To determine the ABI, the systolic hypertension (the top quantity of the blood pressure measurement) of your ankle is divided with the systolic hypertension with the arm.  When atherosclerosis narrows the arteries, blood tends to clot during the narrowed parts, forming a thrombus, a sort of blood clot (plural thrombi). https://www.sgvascularctr.com/types-of-treatments-for-vascular-diseases/