Imaging With Therapeutic Acoustic Wavelets–Short Pulses Enable Acoustic Localization When Time of Arrival is Combined With Delay and Sum

Abstract

Passive acoustic mapping (PAM) is an algorithm that reconstructs the location of acoustic sources using an array of receivers. This technique can monitor therapeutic ultrasound procedures to confirm the spatial distribution and amount of microbubble activity induced. Current PAM algorithms have an excellent lateral resolution, but have a poor axial resolution, making it difficult to distinguish acoustic sources within the ultrasound beams. With recent studies demonstrating that shortlength and low-pressure pulsesacoustic waveletshave therapeutic function, we hypothesized that the axial resolution could be improved with a quasi-pulse-echo approach and that the resolution improvement would depend on the wavelet's pulse length. This paper describes an algorithm that resolves acoustic sources axially using time of flight and laterally using delay-andsum beamforming, which we named axial temporal position passive acoustic mapping (ATP-PAM). The algorithm accommodates a rapid short pulse (RaSP) sequence that can safely deliver drugs across the blood-brain barrier. We developed our algorithm with simulations (k-wave) and in vitro experiments for 1-, 2-, and 5-cycle pulses, comparing our resolution against that of two current PAM algorithms. We then tested ATP-PAM in vivo and evaluated whether the reconstructed acoustic sources mapped to drug delivery within the brain. In simulations and in vitro, ATP-PAM had an improved resolution for all pulse lengths tested. In vivo, experiments in mice indicated that ATP-PAM could be used to target and monitor drug delivery into the brain. With acoustic wavelets and time of flight, ATP-PAM can locate acoustic sources with a vastly improved spatial resolution.

A PZT–PVDF Stacked Transducer for Short-Pulse Ultrasound Therapy and Monitoring

“…Our rough calculation is that 10% signal transmission through the skull would lead to an approximate SNR reduction by 100 times. However, it may be possible to recover this loss in SNR by using hundreds to thousands of receiving elements [47] using passive acoustic mapping algorithms [48][49][50].…”

Passive Cavitation Detection With a Needle Hydrophone Array

“…In this experiment, the microbubbles were diluted to a ratio We used an axial temporal position passive acoustic mapping (ATP-PAM) algorithm to localize the microbubble cloud. This algorithm combines time-of-flight calculations and delay-and-sum beamforming to improve the imaging axial resolution and to remove artifacts [10,38]. The time window used was 6 μs and the sound speed in water was assumed to be 1498 m/s.…”

“…Using a passive cavitation detector (PCD), microbubble activity can be monitored both non-invasively and in real-time [4][5][6][7][8][9]. When combined with an array of detectors and beamforming algorithms (e.g., passive acoustic mapping), such microbubble activities can even be localized [10][11][12][13][14][15][16]. However, PCDs could benefit from several improvements, such as sensors designed to better decipher the unique sounds that cavitation generates and localize the cavitation event.…”

“…Also, due to their large size, focused transducers spatially average the radiated sound on its surface when the acoustic source is outside the focal zone, resulting in less sensitivity and distortion. Linear and hemispherical arrays can localize bubbles [10][11][12][13][14][15], but they are not ideal for detecting broadband microbubble emissions.…”

A Novel Approach for the Detection of Every Significant Collapsing Bubble in Passive Cavitation Imaging

“…Based on this principle, the blood vessels to which stabilized micro-bubbles, what's called, ultrasonic contrast agents, are injected can effectively be observed on the ultrasonic images [14,15]. For the therapeutic purpose, the micro-bubbles may be activated by ultrasound to open blood brain barrier(BBB) and to deliver drugs [12,13].…”