Functional Magnetic Resonance Spectroscopy at 7 T in the Rat Barrel Cortex During Whisker Activation

After checking by blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) that the corresponding somatosensory barrel field cortex area (called S1BF) is correctly activated, the main goal of this study is to quantify lactate content fluctuations in the activated rat brains by localized proton magnetic resonance spectroscopy (¹H-MRS) at 7 T.

This method can help answer key questions in the field of neuro genetics and decipher the metabolic changes that occur between rest and activated states.The main advantage of this technique is to follow this variation in vivo in real time in a non-invasive manner.Notably this technique can be applied to pathological or genetically modified animals, for example to determine the role of a specific protein in metabolic interaction between neurons and lyocells.Begin by placing a breathing sensor on the magnet bed and transferring the rat to the magnet bed in the prone position with its nose in the isoflurane mask and with the breathing sensor located between the rib cage and the magnet bed.Ensure that the right whiskers are free and secure the rat with tape.Use tape to make a sale that traps all of the right whiskers and align the flexible outlet pipe of the air puff system along the rat MRI bed so that the part exiting the tube is perpendicular and about one and a half centimeters from the sale.Then fix the pipe into position with more tape.Next connect the flexible inlet pipe from a compressed air source to a solenoid control valve input and the outlet pipe to the solenoid control valve output, taking care that the solenoid control valve remains outside of the magnet room.Use the transistor transistor-logic port to plug the pulsing device into the solenoid valve and the magnet and configure the device so that the pulsing frequency is eight Hertz, the pulsing time is 20 seconds, and the resting time is 10 seconds.For whisker stimulation place the rat with the brain in an upright position and secure the animal with ear bars.Place the volume array coil above the head and fix the array with tape.Turn on the air puff system to check that the sale moves in the anteroposterior direction with no rotation and no friction.Then switch the air puff system off and place the bed end coil into the center of the magnet.For blood oxygen level dependent functional resonance imaging, check the sale movement again and use a localization sequence to confirm that the rat is well positioned.Drag the localizer sequence tab into the instruction name and click continue.If the location is okay drag the T2 Star FID EPI sequence tab into the instruction name, center the field of view on the middle of the brain, and click adjustment platform to open the edited scan instruction.Then record a B0 map and start the T2 Star FID EPI sequence.Acquire another localization sequence as just demonstrated to compare with the first one and check whether the rat has moved during the T2 Star FID EPI sequence.Then return the bed to its initial position and remove the volume array coil.To process the images open the T2 Star FID EPI file and read the T2 Star FID EPI image in the image display.Open the start-up window of the functional controller and in the processing tab select the functional imaging window.Define the stimulation protocol and select the protocol window, setting the on period to 40 and the off period to 20.Click invert attribution and drag the stimulation state slider to the left to select a value of one.In the pre-processing window set the median filter in plain for pre-processing and the median filter 2D 3D for post-processing.Then click execute.Drag the cursors to adjust the overlay lookup table and visualize the activated brain area.To correctly position the surface coil for proton magnetic resonance spectroscopy or MRS, rotate the head approximately 30 degrees clockwise so that the surface coil can be placed just above the left barrel cortex in a horizontal position and located at the magnet center when inside the magnet.Plug the surface coil and fix the coil into place with tape.Check that the sale can still move correctly when the air puff system is on.Then place the bed into the magnet and check the sale movement again.Confirm the correct position of the animal with the localization sequence as demonstrated and drag the T2-TurboRARE sequence tab into the instruction name window.Then click continue to execute the scan program and to allow the correct localization of the voxel within the somatosensory barrel field cortex.At the end of the scan drag the laser sequence tab into the instruction name window and place the voxel at the center of the somatosensory barrel field cortex area.Click adjustment platform to open the edited scan instruction and click wobble to slightly change the impedance of the receiver coil for tuning.When the tuning is finished, click apply to close the instruction editor and to apply the changes in the edited instruction.Record a B0 map and start the pro diem MRS acquisition during a resting period.Acquire another localization sequence to compare with the first one and confirm that the rat did not move during the laser acquisition.Then turn on the air puff system and use the laser sequence to perform a second proton MRS.Perform a final localization sequence to check whether the rat has moved before returning the bed to its initial position.Then remove the surface coil and return the rat to the bench with monitoring until full recovery.To process the MRS images open the linear combination of model spectra or LC model software and select the appropriate data type and file.Click okay and in the title section manually enter a title and define an adequate parts per milliliter range.Then click run LC model to start the LC model quantification.When right whiskers are stimulated using the homemade air puff system as demonstrated a positive BOLD signal is detected in the left barrel cortex, also called the somatosensory barrel field.Using anatomical magnetic resonance images and a rat brain atlas scheme the activated brain area visualized by BOLD functional MRI allows a voxel to be placed in the somatosensory barrel field area that is activated during the whisker stimulation.When the paradigm for whisker stimulation is turned on an increase in lactate content is observed in the left somatosensory barrel field.To better visualize the metabolic fluctuations between resting and activated periods a spectral subtraction can be performed.From this subtracted spectrum the increase in lactate content with brain activation can be visualized much more easily.For example in this rat the N-Acetylaspartate signal was slightly decreased.While the lactate peak is hardly detected on this in vivo deconvolution spectrum at rest the LC model was able to quantify the peak with a good accuracy and Cram