Using Laser Doppler Imaging and Monitoring to Analyze Spinal Cord Microcirculation in Rat

The overall goal of this procedure is to present a combination of laser doppler imaging and monitoring to measure spinal cord local blood flows and oxygen saturation, as well as a standardized procedure for introducing spinal cord trauma on a rat. This method can help answer key questions in the field of micro circulation such as how blood flow redistributed over time and its impact on the surrounding area after spinal cord injury. The main advantage of this technique is by using both laser doppler imaging and monitoring device we could rapidly derive data of the local distribution of the blood flow as well as the blood flow variation over a period of time, which allows more sophisticated analysis of the blood flow in the problem area.

Begin by checking the anesthetized rat for a surgical plane of anesthesia by the absence of a reaction to a toe pinch. Shave the dorsal area of the rat from the lower back to the neck. Then place the rat dorsal side up on a 40-degrees Celsius heating pad to maintain a constant body temperature.

Sterilize the shaved area by wiping with cotton balls soaked in iodine followed by 75%alcohol. After making a skin incision over the laminectomy site covering thoracic vertebrae T7 to T11, cut the attached muscles on both sides from T8 to T10 to expose the spinous processes, the laminae, and the facet joints. Use the scalpel to make incisions that disconnect the junction between T10 and T11.

Further expose the junction by carefully dissecting the muscle layer away to expose the bone. Use the scissors to further clear muscle away from the lamina and around the pedicle with small snips. This will open a small space between the vertebrae at T10 and T11.

Slowly and delicately insert hemostatic forceps into this gap and break the pedicle. Repeat on the other side. Expose the spinal cord and carefully lift and break off the lamina without leaving free or jagged bone fragments behind.

Repeat the process to further remove T9 and T8 laminae. An impactor may now be used to induce concussion injury in experimental groups. To scan the exposed spinal cord, place the rat dorsal side up on a black, non-reflective background, then set up the scanning parameters in the software by clicking Measure to enter the measurement graphical user interface and then the Scanner Setup button to open the Scanner Setup interface.

To scan small areas such as in this experiment, click Scan Size and Display options and select High Resolution for a fine scanning mode with higher resolution. Click on the Image Scan option to check the scanning parameters. Check the live video image by clicking on the Video and Distance option.

Position the scanner 10 to 13 centimeters above the surgical window and move the background with the animal to center the exposed spinal cord on the scanning window. Select the Auto Distance function to fine adjust the scanning height, which should be kept consistent across all measurements in the experiment. Use a non-reflective cover with a window to expose only the surgical area to further minimize background and mark the animal's direction.

Click on Repeat Scan to set the number of scans, then click Okay to open the Repeat Scan interface. Click the Start button to start scanning. The whole process will take approximately three to four minutes.

A scanner monitor with a VP3 blunt needle and delivery probe is used here to monitor blood flow and oxygen saturation over time. Begin by attaching the probe perpendicular to a stereotaxic instrument to set up the monitoring equipment. Put the rat on the stereotaxic apparatus, dorsal side up, and underlay the animal with a small piece of Styrofoam to level the exposed spinal cord.

Examine the incision and remove any excessive liquid or blood using a cotton pad. Use the apparatus's x-and y-axes to locate the probe to two millimeters rostral to the center point of the exposed spinal cord or lesion point and devoid the central vein. Use the z-axis to slowly lower the probe to the level just touching the surface of the spinal cord.

The probe should just touch the surface of the spinal cord but not be so loose as to allow bright light to escape from the side of the contact point. Proper positioning is critical for adapting and implementing the monitoring protocol. The probe should be perpendicular to the measured surface and excessive pressure should be avoided.

To record data, click on the New Experiment button in the software to open the setup interface. Under the General option, check the system configuration, then click Next. In the Display setup, select the channel for blood flow and oxygen saturation and click Next.

Input file information and click Next to enter the data recording interface. Click on the green triangle button to start recording data from the probe. Once the signal is stable, record data for eight consecutive minutes, then lift the probe and remove the animal from the stereotaxic apparatus.

After suturing the muscle and the skin, place the rat on its side in a clean cage on a heating pad, avoiding contact between the surgery site and the cage bottom. Monitor the animal until it wakes up from anesthesia to ensure no post-surgery bleeding and that the sutures remain closed. These images represent the flux imaging of the spinal cord of the sham group on the left and the spinal cord injury group on the right.

Flux imaging demonstrated that spinal cord injury induced reduction of blood flow and blood flow of the epicenter was lower than rostral cord and caudal cord. These images show the alternating blood flow along the rostral-caudal axis of the spinal cord of the sham group on the left and the SCI group on the right. As shown here, spinal cord blood flow of the spinal cord injury group significantly decreased compared to the sham group.

Simultaneously, the oxygen saturation of the spinal cord was remarkably lower after spinal cord concussion, which was consistent with the change in blood flow after injury. After watching this video, you should have a good understanding of how to measure blood flow of the spinal cord using both laser doppler perfusion imaging and the laser doppler perfusion monitoring. Once mastered, the surgery of introducing spinal cord injury can be done in 20 minutes and the measurements, including use both laser doppler instruments, will take roughly 20 minutes if it is performed properly.

While attempting this procedure, it's important to remember to use a non-reflective cover with a window to expose only the surgical area to further minimize background and mark the animal's direction. This technique paved the way for researchers in the field of microcirculation to explore the changes of blood flow for its pharamacodynamics and pharmacokinetics evaluation.