Sergey Telenkov

Frequency-domain photothermoacoustics: Alternative imaging modality of biological tissues

Frequency-domain photothermoacoustic (FD-PTA) imaging of biological tissues is presented and compared with the conventional time-domain methodology. We demonstrate that tissue imaging can be performed with high axial resolution without the necessity to employ short-pulse and high peak-power laser systems to generate acoustic transients. The presented analysis shows that depth information in the FD-PTA method can be recovered by using linear frequency-modulated (chirped) optical excitation and frequency-domain signal processing algorithms. The signal-to-noise ratio can be increased significantly using correlation processing, which can compensate for the small amplitude of acoustic waves typical to the periodic excitation mode. Additionally, narrow-band signal demodulation enables depth-specific and confocal tissue imaging using the optically induced photothermoacoustic effect. Application of the FD-PTA is demonstrated in experiments with turbid phantoms and ex vivo tissue specimens.

Silica-coated super paramagnetic iron oxide nanoparticles (SPION) as biocompatible contrast agent in biomedical photoacoustics

In this study, we report for the first time the use of silica-coated superparamagnetic iron oxide nanoparticles (SPION) as contrast agents in biomedical photoacoustic imaging. Using frequency-domain photoacoustic correlation (the photoacoustic radar), we investigated the effects of nanoparticle size, concentration and biological media (e.g. serum, sheep blood) on the photoacoustic response in turbid media. Maximum detection depth and the minimum measurable SPION concentration were determined experimentally. The nanoparticle-induced optical contrast ex vivo in dense muscular tissues (avian pectus and murine quadricept) was evaluated and the strong potential of silica-coated SPION as a possible photoacoustic contrast agents was demonstrated.

Signal-to-noise analysis of biomedical photoacoustic measurements in time and frequency domains

Sensitivity analysis of photoacoustic measurements is conducted using estimates of the signal-to-noise ratio (SNR) achieved under two different modes of optical excitation. The standard pulsed time-domain photoacoustic imaging is compared to the frequency-domain counterpart with a modulated optical source. The feasibility of high-SNR continuous wave depth-resolved photoacoustics with frequency-swept (chirp) modulation pattern has been demonstrated. Utilization of chirped modulation waveforms achieves dramatic SNR increase of the periodic signals and preserves axial resolution comparable to the time-domain method. Estimates of the signal-to-noise ratio were obtained using typical parameters of piezoelectric transducers and optical properties of tissue.

Photothermoacoustic imaging of biological tissues: maximum depth characterization comparison of time and frequency-domain measurements

The photothermoacoustic (PTA) or photoacoustic (PA) effect induced in light-absorbing materials can be observed either as a transient signal in time domain or as a periodic response to modulated optical excitation. Both techniques can be utilized for creating an image of subsurface light-absorbing structures (chromophores). In biological materials, the optical contrast information can be related to physiological activity and chemical composition of a test specimen. The present study compares experimentally the two PA imaging modalities with respect to the maximum imaging depth achieved in scattering media with optical properties similar to biological tissues. Depth profilometric measurements were carried out using a dual-mode laser system and a set of aqueous light-scattering solutions mimicking photon propagation in tissue. Various detection schemes and signal processing methods were tested to characterize the depth sensitivity of PA measurements. The obtained results demonstrate the capabilities of both techniques and can be used in specific PTA imaging applications for development of image reconstruction algorithms aimed at maximizing system performance. Our results demonstrate that submillimeter-resolution depth-selective PA imaging can be achieved without nanosecond-pulsed laser systems by appropriate modulation of a continuous laser source and a signal processing algorithm adapted to specific parameters of the PA response.

Photoacoustic correlation signal-to-noise ratio enhancement by coherent averaging and optical waveform optimization

Photoacoustic (PA) imaging of biological tissues using laser diodes instead of conventional Q-switched pulsed systems provides an attractive alternative for biomedical applications. However, the relatively low energy of laser diodes operating in the pulsed regime, results in generation of very weak acoustic waves, and low signal-to-noise ratio (SNR) of the detected signals. This problem can be addressed if optical excitation is modulated using custom waveforms and correlation processing is employed to increase SNR through signal compression. This work investigates the effect of the parameters of the modulation waveform on the resulting correlation signal and offers a practical means for optimizing PA signal detection. The advantage of coherent signal averaging is demonstrated using theoretical analysis and a numerical model of PA generation. It was shown that an additional 5-10 dB of SNR can be gained through waveform engineering by adjusting the parameters and profile of optical modulation waveforms.

