Magnetic resonance imaging of strain in elastic gels

Abstract

Magnetic resonance imaging (MRI) can also be used for studying different material conditions, including strain, which is the subject of this study. A pair of pulsed magnetic field gradients combined with the spin-echo imaging sequence [pulsed-gradient spin-echo (PGSE)] enables the detection of small sample displacements that are induced in the time between the gradient pulses. The displacements are registered in the image phase shift, which is then processed by the phase unwarping algorithm and phase background correction. The processed phase shift can then be used to calculate sample displacements by multiplying it with the corresponding calibration constant. Finally, the strain tensor can be calculated from the map of sample displacements. In the study, the PGSE imaging method was tested on a gelatin sample. While being MRI scanned, the sample was deformed synchronously with the imaging sequence using a special MRI-compatible apparatus for inducing sample deformations. The apparatus was based on the use of compressed air that was directed via a nozzle on the surface of the sample in pulses controlled by transistor-transistor logic pulses of the spectrometer. With the 20 ms pulses of compressed air at a pressure of 0.5 bar, sample displacements of up to 300 μm were detected at the impact point of compressed air, while the detection threshold was at 0.7 μm. Results of the PGSE method were also compared with the tagging method. The study was performed in 2D; however, the method can be implemented also in 3D, thus enabling the measurement of the full strain tensor.