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Figure 1 (A) Transthoracic color Doppler flow mapping in the posterior descending artery (PD) at rest in a patient with a normal right coronary artery (RCA). Note the position (arrowhead) of the distal RCA within the posterior atrioventricular groove. The PD (red-coded blood flow) courses toward the apex within the posterior interventricular sulcus and the middle cardiac vein (MC) is simultaneously imaged with blue-coded blood flow next to the PD. (B) Transthoracic color Doppler flow mapping in the distal left anterior descending coronary artery (LAD) at rest in a patient with a normal left coronary artery. The LAD (red-coded blood flow) courses toward the apex within the anterior interventricular sulcus. A = anterior; I = inferior; L = left; LV = left ventricle; P = posterior; R = right; S = superior.