We have developed reflection-mode multispectral photoacoustic microscopy (PAM) based on a novel optical–acoustic objective that integrates a customized ultrasonic transducer and a commercial reflective microscope objective into one solid piece. This technical innovation provides zero chromatic aberration and convenient confocal alignment of the optical excitation and acoustic detection. With a wavelength-tunable optical-parametric-oscillator laser, we have demonstrated multispectral PAM over an ultrabroad spectral range of 270–1300 nm. A near-constant lateral resolution of ∼2.8 μm is achieved experimentally. Capitalizing on the consistent performance over the ultraviolet, visible, and near-infrared range, multispectral PAM enables label-free concurrent imaging of cell nucleus (DNA/RNA contrast at 270 nm), blood vessel (hemoglobin contrast at 532 nm), and sebaceous gland (lipid contrast at 1260 nm) at the same spatial scale in a living mouse ear.

An intravascular ultrasound (IVUS) and microbubble drug delivery system was evaluated in both ex vivo and in vivo swine vessel models. Microbubbles with the fluorophore DiI embedded in the shell as a model drug were infused into ex vivo swine arteries at a physiological flow rate (105 mL/min) while a 5 MHz IVUS transducer applied ultrasound. Ultrasound pulse sequences consisted of acoustic radiation force pulses to displace DiI-loaded microbubbles from the vessel lumen to the wall followed by higher intensity delivery pulses to release DiI into the vessel wall. Insonation with both the acoustic radiation force pulse and the delivery pulse increased DiI deposition 10-fold compared to deposition with the delivery pulse alone. Localized delivery of DiI was then demonstrated in an in vivo swine model. The theoretical transducer beam width predicted the measured angular extent of delivery to within 11%. These results demonstrate that low frequency IVUS catheters are a viable method for achieving localized drug delivery with microbubbles.

Previous research has demonstrated that acoustic radiation force enhances intravascular microbubble adhesion to blood vessels in the presence of flow for molecular-targeted ultrasound imaging and drug delivery. A prototype acoustic radiation force intravascular ultrasound (ARFIVUS) catheter was designed and fabricated to displace a microbubble contrast agent in flow representative of conditions encountered in the human carotid artery. The prototype ARFIVUS transducer was designed to match the resonance frequency of 1.4- to 2.6-μm-diameter microbubbles modeled by an experimentally verified 1-D microbubble acoustic radiation force translation model. The transducer element was an elongated Navy Type I (hard) lead zirconate titanate (PZT) ceramic designed to operate at 3 MHz. Fabricated devices operated with center frequencies of 3.3 and 3.6 MHz with −6-dB fractional bandwidths of 55% and 50%, respectively. Microbubble translation velocities as high as 0.86 m/s were measured using a high-speed streak camera when insonating with the ARFIVUS transducer. Finally, the prototype was used to displace microbubbles in a flow phantom while imaging with a commercial 45-MHz imaging IVUS transducer. A sustained increase of 31 dB in average video intensity was measured following insonation with the ARFIVUS, indicating microbubble accumulation resulting from the application of acoustic radiation force.

Potent therapeutic compounds with dose dependent side effects require more efficient and selective drug delivery to reduce systemic drug doses. Here, we demonstrate a new platform that combines intravascular ultrasound (IVUS) and drug-loaded microbubbles to enhance and localize drug delivery, while enabling versatility of drug type and dosing. Localization and degree of delivery with IVUS and microbubbles was assessed using fluorophore-loaded microbubbles and different IVUS parameters in ex vivo swine arteries. Using a swine model of neointimal hyperplasia, reduction of neointima formation following balloon injury was evaluated when using the combination of IVUS and sirolimus-loaded microbubbles. IVUS and microbubble enhanced fluorophore delivery was greatest when applying low amplitude pulses in the ex vivo model. In the in vivo model, neointima formation was reduced by 50% after treatment with IVUS and the sirolimus-loaded microbubbles. This reduction was achieved with a sirolimus whole blood concentration comparable to a commercial drug-eluting stent (0.999 ng/mL). We anticipate this therapy will find clinical use localizing drug delivery for numerous other diseases in addition to serving as an adjunct to stents in treating atherosclerosis.

There is interest in examining the potential of modified intravascular ultrasound (IVUS) catheters to facilitate dual diagnostic and therapeutic roles using ultrasound plus microbubbles for localized drug delivery to the vessel wall. The goal of this study was to design, prototype, and validate an IVUS transducer for microbubble-based drug delivery. A 1-D acoustic radiation force model and finite element analysis guided the design of a 1.5-MHz IVUS transducer. Using the IVUS transducer, biotinylated microbubbles were displaced in water and bovine whole blood to the streptavidin-coated wall of a flow phantom by a 1.5-MHz center frequency, peak negative pressure = 70 kPa pulse with varying pulse repetition frequency (PRF) while monitoring microbubble adhesion with ultrasound. A fit was applied to the RF data to extract a time constant (τ). As PRF was increased in water, the time constant decreased (τ = 32.6 s, 1 kHz vs. τ = 8.2 s, 6 kHz), whereas in bovine whole blood an adhesion–no adhesion transition was found for PRFs ≥ 8 kHz. Finally, a fluorophore was delivered to an ex vivo swine artery using microbubbles and the IVUS transducer, resulting in a 6.6-fold increase in fluorescence. These results indicate the importance of PRF (or duty factor) for IVUS acoustic radiation force microbubble displacement and the potential for IVUS and microbubbles to provide localized drug delivery.