Name: switchable acoustic and optical resolution photoacoustic microscopy for in vivo small-animal blood vasculature imaging
Uploaded: 2017-06-26T15:00:00
Description: Watch this Scientific Journal Video about Switchable Acoustic and Optical Resolution Photoacoustic Microscopy for In Vivo Small-animal Blood Vasculature Imaging at JoVE.com

The authors would like to acknowledge the financial support from a Tier 2 grant funded by the Ministry of Education in Singapore (ARC2/15: M4020238). The authors would also like to thank Mr. Chow Wai Hoong Bobby for the machine shop help.

All animal experiments were performed according to the approved regulations and guidelines of the Institutional Animal Care and Use Committee of Nanyang Technological University, Singapore (Animal Protocol Number ARF-SBS/NIE-A0263).
1. AR-OR-PAM System (Figure 1)
<ol>
	<li>System configuration: AR-PAM
	<ol>
		<li>Use a nanosecond tunable laser system consisting of a diode-pumped, solid-state Nd-YAG laser (532 nm) and a dye laser with a tunabilit...

<ol>
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In conclusion, a switchable AR and OR PAM system that can achieve both high-resolution imaging at lower imaging depths and lower-resolution imaging at higher imaging depths has been developed. The lateral resolution and imaging depth of the switchable system was determined. The advantages of this switchable PAM system include: (1) the high-resolution imaging using tight optical focusing; (2) the deep-tissue imaging using acoustic focusing; 3) the dark-field illumination for AR-PAM, which prevents strong PA signals from a...

High-resolution optical imaging modalities, such as optical coherence tomography, confocal microscopy, and multiphoton microscopy, have numerous benefits. However, the spatial resolution decreases significantly as the imaging depth increases. This is because of the diffuse nature of light transport in soft tissues1,2. The integration of optical excitation and ultrasound detection provides a solution to overcome the challenge of high-resolution optical imaging in deep tissues. Photoacoustic microscopy (PAM) is one such modality that can provide deeper imaging than other optical imaging modalities. It has been successfully applied to in vivo structural, functional, molecular, and cell imaging3,4,5,6,7,8,9,10,11,12,13 studies by combining the strong optical absorption contrast with the high spatial resolution from ultrasound.
In PAM, a short laser pulse irradiates the tissue/sample. The absorption of light by chromophores (e.g., melanin, hemoglobin, water etc.) results in a temperature increase, which in turn results in the production of pressure waves in the form of acoustics waves (photoacoustic waves). The generated photoacoustic waves can be detected by a wideband ultrasonic transducer outside the tissue boundary. Utilizing weak optical and tight acoustic focusing, deep-tissue imaging can be achieved in acoustic resolution photoacoustic microscopy (AR-PAM)14,15,16. In AR-PAM, a lateral resolution of 45 &#181;m and an imaging depth up to 3 mm have been demonstrated15. In order to resolve single capillaries (~5 &#181;m) acoustically, ultrasonic transducers operating at &#62;400 MHz central frequencies are required. At such high frequencies, the penetration depth is less than 100 &#181;m. The problem caused by tight acoustic focusing can be resolved using tight optical focusing. Optical resolution photoacoustic microscopy (OR-PAM) is capable of resolving single capillaries, or even a single cell17, and a lateral resolution of 0.5 &#181;m has been achieved18,19,20,21,22,23,24. The use of a photonic nanojet can help to achieve a resolution beyond the diffraction-limited resolution25,26. In OR-PAM, the penetration depth is limited due to light focusing, and it can image up to ~1.2 mm inside the biological tissue23. Therefore, AR-PAM can image deeper, but with a lower resolution, and OR-PAM can image with a very high resolution, but with limited imaging depth. The imaging speed of the AR and OR-PAM system mainly depends upon the pulse repetition rate of the laser source27.
Combining AR-PAM and OR-PAM will be of great benefit to applications that require both a high resolution and deeper imaging. Little effort has been made to combine these systems together. Usually, two different imaging scanners are used for imaging, which requires that the sample be moved between both systems, thus making it difficult to perform in vivo imaging. However, hybrid imaging with both AR and OR PAM enables imaging with scalable resolutions and depths. In one approach, an optical fiber bundle is used to deliver light for both the AR and OR PAM. In this approach, two separate lasers (a high-energy laser at 570 nm for the AR and a low-energy, high-repetition rate laser at 532 nm for the OR) are used, making the system inconvenient and expensive28. The OR-PAM laser wavelength is fixed, and many studies, such as on oxygen saturation, are not possible using this combined system. Comparative studies between AR and OR PAM are also not possible because of the difference in laser wavelengths between the AR and OR. Moreover, AR-PAM uses bright-field illumination; hence, strong photoacoustic signals from the skin surface limit the image quality. For this reason, the system cannot be used for many bioimaging applications. In another approach to perform AR and OR PAM, the optical and ultrasound focus is shifted, which makes the light focus and ultrasound focus unaligned. Thus, the image quality is not optimal29. Using this technique, the AR-PAM and OR-PAM can achieve only 139 &#181;m and 21-&#181;m resolutions, respectively, making it a poor-resolution system. Another approach, which includes changing the optical fiber and collimating optics, was reported to switch between AR and OR PAM, making the alignment process difficult30. In all of these cases, AR-PAM did not use dark-field illumination. The use of dark-field illumination can reduce the generation of strong photoacoustic signals from the skin surface. Therefore, deep-tissue imaging can be performed using ring-shaped illumination, as the detection sensitivity of deep photoacoustic signals will be higher compare to that of bright-field illumination.
This work reports a switchable AR and OR PAM (AR-OR-PAM) imaging system capable of both high-resolution imaging and low-resolution deep-tissue imaging of the same sample, using the same laser and scanner for both systems. The performance of the AR-OR-PAM system was characterized by determining the spatial resolution and imaging depth using phantom experiments. In vivo blood vasculature imaging was performed on a mouse ear to demonstrate its biological imaging capability.

