Mechanically Anisotropic Multi-modal Phantoms for Elastography Imaging

Mechanical anisotropy is a promising biomarker of tissue health because many tissues contain organized fibrous structures that can be disrupted by injury or disease. Elastographic imaging methods have recently emerged to noninvasively quantify anisotropic mechanical properties in soft fibrous tissues like brain white matter or muscle. Calibrated phantoms that reproduce anisotropic tissue behavior are necessary for clinical translation of these methods, but durable, validated, anisotropic, elastography phantoms are not currently available. To address this need, we fabricated mechanically anisotropic lattice composite phantoms by embedding directionally scaled, 3D-printed, polyethylene glycol diacrylate (PEGDA) lattices in polyacrylamide (PAA) gel matrix. Apparent shear moduli governing deformations parallel and perpendicular to the scaling direction in these phantoms were estimated using magnetic resonance elastography (MRE) and ultrasound shear wave elastography (SWE). The properties of the phantom composite materials were independently characterized by benchtop dynamic shear testing (DST). All measurements were repeated over a four week period to assess stability of the phantoms over time. Both elastography imaging methods (MRE and SWE) detected anisotropy in lattice composite phantoms and identified differences in properties between different phantoms. Some quantitative differences were also observed between estimates of shear moduli and anisotropy obtained by MRE, SWE and DST, attributable to different features of each method. These phantoms and their fabrication protocols are available at the National Medical Phantom Library (NMPL) [1], to serve as assessment and calibration tools for anisotropic elastography imaging methods.