Multiple wavelength fringe analysis for surface profile measurements

Interferometry has been widely used for surface metrology because of their precision, reliability, and versatility. Although monochromatic-light interferometery can provide high sensitivity and resolution, but it fails to quantify largediscontinuities. Multiple-wavelength techniques have been successfully used to extend the unambiguous step height measurement rage of single wavelength interferometer. The use of RGB CCD camera allows simultaneous acquisition of fringes generated at different wavelengths. In this work, we discuss details about the fringe analysis of white light interferograms acquired using colour CCD camera. The colour image acquired using RGB camera is decomposed in to red, green, blue components and corresponding interference phase is measured using phase evaluation algorithms. The approach makes the 3D surface measurements faster, cost-effective for industrial applications.NMRC (Natl Medical Research Council, S’pore)Published versio

Pulsed laser diode based optoacoustic imaging of biological tissues

Optoacoustic tomography (OAT) is a promising hybrid imaging modality for many biomedical applications. However, the use of Nd:YAG laser as excitation source makes the OAT system expensive, and non-portable. Miniaturization of OAT system is an immediate task to make it a potential tool for both preclinical and clinical studies. In this work, we present an OA system that uses a miniaturized pulsed laser diode which can provide near infrared pulses at ~803 nm. With laser fluence ~0.3 mJ cm−2 on the tissue surface, we achieved 3 cm deep imaging in chicken tissue. The high pulse repetition rate of the diode laser allowed rapid acquisition of OA cross-sectional images with good image quality. The results promise that the proposed system has potential to be used as an alternative to Nd:YAG based OAT systems for biological imaging applications.MOE (Min. of Education, S’pore)Accepted versio

Dynamic in vivo imaging of small animal brain using pulsed laser diode-based photoacoustic tomography system

We demonstrate dynamic in vivo imaging using a low-cost portable pulsed laser diode based photoacoustic tomography system. The system takes advantage of an 803 nm pulsed laser diode having high repetition rate ~7000 Hz combined with a fast-scanning single-element ultrasound transducer leading to a 5 seconds cross-sectional imaging. Cortical vasculature is imaged in scan-time of 5 seconds with high signal-to-noise ratio ~48. To examine the ability for dynamic imaging, we monitored the fast uptake and clearance process of indo-cyanine green in the rat brain. The system will find applications to study neuro-functional activities, characterization of pharmacokinetic and bio-distribution profiles in the development process of drugs or imaging agents.NMRC (Natl Medical Research Council, S’pore)MOH (Min. of Health, S’pore)Published versio

Performance characterization of low-cost, high-speed, portable pulsed laser diode photoacoustic tomography (PLD-PAT) system

Photoacoustic tomography systems that uses Q-switched Nd:YAG/OPO pulsed lasers are expensive, bulky, and hence limits its use in clinical applications. The low pulse repetition rate of these lasers makes it unsuitable for real-time imaging when used with single-element ultrasound detector. In this work, we present a pulsed laser diode photoacoustic tomography (PLD-PAT) system that integrates a compact PLD inside a single-detector circular scanning geometry. We compared its performance against the traditional Nd:YAG/OPO based PAT system in terms of imaging depth, resolution, imaging time etc. The PLD provides near-infrared pulses at ~803 nm wavelength with pulse energy ~1.4 mJ/pulse at 7 kHz repetition rate. The PLD-PAT system is capable of providing 2D image in scan time as small as 3 sec with a signal-to-noise ratio ~30. High-speed and deep-tissue imaging is demonstrated on phantoms and biological samples. The PLD-PAT system is inexpensive, portable, allows high-speed PAT imaging, and its performance is as good as traditional expensive OPO based PAT system. Therefore, it holds promises for future translational biomedical imaging applications.Accepted versio

Microsphere-aided Optical Microscopy and its Applications for Super-Resolution Imaging

The spatial resolution of a standard optical microscope (SOM) is limited by diffraction. In visible spectrum, SOM can provide ∼200nm resolution. To break the diffraction limit several approaches were developed including scanning near field microscopy, metamaterial super-lenses, nanoscale solid immersion lenses, super-oscillatory lenses, confocal fluorescence microscopy, techniques that exploit non-linear response of fluorophores like stimulated emission depletion microscopy, stochastic optical reconstruction microscopy, etc. Recently, photonic nanojet generated by a dielectric microsphere was used to break the diffraction limit. The microsphere-approach is simple, cost-effective and can be implemented under a standard microscope, hence it has gained enormous attention for super-resolution imaging. In this article, we briefly review the microsphere approach and its applications for super-resolution imaging in various optical imaging modalities.MOE (Min. of Education, S’pore)Accepted versio

Recent advances towards preclinical and clinical translation of photoacoustic tomography: a review

