Name: real-time, two-color stimulated raman scattering imaging of mouse brain for tissue diagnosis
Uploaded: 2022-02-01T14:40:00
Description: Watch this Scientific Journal Video about Real-Time, Two-Color Stimulated Raman Scattering Imaging of Mouse Brain for Tissue Diagnosis at JoVE.com

This study was supported by NIH R35 GM133435 to D.F.

All experimental animal procedures were conducted with 200 &#181;m, fixed, sectioned mouse brains, in accordance with the protocol (# 4395-01) approved by the Institute of Animal Care and Use Committee (IACUC) of the University of Washington. Wild-type mice (C57BL/6J strain) are euthanized with CO2. Then, a craniotomy is performed to extract their brains for fixation in 4% paraformaldehyde in phosphate-buffered saline. The brains are embedded in a 3% agarose and 0.3% gelatin mixture and sectioned into 200 &#95...

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The two-color SRS imaging scheme presented in this protocol hinges on the proper implementation of one-color SRS imaging. In one-color SRS imaging, the critical steps are spatial alignment, temporal alignment, modulation depth, and phase shift. Spatially combining the two beams is accomplished by a dichroic mirror. Several steering mirrors are used for fine adjustment when sending the beams to the dichroic mirror. Once the beams are combined with the dichroic mirror, spatial alignment can be confirmed by picking off the ...

