Passive cavitation imaging of echogenic liposomes insonified with 6 MHz pulsed Doppler ultrasound in a flow phantom.

Microbubble activity has been linked to bioeffects in a variety of therapeutic ultrasound applications including thermal ablation and localized drug delivery. Passive cavitation imaging of microbubble activity has been demonstrated in vitro during continuous‐wave ultrasound exposure [Salgaonkar et al. (2009), JASA 126, 3071–3083; Gyongy and Coussios 2010, IEEE Trans BME 57, 48–56]. Here, we perform passive cavitation imaging of echogenic liposomes (ELIPs) in a flow phantom exposed to 6 MHz pulsed Doppler ultrasound (3.1 μs pulse duration, 1250 Hz pulse repetition frequency) transmitted from a diagnostic scanner (Philips HDI 5000, CL15‐7 transducer). Frames of received ultrasound signal were obtained passively on 64 parallel channels recording for 112 μs (L8‐3 transducer, Zonare z.one ultra scanner). Received echoes were beamformed by frequency‐domain phase shifts to produce cavitation images. Localization of cavitation activity was tested by moving the Doppler exposure volume to three different positions and observing a concurrent spatial shift in cavitation activity. At the highest insonation pressures a loss in echogenicity was observed. Cavitation activity coincided spatially with this loss. Subharmonic and broadband cavitation images for multiple insonation pressures were consistent with previously determined stable and inertial cavitation thresholds for ELIP. [Work supported in part by NIH grants F32HL104916, R01HL074002, R21EB008483, R01HL059586, and R01NS047603.]