<table border="0" cellpadding="0" cellspacing="0" width="939">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Q-switched Nd:YAG laser</td>
			<td style="width:232px;">Edgewave</td>
			<td style="width:177px;">BX80-2-L</td>
			<td style="width:227px;">Pump laser&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Credo-High Repetition Rate Dye Laser</td>
			<td>Spectra physics</td>
			<td style="width:177px;">CREDO-DYE-N</td>
			<td>Dye laser</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Precision Linear Stage</td>
			<td>Physik Instrumente</td>
			<td>PLS 85&#160;</td>
			<td>XY raster scanning stage</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Translation stage</td>
			<td>Physik Instrumente</td>
			<td>VT 80&#160;</td>
			<td>Confocal determine</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Mounted Silicon photodiode</td>
			<td>Thorlabs</td>
			<td>SM05PD1A</td>
			<td>Triggering/Pulse variation</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Motorized continuous Rotational stage&#160;</td>
			<td>Thorlabs</td>
			<td>CR1/M-Z7</td>
			<td>Diverting laser beam</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Mounted Continuously Variable ND Filter</td>
			<td>Thorlabs</td>
			<td>NDC-50C-4M</td>
			<td>Intensity variable</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Fiber Patch Cable</td>
			<td>Thorlabs</td>
			<td>M29L01</td>
			<td>Multimode fiber</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Microscope objective</td>
			<td>Newport</td>
			<td>M-10X</td>
			<td>Objective&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">XY translating mount</td>
			<td>Thorlabs</td>
			<td>CXY1</td>
			<td>Translating mount</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Plano convex lens</td>
			<td>Thorlabs</td>
			<td>LA1951</td>
			<td>Collimating lens</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Conical lens&#160;</td>
			<td>Altechna</td>
			<td>APX-2-B254</td>
			<td>Ring shape beam</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Translation stage</td>
			<td>Thorlabs</td>
			<td>CT1</td>
			<td>Translating stage</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Optical condenser</td>
			<td style="width:232px;">Home made</td>
			<td style="width:177px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ultrasonic transducer</td>
			<td>Olympus-NDT</td>
			<td>V214-BB-RM</td>
			<td>50MHz transducer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Plano concave lens</td>
			<td>Thorlabs</td>
			<td>LC4573</td>
			<td>Acoustic lens</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Pulser/Receiver</td>
			<td>Olympus-NDT</td>
			<td>5073PR</td>
			<td>Pulse echo amplifier&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Mounted standard iris</td>
			<td>Thorlabs</td>
			<td>ID12/M</td>
			<td>Beam shaping</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Plano convex lens</td>
			<td>Thorlabs</td>
			<td>LA4327</td>
			<td>Condenser lens</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Mounted precision pinhole</td>
			<td>Thorlabs</td>
			<td>P50S</td>
			<td>Spatial filtering</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Single mode fiber patch cable</td>
			<td>Thorlabs</td>
			<td>P1-460B-FC-1</td>
			<td>Single mode fiber</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Fiber coupler</td>
			<td>Newport</td>
			<td>F-91-C1</td>
			<td>Single mode coupling</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Achromatic doublet lens</td>
			<td style="width:232px;">Edmund Optics</td>
			<td>32-317</td>
			<td>Achromatic doublet</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Protected silver elliptical mirror</td>
			<td>Thorlabs</td>
			<td>PFE10-P01</td>
			<td>Mirror</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Right angle kinematic mirror mount</td>
			<td>Thorlabs</td>
			<td>KCB1</td>
			<td>Mirror mount</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Z-Axis Translation Mount</td>
			<td>Thorlabs</td>
			<td>SM1Z</td>
			<td>z translator</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Lens tube</td>
			<td>Thorlabs</td>
			<td>SM05L10</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">UV Fused Silica Right-Angle Prism</td>
			<td>Thorlabs</td>
			<td>PS615</td>
			<td>Right angle prism</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Rhomboid prism</td>
			<td style="width:232px;">Edmund Optics</td>
			<td>47-214</td>
			<td>Shear wave</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Dimethylpolysiloxane</td>
			<td>Sigma Aldrich</td>
			<td>DMPS1M</td>
			<td>Silicon oil</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Amplifier</td>
			<td>Mini Circuits</td>
			<td>ZFL-500LN</td>
			<td>Amplifier</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">16 bit high speed digitizer</td>
			<td>Spectrum</td>
			<td>M4i.4420</td>
			<td>Data acquisition card</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Oscilloscope</td>
			<td style="width:232px;">Agilent Technologies</td>
			<td style="width:177px;">DS06014A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Mice&#160;</td>
			<td style="width:232px;">InVivos Pte.Ltd</td>
			<td style="width:177px;">ICR</td>
			<td>Animal model</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Ultrasound gel&#160;</td>
			<td>Progress/parker acquasonic gel</td>
			<td>PA-GEL-CLEA-5000</td>
			<td>Acoustic coupling</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Water tank</td>
			<td style="width:232px;">Home made</td>
			<td style="width:177px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Translation stage</td>
			<td style="width:232px;">Homemade</td>
			<td style="width:177px;"></td>
			<td>Switching AR-OR</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Gold nanoparticles</td>
			<td>Sigma Aldrich</td>
			<td align="right">742031</td>
			<td>Lateral resolution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Sterile ocular ointment</td>
			<td style="width:232px;">Alcon</td>
			<td style="width:177px;">Duratears</td>
			<td>Animal imaging</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">1951 USAF resolution test target</td>
			<td style="width:232px;">Edmund Optics</td>
			<td align="right" style="width:177px;">38257</td>
			<td>Confocal alignment</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Data acquisition software</td>
			<td style="width:232px;">National Instrument</td>
			<td style="width:177px;">Labview</td>
			<td>Home made software using Labview</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:303px;">Image Processing software</td>
			<td style="width:232px;">Mathworks</td>
			<td style="width:177px;">Matlab</td>
			<td>Home made program using Matlab</td>
		</tr>
	</tbody>
</table>

