Hi, I'm JI Pan from Magnetic Residency Imaging of Neurodynamics Laboratory in the drawing biomedical engineering department at Emory University in Georgia Tech. And I'm Garth Thompson, also from the Mind Lab. Today we'll show you a procedure for simultaneously electrophysiology and the FMRI in the rodent.
We use this technique in our lab to study the neural basis of the functional MRI signal. FMRI is sensitive to changes in blood oxygenation, which are linked to fluctuations in neural activity. We are attempting to determine if particular features of the neural signal can be extracted from the MRI data.
So let's get started. Beginning with the operation of a micro electrode implantation. To begin fix an ISO fluorine anesthetized rat into place on a stereotactic surgical system.
Shave the fur from the top of the animal's head with a hair clipper and apply ophthalmic ointment to the eyes to keep them from drying out Before beginning surgery, ensure that the animal is well anesthetized and exhibits no response to a toe pinch. This is a non survival surgery. After removing the fur, separate the muscle and other tissue above the skull and use a cauterizer to block any bleeding on the bone surface.
Now set a small nylon screw into the bone and then apply dental cement to prepare a pier on the skull surface near the midline anterior V-shaped junction. The pier will serve as a fixation point for the shaft of the implanted electrode and should be approximately five millimeters high and three by five millimeters squared in area. At the base under a microscope, use a fine tipped electrical drill to carefully open the skull and expose the dura over the four paw.
Representation in primary somatosensory cortex of each hemisphere. The diameter of each hole should be around one millimeter positioned, one millimeter anterior two and four millimeters lateral from bgma. Next, use a syringe needle tip to cut a tiny opening in the dura.
Being careful to avoid any vessel damage before inserting electrodes. Make sure that no bleeding or exudation is present near the incision. Glass micro electrodes with approximately four centimeter shaft length and impedance of one to five mega ohms should be prepared before surgery.
Fill the capillary of the electrode with artificial CSF and then use a stereotactic arm to insert each electrode into the brain obliquely at approximately 45 to degrees from posterior to anterior, insert the electrodes about 0.4 millimeters from the opened dura before fixing them into check the electrical signal. Tip one end of a ized silver wire into the artificial CSF and connect the other end to the input leads to the amplifier. A silver wire attached subcutaneously at the back of the opened skin serves as the reference electrode.
We record electrical signals at a sampling rate of 12 kilohertz, amplified by a factor of 1000.1 to five kilohertz band pass filtered and 60 hertz notch filtered. Now double check the surgical area, making sure that no bleeding or exudation occurs, and then apply toothpaste to replace the removed skin and muscle on the skull. Using toothpaste improves the MR image quality by reducing the susceptibility mismatch at the skull air interface.
Next, attach the electrode shaft to the prepared with dental cement. After the dental cement cures, transfer the animal to the MRI cradle and fix it in place. Monitor the re's physiological condition for the remainder of the study, including body temperature, respiration rate, oxygenation, and cardiac rate.
When the animal is in place, position a surface coil over the head with the electrodes protruding from the center of the coil. An additional arch shaped hardcover that sits atop the cradle serves as a support for the electrode leads to reduce breathing related motion. The leads used for simultaneous imaging and recording are covered with conductive plastic, which serves as a passive shield.
They extend around five meters to reach the amplifier that is located outside the magnet room. Examine the electrical signal one final time before transferring the anesthetized animal into the magnet. Keep the animal anesthetized throughout the imaging procedure with the electrodes implanted.
Let's see how to perform simultaneous electrophysiology and imaging. Imaging data is collected with a 9.4 Tesla small animal MRI system. Start by acquiring a three plane scout image to position the slices with the gradient.
Echo EPI study. Set up one coronal slice, which includes bilateral four paw primary somatosensory areas in which the electrodes are implanted. Now set the field of view to 1.92 centimeters, matrix size to 64 by 64 in plain resolution to 300 by 300 microns slice thickness to two millimeters, relaxation time to 500 milliseconds and echo time to 15 milliseconds to improve the homogeneity of the magnetic field shim.
The volume of interest with fast map software when the imaging setup is complete. Simultaneous recording and FMRI can begin. We set up a long recording session.
Then start the image acquisition. A TTL Pulse is created at the start of each image acquisition and recorded so that the electrical data can be easily synchronized with the MRI scanner. When the experiment is completed, the rat is euthanized.
Before analysis can begin, the data need to be pre-processed. Let's see how that's done. Begin with the removal of the gradient artifacts from the electrophysiological recordings in matlab.
The noise structure due to scanning can be extracted by first averaging all TR sections, each of which corresponds to the interval between two consecutive FMRI images. This method corrects only the unsaturated recording segments. Each saturated segment is replaced by a line which passes between the time point before and the time point.
After gradient induced saturation. The de-noise recordings are then converted to power time courses, which have the same temporal resolution as the FMRI time course. The sliding window for all frequency bands is moved in 0.5 second increments matching the TR of the FMRI data for imaging data.
Standard FMRI pre-processing is performed including head motion correction, image smoothing, and linear drift removal. Cross correlation analysis is conducted between the LFP power time courses and the time course from each vle of imaging data. Varying time lags allow the examination of time dependent correlation.
Now we'll show some representative results of simultaneous recording and imaging. As an example, we use the combined techniques to determine the relation between LFPs and FMRI Bold signals during spontaneous cortical activity. The LFP power fluctuation significantly correlate to the bold signal changes in the small region, close to the electrode tip with several seconds lag.
The time courses of the region of interest show coherent fluctuation over time between the LFPs and lagged bold signal. We were just showing you how to accomplish simultaneous electrophysiological recording and FMRI imaging During the experiment. It's important to remember to minimize interference between the magnetic field and the electrical recording equipment at each step.
So that's it. Thanks for watching and good luck with your experiments.
