MRI Mapping of Cerebrovascular Reactivity via Gas Inhalation Challenges

The overall goal of this procedure is to measure cerebral vascular reactivity by modulating blood carbon dioxide gas content while recording. Brain perfusion changes with MRI. This is accomplished by first setting up the MRI compatible gas delivery system.

The second step is to set up the equipment needed to record physiological parameters such as end tidal carbon dioxide. Next, the subject is given a nose clip and mouthpiece and is instructed to breathe by mouth with a comfortable breathing rhythm. The final step is to perform an MRI scan during which the inhaled gas is periodically switched between room air and carbon dioxide.

Ultimately, linear regression analysis between the MRI data and tidal CO2 is used to determine cerebral vascular reactivity. The main advantage of this CVR mapping technique over existing methods in the literature is that the gas delivery system is simple. The procedure requires less training time and the equipment is less expensive.

I will be demonstrating this procedure with two of my colleagues and Yame Chan Before the experiment. First, review the gas delivery system diagram as seen here. Then begin set up by filling a 200 liter Douglas bag with a medical grade gas mixture containing 5%carbon dioxide, 21%oxygen, and 74%nitrogen.

Next place, two diaphragms into the two-way non rebreathing valve to ensure one-way gas flow. Then bring this assembled two-way valve and the gas filled Douglas bag into the magnet room. Connect the gas delivery tube to the input end of the two-way valve, and then attach it to the side of the head coil for weight support.

Connect the other end of the delivery tube to the gas filled Douglas bag. Connect the mouthpiece to the U-shaped tube using an elbow connector with a gas sampling port sealed. Then connect the gas sampling tubing to the U-shaped tube via another elbow connector.

Connect a small air filter to the other end of the gas sampling tubing. Then connect the other end of the air filter to the carbon dioxide monitor. In the control room of the MRI suite.

Turn on carbon dioxide and pulse oximetry monitors and perform an auto calibration for the carbon dioxide monitor. Next, connect the monitors to a laptop via USB ports. Then open the hyper terminal software that communicates with the monitors.

To synchronize the monitors, use a timer and write down each monitor's time. The difference in timing can later be accounted for. In data processing, insert one end of a signaling bar into a wave guide so that one end is inside the magnet room and the other is in the control room.

This bar will be used to notify the researcher inside the magnet room when it is necessary to switch the three-way valve during the scan. After screening for MRI safety and obtaining informed consent, have the subject lie on the MRI table. Provide the nurse call button in case they feel discomfort during the scan procedure.

Then ask the subject to wipe his or her nose with a towelette. To remove any oil from the skin, instruct the subject to breathe by mouth and to establish and maintain a breathing rhythm and place the nose clip on the subject. Then connect the open end of the U-shaped tube to the middle port of the two-way valve via the elbow connector.

Next, gently place the mouthpiece so that the subject can breathe through it. Then gently attach the pulse oximetry finger sensor. Ensure that the subject's head is in the ISO center of the head coil.

Then slide the MRI table into the magnet bore. At this point, one researcher should stay inside the magnet room to monitor the subject and to switch the three-way valve on the Douglas bag. Ensure that this researcher is wearing earplugs and a headset to block MRI Noise in the control room.

Check the end tidal CO2 and arterial oxygen saturation fraction parameters displayed on the carbon dioxide and pulse oximetry monitors. Start the recording of the parameters on the laptop. Begin scanning using a blood oxygen level dependent or bold sequence for a three Tesla MRI scanner.

The imaging parameters are seen here closely. Monitor the subject's physiological measurements throughout the scan. Review a pre-prepared sheet on which the timing of the valve switching is listed, and gently swing the signaling bar when a switch is needed.

When signaled, the researcher inside the magnet room should switch on the Douglas bag based on the movement of the signaling bar, which controls the type of gas that the subject inspires. Continue this procedure for the length of the study during the nine minute imaging period. Ensure that the valve switching takes place approximately once every minute.

Note that the timing of the switch does not have to be exactly precise as long as the end title CO2 time course is recorded. Use the intercom to notify the subject when the scan is complete. Then slide out the scanner table and gently remove the nose clip and mouthpiece from the subject while providing cleaning tissue To wipe a saliva, gently remove the finger sensor.

Then allow the subject to sit up and get off the MRI table. Discard the gas sampling tubing, air filter, mouthpiece and nose clip, but clean all the reusable components. Disconnect the two-way valve from the other components, and remove the diaphragms from the valve.

Soak the two-way valve, diaphragm and U-shaped tube in a concentrated phosphate free disinfectant containing surfactants, such as back down detergent disinfectant. Soak in a container for 20 minutes, then rinse thoroughly with distilled water. Dry the U-shaped tube with compressed air.

Place the two-way valve and diaphragms on a clear countertop and allow them to air dry, completely empty the Douglas bag and put away the signaling bar and gray tube. Finally, perform data analysis on the recorded end title CO2 time cores and bold images using MATLAB and statistical parametric mapping software. Conduct a voxel by voxel linear regression using SPM in which the shifted end tidal CO2 time course is the independent variable and the MRI signal time course is the dependent variable.

The output is a cerebral vascular reactivity map. This figure shows representative bold images at different experimental time points. The average signal intensity is also shown in the bottom row.

It can be seen that the bold signal in the brain shows an increase with carbon dioxide inhalation. Note that the signal difference between room air and carbon dioxide periods is on the order of one to 3%In amplitude. This figure shows representative cerebral vascular reactivity maps in units of percent signal change per millimeter mercury.

Carbon dioxide. Change of a healthy subject scanned five different days, demonstrating an excellent reproducibility of the results Once mastered. This technique can be done within nine minutes if it is performed properly and with proper data analysis as described in our paper, a cerebral vascular reactivity map will be obtained.