switchable acoustic and optical resolution photoacoustic microscopy for in vivo small-animal blood vasculature imaging

Photoacoustic microscopy (PAM) is a fast-growing invivo imaging modality that combines both optics and ultrasound, providing penetration beyond the optical mean free path (~1 mm in skin) with high resolution. By combining optical absorption contrast with the high spatial resolution of ultrasound in a single modality, this technique can penetrate deep tissues. Photoacoustic microscopy systems can have either a low acoustic resolution and probe deeply or a high optical resolution and probe shallowly. It is challenging to achieve high spatial resolution and large depth penetration with a single system. This work presents an AR-OR-PAM system capable of both high-resolution imaging at shallow depths and low-resolution deep-tissue imaging of the same sample in vivo. A lateral resolution of 4 &#181;m with 1.4 mm imaging depth using optical focusing and a lateral resolution of 45 &#181;m with 7.8 mm imaging depth using acoustic focusing were successfully demonstrated using the combined system. Here,&#160;in vivo&#160;small-animal blood vasculature imaging is performed to demonstrate its biological imaging capability.

The schematic of the AR-OR-PAM system is shown in Figure 1. In this setup, all components were integrated and assembled in an optical cage setup. The use of a cage system makes the AR-OR-PAM scanning head compact and easily assembled, aligned, and integrated onto a single scanning stage.
Two-dimensional continuous raster scanning of the imaging head was used during image acquisition. The time-resolv...

Watch this Scientific Journal Video about Switchable Acoustic and Optical Resolution Photoacoustic Microscopy for In Vivo Small-animal Blood Vasculature Imaging at JoVE.com

Switchable Acoustic and Optical Resolution Photoacoustic Microscopy for In Vivo Small-animal Blood Vasculature Imaging

Here a switchable acoustic resolution (AR) and optical resolution (OR) photoacoustic microscopy (AR-OR-PAM) system capable of both high resolution imaging at shallow depth and low resolution deep tissue imaging on the same sample in vivo is demonstrated.

Switchable Acoustic and Optical Resolution Photoacoustic Microscopy for In Vivo Small-animal Blood Vasculature Imaging

Photoacoustic microscopy (PAM) is a fast-growing invivo imaging modality that combines both optics and ultrasound, providing penetration beyond the ...

