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Ultrasound cavitation of microbubble contrast agents has a potentialfor therapeutic applications, including sonothrombolysis in acute ischemic stroke. For safety, efficacy, and reproducibility of treatment, it is critical to evaluate the cavitation state (e.g. stable versus inertial forms of cavitation) and intensity in and around a treatment area. Acoustic Passive Cavitation Detectors (PCDs) have been used but lack spatial information. This paper presents a prototype ofa 2D cavitation imager capable of producing images of the dominantcavitation state and intensity in a region of interest at a frame rate of 0.6Hz. The system is based on a commercial ultrasound scannerand imaging probe (iE33 scanner with S5-1 probe, Philips). Cavitation imaging is based on the spectral analysis of acoustic signal radiated by the cavitating microbubbles: ultraharmonics of the excitationfrequency indicate stable cavitation, while noise bands indicate inertial cavitation. The system demonstrates the capability to robustly identify stable and inertial cavitation thresholds of Definity microbubbles (Lantheus) in a vessel phantom through 3 ex-vivo human temporal bones, as well as to spatially discriminate the location of cavitation activities.

Background: Inertial cavitation may cause hazardous bioeffects whileusing ultrasound and microbubble mediated thrombolysis. The purposeof this study was to investigate the influence of ultrasound pulselength and temporal bone on inertial cavitation thresholds within the brain utilizing transtemporal imaging transducers. Methods: A pig temporal bone overlaid with muscle tissue was placed over silastictubing containing a dilute microbubble infusion (0.5% Definity) within Phosphate Buffered Saline at 37 °C. A 1.6 MHz Philips iE33 two-dimensional probe (S5-1) imaged at incremental peak negative pressures. Broadband noise signals were recorded to characterize inertial cavitation using two 20 MHz passive cavitaion detectors (PCD). Backscattered RF signals were recorded by iE33. Results: About half of the acoustic pressure was attenuated by the temporal bone. Peak-negative-pressure thresholds of inertial cavitation were approximately equal to 0.51 and 0.31 MPa, 0.46 and 0.29 MPa for 5 and 20 microsecondspulse durations with and without bone, respectively. RF signals from the S5-1 correlated with inertial cavitation thresholds from the PCD. Conclusion: The threshold of inertial cavitation is influencedby ultrasound pulse length and temporal bone. RF signals can be used to characterize cavitation behavior for bone attenuation estimation and compensation.

Aim: In the past decade ultrasound (US) has become the primary modality for interventional procedures, owing to its low cost, ease of use and real-time soft tissue visualization. The main limitation however, is the visualization of surgical tools due to their artifact prone response. Methods: This paper presents a new method for accurate,robust, inexpensive and real-time 3D tracking of surgical tools. The paper proposes a new sensing technology that utilizes miniature UScrystals that can be easily mounted on a surgical tool. As part ofcurrent clinical workflow, the US imager emits US waves to image thetissue. The sensor then converts this acoustic energy into electrical signals, which the system analyzes to reconstruct the 3D coordinates of the sensor. These coordinates can be used for 3D surgical navigation, similar to current day EM/optical tracking systems. Results: A prototype system with real-time 3D tool tracking and image enhancement was implemented. Extensive phantom experiments with 2mm single-element PZT crystal show robust tracking with a wide range of imaging conditions. The 3D tracking accuracy, tested using a precision robotic stage, was found to be 0.36 ± 0.16 mm in translation throughout the imaging volume. The experiments also show strong robustness to variations in tool position and orientation. Phantom experiments also prove ability to track a tool inside the beating heart. Conclusions: The paper proposes a new tool tracking technology for US-guidedinterventions, with a performance significantly superior to existing tool tracking technologies.

Stroke is the second cause of death and leading cause of disabilityworldwide. Less than 5% of ischemic stroke patients receive the state-of-the art treatment of a thrombolytic drug tPA, and only about 10% of these gain additional benefit from it. Ultrasound (US)-inducedmicrobubble (MB) cavitation has been shown to enhance the efficacy of the tPA drug or dissolve clots without tPA. Such a sonothrombolysis (STL) treatment requires monitoring and control of MB cavitation to ensure its reproducible efficacy and safety. This paper presents a prototype of a US cavitation imaging system. It is a part of an image-guided sonothrombolysis system based on a commercial US scanneriE33 (Philips Healthcare) with an imaging probe S5-1. Backscattereddata from insonified MBs is spectrally analyzed to identify the dominant cavitation state: ultraharmonics indicate Stable Cavitation (SC) and broadband noise indicates Inertial Cavitation (IC). Cavitation at lower levels (neither of SC or IC) are classified as Moderate Oscillations (MO). The system is evaluated in vitro and in vivo. Avessel phantom with Definity microbubbles was imaged through a water path and through a human temporal bone sample. In vivo experimentshave also been conducted for detecting cavitation in real time in the brain transcranially in two pigs. Cavitation images have also been obtained and processed offline on 17 pigs of a swine sonothrombolysis study. The lateral resolution of the system is approximately 3mm at a 6cm depth, and the axial resolution is 3cm for a 20µs pulselength. The maximum frame rate of the prototype system is 2Hz. Cavitation imaging allows assessing the relative importance of the different cavitation states (MO, SC and IC) in the treatment area inside the skull and their changes as a function of acoustic amplitude. Thetemporal evolution of cavitation can also be assessed, showing thatone 20us pulse destroys the majority of the MBs in the treatment area at MIs higher than 1. Such a therapy monitoring system will be critical for the reproducible safe and effective administration of STLtreatment for acute ischemic stroke.