Robust high resolution strain imaging by alternating pulsed field gradient stimulated echo imaging (APGSTEi) at 7 Tesla

A novel displacement-encoding spin-echo-stimulated-echo MRI sequence (APGSTEi) was used to obtain full-volume 3D strain fields in samples of two soft materials, a silicone elastomer and an ovine ligament. The samples were stretched cyclically and imaged synchronously. The multi-slice imaging sequence employed a combination of hard and soft spin-echos with bipolar gradient pulses for spatial encoding and decoding, combined with rapid multi-slice spin echo readouts. The sequence minimized undesirable signal loss due to T₂ ^∗ and T₂ decays, which occur in polymeric materials or in the presence of appreciable air-solid susceptibility contrast, a particular concern for irregularly shaped samples in high magnetic fields. The images’ magnitudes were T₁-weighted; their phase encoded displacements which occurred during a Δ = 400 ms storage interval separating encoding and decoding pulses. Unwanted residual signals were filtered using a Gaussian filter tailored to attain the desired noise floor. The experiments measured 3D deformation with a nominal resolution of 290 μm × 250 μm × 250 μm in a sample volume of 5.6 cm × 1.6 cm × 1.6 cm, in less than an hour.