The overall goal of this procedure is to demonstrate a switchable acoustic and optical resolution photoacoustic microscopy system for in vivo small-animal blood vasculature imaging. Photoacoustic microscopy is a fast-growing in vivo imaging modality that combines optics and ultrasound, providing imaging depth beyond optical path with high resolution. This work present is switchable acoustic optical resolution photo acoustic microscopic system capable of higher-resolution imaging at shallow depth as well as lower-resolution deep tissue imaging of the same sample in vivo.Demonstrating the procedure will be Dr.Mohesh Moothanchery, a research fellow from my laboratory, and Arunima Sharma, a PhD student from my lab. First, construct a nanosecond, tunable laser system from a diode-pumped, solid-state Nd:YAG laser and a ditunable laser with a range of 559 to 576 nanometers. Set the ditunable laser to 570 nan.Mount on an optical table an acrylic tank with a seven-centimeter polyethylene imaging window in the floor of the tank. Fix an imaging stage to an optical post beneath the imaging window. Over the tank, mount an optical cage containing the AR-OR-PAM imaging system on a switching plate fastened to a computer-controlled three-axis motorized translation stage.To configure the system for AR-PAM, use a right angel prism mounted on a motorized continuous rotational stage to direct the beam through a variable neutral-density filter to a multi-mode fiber cable with a 25 NA fiber coupler. Connect the fiber output to an XY translation stage in the imaging system. Direct the beam through a plano convex lens 25 millimeters from the output, and then through a conical lens to create a ring shaped beam.Focus the ring shaped beam on an optical condenser around a 50 megahertz ultrasonic transducer with an acoustic lens mounted at the transducer output. Next, to configure the system for OR-PAM, rotate the first right angle prism by 90 degrees to direct the laser beam through an iris, a variable neutral-density filter, a condenser lens, and a pinhole set at 75 millimeters from the condenser lens. Use a 1 NA single-mode fiber coupler and a single-mode fiber to send the beam to the Z translation stage in the imaging system.Direct the beam through an achromatic doublet lens mounted 50 millimeters from the fiber output. Divert the beam with a kinematic, controllable, elliptical mirror to a second achromatic doublet lens in a lens tube. Focus the beam on a right-angle prism separated by a layer of silicon oil from a rhomboid prism, bearing an acoustic lens and a 50 megahertz ultrasonic transducer, which forms the OR-PAM scanning head.Prior to beginning the system alignment, fill the acrylic tanks with degassed water. Then use the three-axis motorized stage to move the scanning assembly over the tank. Lower the assembly until the acoustic lenses of both systems are submerged.Turn on the laser. Connect the ultrasound transducers to two 25-decibel, fixed-gain amplifiers. Position the AR-PAM scanning head over the imaging window.Place a glass slide wrapped in black electrical tape on the imaging stage and raise the stage to bring the slide into contact with the imaging window. Adjust the conical lens until the photoacoustic signal amplitude generated from the slide reaches a maximum, indicating that the optical and acoustic lenses are confocal. Then manually switch to the OR-PAM system.Adjust the achromatic doublet lens until the optical and acoustic lenses are confocal, as indicated by maximizing the photoacoustic signal amplitude from the tape-covered slide. To determine the lateral resolution of each system, first place 1 milliliter of a dilute solution of 100 nanometer gold nanoparticles in water on a a cover slip. Place the slide on the imaging stage.and raise the stage until the nanoparticle solution contacts the imaging window. Switch the laser and the scanning head to the AR-PAM system. Configure the instrument software for an AR-PAM scan and perform a single raster scan.Repeat this process for the OR-PAM system. Then remove the slide and clean the imaging window with an alcohol swab. Fit the point spread functions determined from the acquired images to a Gaussian curve.The full width at half maximum is the lateral resolution for the corresponding scanning system. Next, to determine the maximum imaging depth in tissue, first insert a sharp metal plate wrapped in black tape into a small section of chicken tissue at a shallow angle. Place the tissue in the water tank under the scanner head.Obtain a single B-scan image for each system. Measure the depth beneath the tissue surface at which the black tape is no longer clearly discernible. Prior to image, ensure that the polyethylene imaging window and the animal imaging stage are both clean.Then obtain a 25-gram, four-week-old female mouse for the imaging procedure after anesthetizing the mouse. Remove hair from the ear surface with depilatory cream. Apply sterile ocular ointment to the mouse's eyes to prevent dryness and block scattered laser beams.Place the mouse on an imaging stage with the plate for positioning the ear to me imaged. Monitor the mouse's physiological condition with a pulse oximeter clamped to the mouses tail or leg. Apply ultrasound gel to the ear to be imaged.Slowly raise the imaging stage to gently bring the ear into contact with the polyethylene imaging window. Acquire AR-PAM and OR-PAM raster scans of the mouse's ear, monitoring the mouse's physiological status throughout. Allow the mouse to recover fully after imaging.The blood vasculature system of a mouse here was imaged in vivo with a switchable AR-OR-PAM system. Blood vessels thicker than 45 micrometers were clearly visible in the AR-PAM image. Single capillaries of about five micrometers wide were resolved with OR-PAM imaging.The combined AR-OR-PAM system has an approximately four micron lateral resolution. And at point, four mm imaging for the OR-PAM and an approximately 45 micrometer lateral resolution and 7.8 mm imaging for the AR-PAM. The switchable combined AR-OR-PAM system allows imaging without moving the sample between different imaging systems.The developed system can be used for pre-clinical imaging. Major pre-clinical applications include imaging of androgenesis, tumor micro-environments, microcirculations, drug response, brain function, bio-markers, and gene activities.