Traditional tissue diagnostics rely on staining protocols followed by examination under an optical microscope. One common staining method used by pathologists is H&#38;E staining: hematoxylin stains cell nuclei a purplish blue, and eosin stains the extracellular matrix and cytoplasm pink. This simple staining remains the gold standard in pathology for many tissue diagnoses tasks, particularly cancer diagnosis. However, H&#38;E histopathology, particularly the frozen sectioning technique used in an intraoperative setting, still has limitations. The staining procedure is a laborious process involving tissue embedding, sectioning, fixation, and staining1. The typical turnaround time is 20 min or longer. Performing H&#38;E during frozen sectioning can sometimes become more challenging when multiple sections are processed at once due to the need to evaluate cellular features or growth patterns in 3D for margin assessment. Moreover, intraoperative histological techniques require skilled technicians and clinicians. Limitation in the number of board-certified pathologists in many hospitals is a constraint for intraoperative consultation in many cases. Such limitations may be alleviated with the fast development interests in digital pathology and artificial intelligence-based diagnosis2. However, the H&#38;E staining results are variable, depending on the experience of the technician, which presents additional challenges for computer-based diagnosis2.
These challenges can potentially be addressed with label-free optical imaging techniques. One such technique is SRS microscopy. SRS uses synchronized pulsed lasers&#8212;pump and Stokes&#8212;to excite molecular vibrations with high efficiency3. Recent reports have demonstrated that SRS imaging of proteins and lipids can generate H&#38;E-equivalent images (also known as stimulated Raman histology or SRH) with intact fresh tissue, which bypasses the need for any tissue processing, significantly shortens the time needed for diagnosis, and has been adapted intraoperatively4. Moreover, SRS imaging can provide 3D images, which offers additional information for diagnosis when 2D images are insufficient5. SRH is unbiased and generates digital images that are readily available for computer-based diagnosis. It quickly emerges as a possible solution for intraoperative cancer diagnosis and tumor margin analysis, especially in brain cancer6,7,8. More recently, SRS imaging of chemical changes of tissue has also been suggested to provide useful diagnostic information that can further help clinicians stratify different cancer types or stages9.
Despite its tremendous potential in tissue diagnosis applications, SRS imaging is mostly limited to academic laboratories specialized in optics due to the complexity associated with the imaging platform, which includes ultrafast lasers, the laser scanning microscope, and sophisticated detection electronics. This protocol provides a detailed workflow to demonstrate the use of a common femtosecond laser source for real-time, two-color SRS imaging and the generation of pseudo-H&#38;E images from mouse brain tissue. The protocol will cover the following procedures:
Alignment and chirp optimization 
Most SRS imaging schemes use either picosecond or femtosecond lasers as the excitation source. With femtosecond lasers, the bandwidth of the laser is much larger than the Raman linewidth. To overcome this limitation, a spectral focusing approach is used to chirp the femtosecond lasers to a picosecond timescale to achieve narrow spectral resolution10. Optimal spectral resolution is only achieved when the temporal chirp (also known as the group delay dispersion or just dispersion) is properly matched for the pump and the Stokes lasers. The alignment procedure and the steps needed to optimize the dispersion of the laser beams using highly dispersive glass rods are demonstrated here.
Frequency calibration 
An advantage of spectral focusing SRS is that the Raman excitation can be quickly tuned by changing the time delay between the pump and the Stokes lasers. Such tuning affords fast imaging and reliable spectral acquisition compared to tuning laser wavelengths. However, the linear relationship between excitation frequency and time delay requires external calibration. Organic solvents with known Raman peaks are used to calibrate the Raman frequency for spectral focusing SRS.
