Name: a high-performance compact photoacoustic tomography system for in vivo small-animal brain imaging
Uploaded: 2017-06-21T14:00:00.000+00:00
Duration: 5 min 32 s
Description: Watch this Scientific Journal Video about A High-performance Compact Photoacoustic Tomography System for In Vivo Small-animal Brain Imaging at JoVE.com

The research is supported by the Tier 2 grant funded by the Ministry of Education in Singapore (ARC2/15: M4020238) and the Singapore Ministry of Health's National Medical Research Council (NMRC/OFIRG/0005/2016: M4062012). The authors would like to thank Mr. Chow Wai Hoong Bobby for the machine shop help.

All animal experiments were performed according to the guidelines and regulations approved by the Institutional Animal Care and Use Committee of Nanyang Technological University, Singapore (Animal Protocol Number ARF-SBS/NIE-A0263).
1. System Description
<ol>
	<li>Mount the PLD inside the circular scanner, as shown in Figure 1a. Connect the PLD to the laser driver unit (LDU). 
	NOTE: The PLD provides ~136-ns pulses at a wavelength of ~803 nm, with a maximum pulse energy of ~1.42 mJ and a repetition rate up to 7 kHz. The laser driving unit (LDU) includes: a temperature controller, a variable power supply, a power supply (12 V), and a function generator; see the Table of Materials. The variable power supply is used to control the laser power, and the function generator is used to change the repetition rate of the PLD.</li>
	<li>Switch on the PLD laser. Set the PLD repetition rate to &#34;7,000&#34; Hz by using the function generator in the LDU. Increase the pulse energy to 1.42 mJ by setting the voltage of the variable power supply to &#34;3.1&#34; V.</li>
	<li>Mount the optical diffuser (OD) in front of the PLD exit window to make the output beam homogenous, as shown in Figure 1a. 
	NOTE: Use a diffuser with a fine grit (i.e., 1,500-grit polish).</li>
	<li>Mount the focused UST on the UST holder, such that it faces the center of the scanning area, as shown in Figure 1a. 
	NOTE: The central frequency of the UST is 2.25 MHz, and the focal length is 1.9 in.</li>
	<li>Place the ultrasound detector inside the acrylic tank, as shown in Figure 1a. Fill the tank with water such that the UST is totally immersed. 
	NOTE: A water medium is used to couple the photoacoustic signal from the brain (sample) to the UST. An acrylic water tank (WT; see the Table of Materials) was custom-designed for small-animal imaging. The schematic of the water tank design is shown in Figure 1b.</li>
	<li>Check the PA signal from the sample using a pulser/receiver unit (PRU; see the Table of Materials). 
	NOTE: These signals were digitized by a 12-bit DAQ card (see the Table of Materials) at a 100 MS/s sampling rate and were saved on a computer.</li>
</ol>
2. Animal Preparation for Rat Brain Imaging
NOTE: The PLD-PAT system described above was demonstrated for imaging small animal brains. For these experiments, healthy female rats (see the Table of Materials) were used.
<ol>
	<li>Anesthetize the animal by intraperitoneally injecting a cocktail of 2 mL of ketamine, 1 mL of xylazine, and 1 mL of saline (dosage of 0.2 mL/100 g).</li>
	<li>Remove the fur on the scalp of the animal using a hair clipper. Gently apply hair removal cream to the shaved area for further depletion of the fur.
	<ol>
		<li>Remove the applied cream after 4-5 min using a cotton swab.</li>
		<li>Apply artificial tear ointment to the eyes of the animal to prevent the dryness due to anesthesia and laser illumination.</li>
	</ol>
	</li>
	<li>Mount the custom-made animal holder (see the Table of Materials) equipped with a breathing mask (see the Table of Materials) on a lab-jack.</li>
	<li>Place the animal in prone position on the holder. Secure it to the holder using surgical tape to avoid movement of the animal during imaging.</li>
	<li>Ensure that the breathing mask covers the nose and mouth of the rat to deliver inhaled anesthetic.</li>
</ol>
3. In Vivo Rat Brain Imaging
<ol>
	<li>Connect the breathing mask to the anesthesia machine. Switch on the anesthesia machine and set it to deliver 1.0 L/min of oxygen with 0.75% isoflurane.
	<ol>
		<li>Clamp the pulse oximeter to its tail to monitor the physiological condition of the animal.</li>
	</ol>
	</li>
	<li>Apply a layer of colorless ultrasound gel to the scalp of the rat. Adjust the lab-jack position to the center of the scanner. The breathing mask is customized to suit the imaging window. 10% of the commercially available nose cone is cut then connected to a piece of glove.
	<ol>
		<li>Adjust the height of the lab-jack manually so that the imaging plane is at the focus of the UST.</li>
	</ol>
	</li>
	<li>Set the parameters in the data acquisition software (see the Table of Materials) as required. Run the data acquisition software program to start acquisition (i.e., imaging). 
	NOTE: The program is used to rotate the UST and collect A-line PA signals. The collected A-lines will be saved on the computer.</li>
	<li>Observe the animal during the entire imaging period and proceed to PAT reconstruction after the imaging is complete.</li>
	<li>After the data acquisition is over, reconstruct the cross-sectional brain image from the A-lines using the reconstruction software program.</li>
	<li>Turn off the anesthesia system, remove the animal from the stage, return it to its cage, and monitor it until it regains consciousness. 
	NOTE: For example, if the UST is rotated for 5 s, the PLD delivers 35,000 (= 5 x 7,000) pulses and the UST collects 35,000 A-lines. The 35,000 A-lines are reduced to 500 by averaging over 70 signals (after averaging A-lines = 35,000/70 = 500). Figure 1c illustrates the illumination of the laser pulses and A-line collection. A reconstruction program based on delay-and-sum back projection algorithm should be used.</li>
</ol>
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/55811/55811fig1.jpg" /> 
Figure 1: Schematics of the PLD-PAT System. (a) Schematic of the PLD-PAT. PLD: pulsed-laser diode, CSP: circular scanning plate, AM: anesthesia machine, M: motor, MPU: motor pulley unit, LDU: laser driver unit, PRU: pulser/receiver unit, UST: ultrasound transducer, WT: water tank, PF: polymer film, and DAQ: data acquisition card. (b) Schematic of the water tank, top view (1) and cross-sectional view (2) for in vivo small-animal brain imaging. A: metric Screw, B: acrylic annular plate, C: silicone &#34;O&#34; ring, D: 100 &#181;m-thick, transparent polythene cover. The tank had a 9 cm diameter hole at its bottom and was sealed with an ultrasonically and optically transparent, 100 &#181;m-thick polyethylene membrane. (c) Schematic of the illumination of laser pulses from the PLD and A-lines, collection in a 5 s continuous scan time. <a href="https://cloudfront-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55811/55811fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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The authors have no relevant financial interests in the manuscript and no other potential conflicts of interest to disclose.

