Target detectability in acoustic elastography

The clinically relevant task of visually detecting low contrast targets in noisy strain images estimated from ultrasonic signals is studied. Detectabi lity is measured quantitatively using contrast-to-noise ratio (CNR) analysi s. Contrast in strain Images is generated by a complex interaction among th e soft tissue elasticity shear modulus distribution, target Shape and locat ion in the stress field, and external boundary conditions. Although a large strain variation is preferred for enhancing the contrast, this also increa ses the signal-dependent noise in strain estimates in a nonlinear fashion. Therefore, understanding the trade-offs between contrast and noise is neces sary for improving the diagnostic performance of strain imaging. In this pa per, targets with slab, cylindrical, and spherical geometries are studied. Strains in the target and background and the precision of their estimates a re described in terms of the corresponding shear modulus values for each ge ometry. These results are then incorporated into the CNR expression to inve stigate the changes in target detectability with the variation of shear mod ulus in the target and the ultrasonic signal parameters (echo signal-to-noi se ratio and inverse fractional bandwidth) as well as the signal processing variables (time-bandwidth product and fractional window overlap). The resu lts include 1) formulas describing target and background strains for the th ree geometries as a function of the applied compression, boundary condition s, and shear modulus values; 2) mathematical description of the consequence s that nonuniformities in tissue elasticity and variations in strain contra st with the target geometry impose upon detectability; and 3) demonstration of the need to choose carefully the values for signal processing variables .