Name: Murine PAUSAT Imaging
Uploaded: 2023-06-02T12:20:00
Description: Watch this Scientific Journal Video about Murine PAUSAT Imaging at JoVE.com

murine pausat imaging

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

Blood transports oxygen (via the hemoglobin protein) and other important nutrients to tissues in our bodies. When the flow of blood through tissues is interrupted (ischemia), severe damage to the tissues can occur, the most immediate effects of which are due to a lack of oxygen (hypoxia). Ischemic stroke is the result of interrupted blood flow to a certain region of the brain. The brain damage resulting from an ischemic stroke can occur within minutes of a vessel blockage, and can often have debilitating and lasting effects1,2. A highly valuable strategy to evaluate the physiopathology after ischemic stroke and identify and test new treatments is the use of small-animal models in the lab. Treatments discovered in the lab aim to be translated to clinical use and improve patients&#39; lives. However, the use of animals in biomedical research needs to be carefully evaluated according to Russell and Burch&#8217;s 3Rs principles: replacement, reduction, and refinement3. The objective of the reduction component is to reduce the number of animals without compromising data collection. With this in mind, being able to longitudinally evaluate the lesion evolution via noninvasive imaging allows a great advantage in reducing the number of animals required, as well as maximizing the information obtained from each animal4.
Photoacoustic tomography (PAT) is a hybrid imaging modality that combines optical absorption contrast with ultrasound imaging spatial resolution5. The imaging mechanism of PAT is as follows. An excitation laser pulse is illuminated on the target being imaged. Assuming the target absorbs light at the wavelength of the excitation laser, it will increase in temperature. This quick increase in temperature results in a thermoelastic expansion of the target. The expansion causes an ultrasound wave to propagate out from the target. By detecting the ultrasound wave at many positions, the time required for the wave to propagate from the target to the detectors can be used to create an image through a reconstruction algorithm. The ability of PAT to detect optical absorption in deep tissue regions differentiates PAT from ultrasound imaging, which detects boundaries of differing acoustic impedances of tissues5. In the visible and near-infrared spectra, the primary highly absorbing biomolecules that are abundant in organisms are hemoglobin, lipids, melanin, and water7. Of particular interest in the study of stroke is hemoglobin. Since oxyhemoglobin and deoxyhemoglobin have different optical absorption spectra, PAT can be used with multiple excitation laser wavelengths to determine the relative concentration of the two states of the protein. This allows the oxygen saturation of hemoglobin (sO2), or blood oxygenation, to be quantified in and outside of the infarct region8,9. This is an important measure in ischemic stroke, as it can indicate the level of oxygen in the damaged brain tissue following ischemia.
Acoustic angiography (AA) is a contrast-enhanced ultrasound imaging method that is particularly useful for imaging the morphology of vasculature in vivo10. The method relies on the use of a dual-element wobbler transducer (a low frequency element and a high frequency element) in conjunction with microbubbles injected into the circulatory system of the imaging subject. The low-frequency element of the transducer is used for transmitting at the resonant frequency of the microbubbles (e.g., 2 MHz), while the high-frequency element is used to receive the super harmonic signals of the microbubbles (e.g., 26&#160;MHz). When excited at a resonant frequency, the microbubbles have a strong nonlinear response, resulting in the production of super harmonic signals that surrounding body tissues do not produce11. By receiving with a high-frequency element, this ensures that only the microbubble signals are detected. Since the microbubbles are confined to the blood vessels, the result is an angiographic image of blood vessel morphology. AA is a powerful method for imaging ischemic stroke, as the microbubbles that flow through the circulatory system are not be able to flow through blocked vessels. This allows AA to detect regions of the brain that are not perfused due to ischemic stroke, which indicates the infarct region.
Preclinical ischemic stroke research generally relies on the use of histology and behavioral testing to assess the location and severity of the stroke. Triphenyltetrazolium chloride (TTC) staining is a common histological analysis used to determine the stroke infarct volume. However, it can only be used at an endpoint, since it requires the animal to be euthanized12. Behavioral tests can be used to determine motor function impairment at multiple time points, but they cannot provide quantitative anatomical or physiological values13. Biomedical imaging provides a more quantitative approach to studying the effects of ischemic stroke noninvasively and longitudinally9,14,15. However, existing imaging technologies (such as small-animal magnetic resonance imaging [MRI]) can come at a high cost, be unable to provide concurrent structural and functional information, or have limited penetration depth (as most optical imaging techniques).
Here, we combine photoacoustic, ultrasound, and angiographic tomography (PAUSAT; see system diagram in Figure 1), which allows for complementary structural and functional information of blood perfusion and oxygenation following ischemic stroke16. These are two important aspects in assessing the severity of injury and monitoring the recovery or response to treatments. Using these integrated imaging methods can increase the amount of information obtained by each animal, reducing the number of animals required and providing more information in the study of potential treatments for ischemic stroke.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65319/65319fig01.jpg" /> 
Figure 1: PAUSAT diagram. (A) Complete schematic of the PAUSAT system, including the laser and OPO used for PAT. (B) Inside view of the PAUSAT system, including two ultrasound transducers. The dual-element wobbler transducer is used for both B-mode ultrasound and AA, and the linear-array transducer is used for PAT. Both transducers are mounted on the same 2D motorized stage, allowing for scanning to generate volumetric data. This figure has been modified from16. <a href="https://www.jove.com/files/ftp_upload/65319/65319fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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

