattering segments, scattering anisotropy factor, divergence angle of the optical fiber, and reflective index of the surrounding medium. These optimized parameters collectively result in uniform light emission along the fiber diffuser and can be flexibly adjusted to accommodate different applications. We fabricated and characterized the prototype fiber diffuser made of agarose gel and intralipid. Equipped with the new fiber diffuser, we performed thorough proof-of-concept studies on ex vivo tissue phantoms and an in vivo swine model to demonstrate the deep-imaging capability (~10 cm achieved ex vivo) of photoacoustic tomography. We believe that the internal light delivery via the optimized fiber diffuser is an effective strategy to image deep targets (e.g., kidney) in large animals or humans.