Hyperpolarized Xenon for NMR and MRI Applications

The overall goal of the following experiment is to produce hyperpolarized, xenon and perform sensitivity enhanced NMR and MRI Measurements. This is achieved by using a process called spin exchange optical pumping with rubidium vapor to achieve high xenon nuclear spin polarization. As a second step acquired NMR signals are compared with those of thermally polarized xenon sample, which allows quantification of the achieved polarization.

Next NMR spectroscopy and imaging experiments are performed in order to demonstrate high sensitivity and molecular specificity. Results are obtained that show that Xenon can be used as an NMR contrast agent, both in gas phase and dissolved in solution state based on novel detection schemes like hyper zest. The main advantage of this technique over existing methods like proton MR Imaging, is that xon can be detected at much lower concentrations and with high specificity for its molecular environment With the ability to design functionalized contrast agents that bind to a specific target molecule.

The implications of this technique extend towards the diagnosis of various diseases. So this technique can provide insight into gas phase R imaging. It can also be applied to solution state experiments for detecting certain analytes.

Generally, people new to this technique will struggle because detailed documentation of many preparation steps is not available. Preventing the rubidium from oxidization during the loading procedure and during operation of the hyperpolarizing are critical steps in this procedure to keep the rubidium under an inert atmosphere. The loading procedure is performed in a glove box and the hyperpolarizing gas mono fold is kept under a protective argon atmosphere.

To begin this protocol, prepare an optical pumping cell in a glove box as described in the written procedure. Accompanying this video, check for unwanted oxygen in the glove box by switching on a light bulb that has a filament exposed to the atmosphere, it should be safe to handle the rubidium if no smoke from the glowing filament is observed. Break a rubidium ampule and melt the alkaline metal with a heat gun.

Soak up some liquid rubidium with a pipe head and inject it into the pumping cell before removing the cell from the glove box. As instructed in the text, attach the heating device to the bottom of the cell and check that the heating device with its thermocouple has proper thermal contact with it. Using a visible light aiming beam, connect the cell to the polarizer manifold, ensuring that it is aligned with the laser beam line illuminating the cell during the pumping process.

Finally, attach a thermocouple to the top of the cell. Evacuate the tube connections up to the inlet and outlet valve of the pumping cell. After reaching a pressure of less than 30 times 10 to the minus three millibar.

Purge the lines with Argonne repeat evacuation and purge three times with the Argonne tank open to the pumping cell inlet. Slowly open the inlet and outlet valve of the cell. Carefully open the polarizer outlet valve a tiny bit to establish an argonne flow of about one standard liter per minute through the manifold, and maintain this flow for two minutes.

After closing the polarizer outlet valve and the inlet connection to the Argonne tank, turn on the heater of the pumping cell to vaporize the rubidium droplet. Next, open the xenon gas mixture connection into the polarizer setup. The tank regulator should be set to approximately 3.5 bar over pressure.

Turn the laser on and then tune Its emitting wavelength to about 794.8 nanometers by adjusting the set temperature of the diode coolant. Monitor the laser profile through an optical spectrometer. Turn on the magnetic field around the pumping cell while monitoring the laser profile transmission should go up immediately as the field causes selective optical pumping wait for all temperatures to stabilize.

The polarizer is now ready to use. This animation summarizes the overall setup for spin exchange optical pumping. The laser beam is first increased in diameter by a primary beam expander and passes through a polarizing beam splitter cube or PBC rotation of this cube changes the relative intensities of the ordinary and extraordinary beam for the position with maximum transmission.

The fast axis of the PBC is aligned with the dominant polarization axis of the incoming light. The linear polarization of the transmitted light can be tested using a second PVC as an analyzer. Insertion of a quarter wave plate converts the linear into circular polarization.

If its fast axis is rotated 45 degrees against the fast axis of the first PBC. The intensity of the transmitted light should now be independent of the rotation of the second cube. Removing the analyzing components and replacing them with a secondary beam expander yields the right beam diameter to illuminate the pumping cell.

A rubidium droplet sitting in this cell is partially vaporized. Once a heater outside the cell is turned on, the xenon gas mixture flowing through the setup in opposite direction to the laser beam distributes this vapor all over the cell without a magnetic field. This causes general D one excitation of the rubidium atoms and strong absorption of the laser light.

Turning the magnetic field on allows for selective pumping of only one transition between the now defined magnetic sublevels. To prepare for an NMR measurement, insert a test tube with water into the NMR probe head. Perform tuning and matching of the radio frequency or RF circuit for the proton and the xenon channel shim on the water signal.

With the automated shim routine of the MRI user interface, replace the water sample with the gas flow phantom. Connect the polarizer outlet tube to the inlet of the test phantom with its five capillaries to inject the xenon and the gas vent tube to the connection with the mass flow controller. Make sure the gas flow controllers are set to close and slowly opened the polarizer outlet valve to pressurize the phantom.