This work presents a protocol for performing in vivo brain imaging on rats using a PLD-PAT system. The protocol includes a detailed description of the imaging system and its alignment, as well as an illustration of brain imaging on rats. The existing OPO-based PAT systems are expensive and bulky and can provide one cross-sectional image in 5-10 min. The PLD-PAT system is compact, portable, and low-cost and can provide good-quality images in 3 s. The performance of the system was previously studied in phantoms and compared with conventional PAT system25. Here, the same PLD-PAT was demonstrated for fast in vivo brain imaging. The outcome demonstrates that the system can provide high-quality in vivo images, even in 5 s.
While there are several advantages, the PLD-PAT system has several drawbacks. The PLD used in this study provides pulses at a single wavelength, so it cannot provide functional imaging, which requires multi-wavelength illumination. For functional imaging, a PLD with multi-wavelength illumination capabilities is required. The low-energy PLD pulses limit the imaging depth. However, using an exogenous contrast agent, it is possible to enhance the imaging depth of the PLD-PAT system.
Usually, the PLD laser beam is not uniform, so a suitable optical diffuser can be used in front of the laser window to improve the image quality. Make sure that the center of the laser beam and the center of the imaging area coincide. While scanning the UST around the brain, ensure that the UST always faces the scan center. When implementing the protocol, additional care needs to be taken: (a) the amount of anesthesia cocktail should be administered according to the weight of the animal; (b) the anesthesia injection must be precise so that the organs (e.g., the urinary bladder, intestine, and kidney) are not affected; (c) during the hair clipping, ensure that the scalp of the animal is not scratched; (d) the pressure of the water tank on the animal must be as minimal as possible; and (e) while positioning the animal under the scanner, make sure that the imaging cross-sectional plane of the brain is at the center of the UST. Future applications of the system include brain tumor imaging, imaging different organs in small animals, high-speed imaging in less than 5 s, investigating biomaterials for contrast agents, and therapy applications. Troubleshooting may be required if the image quality is low.
Laser safety for small-animal in vivo imaging
The maximum permissible exposure (MPE) limit for skin depends on several parameters, such as excitation wavelength, pulse width, time of exposure, illuminating area, etc. The MPE limits for in vivo imaging are governed by the American National Standards Institute (ANSI)32. In the 700 to 1,050 nm wavelength range, the energy density on the skin delivered by a single pulse should be less than 20 x 102(&#955;-700)/1,000 mJ/cm2 (&#955;: excitation wavelength in nm). For the 803 nm PLD wavelength, the limit is ~31 mJ/cm2. If the laser is used continuously over a period of t = 5 s, then the MPE becomes 1.1 x 102(&#955;-700)/1000 &#215; t0.25 J/cm2 (= 2.6 J/cm2). In this experiment, the PLD was operated at 7,000 Hz. In a 5-s scan time, a total of 35,000 (5&#215;7,000) pulses were delivered to the sample, so per pulse, the MPE was 0.07 mJ/cm2. In the described imaging system, the PLD delivers pulses with energy at ~1.05 mJ per pulse, and the laser beam was expanded over a ~12.6 cm2 area. Hence, the laser energy density was ~0.08 mJ/cm2 on the brain area. The ANSI laser safety limit of the PAT system can be changed by reducing the laser power, by expanding the laser beam, or by reducing the pulse repetition rate.