Photoacoustic imaging is an emerging hybrid imaging modality that can provide multicontrast, multiscale imaging of biological features ranging from organelles to organs. The three major embodiments of photoacoustic imaging are microscopy, endoscopy, and computed tomography. Photoacoustic tomography (PAT) or photoacoustic computed tomography allows deep-tissue imaging, and hence it is more suitable for whole body preclinical/clinical imaging applications. Due to fast-growing laser technology and ultrasound detector technology, PAT is evolving rapidly, leading to a quicker translation into clinical trials. We review the recent developments of PAT systems and their applications in preclinical and clinical practices.MOE (Min. of Education, S’pore)Accepted versio

Photoacoustic imaging in the second near-infrared window : a review

Photoacoustic (PA) imaging is an emerging medical imaging modality that combines optical excitation and ultrasound detection. Because ultrasound scatters much less than light in biological tissues, PA generates high-resolution images at centimeters depth. In recent years, wavelengths in the second near-infrared (NIR-II) window (1000 to 1700 nm) have been increasingly explored due to its potential for preclinical and clinical applications. In contrast to the conventional PA imaging in the visible (400 to 700 nm) and the first NIR-I (700 to 1000 nm) window, PA imaging in the NIR-II window offers numerous advantages, including high spatial resolution, deeper penetration depth, reduced optical absorption, and tissue scattering. Moreover, the second window allows a fivefold higher light excitation energy density compared to the visible window for enhancing the imaging depth significantly. We highlight the importance of the second window for PA imaging and discuss the various NIR-II PA imaging systems and contrast agents with strong absorption in the NIR-II spectral region. Numerous applications of NIR-II PA imaging, including whole-body animal imaging and human imaging, are also discussed.NMRC (Natl Medical Research Council, S’pore)Published versio

Photoacoustic monitoring of tissue temperature at high temporal resolution

Monitoring of tissue temperature is necessary for guiding energy-based medical treatments. The local temperature information is also important for the safe deposition of light/heat energy into the surrounding healthy tissue. Existing imaging modalities fail to monitor tissue temperature with high accuracy and high resolution. Photoacoustic sensing of temperature was demonstrated using Q-switched Nd:YAG laser. A temperature sensitivity of ~0.15°C was obtained at a temporal resolution of ~2 s. Photoacoustic imaging is a high-speed, high-resolution, deep tissue imaging modality for both preclinical and clinical applications. In this work, we demonstrate photoacoustic sensing of temperature at high temporal resolution order of microseconds using high repetition rate (7000 Hz) near-infrared (~803 nm) pulsed laser diodes. The system will find applications in radiation therapy, photothermal therapy, photodynamic therapy, etc.NMRC (Natl Medical Research Council, S’pore)Published versio

High-speed pre-clinical brain imaging using pulsed laser diode based photoacoustic tomography (PLD-PAT) system

Photoacoustic tomography (PAT) is a promising biomedical imaging modality for small animal imaging, breast cancer imaging, monitoring of vascularisation, tumor angiogenesis, blood oxygenation, total haemoglobin concentration etc. The existing PAT systems that uses Q-switched Nd:YAG and OPO nanosecond lasers have limitations in clinical applications because they are expensive, non-potable and not suitable for real-time imaging due to their low pulse repetition rate. Low-energy pulsed near-infrared diode laser which are low-cost, compact, and light-weight (<200 grams), can be used as an alternate. In this work, we present a photoacoustic tomography system with a pulsed laser diode (PLD) that can nanosecond pulses with pulse energy 1.3 mJ/pulse at ~803 nm wavelength and 7000 Hz repetition rate. The PLD is integrated inside a single-detector circular scanning geometric system. To verify the high speed imaging capabilities of the PLD-PAT system, we performed in vivo experimental results on small animal brain imaging using this system. The proposed system is portable, low-cost and can provide real-time imaging.MOE (Min. of Education, S’pore)Published versio

Fast photoacoustic imaging systems using pulsed laser diodes: a review

Photoacoustic imaging (PAI) is a newly emerging imaging modality for preclinical and clinical applications. The conventional PAI systems use Q-switched Nd:YAG/OPO (Optical Parametric Oscillator) nanosecond lasers as excitation sources. Such lasers are expensive, bulky, and imaging speed is limited because of low pulse repetition rate. In recent years, the semiconductor laser technology has advanced to generate high-repetitions rate near-infrared pulsed lasers diodes (PLDs) which are reliable, less-expensive, hand-held, and light-weight, about 200 g. In this article, we review the development and demonstration of PLD based PAI systems for preclinical and clinical applications reported in recent years.NMRC (Natl Medical Research Council, S’pore)Accepted versio