Real-time, two-color imaging 
It is important to increase the imaging speed in tissue diagnosis applications to shorten the time needed for analyzing large tissue specimens. Simultaneous two-color SRS imaging of lipids and proteins obviates the need to tune the laser or time delay, which increases the imaging speed by more than two-fold. This is achieved by using a novel orthogonal modulation technique and dual-channel demodulation with a lock-in amplifier11. This paper describes the protocol for orthogonal modulation and dual-channel image acquisition.
Epi-mode SRS imaging 
The majority of SRS imaging shown to date is performed in transmission mode. Epi-mode imaging detects backscattered photons from tissue12. For pathology applications, surgical specimens can be quite large. For transmission mode imaging, tissue sectioning is often necessary, which undesirably requires extra time. In contrast, epi-mode imaging can work with intact surgical specimens. Because the same objective is used to collect backscattered light, there is also no need for aligning a high numerical-aperture condenser required for transmission imaging. Epi-mode is also the only option when tissue sectioning is difficult, such as with bone. Previously we have demonstrated that for brain tissue, epi-mode imaging offers superior imaging quality for tissue thickness &#62; 2 mm13. This protocol uses a polarizing beam splitter (PBS) to collect scattered photons depolarized by tissue. It is possible to collect more photons with an annular detector at the expense of the complexity of customized detector assembly12. The PBS approach is simpler to implement (similar to fluorescence), with the standard photodiode already being used for transmission mode detection.
Pseudo-H&#38;E image generation 
Once two-color SRS images are collected, they can be recolored to simulate H&#38;E staining. This paper demonstrates the procedure for converting lipid and protein SRS images to pseudo-H&#38;E SRS images for pathology applications. The experimental protocol details critical steps needed to generate high-quality SRS images. The procedure shown here is not only applicable to tissue diagnosis but also can be adapted for many other hyperspectral SRS imaging applications such as drug imaging and metabolic imaging14,15.
General system requirements 
The laser system for this protocol must be able to output 2 synchronized femtosecond laser beams. Systems ideally feature an Optical Parametric Oscillator (OPO) for broad wavelength tuning of one of the laser beams. The setup in this protocol uses a commercial laser system Insight DS+ that outputs two lasers (one fixed beam at 1,040 nm and one OPO-based tunable beam, ranging from 680 to 1,300 nm) with a repetition rate of 80 MHz. Laser scanning microscopes, either from major microscope manufacturers or home-built, can be used for SRS imaging. The utilized microscope is an upright laser scanning microscope built on top of a commercial upright microscope frame. A pair of 5 mm galvo mirrors are used to scan the laser beam. For users choosing to adopt a homebuilt laser scanning microscope, refer to a previously published protocol for the construction of a laser scanning microscope16.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>100 mm Achromatic Lens</td>
			<td>THORLABS</td>
			<td>AC254-100-B</td>
			<td>Broadband, 650 - 1,050 nm, achromatic lens focal length, 100 mm</td>
		</tr>
		<tr>
			<td>20 MHz bandpass filter</td>
			<td>Minicircuits</td>
			<td>BBP-21.4+</td>
			<td>Lumped LC Band Pass Filter, 19.2 - 23.6 MHz, 50 &#937;</td>
		</tr>
		<tr>
			<td>200 mm Achromatic Lens</td>
			<td>THORLABS</td>
			<td>AC254-200-B</td>
			<td>Broadband, 650 - 1,050 nm, achromatic lens focal length, 200 mm</td>
		</tr>
		<tr>
			<td>Achromatic Half Waveplate</td>
			<td>Union Optic</td>
			<td>WPA2210-650-1100-M25.4</td>
			<td>Broadband half waveplate</td>
		</tr>
		<tr>
			<td>Achromatic Quarter Waveplate</td>
			<td>Union Optic</td>
			<td>WPA4210-650-1100-M25.4</td>
			<td>Broadband quarter waveplate</td>
		</tr>
		<tr>
			<td>Beam Sampler</td>
			<td>THORLABS</td>
			<td>BSN11</td>
			<td>10:90 Plate Beamsplitter</td>
		</tr>
		<tr>
			<td>Dichroic Mirror</td>
			<td>THORLABS</td>
			<td>DMSP1000</td>
			<td>Other dichroics with a center wavelength around 1,000 nm can be used.</td>
		</tr>
		<tr>
			<td>DMSO (Dimethyl sulfoxide)</td>
			<td>Sigma Aldrich</td>
