Xiaohua Feng

Thermoacoustic resonance effect and circuit modelling of biological tissue

In this letter, thermoacoustic resonance effect is predicted from theoretical analysis with series resistor-inductor-capacitor resonance circuit model and then observed experimentally using muscle tissue illuminated by multi-pulse microwave source. Through model fitting, the circuit parameters are extracted to characterize quantitatively the resonant response of the tissue. Coherent demodulation is applied to obtain the enhanced signal-to-noise ratio and spatial information by treating tissue as a communication channel. This physical phenomenon shows significantly higher sensitivity than conventional single microwave pulse induced thermoacoustic effect, enabling the potential design of low-power thermoacoustic imaging device for portable and on-site diagnosis.

Magnetically mediated thermoacoustic imaging toward deeper penetration

Magnetically mediated thermo-acoustic effect is predicted in theory and demonstrated in phantom studies in this letter. By applying transient current to a compact magnetically resonant coil at radio frequency below 20 MHz, large electric field is inducted by magnetic field inside conductive objects which then undergoes joule heating and emanates acoustic signal thermo-elastically. The magnetic mediation approach with low radio frequency can provide deeper penetration into conductive objects which may extend thermoacoustic imaging to deep laid human organs. Both incoherent time domain and coherent frequency domain approaches are discussed with the latter demonstrated potential for portable imaging system.

Coherent photoacoustic-ultrasound correlation and imaging

Both photoacoustics and ultrasound have been researched extensively but separately. In this letter, we report an initial study on the coherent correlation between pulsed photoacoustic wave and pulse-echo ultrasound wave. By illuminating an object with a pulsed laser and external ultrasound sequentially, both the endogenous photoacoustic wave and pulse-echo ultrasound wave are received and coherently correlated, demonstrating enhanced signal-to-noise ratio. Image contrast of the proposed coherent photoacoustic-ultrasound imaging is also demonstrated to be improved significantly on vessel-mimicking phantom, due to fusion of the optical absorption and ultrasound reflection contrasts by coherent correlation of either conventional laser-induced photoacoustic imaging or pulse-echo ultrasound imaging separately.

Surface engineering of semiconducting polymer nanoparticles for amplified photoacoustic imaging

Despite the deeper tissue penetration of photoacoustic (PA) imaging, its sensitivity is generally lower than optical imaging. This fact partially restricts the applications of PA imaging and greatly stimulates the development of sensitive PA imaging agents. We herein report that the surface coating of semiconducting polymer nanoparticles (SPNs) with the silica layer can simultaneously amplify fluorescence and PA brightness while maintaining their photothermal conversion efficiency nearly unchanged. As compared with the bare SPNs, the silica-coated SPNs (SPNs-SiO2) have higher photothermal heating rate in the initial stage of laser irradiation due to the higher interfacial thermal conductance between the silica layer and water relative to that between the SP and water. Such an interfacial effect consequently results in sharp temperature increase and in turn amplified PA brightness for SPNs-SiO2. By conjugating poly(ethylene glycol) (PEG) and cyclic-RGD onto SPNs-SiO2, targeted PA imaging of tumor in living mice is demonstrated after systemic administration, showing a high signal to background ratio. Our study provides a surface engineering approach to amplify the PA signals of organic nanoparticles for molecular imaging.

Photoacoustic resonance spectroscopy for biological tissue characterization

Abstract. By “listening to photons,” photoacoustics allows the probing of chromosomes in depth beyond the optical diffusion limit. Here we report the photoacoustic resonance effect induced by multiburst modulated laser illumination, which is theoretically modeled as a damped mass-string oscillator and a resistor-inductor-capacitor (RLC) circuit. Through sweeping the frequency of multiburst modulated laser, the photoacoustic resonance effect is observed experimentally on phantoms and porcine tissues. Experimental results demonstrate different spectra for each phantom and tissue sample to show significant potential for spectroscopic analysis, fusing optical absorption and mechanical vibration properties. Unique RLC circuit parameters are extracted to quantitatively characterize phantom and biological tissues.

