Analyzing the impact of increasing Mechanical Index (MI) and energy deposition on shear wave speed (SWS) reconstruction in human liver
Shear wave elasticity imaging (SWEI) has found success in liver fibrosis staging. However, technical failure and unreliable shear wave speed (SWS) estimation have been reported to increase both with elevated patient body mass index (BMI) and in the presence of significant hepatic fibrosis. Elevated BMI results in a significant amount of subcutaneous fat which attenuates acoustic radiation force (ARF) and abberates tracking beams. Advanced fibrosis results in small displacement amplitudes in stiff livers. This work evaluates hepatic SWEI measurement success as a function of push pulse energy using 2 Mechanical Index (MI) values (1.6 and 2.2) over a range of pulse durations. The rate of successful SWS estimation for 8 repeated measurements is linearly proportional to the push energy level. As expected, elevated push energy in SWEI measurements results in higher displacement signal-to-noise ratio (SNR). SWEI measurements with elevated push energy are successful in patients for whom standard push energy levels failed. Deep liver capsule is shown to be an indicator for lower yield of SWS estimation. Patients with deep liver capsules are likely to benefit from elevated push energies. We conclude that there is clinical benefit to using elevated acoustic output for hepatic SWS measurement in 'difficult to image' patients.
