Example of a transthoracic Doppler finding of retrograde collateral blood flow in a septal perforating branch, compared with coronary angiography. (A) In a modified parasternal short-axis view, a septal perforating branch from the left anterior descending coronary artery (Sb-LAD) is seen with retrograde blood flow. (B) Angiogram of the right coronary artery (RCA) demonstrates collateral (CL) blood supply from the posterior descending coronary artery (PDA) through the septal perforating branches to the left anterior descending coronary artery (LAD). Angiogram of the left coronary artery demonstrated proximal LAD occlusion (not shown). AW = anterior wall of the left ventricle, IS = interventricular septum, IW = inferior wall of the left ventricle, LV = left ventricle, RV = right ventricle.