			<td>472301</td>
			<td>Solvent for calibration of Raman shift. Other solvents with known Raman peaks can be used.</td>
		</tr>
		<tr>
			<td>Electrooptic Amplitude Modulator</td>
			<td>THORLABS</td>
			<td>EO-AM-NR-C1</td>
			<td>Two EOMs are needed for orthogonal modulation and dual-channel imaging. Resonant version is recommended so lower driving voltage can be used.</td>
		</tr>
		<tr>
			<td>False H&#38;E Staining Script</td>
			<td>Matlab</td>
			<td></td>
			<td>https://github.com/TheFuGroup/HE_Staining</td>
		</tr>
		<tr>
			<td>Fanout Buffer</td>
			<td>PRL-414B</td>
			<td>Pulse Research Lab</td>
			<td>1:4 TTL/CMOS Fanout Buffer and Line Driver, for generating the EOM driving frequency and the reference to the lock-in</td>
		</tr>
		<tr>
			<td>Fast Photodiode</td>
			<td>THORLABS</td>
			<td>DET10A2</td>
			<td>Si Detector, 1 ns Rise Time</td>
		</tr>
		<tr>
			<td>Frequency Divider</td>
			<td>PRL-220A</td>
			<td>Pulse Research Lab</td>
			<td>TTL Freq. Divider (f/2, f/4, f/8, f/16), for generating 20MHz from the laser output.</td>
		</tr>
		<tr>
			<td>Highly Dispersive Glass Rods</td>
			<td>Union Optic</td>
			<td>CYLROD01</td>
			<td>High dispersion H-ZF52A Rod lens 120 mm, SF11 Rod lens 100 mm</td>
		</tr>
		<tr>
			<td>Insight DS+</td>
			<td>Newport</td>
			<td></td>
			<td>Laser system capable of outputting two synchronzied pulsed lasers (one fixed beam at 1, 040 nm and one tunable beam, ranging from 680-1,300 nm) with a repetition rate of 80 MHz.&#160;</td>
		</tr>
		<tr>
			<td>Lock-in Amplifier</td>
			<td>Liquid Instruments</td>
			<td>Moku Lab</td>
			<td>Lock-in amplifier to extract SRS signal from the photodiode. A Zurich Instrument HF2LI or similar instrument can be used as well.</td>
		</tr>
		<tr>
			<td>Mirrors</td>
			<td>THORLABS</td>
			<td>BB05-E03-10</td>
			<td>Broadband Dielectric Mirror, 750 - 1,100 nm. Silver mirrors can also be used.</td>
		</tr>
		<tr>
			<td>Motorized Delay Stage</td>
			<td>Zaber</td>
			<td>X-DMQ12P-DE52</td>
			<td>Delay stage for fine control of the temporal overlap of the pump and the Stokes lasers. Any other motorized stage should work.</td>
		</tr>
		<tr>
			<td>Oil Immersion Condensor</td>
			<td>Nikon</td>
			<td>CSC1003</td>
			<td>1.4 NA. Other condensers with NA&#62;1.2 can be used.</td>
		</tr>
		<tr>
			<td>Oscilloscope</td>
			<td>Tektronix</td>
			<td>TDS7054</td>
			<td>Any other oscilloscope with 400 MHz bandwdith or higher should work.</td>
		</tr>
		<tr>
			<td>Phase Shifter</td>
			<td>SigaTek</td>
			<td>SF50A2</td>
			<td>For shifting the phase of the modulation frequency</td>
		</tr>
		<tr>
			<td>Photodiode</td>
			<td>Hamamatsu Corp</td>
			<td>S3994-01</td>
			<td>Silicon PIN diode with large area (10 x 10 cm2). Other diodes with large area and low capacitance can be used.</td>
		</tr>
		<tr>
			<td>Polarizing Beam Splitter</td>
			<td>Union Optic</td>
			<td>PBS9025-620-1000</td>
			<td>Broadband polarizing beamsplitter</td>
		</tr>
		<tr>
			<td>Refactive Index Database</td>
			<td></td>
			<td></td>
			<td>refractiveindex.info</td>
		</tr>
		<tr>
			<td>Retro-reflector</td>
			<td>Edmund Optics</td>
			<td>34-408</td>
			<td>BBAR Right Angle Prism. Other prisms or retroreflector can be used.</td>
		</tr>
		<tr>
			<td>RF Power Amplifier</td>
			<td>Minicircuits</td>
			<td>ZHL-1-2W+</td>
			<td>Gain Block, 5 - 500 MHz, 50 &#937;</td>
		</tr>
		<tr>
			<td>Scan Mirrors</td>
			<td>Cambridge Technologies</td>
			<td>6215H</td>
			<td>We used a 5mm mirror set with silver coating</td>
		</tr>
		<tr>
			<td>ScanImage</td>
			<td>Vidrio</td>
			<td>ScanImage Basic</td>
			<td>Laser scanning microscope control software</td>
		</tr>
		<tr>
			<td>Shortpass Filter</td>
			<td>THORLABS</td>
			<td>FESH1000</td>
			<td>25.0 mm Premium Shortpass Filter, Cut-Off Wavelength: 1,000 nm. For efficient suppression of the Stokes, two filters may be necessary.</td>
		</tr>
		<tr>
			<td>Upright Microscope</td>
			<td>Nikon</td>
			<td>Eclipse FN1</td>
			<td>Any other microscope frame can be used. If a laser scanning microscope is available, it can be used directly. Otherwise, a galvo scanner and scan lens needed to be added to the microscope.</td>
		</tr>
		<tr>
			<td>Water Immersion Objective</td>
			<td>Olympus</td>
			<td>XLPLN25XWMP2</td>
			<td>The multiphoton 25X Objective has a NA of 1.05. Other similar objectives can be used.</td>
		</tr>
	</tbody>
</table>

