We report a head-mounted photoacoustic fiberscope for cerebral imaging in a freely behaving mouse. The 4.5-gram imaging probe has a 9-µm lateral resolution and 0.2-Hz frame rate over a 1.2-mm wide area. The probe can continuously monitor cerebral oxygenation and hemodynamic responses at single-vessel resolution, showing significantly different cerebrovascular responses to external stimuli under anesthesia and in the freely moving state. When mice subjected to high-concentration CO2 respiration, enhanced oxygenation to compensate for hypercapnia can be visualized in freely moving state. Comparative studies exhibit significantly weakened compensation capabilities in obese mice. This new imaging modality shows promise in neuroscience studies.
Optical fiber enables the implementation of flexible medical endoscopes. Here, we present the development of fiber-optic endoscopic ultrasound, which utilizes laser pulse absorption to generate ultrasound waves and a fiber-optic acoustic sensor to detect echo waves. Compared to its piezoelectric counterpart, the fiber-optic sensor has a significantly higher detection sensitivity and broader bandwidth. As a result, we were able to perform in vivo rotational-scanning (or B-mode) imaging of the gastrointestinal tract and extraluminal structures of a rat with an operating frequency of 20 MHz, an imaging depth of 2 cm, and a frame rate of 1 Hz.
We report wide-field polygon-scanning optical-resolution photoacoustic microscopy (OR-PAM) that achieves 1-MHz A-line rate of oxygen saturation in vivo. We develop a polygon-scanning imaging probe. Using stimulated Raman scattering and optical delay in fiber, we develop a dual-wavelength pulsed laser that has proper wavelengths, sufficient pulse energy, high pulse repetition rate, and sub-microseconds wavelength switching time for fast imaging of oxygen saturation. The OR-PAM system offers a 478-Hz B-scan rate and a 1-Hz volumetric imaging rate over a 12×5 mm2 scanning area. We demonstrate dynamic imaging of oxygen saturation of fast response to epinephrine injection in the mouse ear. Results show fast hemodynamic and functional changes in centimeter-sized areas.
Here we report a high performance fiber-based photoacoustic microscope for brain sO2 imaging. The critical element is a fiber-optic ultrasound sensor, which translates the megahertz acoustic waves into detectable lasing frequency variations. Its working bandwidth has been extended to 40 MHz, by reducing the fiber diameter to 58 μm via chemical etching, this sensor should good result in brain sO2 imaging. The fiber-based PAM offers a new strategy to implement a head-mounted microscope to continuously monitor the brain activities of an awake, free-moving small animal.
We present the all-fiber approach towards building a miniatured, optical-resolution photoacoustic endoscope. The catheter encapsulates two optical fibers for optical excitation and ultrasound detection, respectively. The ultrasound waves are detected with the laser-based fiber optic sensor, with a diameter of 125 μm, instead of a focused piezoelectric transducer. Photoacoustic endoscopic images from a rat rectum have been acquired in vivo with a range of 6.3 mm, a lateral resolution of 10 μm, and a 285° angular field-of-view. The catheter has a diameter of 2.3 mm and can be further reduced by replacing the bulk prism reflector.
KEYWORDS: Ultrasonography, Sensors, Fiber optics sensors, Photoacoustic tomography, Signal to noise ratio, Birefringence, In vivo imaging, Photoacoustic spectroscopy, Brain, Neuroimaging
Optical detection of ultrasound for photoacoustic imaging has received great interest. Recently, we have developed a new fiber-optic ultrasound sensor by exploiting dual-polarization fiber laser. It offers high sensitivity (40 Pa over 50 MHz) as well as good stability as a result of the self-heterodyning detection. In this work, the signal-to-noise ratio has been enhanced by suppressing the noise of the ultrasound sensing system via signal averaging. As a result of multiple measurements of a single photoacoustic signal, the total noise was reduced by 40%. With the enhanced detection capability, the sensors have been deployed as photoacoustic probes in different imaging modalities. We demonstrate fastscanning photoacoustic microscopy with a field-of-view 2×2 mm2, a frame rate of 2 Hz to visualize the blood flow dynamics. By bending the flexible fiber optic sensor for geometrical focusing, PACT was realized to image a mouse brain with a spatial resolution of 70 μm. An all-fiber photoacoustic endoscope was built to in vivo image the vascular network of a rat rectum, with a lateral resolution of 10 μm, with a 2.3-mm probe diameter.
Optical-resolution photoacoustic microscopy can measure oxygen saturation () in vivo, offering an important tool to assess tissue oxygenation and health condition. Limited by available wavelengths for fast OR-PAM, the accuracy of sO2 imaging may be degraded by absorption saturation due to high absorption in the blood. Here, we report a nonlinear model to solve the saturation problem and increase the accuracy of measurement. The absorption saturation is analyzed by comparing a nonlinear and linear photoacoustic model using numerical simulation, which shows the nonlinear model has an improved accuracy than the linear model when the absorption is high. Phantom experiments on bovine blood further validate the accuracy of the nonlinear sO2 measurement method. In vivo experiments are conducted in the mouse ear. The values in a pair of arteries and veins are calculated using both linear and nonlinear methods, showing that the nonlinear method measures the arterial value closer to normal physiological condition than the conventional linear model. The nonlinear model requires the use of three or more wavelengths (532nm, 545nm, and 558nm in this work). As a result, we demonstrate the saturation effect in OR-PAM can be compensated via a nonlinear model, which may advance the application of functional optical-resolution photoacoustic microscopy.