Photoacoustic tomography (PAT) is a hybrid imaging modality that has many applications in both clinical and preclinical studies1,2,3,4,5. In PAT, nanosecond laser pulses irradiate biological tissue. The absorption of incident light by the tissue chromophores leads to a local temperature rise, which then yields pressure waves emitted in the form of sound waves. An ultrasound detector collects the photoacoustic signals at various positions around the sample. The photoacoustic (PA) signals are reconstructed using various algorithms (such as a delay-and-sum algorithm)6 to generate the photoacoustic image.
This hybrid imaging modality offers high-resolution, deep-tissue imaging and high optical absorption contrast7,8. Recently, a ~12-cm imaging depth9 was achieved in chicken breast tissue with the aid of a longer wavelength (~1,064 nm) and an exogenous contrast agent calledphosphorus phthalocyanine. This depth sensitivity is much higher than the depth sensitivity of other optical methods, such as confocal fluorescence microscopy, two-photon fluorescence microscopy,10 optical coherence tomography, 11etc. Using more than one wavelength, PAT can demonstrate structural and functional changes in organs. For many human diseases, small-animal models have been well-established12,13,14,15. For the imaging of small animals, several modalities have been demonstrated. Out of all those approaches, PA imaging has gained attention rather quickly due to the advantages mentioned above. PAT has shown its potential for imaging blood vessels in the tissues and organs (i.e., heart, lungs, liver, eyes, spleen, brain, skin, spinal cord, kidney, etc.) of small animals4,16,17,18. PAT is a well-established modality for small-animal brain imaging. PA waves are produced due to the light absorption by the chromophores, so multiple-wavelength PAT allows for the mapping of total hemoglobin concentration (HbT) and oxygen saturation (SO2)19,20,21,22. Brain neurovascular imaging was achieved with the aid of exogenous contrast agents12,23,24. PA modality can help to give a better understanding of brain health by providing information at the molecular and genetic levels.
For small-animal imaging, Nd : YAG/OPO lasers are widely used as PAT excitation sources. These lasers deliver ~5 ns near-infrared pulses with energy (~100 mJ at the OPO output window) at a ~10-Hz repetition rate25. The PA system equipped with such lasers is costly and bulky and allows for low-speed imaging with single-element ultrasound transducers (UST) due to the low repetition rate of the laser source. A typical A-line acquisition time in such PA systems is ~5 min per cross-section25. An imaging system with such a lengthy measurement time is not ideal for small-animal imaging, because it is difficult to control the physiological parameters for full-body imaging, time-resolved functional imaging, etc. By adopting multiple single-element USTs, array-based USTs, or a high-repetition-rate laser, it is possible to increase the imaging speed of PA systems. Using only one single-element UST to collect all PA signals around the sample will limit the imaging speed of the system. Multiple single-element USTs arranged in circular or semi-circular geometry are demonstrated for high-speed, highly sensitive imaging techniques. Array-based USTs26, such as linear, semi-circular, circular, and volumetric arrays, have been successfully used for real-time imaging1. These array-based USTs will increase the imaging speed and reduce the measurement sensitivity, but they are expensive. However, the imaging speed of PA systems that use array-based USTs is still limited by the repetition rate of the laser.
Pulsed-laser technology advanced to make high-repetition-rate pulsed-laser diodes (PLDs). 7,000 frames/s B-scan photoacoustic imaging was demonstrated with PLDs using a clinical ultrasound platform27. Such PLDs can improve the imaging speed of the PAT system, even with single-element UST circular scanning geometry. Single-element USTs are less expensive and highly sensitive, unlike array-based USTs. Over the last decade, little research was reported on the use of high-repetition-rate PLDs as the excitation source for PA imaging. A fiber-based near-infrared PLD was demonstrated for PA imaging of phantoms28. The in vivo imaging of blood vessels at a ~1 mm depth below the human skin was demonstrated using low-energy PLDs29. A PLD-based optical resolution photoacoustic microscope (ORPAM) was reported. Using PLDs, ~1.5 cm deep imaging at a frame rate of 0.43 Hz was demonstrated30. Very recently, a PLD-PAT system was reported that provided images in as short as ~3 s and at a ~2 cm imaging depth in biological tissue25,31. This study proved that such a low-cost, compact system can provide high-quality images, even at high speeds. The PLD-PAT system can be used for high-frame-rate (7,000 fps) photoacoustic imaging, surficial blood vessel imaging, finger joint imaging, 2 cm-deep tissue imaging, small-animal brain imaging, etc. The single-wavelength and low-pulse-energy pulses from PLD limit its application to multi-spectral and deep-tissue imaging. Experiments have been carried out on small animals using the same PLD-PAT system used for pre-clinical applications. The purpose of this work is to provide the visualized experimental demonstration of the PLD-PAT system for in vivo 2D cross-sectional brain imaging of small animals.