switchable-acoustic-optical-resolution-photoacoustic-microscopy-for

Research

JoVE Journal

Bioengineering

Three-dimensional Optical-resolution Photoacoustic Microscopy

Optical microscopy, providing valuable insights at the cellular and organelle levels, has been widely recognized as an enabling biomedical technology. As the mainstays of in vivo three-dimensional (3-D) optical microscopy, single-/multi-photon fluorescence microscopy and optical coherence tomography (OCT) have demonstrated their extraordinary sensitivities to fluorescence and optical scattering contrasts, respectively. However, the optical absorption contrast of biological tissues, which encodes essential physiological/pathological information, has not yet been assessable. 
The emergence of biomedical photoacoustics has led to a new branch of optical microscopy optical-resolution photoacoustic microscopy (OR-PAM)1, where the optical irradiation is focused to the diffraction limit to achieve cellular1 or even subcellular2 level lateral resolution. As a valuable complement to existing optical microscopy technologies, OR-PAM brings in at least two novelties. First and most importantly, OR-PAM detects optical absorption contrasts with extraordinary sensitivity (i.e., 100%). Combining OR-PAM with fluorescence microscopy3 or with optical-scattering-based OCT4 (or with both) provides comprehensive optical properties of biological tissues. Second, OR-PAM encodes optical absorption into acoustic waves, in contrast to the pure optical processes in fluorescence microscopy and OCT, and provides background-free detection. The acoustic detection in OR-PAM mitigates the impacts of optical scattering on signal degradation and naturally eliminates possible interferences (i.e., crosstalks) between excitation and detection, which is a common problem in fluorescence microscopy due to the overlap between the excitation and fluorescence spectra. 
Unique for optical absorption imaging, OR-PAM has demonstrated broad biomedical applications since its invention, including, but not limited to, neurology5, 6, ophthalmology7, 8, vascular biology9, and dermatology10. In this video, we teach the system configuration and alignment of OR-PAM as well as the experimental procedures for in vivo functional microvascular imaging.

Optical-resolution photoacoustic microscopy (OR-PAM) is an emerging technology capable of imaging optical absorption contrasts in vivo with cellular resolution and sensitivity. Here, we provide a visualized instruction on the experimental protocols of OR-PAM, including system configuration, system alignment, typical in vivo experimental procedures, and functional imaging schemes.

Optical microscopy, providing valuable insights at the cellular and organelle levels, has been widely recognized as an enabling biomedical technology. As ...

A Cre-Lox P Recombination Approach for the Detection of Cell Fusion In Vivo

The ability of two or more cells of the same type to fuse has been utilized in metazoans throughout evolution to form many complex organs, including skeletal muscle, bone and placenta. Contemporary studies demonstrate fusion of cells of the same type confers enhanced function. For example, when the trophoblast cells of the placenta fuse to form the syncytiotrophoblast, the syncytiotrophoblast is better able to transport nutrients and hormones across the maternal-fetal barrier than unfused trophoblasts1-4. More recent studies demonstrate fusion of cells of different types can direct cell fate. The "reversion" or modification of cell fate by fusion was once thought to be limited to cell culture systems. But the advent of stem cell transplantation led to the discovery by us and others that stem cells can fuse with somatic cells in vivo and that fusion facilitates stem cell differentiation5-7. Thus, cell fusion is a regulated process capable of promoting cell survival and differentiation and thus could be of central importance for development, repair of tissues and even the pathogenesis of disease.
Limiting the study of cell fusion, is lack of appropriate technology to 1) accurately identify fusion products and to 2) track fusion products over time. Here we present a novel approach to address both limitations via induction of bioluminescence upon fusion (Figure 1); bioluminescence can be detected with high sensitivity in vivo8-15. We utilize a construct encoding the firefly luciferase (Photinus pyralis) gene placed adjacent to a stop codon flanked by LoxP sequences. When cells expressing this gene fuse with cells expressing the Cre recombinase protein, the LoxP sites are cleaved and the stop signal is excised allowing transcription of luciferase. Because the signal is inducible, the incidence of false-positive signals is very low. Unlike existing methods which utilize the Cre/LoxP system16, 17, we have incorporated a "living" detection signal and thereby afford for the first time the opportunity to track the kinetics of cell fusion in vivo.
To demonstrate the approach, mice ubiquitously expressing Cre recombinase served as recipients of stem cells transfected with a construct to express luciferase downstream of a floxed stop codon. Stem cells were transplanted via intramyocardial injection and after transplantation intravital image analysis was conducted to track the presence of fusion products in the heart and surrounding tissues over time. This approach could be adapted to analyze cell fusion in any tissue type at any stage of development, disease or adult tissue repair.

A Cre-Lox P Recombination Approach for the Detection of Cell Fusion In Vivo

A method to track cell fusion in living organisms over time is described. The approach utilizes Cre-LoxP recombination to induce luciferase expression upon cell fusion. The luminescent signal generated can be detected in living organisms using biophotonic imaging systems with a sensitivity of detection of &tilde;1,000 cells in peripheral tissues.

The ability of two or more cells of the same type to fuse has been utilized in metazoans throughout evolution to form many complex organs, including ...