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

There are a few experimental challenges. For example, the presence of hair in the scalp can cause artifact in the acquired images. The age of the animal also plays a key role since the thickness of the skull is currently a limitation for the detection of deeper signals.Many purely optical imaging techniques require an invasive procedure and can only image about one millimeter deep. With PAUSAT, we combine the advantages of both ultrasound and optical imaging. We can get both structural and functional information surrounding ischemic stroke in a completely safe and non-invasive way.Being able to evaluate the evolution of an injury with or without treatment from the same animal at different time points allows us to collect very valuable information. And also to reduce the number of animals required for different studies. We are constantly working on ways to improve the imaging speed, depth, resolution and contrast for both photoacoustic and ultrasound imaging.With better imaging systems, we can answer more fundamental biological questions and evaluate new experimental treatments.

Watch this Scientific Journal Video about Murine PAUSAT Imaging at JoVE.com

Murine PAUSAT Imaging  

To set up the imaging system, start by leaving the 532 nanometer laser on for 15 minutes to warm up. Next, place a customized ramp attached to a manually adjustable stage next to the imaging membrane. Attach a mouse tooth holder with a connected breathing tube to the ramp, and secure a heating pad on the ramp surface.Once the laser has warmed up, place a near IR detector card in front of the fiber bundle input and check the laser alignment by ensuring that the laser light enters the bundle. Begin animal preparation by trimming the hair of the anesthetized mouse's head with an electric shaver from near the eyes to behind the ears. Apply a hair removal cream to the top of the head.After leaving it on for five minutes, wipe the cream off with a wet cotton swab. Repeat the application until the skin of the head is fully depilated. Just before transferring the animal to the system's platform, use a 27 gauge needle to retro-orbitally inject 100 microliters of microbubbles at stock concentration.Now, put one drop of the eye protection lotion on the eyes. Begin PAUSAT imaging by filling the imaging window with distilled water on the surface for acoustic coupling. Next, secure the mouse's head in the tooth holder and ensure proper anesthesia and air flow.Using a heating lamp and a rectal probe connected to a temperature controller, keep the animal's body temperature at 37 degrees Celsius. Open the imaging application and navigate to the B mode ultrasound. Use the live ultrasound window to adjust the mouse's head to the desired position manually and the stage height accordingly.Adjust the value of the ultrasound transmission frequency in B mode to 16 megahertz. Then, input the save directory information in the imaging application. Next, select the desired region of the brain scan by using the floating box.Press the acquire static button and check the scan results once the image acquisition is completed to ensure the desired region imaging. To perform acoustic angiography imaging, return to Image Acquisition, then change the mode to Acoustic Angiography. Next, input the desired scan protocol parameters, setting the frame spacing at 0.2 millimeters and 10 frames per position.Check scan results under image analysis to ensure image quality. To image with photoacoustic tomography, open the optical parametric oscillator, or OPO application, and set it to 756 nanometers. Then, manually translate the linear array transducer to the previously determined coordinates to automatically co-register the wobbler volumes and the linear array volumes.Open the laser application and turn on the 532 nanometer laser. Set the scanning parameters as 0.4 millimeters step size, 20 millimeters scan length, and 10 frames averaged per position. Now, open the ultrasound data acquisition system MATLAB program, and press the run button.Next, press start in the scanning application. Once the scan is complete, open the MATLAB saving program. Change the save name to the desired file name and press Run.Then change the OPO wavelength to 798 nanometers and repeat scanning. Acoustic angiographic, or AA imaging, showed that in the uninjured brain, both hemispheres present a similar distribution of blood vessels. Similar results are observed in the photoacoustic images at two different wavelengths.Notable signal reduction in the right lateral cortex was observed in the AA images of electrocauterized mice. The same region also showed reduced tissue oxygenation suggesting ischemia. PAUSAT imaging can identify stroke areas based on the photo thrombotic stroke model by identifying upper cortex region with reduced blood flow supply and decreased oxygen saturation.Mouse age can affect imaging since the skull becomes thicker and results in different acoustic impedance resulting in decreased imaging depth for older mice.

Research

JoVE Journal

Neuroscience

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