A novel fiber laser development for photoacoustic microscopy

Photoacoustic microscopy, as an imaging modality, has shown promising results in imaging angiogenesis and cutaneous malignancies like melanoma, revealing systemic diseases including diabetes, hypertension, tracing drug efficiency and assessment of therapy, monitoring healing processes such as wound cicatrization, brain imaging and mapping. Clinically, photoacoustic microscopy is emerging as a capable diagnostic tool. Parameters of lasers used in photoacoustic microscopy, particularly, pulse duration, energy, pulse repetition frequency, and pulse-to-pulse stability affect signal amplitude and quality, data acquisition speed and indirectly, spatial resolution. Lasers used in photoacoustic microscopy are typically Q-switched lasers, low-power laser diodes, and recently, fiber lasers. Significantly, the key parameters cannot be adjusted independently of each other, whereas microvasculature and cellular imaging, e.g., have different requirements. Here, we report an integrated fiber laser system producing nanosecond pulses, covering the spectrum from 600 nm to 1100 nm, developed specifically for photoacoustic excitation. The system comprises of Yb-doped fiber oscillator and amplifier, an acousto-optic modulator and a photonic-crystal fiber to generate supercontinuum. Complete control over the pulse train, including generation of non-uniform pulse trains, is achieved via the AOM through custom-developed field-programmable gate-array electronics. The system is unique in that all the important parameters are adjustable: pulse duration in the range of 1-3 ns, pulse energy up to 10 μJ, repetition rate from 50 kHz to 3 MHz. Different photocoustic imaging probes can be excited with the ultrabroad spectrum. The entire system is fiber-integrated; guided-beam-propagation rendersit misalignment free and largely immune to mechanical perturbations. The laser is robust, low-cost and built using readily available components. © 2013 Copyright SPIE

Investigation on the effect of spatial compounding on photoacoustic images of carotid plaques in the in vivo available rotational range

Photoacoustic imaging (PAI) is a promising imaging modality due to its high optical specificity. However, the low signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of in vivo PA images are major challenges that prevent PAI from finding its place in clinics. This paper investigates the merit of spatial compounding of PA images in arterial phantoms and the achievable improvements of SNR, when in vivo conditions are mimicked. The analysis of the compounding technique was performed on a polyvinyl alcohol vessel phantom with black threads embedded in its wall. The in vivo conditions were mimicked by limiting the rotation range in ±30°, adding turbid surrounding medium, and filling the lumen with porcine blood. Finally, the performance of the technique was evaluated in ex vivo human carotid plaque samples. Results showed that spatial compounding elevates the SNR by 5-10 dB and CNR by 1-5 dB, depending on the location of the absorbers. This paper elucidates prospective in vivo PA characterization of carotid plaques by proposing a method to enhance PA image quality

Ex vivo photoacoustic imaging of atherosclerotic carotid plaques

Vulnerability assessment of carotid plaques is vital to prevent atherosclerosis-related mortality and disability. Photoacoustic imaging (PAI) in combination with plane-wave ultrasound (PUS) may have the ability to reveal the composition and the anatomical structure of the plaque, which infers its mechanical properties and vulnerability. In this study, we used PAI and PUS imaging to scan endarterectomy samples ex vivo, targeting intraplaque hemorrhage, and compared the results with those obtained in healthy (porcine) carotids and histology. A fully integrated hand-held photoacoustic probe was used, consisting of a pulsed diode laser (tp = 130 ns, Ep = 1 mJ, λ = 808 nm) and a linear array transducer (fc = 7.5 MHz). Three porcine carotid arteries and six carotid plaque samples were obtained from a local slaughterhouse and hospital respectively, and were mounted to the imaging setup. Data of endarterectomy samples revealed that PAI of carotid plaques at 808 nm wavelength is capable of detecting blood clots, which can be extensions of vasculature in the plaque, intra-plaque hemorrhage, or the result of trauma inflicted on the medial vascularization. Due to calcification and the limited optical penetration, imaging depth was mostly limited to the proximal wall of the samples. The porcine carotids revealed no hemorrhaging, which was corroborated by the lack of PAI contrast

Characterization of human carotid plaques using multi-wavelength photoacoustic imaging

Recently, multi-spectral photoacoustic (PA) imaging has been explored to aid in the diagnosis of atherosclerosis in carotid arteries. Using multiple wavelengths, PA has the potential to reveal vital morphological information in plaques, such as intraplaque hemorrhages, lipid pools, and the fibrous cap. In this study, we used multispectral PA and plane wave ultrasound (US) hybrid-imaging to reveal the composition of human plaques ex-vivo

Investigation of the effects of multi-angle compounding in photoacoustic imaging

The signal-to-noise ratio (SNR) of photoacoustic (PA) images of carotid arteries is considerably low for in vivo measurements. Compounding of the acquisitions from multiple locations might improve SNR of PA images. In this study, we investigated the effects of spatial compounding based on SNR comparison of PA images of a polyvinyl alcohol (PVA) phantom and an ex vivo carotid plaque, both imaged in an experimental setting. PA and plane wave ultrasound (PUS) data were acquired in a cross-section of each sample. The sample was rotated by 10° and measurements were repeated for 36 angles. Results showed that PA compounding elevated the SNR by 28.7±5.1 dB for the PVA sample and 5.5±2.4 dB for the plaque sample. Additionally, the compounding results for the limited range of rotation as would be feasible in vivo ( -30 to +30) showed an enhancement in SNR of 7.02 ± 2.73 dB. For the future, extra characterization parameters such as resolution, contrast, and sensitivity will be investigated under in vivo conditions