real-time, two-color stimulated raman scattering imaging of mouse brain for tissue diagnosis

Stimulated Raman scattering (SRS) microscopy has emerged as a powerful optical imaging technique for tissue diagnosis. In recent years, two-color SRS has been shown to be able to provide hematoxylin and eosin (H&amp;E)-equivalent images that allow fast and reliable diagnosis of brain cancer. Such capability has enabled exciting intraoperative cancer diagnosis applications. Two-color SRS imaging of tissue can be done with either a picosecond or femtosecond laser source. Femtosecond lasers have the advantage of enabling flexible imaging modes, including fast hyperspectral imaging and real-time, two-color SRS imaging. A spectral-focusing approach with chirped laser pulses is typically used with femtosecond lasers to achieve high spectral resolution. 
Two-color SRS acquisition can be realized with orthogonal modulation and lock-in detection. The complexity of pulse chirping, modulation, and characterization is a bottleneck for the widespread adoption of this method. This article provides a detailed protocol to demonstrate the implementation and optimization of spectral-focusing SRS and real-time, two-color imaging of mouse brain tissue in the epi-mode. This protocol can be used for a broad range of SRS imaging applications that leverage the high speed and spectroscopic imaging capability of SRS.

Optimizing spectral resolution: 
Dispersion through a material is affected by the dispersive medium (length and material) and wavelength. Changing the dispersion rod length affects the spectral resolution and the signal size. It is a give-and-take relationship that can be weighed differently depending on the application. The rods stretch out the beam pulse from being wide in frequency and narrow in time to being narrow in frequency and broad in time. Figure 7 shows the ...

Watch this Scientific Journal Video about Real-Time, Two-Color Stimulated Raman Scattering Imaging of Mouse Brain for Tissue Diagnosis at JoVE.com

Real-Time, Two-Color Stimulated Raman Scattering Imaging of Mouse Brain for Tissue Diagnosis

Stimulated Raman scattering (SRS) microscopy is a powerful, nondestructive, and label-free imaging technique. One emerging application is stimulated Raman histology, where two-color SRS imaging at the protein and lipid Raman transitions are used to generate pseudo-hematoxylin and eosin images. Here, we demonstrate a protocol for real-time, two-color SRS imaging for tissue diagnosis.

Stimulated Raman scattering (SRS) microscopy has emerged as a powerful optical imaging technique for tissue diagnosis. In recent years, two-color SRS has ...