Frequency domain photoacoustic correlation (radar) imaging: a novel methodology for non-invasive imaging of biological tissues

We report the development of a novel frequency-domain biomedical photoacoustic (PA) system that utilizes a continuous-wave laser source with a custom intensity modulation pattern for spatially-resolved imaging of biological tissues. The feasibility of using relatively long duration and low optical power laser sources for spatially-resolved PA imaging is presented. We demonstrate that B-mode PA imaging can be performed using an ultrasonic phased array coupled with multi-channel correlation processing and a frequency-domain beamforming algorithm. Application of the frequency-domain PA correlation methodology is shown using tissue-like phantoms with embedded optical contrast, tissue ex-vivo samples and a small animal model in-vivo.

Trends in biothermophotonics and bioacoustophotonics of tissues

Recent trends in bioacoustophotonics and biothermophotonics of tissues are presented. The presentation is centered on the development of well-known frequency-domain photothermal and photoacoustic techniques to address issues associated with diffuse photon density waves during optical excitation of turbid media, both in hard tissues (teeth) and soft tissues. These methods have concrete advantages over the conventional pulsed-laser counterparts. In Part I we present biothermophotonic principles and applications to the detection of the carious state in human teeth as embodied by laser photothermal radiometry supported by modulated luminescence. The emphasis is on the abilities of these techniques to approach important problems such as the diagnosis of occlusal pits and fissures and interproximal lesions between teeth which normally go undetected by x-ray radiographs. In Part II we present theoretical and experimental results in frequency-domain bioacoustophotonics of turbid media, such as soft tissues, and we describe the development of sensitive sub-surface imaging methodologies which hold the promise for sensitive diagnostics of cancerous lesions in e.g. a human breast. Results using tissue phantoms and ex-vivo specimens are discussed and the current level of subsurface lesion sensitivity compared to state-of-the-art pulsed photoacoustic techniques is examined. In summary, advances in coupled frequency-domain diffuse-photon-density-wave and thermal or thermoelastic responses of turbid media constitute new trends in bioacoustophotonics and biothermophotonics promising for their signal quality and high dynamic range.

Photoacoustic sonar: principles of operation, imaging, and signal-to-noise analysis in time and frequency domains

A photoacoustic (PA) imaging methodology utilizing coded optical excitation and correlation signal processing has been described. The basic principles of using relatively long coded waveforms and a matched filter signal compression to increase signal-to-noise ratio (SNR) and axial resolution are common in conventional radar and sonar systems. To emphasize these similarities, the proposed technique is called the photoacoustic sonar (or radar). We describe the implementation of the PA sonar using a near-IR intensity modulated continuous wave laser source and frequency-domain correlation processing of the acoustic response. Application of the PA sonar for imaging of biological materials with discrete chromophores was studied using tissue mimicking phantoms. The SNR gain achieved with linear chirps is analyzed and compared with conventional time-domain photoacoustics.

Combined frequency domain photoacoustic and ultrasound imaging for intravascular applications.

Intravascular photoacoustic (IVPA) imaging has the potential to characterize lipid-rich structures based on the optical absorption contrast of tissues. In this study, we explore frequency domain photoacoustics (FDPA) for intravascular applications. The system employed an intensity-modulated continuous wave (CW) laser diode, delivering 1W over an intensity modulated chirp frequency of 4-12MHz. We demonstrated the feasibility of this approach on an agar vessel phantom with graphite and lipid targets, imaged using a planar acoustic transducer co-aligned with an optical fibre, allowing for the co-registration of IVUS and FDPA images. A frequency domain correlation method was used for signal processing and image reconstruction. The graphite and lipid targets show an increase in FDPA signal as compared to the background of 21dB and 16dB, respectively. Use of compact CW laser diodes may provide a valuable alternative for the development of photoacoustic intravascular devices instead of pulsed laser systems.