<table><tbody><tr><td>Pulsed laser diode</td><td>Quantel, France</td><td>QD-Q1910-SA-TEC</td><td>It is the excitation laser source with specifications 803 nm, 1.4 mJ per pulse, 136 ns pulse, 7 kHz maximum, dimentions : 11.0 x 6.0 x 3.6 cm, weight: ~150 gm</td></tr><tr><td>Stepper motor with gearbox</td><td>LIN Engineering (Servo Dynamics)</td><td>Motor: CO-5718-01P, Gearbox: DPL64/1, I = 10 for NEMA 23; power supply PW100-48</td><td>To move the detector holder in a circular geometry. Torque: 2.08 N-m, Rotor inertia: 2.6 kg-cm2</td></tr><tr><td>Ultrasonic pulser/receiver</td><td>Olympus</td><td>5072PR</td><td>To receive, filter and ampligy the PA signal from UST. Its bandwidth is 35 MHz, and gain is &amp;plusmn;59 dB.</td></tr><tr><td>Ultrasound Transducer</td><td>Olympus</td><td>V306-SU-NK-CF1.9IN/Q4200069</td><td>Ultrasonic sensors used for photoacoustic detection. Central freqency 2.25 MHz, 0.5 in, Cylindrical focus 1.9 inch</td></tr><tr><td>PCIe DAQ (Data acquisition) Card</td><td>GaGe</td><td>CSE4227/ A6000610/B0E00610</td><td>12 bit, 100 Ms/s, 2 channels, 1 Gs on board memory, PCIe x16 interface</td></tr><tr><td>Rats</td><td>In Vivos Pte Ltd, Singapore</td><td>NTac:SD, Sprague Dawley / SD</td><td>Female, weight 100 &amp;plusmn;10g</td></tr><tr><td>Acrylic water tank&amp;nbsp;</td><td>NTU workshop</td><td>Custom-made</td><td>It contains the water that acts as an acoustic coupling medium between brain and detector</td></tr><tr><td>Circular Scanner&amp;nbsp;</td><td>NTU workshop</td><td>Custom-made</td><td>Scanner is made out of Alluminum&amp;nbsp;</td></tr><tr><td>Anesthetic Machine</td><td>medical plus pte ltd</td><td>Non-Rebreathing Anaesthesia machine with oxygen concentrator.</td><td>Supplies oxygen and isoflurane to animal</td></tr><tr><td>Pulse Oxymeter portable</td><td>Medtronic</td><td>PM10N with veterinary sensor</td><td>Monitors the pulse oxymetry of the animal</td></tr><tr><td>Ultrasound gel</td><td>Progress/parker acquasonic gel</td><td>PA-GEL-CLEA-5000</td><td>Clear ultrasound gel</td></tr><tr><td>Data acqusison software</td><td>National Instruments Corporation,Austin,TX,USA)</td><td>NI LabVIEW 2015 SP1</td><td>LabVIEW based program was developed in our laboratory for controlling the stepper motor and acquring the PA singnals from the detector</td></tr><tr><td>Data processing software</td><td>Matlab (Mathworks, Natick, MA, USA)</td><td>Matlab R2012b</td><td>Matlab code for reconstruction of PA images was developed in our lab</td></tr><tr><td>Temperature controller&amp;nbsp;</td><td>LaridTech, MO,USA</td><td>MTTC1410</td><td>It will constantly control temperature of the PLD&amp;nbsp;</td></tr><tr><td>12 V power supply&amp;nbsp;</td><td>Voltcraft&amp;nbsp;</td><td>PPS-11810</td><td>To supply operating voltage for PLD</td></tr><tr><td>Variable power supply&amp;nbsp;</td><td>BASETech</td><td>BT-153</td><td>To change the laser output power</td></tr><tr><td>Funtion generator&amp;nbsp;</td><td>Funktionsgenerator</td><td>FG250D</td><td>To change the repetetion rate of the PLD. It will provide TTL signal to synchronize the DAQ with the laser excitation.</td></tr><tr><td>Animal distributor</td><td>In Vivos Pte Ltd, Singapore</td><td></td><td>Animal distributor that supplies small animals for research purpose.</td></tr><tr><td>Animal holder</td><td>NTU workshop</td><td>Custom-made</td><td>Used for holding the animal on its abdomen</td></tr><tr><td>Breathing mask</td><td>NTU workshop</td><td>Custom-made</td><td>Used along with animal holder to supply anesthesia mixture to the animal</td></tr><tr><td>Pentobarbital sodium</td><td>Valabarb</td><td></td><td>Used for euthanizing the animal after the expeirment.</td></tr><tr><td>Optical diffuser</td><td>Thorlabs</td><td>DG10-1500</td><td>Used to to make the laser beam homogeneous</td></tr></tbody></table>

