Detection of weak optical absorption by optical-resolution photoacoustic microscopy.

Optical-resolution photoacoustic microscopy (OR-PAM) is one of the major implementations of photoacoustic (PA) imaging. With tightly focused optical illumination and high-frequency ultrasound detection, OR-PAM provides micrometer-level resolutions as well as high sensitivity to optical absorption contrast. Traditionally, it is assumed that the detected PA signal in OR-PAM has a linear dependence on the target's optical absorption coefficient, which is the basis for quantitative functional and molecular PA imaging. In this paper, we demonstrate that, due to the limited detection bandwidth and detection view, OR-PAM can have a strong nonlinear dependence on the optical absorption, especially for weak optical absorption (<10 cm-1). We have investigated the nonlinear dependence in OR-PAM using numerical simulations, analyzed the underlining mechanisms, proposed potential solutions, and experimentally confirmed the results on phantoms. This work may correct a traditional misunderstanding of the OR-PAM signals and improve quantitative accuracy for functional and molecular applications.

Low-Cost Multi-Wavelength Photoacoustic Imaging Based on Portable Continuous-Wave Laser Diode Module.

Photoacoustic imaging (PAI), an emerging imaging technique, exploits the merits of both optical and ultrasound imaging, equipped with optical contrast and deep penetration. Typical linear PAI relies on a nanosecond laser pulse to induce photoacoustic signals. To construct a multi-wavelength PAI system, a multi-wavelength nano-second laser source is required, which greatly increases the cost of the PAI system. However, according to the nonlinear photoacoustic effect, the amplitude of the photoacoustic signals will vary with different base temperatures of the tissue. Therefore, using continuous-wave lasers with different wavelengths to induce different temperature variations at the same point of the tissue, and then using a single-wavelength pulsed laser to induce photoacoustic signals has been an alternative method to achieve multi-wavelength PAI. In this paper, based on the nonlinear photoacoustic effect, we developed a continuous-wave multi-wavelength laser source to cut down the cost of the conventional multi-wavelength PAI system. The principle will be introduced firstly, followed by qualitative and quantitative experiments.

Enabling both time-domain and frequency-domain photoacoustic imaging by a fingertip laser diode system.

Photoacoustic imaging has attracted increasing research interest in recent years, due to its unique merit of combining light and sound. Enabling deep tissue imaging with high ultrasound spatial resolution and optical absorption contrast, photoacoustic imaging has been applied in various application scenarios including anatomical, functional and molecular imaging. However, the bulky and expensive laser source is one of the key bottlenecks that needs to be addressed for further compact system development. A photoacoustic imaging system based on a low-cost laser diode (LD) is one of the promising solutions. In this paper, we report a custom-made fingertip laser diode system enabling both pulsed and continuous modulation modes with shortest pulse-width of 40 ns, largest driving current of 13 A, and highest modulation frequency of 3 MHz, which is suitable for both time and frequency domain photoacoustic imaging. To the best of our knowledge, this may be the most compact laser source reported for photoacoustic imaging enabling both two modulation modes. Owing to its super-compact size, the proposed LD system could pave the pathway to a low-cost photoacoustic sensing and imaging device, even wearable photoacoustic biomedical sensors.

Optical spectroscopic ultrasound displacement imaging.

Photoacoustic imaging has been intensively studied in recent years, and many of the achievements have already been applied in important biomedical and clinical applications, e.g. spectroscopic photoacoustic imaging to extract functional and molecular information. However, spectroscopic photoacoustic imaging requires expensive and bulky tunable laser source, which severely hinder its further development towards portable device. In this paper, we propose a novel imaging method, named optical spectroscopic ultrasound displacement (OSUD) imaging, which enables optical spectroscopic imaging in deep scattering tissue using multiple low-cost continuous-wave laser sources and ultrasound imaging equipment. The principle of the OSUD imaging method will be introduced, and followed by preliminary experimental results. The OSUD imaging may provide another pathway to provide spectroscopic optical absorption contrast in deep scattering tissue beyond commonly used photoacoustic imaging.

Wavelet de-noising method with adaptive threshold selection for photoacoustic tomography.