This protocol will demonstrate implementation and optimization of spectral focusing SRS microscopy and how it can be used for real-time stimulated Raman histology applications. The main advantage of this technique is the speed at which samples can be processed once the instrumentation is properly aligned, as SRS does not require tissue processing and staining. This protocol is applicable to other SRS applications, not just limited to histology.Such applications include small molecules, drugs, cells, and tissues. Begin by installing a pair of achromatic lenses for the pump beam to magnify the laser beam size by twofold as a starting point. Place a 100 and 200 millimeter lens at roughly 300 millimeters from each other.Then align the pump beam through the center of both lenses. Next place a mirror after the second lens to send the beam towards a wall more than one meter away. Trace the beam from the mirror to the wall with an IR card and check if the beam changes in size.Collimate the beam if the beam changes in size as a function of distance. Combine the two laser beams by installing a dichroic mirror with several steering mirrors for adjustment. Optimize spatial overlap of the pump and Stokes by monitoring beams after the dichroic mirror at two different positions around one meter apart.Iteratively adjust the steering mirror followed by the dichroic mirror to align the Stokes beam with the pump beam. Ensure that the combined beams are sent to the center of the scan mirrors of the laser scanning microscope by adjusting a pair of steering mirrors when the scan mirrors are in the parked position. After the condenser, use another pair of lenses with focal lengths of 100 millimeters and 30 millimeters, respectively, to relay the transmitted beam onto the photodiode, ensuring both beams are contained within the photodiode.Then install two low pass filters to block out the modulated Stokes beam. Place a beam sampler in the Stokes beam path to pick up 10%of the beam and send it to a fast photodiode to detect the 80 megahertz pulse train. Take one of the outputs of the fanout buffer and filter it with a band pass filter to obtain a 20 megahertz sinusoidal wave.Then use an RF attenuator to adjust the output peak to peak voltage to approximately 500 millivolts. Send the resulting output to a phase shifter, which allows fine adjustment of the RF phase with a voltage source. Send this output to an RF power amplifier and connect the output of the amplifier to the EOM.After unblocking the Stokes beam, optimize the modulation depth of the first EOM by placing a photodiode in the beam path. Adjust the EOM voltage and quarter wave plate until the modulation depth appears satisfactory. Place a photodiode after the dichroic mirror to detect the laser beam.Block the Stokes beam first, then zoom in on one of the pump pulse peaks on the oscilloscope. Place a vertical cursor to mark the temporal position of this peak with the oscilloscope. Now block the pump beam, and unblock the Stokes beam.Translate the delay stage to temporarily match the peak positions on the oscilloscope to the marked position in the previous step. By calculating the delay distance required to match the two beams followed by elongating the beam path of the faster beam or shortening the beam path of the slower beam. Next prepare a microscope slide sample with DMSO and double-sided tape as a spacer to hold the sample between the slide and a coverslip.Place the sample on the microscope with the coverslip side facing the microscope objective and observe the sample from the I piece under brightfield illumination. Find the focus of the sample at both the top and bottom layer of air bubbles at the glass tape interface, then move the focus to be in between the two layers of the tape. Next set the tunable beam output to 798 nanometers.Based on the optical throughput of the condenser, adjust the optical power to be approximately 40 milliwatts each for the pump and Stokes beams at the objective focus. Then open scan image in MATLAB and click on the button labeled focus to start scanning. Adjust the steering mirror before the galvo scanner to center the DC signal on the channel one display.Move the motorized delay stage and closely observe the lock-in output shown on the channel two display. Finally adjust the time delay to maximize the AC signal intensity. Adjust the dichroic mirror to center the SRS signal on the AC channel.Then adjust the phase of the lock-in amplifier to maximize the signal. After mounting the sample slide onto the microscope and adjusting the power of both beams to approximately 40 milliwatts each at sample focus as demonstrated earlier. Open scan image from MATLAB.Then find the maximal SRS signal by scanning through the delay stage corresponding to the 2, 913 inverse centimeter Raman peak of DMSO. Acquire an SRS image corresponding to the 2, 913 inverse centimeter Raman peak of DMSO. Open the image in ImageJ and select a small area in the center of the frame.Use the measure function to calculate the mean and standard deviation of values in the selected area. For frequency access calibration, save a hyperspectral scan with the delay stage range covering the 2, 913 and 2, 994 inverse centimeter Raman peaks of DMSO. Then save the stage positions corresponding to the hyperspectral data set.Perform linear regression for the stage positions and Raman shifts at 2, 913 and 2, 994 inverse centimeters. Using the linear regression equation, convert the delay positions to the corresponding Raman frequencies. For calibration of spectral resolution, estimate the stage position based on the previous position with the increased optical path length due to the insertion of rods.Realign the beams spatial overlap because of the slight deviation of the beam when glass rods are added. Install a polarizing beam splitter, a quarter wave plate, and a second EOM into the soaks beam path after the first EOM. Unplug the signal input to the second EOM, then plug in the signal input to the first EOM, and turn it on.Modulate the Stokes beam at 20 megahertz by sending the beam through the first EOM. Adjust the tilt and position of the first EOM to ensure that the beam is centered through the EOM crystal. Prepare a tissue slide sample with double-sided tape, microscope slide and cover glass.Place the sample on the microscope with the coverslip side facing the microscope objective. For epi-mode SRS imaging, install a half-wave plate before the beam enters the microscope to change the polarization of the beam going into the microscope. Place a polarizing beam splitter above the objective to allow the depolarized back reflected beam to reach the detector.Next, use a 75 millimeter achromate lens and a 30 millimeter aspheric lens to relay the back scattered photons from the back aperture of the objective to the photo detector. Mount the detector to collect the back scattered light directed by the polarizing beam splitter. Then install a filter to block out the modulated beam from entering the detector.Varying the rod lengths affect spectral resolution. When no glass chirping rods are used, the two Raman peaks from DMSO are not resolved at all. An increase in the number of glass chirping rods starts to resolve the two peaks at a satisfactory point.Matched chirping resolves both peaks and can be used to calibrate stage positions to frequency. Two time delayed pulse trains of orthogonal polarization used to image DMSO spectra show the time delay between the SRS excitations with acceptable modulation depth and temporal separation. In contrast, a poor two color SRS phase shift, gives rise to inverted or negative peaks.Real-time two color SRS imaging at 2, 850 and 2, 930 inverse centimeters of ex vivo mouse brain tissue are shown here. The raw lipid and protein images were color coded to produce a single image depicting lipid and protein contributions. False hematoxylin and eosin recoloring, were also performed to mimic hematoxylin and eosin staining for pathological applications.Spatial temporal overlap and spatial resolution optimization are the most important steps for the spatial focusing approach of SRS. This method is generally applicable for other pump probe microscopy experiments such as transient absorption microscopy. The method also allows for imaging of non-fluorescent molecules in cells and tissue.The method presented here speeds up the image acquisition time for future translation of stimulated Raman histology in the clinic.

Research

JoVE Journal

Bioengineering

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