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.

The in vivo brain imaging results that demonstrate the capabilities of the described PLD-PAT system are showcased in this section. To demonstrate the high-speed imaging capabilities of the PLD-PAT system, the in vivo brain imaging of two different healthy rats was performed. Figure 2 shows the brain images of a female rat (93 g) at various scan speeds. Figure 2a and b show the photographs of the rat brain before and after removing the scalp over the brain area. PAT imaging was done non-invasively (i.e., with the skin and skull intact). PA signals from the cross-section of the brain were collected by circularly rotating the UST for 5 s, 10 s, 20 s, and 30 s. Figure 2c-f show the PAT-reconstructed cross-sectional images of the rat brain, obtained in 5 s, 10 s, 20 s, and 30 s scan times. In all these brain images, the transverse sinus (TS), superior sagittal sinus (SS), and cerebral veins (CV), including branches, are clearly visible. These features are indicated on the image shown in Figure 2f. These results promise that the system can provide high-quality in vivo images, even at high scanning speeds.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/55811/55811fig2.jpg" /> 
Figure 2: Non-invasive In Vivo PLD-PAT Images. Non-invasive PLD-PAT images of the vasculature in a 93 g female rat brain. Photograph of the rat brain before (a) and after (b) removing the scalp. In vivo brain images at different scan times: (c) 5 s, (d) 10 s, (e) 20 s, and (f) 30 s. SS: sagittal sinus, TS: transverse sinus, and CV: cerebral veins. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55811/55811fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
A similar imaging experiment was carried out on another female rat (95 g), and corresponding brain images obtained in 5 s, 10 s, 20 s, and 30 s are shown in Figure 3.
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/55811/55811fig3.jpg" /> 
Figure 3: Non-invasive In Vivo PLD-PAT Images. Non-invasive PLD-PAT images of the vasculature in a 95 g female rat brain. Photograph of the rat brain before (a) and after (b) removing the scalp. In vivo brain images at different scan times: (c) 5 s, (d) 10 s, (e) 20 s, and (f) 30 s. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55811/55811fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about A High-performance Compact Photoacoustic Tomography System for In Vivo Small-animal Brain Imaging at JoVE.com

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.

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 ...

a-high-performance-compact-photoacoustic-tomography-system-for-vivo

Nanyang Technological University

Research

JoVE Journal

Bioengineering

10.4K Views.  Nanyang Technological University.  Een compact pulsed-laser diode gebaseerde foto-akoestische tomografie (PLD-PAT) systeem voor high-speed in vivo hersenbeelden in kleine dieren wordt aangetoond. 