Photoacoustic phasoscopy super-contrast imaging

Phasoscopy is a recently proposed concept correlating electromagnetic (EM) absorption and scattering properties based on energy conservation. Phase information can be extracted from EM absorption induced acoustic wave and scattered EM wave for biological tissue characterization. In this paper, an imaging modality, termed photoacoustic phasoscopy imaging (PAPS), is proposed and verified experimentally based on phasoscopy concept with laser illumination. Both endogenous photoacoustic wave and scattered photons are collected simultaneously to extract the phase information. The PAPS images are then reconstructed on vessel-mimicking phantom and ex vivo porcine tissues to show significantly improved contrast than conventional photoacoustic imaging.

Thermally modulated photoacoustic imaging with super-paramagnetic iron oxide nanoparticles

Thermally modulated photoacoustic imaging (TMPI) is reported here for contrast enhancement when using nanoparticles as contrast agents. Exploiting the excellent sensitivity of the photoacoustic (PA) process on temperature and the highly selective heating capability of nanoparticles under electromagnetic field, the PA signals stemming from the nanoparticles labeled region can be efficiently modulated whereas those from highly light absorptive backgrounds are minimally affected. A coherent difference imaging procedure reduces the background signal and thus improves the imaging contrast. Phantom experiments with super-paramagnetic iron oxide nanoparticles (SPIONs) as contrast agents and alternating magnetic fields for heating are demonstrated. Further improvements toward clinical applications are also discussed.

Fast photoacoustic-guided depth-resolved Raman spectroscopy: a feasibility study

In this Letter, photoacoustic-guided Raman spectroscopy (PARS) is proposed for a fast depth-resolved Raman measurement with accurate depth localization. The approach was experimentally demonstrated to receive both photoacoustic and Raman signals from a three-layer agar phantom based on a developed synergic photoacoustic-Raman probe, showing strong depth correlation and achieving magnitude of faster operation speed due to photoacoustic time-of-flight measurement and guidance, compared with the conventional depth-resolved Raman spectroscopy method. In addition, further combination with advanced optical-focusing techniques in biological-scattering medium could potentially enable the proposed approach for cancer diagnostics with both tight and fast optical focusing at the desired depth of tumor.

Self temperature regulation of photothermal therapy by laser-shared photoacoustic feedback

This article describes a laser-shared photothermal system that achieves tight temperature regulation by frequency-domain photoacoustic (FD-PA) feedback. To this end, a continuous-wave laser system was designed with arbitrarily modulatable laser intensity. And, by fast alternating in the time domain between a constant laser intensity for photothermal heating and a modulated laser intensity for FD-PA temperature measurement, photothermal temperature variations are captured by FD-PA in real time. A proportional-integral-derivative (PID) controller monitors the feedback from FD-PA measurements and controls photothermal heating dose accordingly, thus stabilizing the temperature at preset values. The proposed system is demonstrated to achieve ultrafast temperature measurement at a 4 kHz rate, and with proper averaging, the measurement and regulation accuracy are 0.75 deg and 0.9 deg respectively.

Modulatable magnetically mediated thermoacoustic imaging with magnetic nanoparticles

Modulatable magnetically mediated thermoacoustic imaging with magnetic nanoparticles is reported here. Under a pulsed radio frequency magnetic field, magnetic nanoparticles absorb energy strongly from the field and then emanate ultrasound signal thermoelastically. The energy absorption and, consequently, generated thermoacoustic signal strength depend sensitively on the magnetization state of magnetic nanoparticles, which can therefore be modulated effectively by a “bias” magnetic field. The magnetic modulation is demonstrated with a static magnet and modulated phantom imaging results are presented. This method offers an alternative modality for mapping magnetic nanoparticles and its unique modulation capability is demonstrated to be useful for contrast enhancement.