The overall goal of this procedure is to measure the conversion of hyperpolarized pyruvate to lactate by magnetic resonance imaging or MRI in a controlled phantom environment. This method can help answer key questions in the hyperpolarized MRI field such as the ability of a system to detect chemical conversion of pyruvate by magnetic resonance. The main advantage of this technique is that chemical conversion of pyruvate proceeds similarly to metabolism in vivo but is more controllable and repeatable than in living systems.
The implications of this technique extend towards diagnosis of cancer because the elevated conversion of pyruvate to lactate common in most cancers is simulated by the phantom environment. Though this method can provide insight into cancer it can also be applied to other metabolic imaging such as cardiac metabolism. Generally individuals new to this method may struggle because the time constraints inherent to hyperpolarized media are short.
Visual demonstration of this method is critical as dissolution and ejection steps are difficult to learn because they have to happen rapidly and must be performed precisely. In a sample cup for a dynamic nuclear polarization or DNP system pipette 0.3 micro-liters of the gadoteridol solution and 13 milligrams of the pyruvic acid solution. Briefly stir this mixture in the sample cup with a pipette tip.
Begin the sample insertion process by clicking the insert sample button on the DNP system console. On the sample wizard select normal sample and click next. Keeping the sample cup vertical gently place the insertion rod over the top of the sample cup.
When prompted open the DNP system and insert the cup into the variable temperature insert using the insertion rod. Pull on the plunger at the end of the sample insertion rod to release the sample in the variable temperature insert. Remove the sample insertion rod from the system and click the next button on the DNP system console.
Then initiate the polarization by clicking the polarise sample button on the DNP system console. In the RINMR software type HYPERSENSENMR to polarization monitoring software. Set build up configuration to one and press enter.
Then click solid build up. After setting the location and name of the save file select the profile for carbon 13 in the drop down tab on the DNP system console. Click next.
Check the box to enable sampling during the build up. Set sample time to 300 seconds and click finish up. Finally measure out 3.85 grams of the dissolution media either by volume with a five milliliter syringe or by weight using a scale.
Place the phantom in the center of the magnet with easy access to the injection lines. Ensure that there is some container to catch the liquid that will vent out to the exhaust line. Prepare the high activity enzyme mixture by mixing 240 micro-liters of NADH solution 125 micro-liters of LDH solution and 335 micro-liters of buffer.
Keep the solution in a three milliliter syringe that can be attached to the injection line. Then prepare the low activity enzyme mixture by mixing together 240 micro-liters of NADH solution 75 micro-liters of LDH solution and 385 micro-liters of buffer. Keep this mixture in a separate three milliliter syringe that can be attached to the injection line.
To perform initial positioning load a new localizer scan. Wobble the proton coil by selecting Acq/Reco. Display and open the adjustment platform.
On the adjustment panel select Wobble Adjust and click ppen. Set the sweep width to ten megahertz and click set up. After a moment the protons coils tuning and matching should appear in the acquisition window.
Wobble the carbon coil by changing the coil element to 13C or element two and setting the sweep width to five megahertz. After a moment the carbons coils tuning and matching should appear in the acquisition window and if tuned properly, hit stop. To return to scan control press apply then back and finally press continue to begin the scan.
It is critical that the scan in this step is fully set up before beginning dissolution. After dissolution has begun it should not be stopped and their will be little time to adjust sequence parameters before the hyperpolarized pyruvate is delivered. Load a new radio echo planar spectroscopic imaging scan.
Set the slice thickness to 30 millimeters so as to cover the whole reaction chamber. Set the operation mode to carbon 13 by selecting the system tab and changing the operation mode to 13C transmit receive. Once the pyruvate has attained greater than 90%polarization the solutions and phantom are ready and the scan is configured.
Click the run dissolution button on the DNP system console. When prompted move the dissolution stick into its operating position and inject the dissolution media. Close the DNP system and click the finished button on the DNP system console.
It is important that injection of pyruvate and enzyme is done smoothly. This will ensure the enzyme mixture is properly mixed and delivered to the phantom chamber before the chemical conversion has completed. When the DNP system delivers the hyperpolarized pyruvate aspirate 500 micro-liters of the pyruvate solution into each of the high and low enzyme concentration solution syringes.
Slowly inject each syringe into an injection line. Scanning can be initiated prior to injection or immediately after injection depending on the scan protocol used. After the dissolution is complete move the dissolution stick back to the resting position when prompted and then click finish.
Representative results of a radio echo planar spectral imaging sequence are shown here. The pyruvate image shows the strong pyruvate signal in both chambers. The lactate image shows a weaker lactate signal but is still localized to the chambers.
The signal ratio of hyperpolarized lactate and pyruvate can be used to estimate the enzyme activity in each chamber. The signal ratios in each chamber match the enzyme activity present. Once mastered, this technique can be done in one hour.
While attempting this procedure it is important to remember to have the phantom mixture and imaging sequence ready before beginning the dissolution process. Following this procedure other hyperpolarized agents and enzymes can be used in order to answer additional questions like how well a sequence can image other chemical reactions. This technique paves the way for researchers in the field of hyperpolarized MRI to explore sequence performance using reproduceable phantoms.
After watching this video you should have a good understanding of how to prepare an enzyme mixture to facilitate the conversion of hyperpolarized pyruvate to lactate and to polarize carbon 13 enriched pyruvate for imaging. Don't forget that working with strong magnetic fields can be extremely hazardous and precautions such as controlled access to the scan room should always be taken.
