Interpretation of Shear Wave Propagation Maps (Elastogram) Using Transient Elastography

Abstract

This chapter focuses on the interpretation of shear wave propagation maps (briefly called elastograms) that are typically obtained by transient elastography (TE). In such an elastogram, the propagation of the shear wave within the liver tissue is depicted in milliseconds per millimeter. The resulting slope allows direct calculation of the shear wave speed from which the Young’s modulus or liver stiffness is derived. Since shear wave propagation is followed in parallel to the vibration of the probe, compression wave artifacts can be also seen. It should be noted that ultrasound propagation in biological tissues, especially the behavior of near field shear waves or poro-elastic properties, is still controversially discussed among ultrasound physicists. Although the high standardization and specifications of TE (FibroScan device) have been one reason for its enormous success and worldwide use, elastograms contain useful information and knowledge of their interpretation can further improve fibrosis assessment in patients. However, some wave disturbances such as diffractions, superimpositions, reflections, or dispersions are hard to interpret for experts. This chapter briefly discusses a normal elastogram and pitfalls due to wrong probe positioning such as non-perpendicular positioning or rib artifacts. It also provides a brief classification of typical elastogram disturbances that can lead to incorrect shear wave speed calculations by the internal regression algorithm. These disturbances include interruptions, reflections, bifurcations, and wave broadening due to dispersions or artifacts. It is suggested to implement a so-called expert mode on the FibroScan device to allow a manual selection or adaptation of shear wave slopes to further improve fibrosis assessment namely in patients with suboptimal examination conditions and for experienced users. Finally, and this is one of the still not well appreciated advantages of TE, all shear wave disturbances or probe positioning artifacts always lead to higher LS values. Therefore, TE has an excellent negative predictive value while a normal LS rules out fibrosis, confounders and measuring errors.