Integrated Photoacoustic Ophthalmoscopy and Spectral-domain Optical Coherence Tomography

Both the clinical diagnosis and fundamental investigation of major ocular diseases greatly benefit from various non-invasive ophthalmic imaging technologies. Existing retinal imaging modalities, such as fundus photography1, confocal scanning laser ophthalmoscopy (cSLO)2, and optical coherence tomography (OCT)3, have significant contributions in monitoring disease onsets and progressions, and developing new therapeutic strategies. However, they predominantly rely on the back-reflected photons from the retina. As a consequence, the optical absorption properties of the retina, which are usually strongly associated with retinal pathophysiology status, are inaccessible by the traditional imaging technologies.
Photoacoustic ophthalmoscopy (PAOM) is an emerging retinal imaging modality that permits the detection of the optical absorption contrasts in the eye with a high sensitivity4-7 . In PAOM nanosecond laser pulses are delivered through the pupil and scanned across the posterior eye to induce photoacoustic (PA) signals, which are detected by an unfocused ultrasonic transducer attached to the eyelid. Because of the strong optical absorption of hemoglobin and melanin, PAOM is capable of non-invasively imaging the retinal and choroidal vasculatures, and the retinal pigment epithelium (RPE) melanin at high contrasts 6,7. More importantly, based on the well-developed spectroscopic photoacoustic imaging5,8 , PAOM has the potential to map the hemoglobin oxygen saturation in retinal vessels, which can be critical in studying the physiology and pathology of several blinding diseases 9 such as diabetic retinopathy and neovascular age-related macular degeneration.
Moreover, being the only existing optical-absorption-based ophthalmic imaging modality, PAOM can be integrated with well-established clinical ophthalmic imaging techniques to achieve more comprehensive anatomic and functional evaluations of the eye based on multiple optical contrasts6,10 . In this work, we integrate PAOM and spectral-domain OCT (SD-OCT) for simultaneously in vivo retinal imaging of rat, where both optical absorption and scattering properties of the retina are revealed. The system configuration, system alignment and imaging acquisition are presented.

Photoacoustic ophthalmology (PAOM), an optical-absorption-based imaging modality, provides the complementary evaluation of the retina to the currently available ophthalmic imaging technologies. We report the using of PAOM integrated with spectral-domain optical coherence tomography (SD-OCT) for simultaneous multimodal retinal imaging in rats.

Both the clinical diagnosis and fundamental investigation of major ocular diseases greatly benefit from various non-invasive ophthalmic imaging ...

High-resolution Spatiotemporal Analysis of Receptor Dynamics by Single-molecule Fluorescence Microscopy

Single-molecule microscopy is emerging as a powerful approach to analyze the behavior of signaling molecules, in particular concerning those aspect (e.g., kinetics, coexistence of different states and populations, transient interactions), which are typically hidden in ensemble measurements, such as those obtained with standard biochemical or microscopy methods. Thus, dynamic events, such as receptor-receptor interactions, can be followed in real time in a living cell with high spatiotemporal resolution. This protocol describes a method based on labeling with small and bright organic fluorophores and total internal reflection fluorescence (TIRF) microscopy to directly visualize single receptors on the surface of living cells. This approach allows one to precisely localize receptors, measure the size of receptor complexes, and capture dynamic events such as transient receptor-receptor interactions. The protocol provides a detailed description of how to perform a single-molecule experiment, including sample preparation, image acquisition and image analysis. As an example, the application of this method to analyze two G-protein-coupled receptors, i.e., &#946;2-adrenergic and &#947;-aminobutyric acid type B (GABAB) receptor, is reported. The protocol can be adapted to other membrane proteins and different cell models, transfection methods and labeling strategies.

This protocol describes how to use total internal reflection fluorescence microscopy to visualize and track single receptors on the surface of living cells and thereby analyze receptor lateral mobility, size of receptor complexes as well as to visualize transient receptor-receptor interactions. This protocol can be extended to other membrane proteins.

Single-molecule microscopy is emerging as a powerful approach to analyze the behavior of signaling molecules, in particular concerning those aspect (e.g., ...

Preparation of Mica Supported Lipid Bilayers for High Resolution Optical Microscopy Imaging

Supported lipid bilayers (SLBs) are widely used as a model for studying membrane properties (phase separation, clustering, dynamics) and its interaction with other compounds, such as drugs or peptides. However SLB characteristics differ depending on the support used. 
Commonly used techniques for SLB imaging and measurements are single molecule fluorescence microscopy, FCS and atomic force microscopy (AFM). Because most optical imaging studies are carried out on a glass support, while AFM requires an extremely flat surface (generally mica), results from these techniques cannot be compared directly, since the charge and smoothness properties of these materials strongly influence diffusion. Unfortunately, the high level of manual dexterity required for the cutting and gluing thin slices of mica to the glass slide presents a hurdle to routine use of mica for SLB preparation. Although this would be the method of choice, such prepared mica surfaces often end up being uneven (wavy) and difficult to image, especially with small working distance, high numerical aperture lenses. Here we present a simple and reproducible method for preparing thin, flat mica surfaces for lipid vesicle deposition and SLB preparation. Additionally, our custom made chamber requires only very small volumes of vesicles for SLB formation. The overall procedure results in the efficient, simple and inexpensive production of high quality lipid bilayer surfaces that are directly comparable to those used in AFM studies.

