Analysis of microcantilevers excited by pulsed-laser-induced photoacoustic waves.

This study presents a simulation-based analysis on the excitation of microcantilever in air using pulsed-laser-induced photoacoustic waves. A model was designed and coded to investigate the effects of consecutive photoacoustic waves, arising from a spherical light absorber illuminated by short laser pulses. The consecutiveness of the waves were adjusted with respect to the pulse repetition frequency of the laser to examine their cumulative effects on the oscillation of microcantilever. Using this approach, oscillation characteristics of two rectangular cantilevers with different resonant frequencies (16.9 kHz and 505.7 kHz) were investigated in the presence of the random oscillations. The results show that the effective responses of the microcantilevers to the consecutive photoacoustic waves provide steady-state oscillations, when the pulse repetition frequency matches to the fundamental resonant frequency or its lower harmonics. Another major finding is that being driven by the same photoacoustic pressure value, the high frequency cantilever tend to oscillate at higher amplitudes. Some of the issues emerging from these findings may find application area in atomic force microscopy actuation and photoacoustic signal detection.

Analytical reconstruction of the bioluminescent source with priors.

Bioluminescence imaging has been a popular tool in small animal imaging. During the last decade, the efforts have focused on the development of tomographic systems. However, due to the difficulties in the nature of inverse source problem, multi-modal systems have been the center of attention for the last couple of years. These systems provide complementary information such that the difficulties of the inverse source problem could be overcome using the a priori information obtained. Motivated by these advances in multi-modal systems, this work presents a novel analytical reconstruction of the bioluminescent source. It is shown that if source strength is known a priori then source position could be calculated or vice versa, if source location is known a priori, source strength could be calculated as well as the photon fluence rate. The determination of the source location can be achieved by another imaging system such as X-ray computed tomography. Therefore, in bioluminescence tomography together with an imaging system can be utilized as a multi-modal system. In this work, conventional finite element based simulations are also performed and the numerical results are compared with the analytical ones. It turns out to be that the analytical results are in a good accordance with the numerical results.

Comparison of noise reduction methods in photoacoustic microscopy.

Photoacoustic microscopy (PAM) is classified as a hybrid imaging technique based on the photoacoustic effect and has been frequently studied in recent years. Photoacoustic (PA) signals are inherently recorded in a noisy environment and are also exposed to noise by system components. Therefore, it is essential to reduce the noise in PA signals to reconstruct images with less error. In this study, an image reconstruction algorithm for PAM system was implemented and different filtering approaches for denoising were compared. Studies were carried out in three steps: simulation, experimental phantom and blood cell studies. FIR low-pass and band-pass filters and Discrete Wavelet Transform (DWT) based filters (mother wavelets: "bior3.5″, "bior3.7″, "sym7″) with four different thresholding techniques were examined. For the evaluation purposes, Root Mean Square Error (RMSE), Signal to Noise Ratio (SNR) and Contrast to Noise Ratio (CNR) metrics were calculated. In the simulation studies, the most effective methods were obtained as: sym7/heursure/hard thresh. combination (low and medium level noise) and bior3.7/sqtwolog/soft thresh. combination (high-level noise). In experimental phantom studies, noise was classified into five levels. Different filtering approaches perform better depending on the SNR of PA images. For the blood cell study, based on the standard deviation in the background, sym7/sqtwolog/soft thresh. combination provided the best improvement and this result supported the experimental phantom results.

Development of a Fiber Laser with Independently Adjustable Properties for Optical Resolution Photoacoustic Microscopy.

Photoacoustic imaging is based on the detection of generated acoustic waves through thermal expansion of tissue illuminated by short laser pulses. Fiber lasers as an excitation source for photoacoustic imaging have recently been preferred for their high repetition frequencies. Here, we report a unique fiber laser developed specifically for multiwavelength photoacoustic microscopy system. The laser is custom-made for maximum flexibility in adjustment of its parameters; pulse duration (5-10 ns), pulse energy (up to 10 µJ) and repetition frequency (up to 1 MHz) independently from each other and covers a broad spectral region from 450 to 1100 nm and also can emit wavelengths of 532, 355, and 266 nm. The laser system consists of a master oscillator power amplifier, seeding two stages; supercontinuum and harmonic generation units. The laser is outstanding since the oscillator, amplifier and supercontinuum generation parts are all-fiber integrated with custom-developed electronics and software. To demonstrate the feasibility of the system, the images of several elements of standardized resolution test chart are acquired at multiple wavelengths. The lateral resolution of optical resolution photoacoustic microscopy system is determined as 2.68 µm. The developed system may pave the way for spectroscopic photoacoustic microscopy applications via widely tunable fiber laser technologies.