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

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 ...

Computed Tomography-guided Time-domain Diffuse Fluorescence Tomography in Small Animals for Localization of Cancer Biomarkers

Small animal fluorescence molecular imaging (FMI) can be a powerful tool for preclinical drug discovery and development studies1. However, light absorption by tissue chromophores (e.g., hemoglobin, water, lipids, melanin) typically limits optical signal propagation through thicknesses larger than a few millimeters2. Compared to other visible wavelengths, tissue absorption for red and near-infrared (near-IR) light absorption dramatically decreases and non-elastic scattering becomes the dominant light-tissue interaction mechanism. The relatively recent development of fluorescent agents that absorb and emit light in the near-IR range (600-1000 nm), has driven the development of imaging systems and light propagation models that can achieve whole body three-dimensional imaging in small animals3.
Despite great strides in this area, the ill-posed nature of diffuse fluorescence tomography remains a significant problem for the stability, contrast recovery and spatial resolution of image reconstruction techniques and the optimal approach to FMI in small animals has yet to be agreed on. The majority of research groups have invested in charge-coupled device (CCD)-based systems that provide abundant tissue-sampling but suboptimal sensitivity4-9, while our group and a few others10-13 have pursued systems based on very high sensitivity detectors, that at this time allow dense tissue sampling to be achieved only at the cost of low imaging throughput. Here we demonstrate the methodology for applying single-photon detection technology in a fluorescence tomography system to localize a cancerous brain lesion in a mouse model.
The fluorescence tomography (FT) system employed single photon counting using photomultiplier tubes (PMT) and information-rich time-domain light detection in a non-contact conformation11. This provides a simultaneous collection of transmitted excitation and emission light, and includes automatic fluorescence excitation exposure control14, laser referencing, and co-registration with a small animal computed tomography (microCT) system15. A nude mouse model was used for imaging. The animal was inoculated orthotopically with a human glioma cell line (U251) in the left cerebral hemisphere and imaged 2 weeks later. The tumor was made to fluoresce by injecting a fluorescent tracer, IRDye 800CW-EGF (LI-COR Biosciences, Lincoln, NE) targeted to epidermal growth factor receptor, a cell membrane protein known to be overexpressed in the U251 tumor line and many other cancers18. A second, untargeted fluorescent tracer, Alexa Fluor 647 (Life Technologies, Grand Island, NY) was also injected to account for non-receptor mediated effects on the uptake of the targeted tracers to provide a means of quantifying tracer binding and receptor availability/density27. A CT-guided, time-domain algorithm was used to reconstruct the location of both fluorescent tracers (i.e., the location of the tumor) in the mouse brain and their ability to localize the tumor was verified by contrast-enhanced magnetic resonance imaging.
Though demonstrated for fluorescence imaging in a glioma mouse model, the methodology presented in this video can be extended to different tumor models in various small animal models potentially up to the size of a rat17.

Diffuse fluorescence tomography offers a relatively low-cost and potentially high-throughout approach to preclinical in vivo tumor imaging. The methodology of optical data collection, calibration, and image reconstruction is presented for a computed tomography-guided non-contact time-domain system using fluorescent targeting of the tumor biomarker epidermal growth factor receptor in a mouse glioma model.

Small animal fluorescence molecular imaging (FMI) can be a powerful tool for preclinical drug discovery and development studies1. However, light ...

Medicine

Universal Hand-held Three-dimensional Optoacoustic Imaging Probe for Deep Tissue Human Angiography and Functional Preclinical Studies in Real Time

The exclusive combination of high optical contrast and excellent spatial resolution makes optoacoustics (photoacoustics) ideal for simultaneously attaining anatomical, functional and molecular contrast in deep optically opaque tissues. While enormous potential has been recently demonstrated in the application of optoacoustics for small animal research, vast efforts have also been undertaken in translating this imaging technology into clinical practice. We present here a newly developed optoacoustic tomography approach capable of delivering high resolution and spectrally enriched volumetric images of tissue morphology and function in real time. A detailed description of the experimental protocol for operating with the imaging system in both hand-held and stationary modes is provided and showcased for different potential scenarios involving functional and molecular studies in murine models and humans. The possibility for real time visualization in three dimensions along with the versatile handheld design of the imaging probe make the newly developed approach unique among the pantheon of imaging modalities used in today&#8217;s preclinical research and clinical practice.