Set the flow rate to about 0.5 standard leaders per minute. To start a continuous flow through the phantom for determining the required excitation pulse length, acquire a series of free induction decay or FID signals with the NMR spectrometer. Use a hard pulse that is approximately 50%of the maximum rated power for the probe and various excitation lengths in this case, a hard pulse of 10 decibels attenuation is used.

Enter experiment parameters as listed in the written protocol accompanying this video. After decreasing the flow to 0.1 standard leaders per minute, increasing repetition time to 15 seconds and leaving the other parameters unchanged. Acquire a data set with 16 FID scans while the hyperpolarized xenon is flowing through the sample.

Perform the Fourier transformation and measure the peak integral in the spectrum. Note the resonance frequency of the gas peak in Hertz. Next, evacuate a heavy wall NMR tube equipped with a valve for low pressure ceiling and fill it with about two bar over pressure of pure xenon.

Evacuate the gas manifold holding the NMR tube and fill approximately 0.2 bar of pure oxygen on top of the xenon into the NMR tube. Replace the previously used gas flow phantom in the NMR magnet with this low pressure NMR tube and repeat the same measurement taken with the gas flow phantom. This will provide the reference NMR signal intensity for thermally polarized high concentration Xon.

To perform xenon solution, state spectroscopy first prepare a 50 to 200 micromolar sample of Khan as discussed in the written protocol. Take approximately 1.5 milliliters of this solution and fill it into the gas flow phantom, ensuring that the five fused silica capillaries allow sufficient bubbling of the solution with the xenon gas mixture. After preparation of the phantom into the NMR probe, use an FID acquisition with appropriate delay.

Trigger pulses from the spectrometer to open and close the mass flow controllers. Perform 16 or 32 repetitions with spectral width of 40 kilohertz centered at around 11 kilohertz downfield from the gas resonance frequency as determined earlier, FID readout should be 500 to 1000 milliseconds. Fourier transform the FID to get the spectrum set.

The chemical shift value for gas phase, which is the most right signal to zero PPM. Write down the frequency of the intense solution signal, which is the most left signal in Hertz and PPM. Also note the offset between this signal at delta solution and the signal of encapsulated xenon at delta cage.

The animation shown here explains the sess effect. K Khan cages serve as molecular hosts to trap xenon atoms, which change their resonance frequency upon this binding event. Depicted here as a transition from blue to green, A first NMR acquisition determines the amount of unbound xenon as a reference signal.

Next, a selective saturation pulse affecting only the caged destroys their magnetization. Since the xenon binding is a reversible process, a long pulse cancels the magnetization of many atoms, and a second NMR acquisition reveals a significant signal decrease from free xenon compared to the reference signal. After preparing the NMR sample as discussed in the written protocol, select a single shot echo planer imaging sequence for fast imaging.

This sequence may need to be modified to include delays and trigger pulses from the spectrometer to open and close. The mass flow controllers allow for approximately 15 to 20 seconds of bubbling with 0.1 standard liter per minute and subsequent five to eight second waiting delay for the bubbles to disappear, followed by MRI and coating. Set the detection nucleus to xenon 1 29 on the X Channel and the transmitter observer frequency to the value determined for the solution signal.

Using the RF pulse calculator tool, convert the pulse parameters used in the spectroscopy measurements into the excitation used in the imaging sequence. Open the CST preparation module, a modified magnetization transfer module for signal preparation and enable a continuous wave pre-SAT pulse. Perform two scans in transverse orientation with the carrier frequency of this saturation Pulse being set once to delta cage equals delta solution minus delta omega and once to delta control equals delta solution plus delta omega.

Using an image post-processing tool. Generate the hyper CET difference image by subtracting the image with saturation at delta cage from the one with saturation at delta control. The result should only highlight areas where the xenon host was present.

The quality of the optical pumping process can be monitored by switching the magnetic field around the cell on and off. Depending on the laser power and cell temperature, almost complete absorption is observed with the magnetic field switched off and approximately 30%transmission occurs with the field on. For an NMR system operating at 9.4 Tesla.

The signal enhancement should be approximately 16, 000 fold when comparing thermally polarized xenon with hyperpolarized xenon, which corresponds to a spin polarization of about 15%The XENON 1 29 NMR spectrum of A-D-M-S-O solution containing 213 micromolar of a molecular host should exhibit a signal of caged xenon with a signal to noise ratio of approximately 10 for 16 acquisitions. The hyper zest MRI dataset shows full signal intensity for the off resonant control image and signal depletion in areas containing the xenon host molecule. In the on resonant saturation image, the difference image exclusively displays the areas that responded to the saturation pulse.

While attempting this procedure, it is important to remember to ensure a proper laser illumination of the pumping cell. This technique can pave for the way for researchers in the field of NMR imaging to explore high sensitivity detection of biochemical analytes and test solutions, cell cultures, and potentially life animals. After watching this video, you should have a good understanding of how to produce hyperpolarized xon for NMR experiments.

Don't forget that working with rubidium can be extremely hazardous and precautions such as protection from uncontrolled oxidation should always be taken throughout this procedure.