Viscoelastic Characterization of Soft Tissue-Mimicking Gelatin Phantoms using Indentation and Magnetic Resonance Elastography

This article presents a demonstration and summary of protocols of making gelatin phantoms that mimic soft tissues, and the corresponding viscoelastic characterization using indentation and magnetic resonance elastography.

This protocol provides a way to prepare tissue-mimicking gel functions in the ex vivo and the in vivo biomechanic characterization methods, which are critical for understanding and finding new biomarkers for tissue The indentation can measure small-sized tissue samples and is easy to carry out while the same sample can be tested using MRE, providing a direct estimation of in vivo test scenarios. This method could provide insight into viscoelastic frequency-dependent mechanical properties of soft biological samples, such as the brain, liver, tumor tissues, et cetera. Similar properties of other sample phantoms can also be measured.

Begin by mixing the gelatin powder with water to obtain the gelatin solution. Heat the gelatin solution to 60 degrees Celsius in a water bath and add glycerol to the solution while maintaining the temperature. Stir the solution and heat it to 60 degrees Celsius again.

Pour the mixed solution into a container that will be used for MRE and indentation tests. Cool the solution to room temperature and wait till the solution is solidified. Place the gelatin phantom into the head coil.

Then, put the vibration plate on top of the gelatin phantom. Ensure that the contact between the phantom and the vibration plate is firm. Put sponges and sandbags around the gelatin phantom to make sure the phantom is firmly placed.

Mount an electromagnetic actuator on the head coil and connect the transmission bar to the vibration plate. Connect the power lines of the actuator with the amplifier and then connect the control lines with the controller. Set the wave form, vibration frequency, and amplitude in the function generator.

Set the desired vibration amplitude by adjusting the power amplifier. Then, set the function generator to work in the trigger mode. Connect the trigger line to the external trigger port of the MRI machine.

Set the MRE scanning frequency the same as that from the function generator, so that the motion encoding gradient is synchronized with the motion of the vibration plate. Next, set the flip angle to 30 degrees, TR and TE to 50 and 31 milliseconds, field-of-view to 300 millimeters, slice thickness to five millimeters, and voxel size to 2.34 by 2.34 square millimeter. Measure the phase images at four temporal points in one sinusoidal cycle.

Apply both positive and negative motion and coding gradients at each time point. Based on the phase image acquired, remove the background phase by subtracting the positive and negative encoded phase images. Unwrap the phase with a reliability sorting-based algorithm.

Extract the principle components of the motion by applying fast Fourier transform to the unwrapped phase images. Filter the phase image with a digital band pass filter and estimate the shear modulus with a 2D direct inversion algorithm to obtain storage modulus G-prime and loss modulus G-double prime. Use a circular punch to trim the gelatin phantom into a cylindrical sample and use a surgical blade to trim it into a cuboid sample.

Trim the sample's surface with a sharp blade to make it as smooth as possible for indentation. Turn on the power of the indentation tester and click on the Back Off button in the GUI to initialize the calibration process. Read the value from the laser sensor and type the value in the BaseLine box.

Place a glass slide on the baffle plate and record the value shown by the laser sensor. Next, put the sample on the glass slide and place them together on the baffle plate. Read the value from the laser sensor and type this value in the Sample Slide box.

The difference between these two values is the sample thickness at the region of interest. Carefully place the sample along with the underlying glass slide right below the indentor and then click on the Contact button to initiate automatic contact between the indentor and the sample surface. Based on the measured sample thickness, estimate the indentation displacement by multiplying the thickness with the indented testing strain.

Type the displacement values in the Displacement box. Set the relaxation time to 180 seconds in the Dwell time box and click on the Indentation button. The displacement and the reactive force during the ramp/hold procedure will be automatically recorded and saved in a file at the specified file path.

Wave propagation images for the two gelatin phantoms at 40 and 50 hertz are shown here. The four phases correspond to the four temporal points at one sinusoidal cycle. Viscoelastic properties measured from MRE and indentation experiments are shown here.

The representative images depict typical estimated G-prime and G-double prime maps at 40 and 50 hertz for the two gelatin phantoms from MRE. Mean and standard deviation of the G-zero and G-infinity values for the two phantoms from the six repeated indentation tests are presented here. The graphical images shown on the screen represent the mean and standard deviation of the G-prime and G-double prime values at 40 and 50 hertz for the two phantoms from the six repeated MRE tests.

The asterisk symbol indicates a significant difference. When attempting this procedure, make sure the vibrating plate is firmly pressed on top of the phantom and not to over-press the plate. When treating the sample, make sure the surface is as flat as possible.

This technique paves the way to explore biomechanical properties related to pathological studies, and develop biomechanics-based biomarkers for disease diagnosis and prognosis.