The synthesis of nanomaterials through pulsed-laser ablation of a bulk solids can be applied for the generation of reference aerosols for applications in aerosol science, where no reliable source of reference aerosol samples currently exists. To develop aerosol generators based on laser ablation that are capable of reliable aerosol generation with well-defined properties, more data is needed on how the aerosol properties depend on the laser ablation parameters, such as wavelength, power, repetition rate, and surrounding gas. We present an aerosol generation device based on laser ablation from a nanosecond pulsed Nd:YAG laser and report initial data from our systematic study of the effect of ablation parameters on aerosol formation from different metals and graphite. The size and concentration distribution of the resulting laser-ablation-generated aerosols were measured using a scanning mobility particle sizer. The primary nanoparticles formed in the ablation chamber directly as a result of ablation were collected and imaged using scanning electron microscopy and compared to the ensuing aerosols formed after agglomeration of the nanoparticles. We found that the aerosol size and concentration increases with increasing laser irradiance and increasing repetition rate. Different gases were used for the aerosol generation, and our results shed light on how the plasma development and resulting nanoparticle formation depend on the gas environment. Certain metal aerosols formed in a synthetic air environment were larger and more concentrated than in argon and nitrogen. The charge of the aerosols was also monitored using a parallel plate capacitor.
We propose a new method for biochemical sensing using photoacoustic (PA) excitation of gold nanoparticles (GNPs) to achieve real-time detection of transient biomarkers. Our approach is based on the effect of a particle coating on nonlinear PA signal generation combined with our recently-developed serial PA tomographic imaging method. We have shown the ability to image the three-dimensional spatial distribution of GNPs that produce nonlinear PA signal with respect to fluence. If the occurrence of this nonlinearity is made dependent on the presence of transient biochemical markers, for example through the degradation of a nonlinearity-quenching particle coating, our serial PA tomographic imaging method can be extended for real-time three-dimensional in situ imaging of these biomarkers. Our early-stage proof-ofconcept experimental results presented here show that coating GNPs that exhibit nonlinear PA signal generation behavior is an effective method to remove the nonlinear effect. The coefficient of determination (R2 ) of a linear fit to the PA signal as a function of fluence can be effectively used to differentiate the coated GNPs and the GNPs without coating. This differentiation can also be achieved using the second order coefficient to a quadratic fit to the PA signal vs. fluence data.
Optoacoustic techniques rely on ultrasound transmission between optical absorbers within tissues and the measurement location. Much like in echography, commonly used piezoelectric transducers require either direct contact with the tissue or through a liquid coupling medium. The contact nature of this detection approach then represents a disadvantage of standard optoacoustic systems with respect to other imaging modalities (including optical techniques) in applications where non-contact imaging is needed, e.g. in open surgeries or when burns or other lesions are present in the skin. Herein, non-contact optoacoustic imaging using raster-scanning of a spherically-focused piezoelectric air-coupled ultrasound transducer is demonstrated. When employing laser fluence levels not exceeding the maximal permissible human exposure, it is shown possible to attain detectable signals from objects as small as 1 mm having absorption properties representative of blood at near-infrared wavelengths with a relatively low number of averages. Optoacoustic images from vessel-mimicking tubes embedded in an agar phantom are further showcased. The initial results indicate that the air-coupled ultrasound detection approach can be potentially made suitable for non-contact biomedical imaging with optoacoustics.
Current radiofrequency cardiac ablation procedures lack real-time lesion monitoring guidance, limiting the reliability and efficacy of the treatment. The objective of this work is to demonstrate that optoacoustic imaging can be applied to develop a diagnostic technique applicable to radiofrequency ablation for cardiac arrhythmia treatment with the capabilities of real-time monitoring of ablated lesion size and geometry. We demonstrate an optoacoustic imaging method using a 256-detector optoacoustic imaging probe and pulsed-laser illumination in the infrared wavelength range that is applied during radiofrequency ablation in excised porcine myocardial tissue samples. This technique results in images with high contrast between the lesion volume and unablated tissue, and is also capable of capturing time-resolved image sequences that provide information on the lesion development process. The size and geometry of the imaged lesion were shown to be in excellent agreement with the histological examinations. This study demonstrates the first deep-lesion real-time monitoring for radiofrequency ablation generated lesions, and the technique presented here has the potential for providing critical feedback that can significantly impact the outcome of clinical radiofrequency ablation procedures.
Lack of sensory feedback during laser surgery prevents surgeons from keeping track of the exact lesion profile and cutting depth. As a result, duration and complexity of the treatments are significantly increased. In this study we propose a new method for enabling three-dimensional tracking of the exact lesion profile, based on detection of shock waves emanating from the ablated tissue and subsequent reconstruction of the incision location using time-of-flight data obtained from multiple acoustic detectors. Ablation was performed in fresh bovine tissue samples using a Q-switched Nd-YAG laser, delivering 8 ns duration 150mJ pulses at a wavelength of 1064nm and repetition rate of 5Hz. The beam was focused by a 50mm lens on the tissue surface, which resulted in a deep cut of up to 9mm depth. The generated shock waves were detected using a spherical matrix ultrasonic array. The exact cutting profile was subsequently rendered by reconstructing the origin of shockwaves detected during the entire procedure. Different combinations of the detector positions were considered with respect to the resulting reconstruction quality. It was observed that, by utilizing at least 12 detection elements, the lesion profile could be characterized with high accuracy in all three dimensions, which was confirmed by histological evaluations. The proposed method holds promise for delivering highly precise and accurate real-time feedback during laser surgeries.
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