In fast functional photoacoustic microscopy (FPAM), the detection and monitoring of the oxygen saturation are important to monitor tissue functionality and disease progress. FPAM needs multi-wavelength pulsed laser sources with high pulse repetition rates, sufficient pulse energies and short wavelength switching time. Here, we develop a multi-wavelength pulsed laser source based on the stimulated Raman-scattering effect. The new laser is based on a 532-nm 1-MHz pulsed laser. The 532-nm laser pulse is split into two beams: one pumps a 5-m optical fiber to excite a 558-nm wavelength via stimulated Raman scattering; the other one propagates through a 50-m optical fiber to delay the pulse by 220 nano second so that the excitation wavelengths can be separated in time for fast functional photoacoustic imaging. The two beams are spatially combined and coupled into an optical fiber for photoacoustic excitation. Consequently, the new laser source can generate 2 million pulses per second, switch wavelengths in 220 ns, and provide hundreds of nano-Joules pulse energy for each wavelength. Using this laser source, we demonstrate optical-resolution photoacoustic imaging of microvascular structure and oxygen saturation in the mouse ear. The ultrashort wavelength switching time enables oxygen saturation imaging of flowing single red blood cells.
A compact fiber grating laser has been exploited as an ultrasound sensor to probe optically induced spherical elastic waves, taking advantage of its response in beat-frequency variation of the laser output. Optical-resolution photoacoustic microscopy (PAM) is further implemented with such a sensor by raster scanning the excitation light with a 2-axis galvo scanner. A PAM image of mouse ear with a field width of 2 mm is demonstrated. The wide field-of-view of the sensor allows the implementation of fast-scanning PAMs which is attractive for in vivo imaging applications.
A fiber-optic Doppler velocimeter based on a dual-polarization fiber grating laser is demonstrated. The fiber grating laser produces two orthogonally polarized laser outputs with their frequency difference proportional to the intra-cavity birefringence. When the laser outputs are reflected from a moving targets, the laser frequencies will be shifted due to the Doppler effect. It shows that the frequency difference between the beat note of the laser outputs and the beat note of the reflected lasers is proportional to the velocity. The proposed fiber-optic Doppler velocimeter shows a high sensitivity of 0.64 MHz/m/s and is capable of measurement of wide range of velocity.
We demonstrate an accelerometer based on a dual-frequency DBR fiber laser with a resolution of 6 μg/Hz1/2 at 1 kHz. The accelerometer is implemented by mounting a 250-milligram proof mass onto the laser cavity and converting the vibration into change in beat frequency between the two orthogonal polarization lasing modes. Experimental result shows that the sensitivity reaches 1.7 MHz/g at 1 kHz with a working bandwidth over 1 kHz. The high resolution is also a result of the noise level as low as 10 Hz/Hz1/2 due to the compensation between the two lasing modes. The present accelerometer with extremely high resolution and light weight is promising for geophysical applications.
The beat frequency dependence of the sensitivity for a Faraday-rotation based heterodyning fiber laser magnetic field sensor is studied, which shows that lower beat frequency results in higher sensitivity. By lowering the beat frequency to 2 MHz, the maximum sensitivity of about 43 Hz/μT to magnetic field has been achieved for a heterodyning fiber laser inscribed on an Erbium doped fiber. It also shows that the beat frequency is dependent on the polarization of the 980 nm pump. Therefore, dynamical tuning of the sensitivity optically has also been demonstrated by tuning the polarization of the 980 nm pump.
A novel fiber-optic magnetic field sensor is proposed by embedding a heterodyning fiber laser into an epoxy resin bonded magnetostrictive composite material with Terfenol-D particles doped. The magnetic field induced strain in the magnetostrictive composite material is converted to transversal stress by a structure which is applied to the fiber laser to produce beat note frequency changes for measurement. The response of the proposed sensor is measured, which shows a quite good directivity with a sensitivity of 10.5 Hz/μT to magnetic field and a large measurable range up to about 0.3 T.
The noise performance of the beat note generated by a dual-polarization fiber grating laser is very critical for sensing applications. To reduce the noise of the beat note, external optical feedback is employed with a fiber Bragg grating as a reflector. It then shows that a longer feedback time results in larger noise reduction. With a 50 m single mode fiber as the delay line, more than 20 dB phase noise reduction has been achieved for a dual-polarization fiber grating laser which shows a phase noise of -92 dBc/Hz @ 10 kHz offset with external optical feedback applied.
We demonstrate the ability of a fiber grating laser with dual-polarization, single-longitudinal-mode output to measure an extremely small mass (or transverse load). The minimum detectable mass is 0.28 milligram by reducing the noise level of the output beat signal.
In this paper, the output beat signal of the polarimetric heterodyning fiber grating laser sensor has been stabilized based on the investigation of polarization effect on the beat frequency. The short-term frequency fluctuation has been reduced from 1.5 MHz to about 0.1 MHz and the resolution of the sensors is greatly improved.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.