We present a method of preparing mica supported lipid bilayers for high resolution microscopy. Mica is transparent and flat on an atomic scale, but rarely used in imaging because of handling difficulties; our preparation results in even deposition of the mica sheet, and reduces the material used in bilayer preparation.

Supported lipid bilayers (SLBs) are widely used as a model for studying membrane properties (phase separation, clustering, dynamics) and its interaction ...

Fluorescence-quenching of a Liposomal-encapsulated Near-infrared Fluorophore as a Tool for In Vivo Optical Imaging

Optical imaging offers a wide range of diagnostic modalities and has attracted a lot of interest as a tool for biomedical imaging. Despite the enormous number of imaging techniques currently available and the progress in instrumentation, there is still a need for highly sensitive probes that are suitable for in vivo imaging. One typical problem of available preclinical fluorescent probes is their rapid clearance in vivo, which reduces their imaging sensitivity. To circumvent rapid clearance, increase number of dye molecules at the target site, and thereby reduce background autofluorescence, encapsulation of the near-infrared fluorescent dye, DY-676-COOH in liposomes and verification of its potential for in vivo imaging of inflammation was done. DY-676 is known for its ability to self-quench at high concentrations. We first determined the concentration suitable for self-quenching, and then encapsulated this quenching concentration into the aqueous interior of PEGylated liposomes. To substantiate the quenching and activation potential of the liposomes we use a harsh freezing method which leads to damage of liposomal membranes without affecting the encapsulated dye. The liposomes characterized by a high level of fluorescence quenching were termed Lip-Q. We show by experiments with different cell lines that uptake of Lip-Q is predominantly by phagocytosis which in turn enabled the characterization of its potential as a tool for in vivo imaging of inflammation in mice models. Furthermore, we use a zymosan-induced edema model in mice to substantiate the potential of Lip-Q in optical imaging of inflammation in vivo. Considering possible uptake due to inflammation-induced enhanced permeability and retention (EPR) effect, an always-on liposome formulation with low, non-quenched concentration of DY-676-COOH (termed Lip-dQ) and the free DY-676-COOH were compared with Lip-Q in animal trials.

Fluorescence-quenching of a Liposomal-encapsulated Near-infrared Fluorophore as a Tool for In Vivo Optical Imaging

The use of fluorophores for in vivo imaging can be greatly limited by opsonization, rapid clearance, low detection sensitivity and cytotoxic effects on the host. Encapsulation of fluorophores in liposomes by film hydration and extrusion leads to fluorescence quenching and protection which enables in vivo imaging with high detection sensitivity.

Optical imaging offers a wide range of diagnostic modalities and has attracted a lot of interest as a tool for biomedical imaging. Despite the enormous ...

Visualizing Angiogenesis by Multiphoton Microscopy In Vivo in Genetically Modified 3D-PLGA/nHAp Scaffold for Calvarial Critical Bone Defect Repair

The reconstruction of critically sized bone defects remains a serious clinical problem because of poor angiogenesis within tissue-engineered scaffolds during repair, which gives rise to a lack of sufficient blood supply and causes necrosis of the new tissues. Rapid vascularization is a vital prerequisite for new tissue survival and integration with existing host tissue. The de novo generation of vasculature in scaffolds is one of the most important steps in making bone regeneration more efficient, allowing repairing tissue to grow into a scaffold. To tackle this problem, the genetic modification of a biomaterial scaffold is used to accelerate angiogenesis and osteogenesis. However, visualizing and tracking in vivo blood vessel formation in real-time and in three-dimensional (3D) scaffolds or new bone tissue is still an obstacle for bone tissue engineering. Multiphoton microscopy (MPM) is a novel bio-imaging modality that can acquire volumetric data from biological structures in a high-resolution and minimally-invasive manner. The objective of this study was to visualize angiogenesis with multiphoton microscopy in vivo in a genetically modified 3D-PLGA/nHAp scaffold for calvarial critical bone defect repair. PLGA/nHAp scaffolds were functionalized for the sustained delivery of a growth factor pdgf-b gene carrying lentiviral vectors (LV-pdgfb) in order to facilitate angiogenesis and to enhance bone regeneration. In a scaffold-implanted calvarial critical bone defect mouse model, the blood vessel areas (BVAs) in PHp scaffolds were significantly higher than in PH scaffolds. Additionally, the expression of pdgf-b and angiogenesis-related genes, vWF and VEGFR2, increased correspondingly. MicroCT analysis indicated that the new bone formation in the PHp group dramatically improved compared to the other groups. To our knowledge, this is the first time multiphoton microscopy was used in bone tissue-engineering to investigate angiogenesis in a 3D bio-degradable scaffold in vivo and in real-time.

