Cryogel tissue phantoms with uniform elasticity for medical imaging

Elastography-based medical imaging techniques such as ultrasound elastography, optical coherence elastography and magnetic resonance elastography are valuable tools to quantitatively assess the mechanical properties of biological tissues in vivo. Estimating the local tissue stiffness or elasticity can be an effective noninvasive way of detecting tumors, particularly for parts of the body that are not accessible for physical examination by palpation. However, there are significant differences in the absolute values of the reported modulus for biological tissues in clinical elastography studies. This is usually attributed to differences in experimental methodology and reconstruction algorithms, as well as the age and gender of volunteers. Tissue-mimicking phantoms with well-defined properties can help in identifying the potential weaknesses in elastography systems. This is primarily because a calibrated phantom can be characterized by independent measurements to directly estimate its mechanical properties. This chapter gives an overview of the current strategies in developing tissue-mimicking materials (TMMs) as elastography phantoms, with a focus on polymeric cryogels. Among these materials, polyvinyl alcohol cryogel (PVA-C) has been extensively used as a phantom material by medical imaging researchers. Since large cryogel phantoms suffer from variations in properties due to the inhomogeneous thawing rates during freeze-thaw cycles, this chapter also briefly covers some of the recent efforts to improve the homogeneity of cryogel phantoms.