Ultrasound Imaging of Cardiac Fiber Orientation: What are We Looking at?

Myocardial tissue is very complex: sarcomeres are linked to form myofibrils that combine into myocytes that, in turn, group into muscle fibers; the latter are organized in a complex 3D network governing cardiac mechanics and thereby function. As such, cardiac fiber imaging might be an important diagnostic tool. Hereto, Papadacci et al. enabled the assessment of the transmural fiber orientation during the cardiac cycle by 3D Backscatter Tensor Imaging; however, which histologic structure it detects remains unclear. The aim of this work was therefore to propose and validate, by computer simulation, a theoretical framework that addresses this issue. Field II simulations were carried out by modeling a 32⨯32 array while numeric phantoms were developed to mimic cardiac fibers and their orientation. Examples of spatial coherence (SC)maps are presented clearly showing their dependency on fiber pitch (P)and size (S). Indeed, the main-to-secondary lobe distance of the SC function linearly correlates with P (R 2 =99 %)while the width of the main lobe linearly correlates with S (R 2 =92 %). Transmural fiber orientation was also assessed with an overall RMSE and absolute error lower than 3.3° and 1.1° respectively. In conclusion, as predicted by the proposed theory, the SC maps not only allow assessing the local fiber direction but also allow estimating fiber pitch and size thus providing information on the microstructures being investigated.