Visualizing Angiogenesis by Multiphoton Microscopy In Vivo in Genetically Modified 3D-PLGA/nHAp Scaffold for Calvarial Critical Bone Defect Repair

Here, we present a protocol to visualize blood vessel formation in vivo and in real-time in 3D scaffolds by multiphoton microscopy. Angiogenesis in genetically modified scaffolds was studied in a murine calvarial critical bone defect model. More new blood vessels were detected in the treatment group than in controls.

The reconstruction of critically sized bone defects remains a serious clinical problem because of poor angiogenesis within tissue-engineered scaffolds ...

A High-performance Compact Photoacoustic Tomography System for In Vivo Small-animal Brain Imaging

In vivo small-animal imaging has an important role to play in preclinical studies. Photoacoustic tomography (PAT) is an emerging hybrid imaging modality that shows great potential for both preclinical and clinical applications. Conventional optical parametric oscillator-based PAT (OPO-PAT) systems are bulky and expensive and cannot provide high-speed imaging. Recently, pulsed-laser diodes (PLDs) have been successfully demonstrated as an alternative excitation source for PAT. Pulsed-laser diode PAT (PLD-PAT) has been successfully demonstrated for high-speed imaging on photoacoustic phantoms and biological tissues. This work provides a visualized experimental protocol for in vivo brain imaging using PLD-PAT. The protocol includes the compact PLD-PAT system configuration and its description, animal preparation for brain imaging, and a typical experimental procedure for 2D cross-sectional rat brain imaging. The PLD-PAT system is compact and cost-effective and can provide high-speed, high-quality imaging. Brain images collected in vivo at various scan speeds are presented.

A High-performance Compact Photoacoustic Tomography System for In Vivo Small-animal Brain Imaging

A compact pulsed-laser diode-based photoacoustic tomography (PLD-PAT) system for high-speed in vivo brain imaging in small animals is demonstrated.

In vivo small-animal imaging has an important role to play in preclinical studies. Photoacoustic tomography (PAT) is an emerging hybrid imaging modality ...

Hand-held Clinical Photoacoustic Imaging System for Real-time Non-invasive Small Animal Imaging

Translation of photoacoustic imaging into the clinic is a major challenge. Handheld real-time clinical photoacoustic imaging systems are very rare. Here, we report a combined photoacoustic and clinical ultrasound imaging system by integrating an ultrasound probe with light delivery for small animal imaging. We demonstrate this by showing sentinel lymph node imaging in small animals along with minimally invasive real-time needle guidance. A clinical ultrasound platform with access to raw channel data allows the integration of photoacoustic imaging leading to a handheld real-time clinical photoacoustic imaging system. Methylene blue was used for sentinel lymph node imaging at 675 nm wavelength. Additionally, needle guidance with dual modal ultrasound and photoacoustic imaging was shown using the imaging system. Depth imaging of up to 1.5 cm was demonstrated with a 10 Hz laser at a photoacoustic imaging frame rate of 5 frames per second.

A clinical handheld photoacoustic imaging system will be demonstrated for real-time non-invasive small animal imaging.

Translation of photoacoustic imaging into the clinic is a major challenge. Handheld real-time clinical photoacoustic imaging systems are very rare. Here, ...

Rigid Embedding of Fixed and Stained, Whole, Millimeter-Scale Specimens for Section-free 3D Histology by Micro-Computed Tomography

For over a hundred years, the histological study of tissues has been the gold standard for medical diagnosis because histology allows all cell types in every tissue to be identified and characterized. Our laboratory is actively working to make technological advances in X-ray micro-computed tomography (micro-CT) that will bring the diagnostic power of histology to the study of full tissue volumes at cellular resolution (i.e., an X-ray Histo-tomography modality). Toward this end, we have made targeted improvements to the sample preparation pipeline. One key optimization, and the focus of the present work, is a straightforward method for rigid embedding of fixed and stained millimeter-scale samples. Many of the published methods for sample immobilization and correlative micro-CT imaging rely on placing the samples in paraffin wax, agarose, or liquids such as alcohol. Our approach extends this work with custom procedures and the design of a 3-dimensional printable apparatus to embed the samples in an acrylic resin directly into polyimide tubing, which is relatively transparent to X-rays. Herein, sample preparation procedures are described for the samples from 0.5 to 10 mm in diameter, which would be suitable for whole zebrafish larvae and juveniles, or other animals and tissue samples of similar dimensions. As proof of concept, we have embedded the specimens from Danio, Drosophila, Daphnia, and a mouse embryo; representative images from 3-dimensional scans for three of these samples are shown. Importantly, our methodology leads to multiple benefits including rigid immobilization, long-term preservation of laboriously-created resources, and the ability to re-interrogate samples.

We developed protocols and designed a custom apparatus to enable embedding of millimeter-scale specimens. We present sample preparation procedures with an emphasis on embedding in acrylic resin and polyimide tubing to achieve rigid immobilization and long-term storage of specimens for the interrogation of tissue architecture and cell morphology by micro-CT.

For over a hundred years, the histological study of tissues has been the gold standard for medical diagnosis because histology allows all cell types in ...