We provide herein a detailed description of the experimental protocol for imaging with a newly developed hand-held optoacoustic (photoacoustic) system for three-dimensional functional and molecular imaging in real time. The demonstrated powerful performance and versatility may define new application areas of the optoacoustic technology in preclinical research and clinical practice.

The exclusive combination of high optical contrast and excellent spatial resolution makes optoacoustics (photoacoustics) ideal for simultaneously ...

Non-invasive Parenchymal, Vascular and Metabolic High-frequency Ultrasound and Photoacoustic Rat Deep Brain Imaging

Photoacoustics and high frequency ultrasound stands out as powerful tools for neurobiological applications enabling high-resolution imaging on the central nervous system of small animals. However, transdermal and transcranial neuroimaging is frequently affected by low sensitivity, image aberrations and loss of space resolution, requiring scalp or even skull removal before imaging. To overcome this challenge, a new protocol is presented to gain significant insights in brain hemodynamics by photoacoustic and high-frequency ultrasounds imaging with the animal skin and skull intact. The procedure relies on the passage of ultrasound (US) waves and laser directly through the fissures that are naturally present on the animal cranium. By juxtaposing the imaging transducer device exactly in correspondence to these selected areas where the skull has a reduced thickness or is totally absent, one can acquire high quality deep images and explore internal brain regions that are usually difficult to anatomically or functionally describe without an invasive approach. By applying this experimental procedure, significant data can be collected in both sonic and optoacoustic modalities, enabling to image the parenchymal and the vascular anatomy far below the head surface. Deep brain features such as parenchymal convolutions and fissures separating the lobes were clearly visible. Moreover, the configuration of large and small blood vessels was imaged at several millimeters of depth, and precise information were collected about blood fluxes, vascular stream velocities and the hemoglobin chemical state. This repertoire of data could be crucial in several research contests, ranging from brain vascular disease studies to experimental techniques involving the systemic administration of exogenous chemicals or other objects endowed with imaging contrast enhancement properties. In conclusion, thanks to the presented protocol, the US and PA techniques become an attractive noninvasive performance-competitive means for cortical and internal brain imaging, retaining a significant potential in many neurologic fields.

The present work describes a new protocol to perform non-invasive high-frequency ultrasound and photoacoustic based imaging on rat brain, to efficiently visualize deep subcortical regions and their vascular patterns by directing signals on skull foramina naturally present on animal cranium.

Photoacoustics and high frequency ultrasound stands out as powerful tools for neurobiological applications enabling high-resolution imaging on the central ...

Neuroscience

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.

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.

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

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, ...

Pulsed Laser Diode-Based Desktop Photoacoustic Tomography for Monitoring Wash-In and Wash-Out of Dye in Rat Cortical Vasculature

Photoacoustic (PA) tomography (PAT) imaging is an emerging biomedical imaging modality useful in various preclinical and clinical applications. Custom-made circular ring array-based transducers and conventional bulky Nd:YAG/OPO lasers inhibit translation of the PAT system to clinics. Ultra-compact pulsed laser diodes (PLDs) are currently being used as an alternative source of near-infrared excitation for PA imaging. High-speed dynamic in vivo imaging has been demonstrated using a compact PLD-based desktop PAT system (PLD-PAT). A visualized experimental protocol using the desktop PLD-PAT system is provided in this work for dynamic in vivo brain imaging. The protocol describes the desktop PLD-PAT system configuration, preparation of animal for brain vascular imaging, and procedure for dynamic visualization of indocyanine green (ICG) dye uptake and clearance process in rat cortical vasculature.&#160;

A compact pulsed laser diode-based desktop photoacoustic tomography (PLD-PAT) system is demonstrated for high-speed dynamic in vivo imaging of small animal cortical vasculature.

Photoacoustic (PA) tomography (PAT) imaging is an emerging biomedical imaging modality useful in various preclinical and clinical applications. ...

Dual Raster-Scanning Photoacoustic Small-Animal Imager for Vascular Visualization

Imaging of vascular networks on small animals has played an important role in basic biomedical research. Photoacoustic imaging technology has great potential for application in the imageology of small animals. The wide-field photoacoustic imaging of small animals can provide images with high spatiotemporal resolution, deep penetration, and multiple contrasts. Also, the real-time photoacoustic imaging system is desirable to observe the hemodynamic activities of small-animal vasculature, which can be used to research the dynamic monitoring of small-animal physiological features. Here, a dual-raster-scanning photoacoustic imager is presented, featuring a switchable double-mode imaging function. The wide-field imaging is driven by a two-dimensional motorized translation stage, while the real-time imaging is realized with galvanometers. By setting different parameters and imaging modes, in vivo visualization of small-animal vascular network can be performed. The real-time imaging can be used to observe pulse change and blood flow change of drug-induced, etc. The wide-field imaging can be used to track the growth change of tumor vasculature. These are easy to be adopted in various areas of basic biomedicine research.

