Fat-Water Phantoms for Magnetic Resonance Imaging Validation: A Flexible and Scalable Protocol

This method can help to optimize magnetic resonance imaging sequences and sequence parameters for quantitative fat-water imagining of tissues with high fat fractions. The main advantage of this technique is that it can be performed using commonly available lab supplies that can be purchased at a relatively low cost. Generally, individuals new to this method will struggle because creating stable phantoms with fat fractions greater than 50%is difficult due to the hydrophobic nature of the lipids.

To prepare the water solution, first place a stir bar into a 400 milliliter beaker. Then, use a 100 or 200 milliliter graduated cylinder to measure 300 milliliters of distilled water, and pour the water into the beaker. Place the beaker on the hot plate and set at 90 degrees Celsius with a stir rate of 100 RPM.

Use a calibrated scale to measure 0.30 grams of sodium benzoate into a weigh boat. Add the sodium benzoate to the water solution. Next, use a syringe to measure 0.6 milliliters of the water soluble surfactant.

Make sure there are no air bubbles. Hold the needle a few milliliters over the center of the solution, and slowly release the water soluble surfactant to avoid splatter on the walls of the beaker. Using a clean syringe, measure 0.24 milliliters of the gadolinium DTPA contrast agent.

Add it to the beaker using the same technique. Then, slowly spoon the 9.0 grams of agar with a spatula into the beaker with water. Once everything has been added to the water solution, increase the hot plate temperature to 350 degrees Celsius and stir bar speed to 1100 RPM for five to 10 minutes to melt the agar.

To check if the agar is melted, briefly remove the water solution from the hot plate, stop stirring, and check the color of the solution. Melted agar should be clear and yellow or amber in color. Once the agar is fully melted, remove the water solution beaker from the hot place and pour about 3.5 milliliters of the water solution into a one dram vial.

Check the solution to ensure it is clear and homogenous. Leave the water solution on the hot place at 50 degrees Celsius and 100 RPM. Clean the workspace and prepare for the oil solution.

Peanut oil is used for the oil solution because it has a similar nuclear magnetic resonance spectrum as compared to triglycerides and human adipose tissue. Place a new stir bar into a clean 400 milliliter beaker. Use a graduated cylinder to measure 300 milliliters of peanut oil, and pour into the beaker.

Remove the beaker containing the water solution and place the oil solution beaker on the hot plate. Set the hot plate to 90 degrees Celsius with a stir of 100 RPM for one minute. Do not leave the oil on the hot plate unattended.

Measure 3.0 milliliters of the oil soluble surfactant with a clean syringe. Using the same technique as before, add the oil soluble surfactant to the beaker. Set the hot plate to 150 degrees Celsius and 1100 RPM for five minutes to fully mix the oil solution.

Take the oil solution off the hot place and clean the workspace in preparation for creating the phantom. To create the phantom emulsion, prepare volumetric pipettes for the oil and water solution. Volumetric pipettes should only be used for the respective solution to prevent cross-contamination.

Match the size of the pipette to the volume being used in the protocol. For example, use two 50 milliliter volumetric pipettes to create a 100 milliliter phantom with a target fat fraction of 50%fat. Place the water solution on the hot plate and set the hot plate to 300 degrees Celsius and 1100 RPM.

After four to five minutes, turn the stir off. Using a volumetric pipette, check if the water solution is ready for extraction by partially filling the pipette with a small amount of the solution and releasing it back into the beaker. If the water solution can be easily removed and released without excessive remnants in the pipette, move onto the next step.

Otherwise, leave it on the hot plate and check again in two to three minutes. Now, carefully add a clean stir bar to a 250 milliliter Erlenmeyer flask. Take the water solution off the hot plate, measure the proper volume, and transfer it to the Erlenmeyer flask.

Place the oil solution on the hot plate and set at 90 degrees Celsius and 1100 RPM to ensure the solution is homogenous. After one to two minutes, remove the oil solution from the hot plate and replace it with the Erlenmeyer flask. Measure the proper amount of the oil solution and slowly add to the water solution into the Erlenmeyer flask.

Now you'll be able to find if your previous steps in creating the emulsion have worked. Once all oil solution has been added, increase the temperature to 300 degrees Celsius and maintain the stirring at 1100 RPM for four to five minutes. There should be a vortex from the stir bar.

The emulsion should be white with a creamy texture. Now, remove the stir bar with a magnetic stir bar retriever. Use heat resistant gloves to carefully pour the mixture in the Erlenmeyer flask into a clean 120 milliliter glass jar.

Slowly pour the mixture down the side of the glass jar to prevent bubbles in the mixture as it cools. After cleaning the Erlenmeyer flask and stir bar, repeat these steps, adjusting the amounts of water and oil solutions, until all phantoms are created. Successful phantoms will congeal to form a homogenous mixture, which can be imaged and measured via magnetic resonance imaging, or MRI.

Shown here are pictures of slight color differences in the constructed phantoms, with fat fractions of 0%25%50%75%and 100%Proton density fat signal fraction, or FSF maps, obtained from magnetic resonance imaging, reveal a homogenous FSF measurement similar to the target fat content. A high concordance correlation coefficient and the inclusion of the line of identity within the 95%confidence band of the regression line suggests the mean MRI-observed FSF values did not differ significantly from the known FF values in the fat water phantoms. We first had the idea for this method when we attempted to make some of our own high fat fraction phantoms for our studies of human brown adipose tissue, and we found that many of the methods described in the literature were either incompletely described or required expensive equipment.

After watching this video, you should have a good idea how to create phantoms with fat fractions greater than 50%While attempting this procedure, it is critical to have accurately measured the amount of oil and water solution that will be used in the emulsion. This is important because the ratio ultimately determines the fat fraction of the phantom.