Photoacoustic (PA) tomography enables imaging of optical absorption property in deep scattering tissue by listening to the PA wave. However, it is an open challenge that the conversion efficiency from light to sound based on PA effect is extremely low. The consequence is the poor signal-to-noise ratio (SNR) of PA signal especially in scenarios of low laser power and deep penetration. The conventional way to improve PA signal's SNR is data averaging, which however severely limits the imaging speed. In this paper, we propose a new adaptive wavelet threshold de-noising (aWTD) algorithm, and apply it in photoacoustic tomography to increase the PA signal's SNR without sacrificing the signal fidelity and imaging speed. PA image quality in terms of contrast is also significantly improved. The proposed method provides the potential to develop real-time low-cost PA tomography system with low-power laser source.

Hybrid multi-wavelength photoacoustic imaging.

Multi-wavelength photoacoustic (PA) imaging has been studied extensively to explore the spectroscopic absorption contrast of biological tissues. To generate strong PA signals, a high-power wavelength-tunable pulsed laser source has to be employed, which is bulky and quite expensive. In this paper, we propose a hybrid multi-wavelength PA imaging (hPAI) method based on combination of single-wavelength pulsed and multi-wavelength continuous-wave (CW) laser sources. By carefully controlling laser illumination sequence (pulse-CW-pulse), and extracting the PA signals' difference before and after heating of CW lasers, the optical absorption property of multi-wavelength CW lasers could be obtained. Compared with conventional PA imaging, the proposed hPAI shows much lower system cost due to the usage of single-wavelength pulsed laser and cheap CW lasers. Theoretical analysis and analytical model are presented in this paper, followed by proof-of-concept experimental results.

Pattern-learning-based Noise Elimination Algorithm in Photoacoustic Sensing and Imaging.

As one of the fastest-growing imaging modalities in recent years, photoacoustic (PA) imaging has attracted tremendous research interest for various applications including anatomical, functional and molecular imaging. However, the PA signal's amplitude is usually quite weak and can be easily distorted by instrumental noise and interference, which can severely degrade the image quality. To improve the PA signal's signal-to-noise ratio efficiently, this paper introduces a pattern-learning based PA (PLPA) detection method to eliminate the periodically interference noise for PA sensing and imaging. Both simulation and experimental results are demonstrated to prove the validity of the proposed algorithm.

Hybrid multi-wavelength nonlinear photoacoustic sensing and imaging.

Multi-wavelength photoacoustic (PA) imaging has been studied extensively to explore the spectroscopic absorption contrast of biological tissues. To generate strong PA signals, a high-power wavelength tunable pulsed laser source has to be employed, which is bulky and quite expensive. In this Letter, we propose a hybrid multi-wavelength PA imaging (hPAI) method based on the combination of a single-wavelength pulsed laser source and multi-wavelength continuous-wave (CW) laser sources. By carefully controlling the laser illumination sequence (pulse-CW-pulse) and extracting the PA signal difference before and after the heating of CW lasers, the optical absorption property of multi-wavelength light illumination could be obtained. Compared with conventional PA imaging, the proposed hPAI shows a much lower system cost due to the usage of single-wavelength pulsed lasers and multiple inexpensive CW lasers. As the preliminary results show in this Letter, hPAI imaging has the potential to provide another pathway for high spectroscopic optical absorption contrast in PA imaging.

Review of Low-Cost Photoacoustic Sensing and Imaging Based on Laser Diode and Light-Emitting Diode.

Photoacoustic tomography (PAT), a promising medical imaging method that combines optical and ultrasound techniques, has been developing for decades mostly in preclinical application. A recent trend is to utilize the economical laser source to develop a low-cost sensing and imaging system, which aims at an affordable solution in clinical application. These low-cost laser sources have different modulation modes such as pulsed modulation, continuous modulation and coded modulation to generate different profiles of PA signals in photoacoustic (PA) imaging. In this paper, we review the recent development of the photoacoustic sensing and imaging based on the economical laser sources such as laser diode (LD) and light-emitting diode (LED) in different kinds of modulation types, and discuss several representative methods to improve the performance of such imaging systems based on low-cost laser sources. Finally, some perspectives regarding the future development of portable PAT systems are discussed, followed by the conclusion.