A dual raster-scanning photoacoustic imager was designed, which integrated wide-field imaging and real-time imaging.

Imaging of vascular networks on small animals has played an important role in basic biomedical research. Photoacoustic imaging technology has great ...

A High-Throughput Image-Guided Stereotactic Neuronavigation and Focused Ultrasound System for Blood-Brain Barrier Opening in Rodents

The blood-brain barrier (BBB) has been a major hurdle for the treatment of various brain diseases. Endothelial cells, connected by tight junctions, form a physiological barrier preventing large molecules (&#62;500 Da) from entering the brain tissue. Microbubble-mediated focused ultrasound (FUS) can be used to induce a transient local BBB opening, allowing larger drugs to enter the brain parenchyma.
In addition to large-scale clinical devices for clinical translation, preclinical research for therapy response assessment of drug candidates requires dedicated small animal ultrasound setups for targeted BBB opening. Preferably, these systems allow high-throughput workflows with both high-spatial precision as well as integrated cavitation monitoring, while still being cost effective in both initial investment and running costs.
Here, we present a bioluminescence and X-ray guided stereotactic small animal FUS system that is based on commercially available components and fulfills the aforementioned requirements. A particular emphasis has been placed on a high degree of automation facilitating the challenges typically encountered in high-volume preclinical drug evaluation studies. Examples of these challenges are the need for standardization in order to ensure data reproducibility, reduce intra-group variability, reduce sample size and thus comply with ethical requirements and decrease unnecessary workload. The proposed BBB system has been validated in the scope of BBB opening facilitated drug delivery trials on patient-derived xenograft models of glioblastoma multiforme and diffuse midline glioma.

The blood-brain barrier (BBB) can be temporarily disrupted with microbubble-mediated focused ultrasound (FUS). Here, we describe a step-by-step protocol for high-throughput BBB opening in vivo using a modular FUS system accessible for non-ultrasound experts.

The blood-brain barrier (BBB) has been a major hurdle for the treatment of various brain diseases. Endothelial cells, connected by tight junctions, form a ...

Non-Invasive Integrated PAUSAT Whole-Brain Imaging of Ischemic Stroke

Integrated Photoacoustic, Ultrasound, and Angiographic Tomography (PAUSAT) for NonInvasive Whole-Brain Imaging of Ischemic Stroke

Presented here is an experimental ischemic stroke study using our newly developed noninvasive imaging system that integrates three acoustic-based imaging technologies: photoacoustic, ultrasound, and angiographic tomography (PAUSAT). Combining these three modalities helps acquire multi-spectral photoacoustic tomography (PAT) of the brain blood oxygenation, high-frequency ultrasound imaging of the brain tissue, and acoustic angiography of the cerebral blood perfusion. The multi-modal imaging platform allows the study of cerebral perfusion and oxygenation changes in the whole mouse brain after stroke. Two commonly used ischemic stroke models were evaluated: the permanent middle cerebral artery occlusion (pMCAO) model and the photothrombotic (PT) model. PAUSAT was used to image the same mouse brains before and after a stroke and quantitatively analyze both stroke models. This imaging system was able to clearly show the brain vascular changes after ischemic stroke, including significantly reduced blood perfusion and oxygenation in the stroke infarct region (ipsilateral) compared to the uninjured tissue (contralateral). The results were confirmed by both laser speckle contrast imaging and triphenyltetrazolium chloride (TTC) staining. Furthermore, stroke infarct volume in both stroke models was measured and validated by TTC staining as the ground truth. Through this study, we have demonstrated that PAUSAT can be a powerful tool in noninvasive and longitudinal preclinical studies of ischemic stroke.

Author Spotlight: Integrated Photoacoustic, Ultrasound, and Angiographic Tomography (PAUSAT) for NonInvasive Whole-Brain Imaging of Ischemic Stroke  

This work demonstrates the use of a multimodal ultrasound-based imaging platform for noninvasive imaging of ischemic stroke. This system allows for the quantification of blood oxygenation through photoacoustic imaging and impaired perfusion in the brain through acoustic angiography.

Presented here is an experimental ischemic stroke study using our newly developed noninvasive imaging system that integrates three acoustic-based imaging ...

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

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