▁algorithm . ▁There ▁are ▁different ▁search ▁algorithms , ▁the ▁most ▁commonly ▁used ▁is ▁" sum ▁of ▁absolute ▁differences ", ▁shown ▁to ▁be ▁similarly ▁accurate ▁as ▁cross - cor relation , ▁which ▁is ▁an ▁alternative . ▁The ▁movement ▁of ▁the ▁kernel ▁across ▁the ▁image ▁can ▁thus ▁be ▁track ed , ▁in ▁principle ▁independent ▁of ▁the ▁beam ▁angle , ▁as ▁opposed ▁to ▁t issue ▁Do pp ler . ▁Spe ck le ▁tracking ▁can ▁thus ▁track ▁in ▁two ▁dimensions . ▁However , ▁as ▁the ▁ax ial ▁( in ▁the ▁direction ▁of ▁the ▁beam ) ▁resolution ▁of ▁the ▁ul tras ound ▁is ▁far ▁better ▁than ▁the ▁trans verse , ▁the ▁tracking ▁ability ▁is ▁less ▁in ▁the ▁trans verse ▁direction . ▁Also , ▁the ▁trans verse ▁resolution ▁( and ▁hence , ▁tracking ▁ability ) ▁decre ases ▁with ▁depth , ▁in ▁a ▁sector ▁scan ▁were ▁ul tras ound ▁be ams ▁diver ge . ▁ ▁D ifferent ▁commercial ▁and ▁non ▁commercial ▁operators ▁then ▁use ▁different ▁approaches ▁to ▁derive ▁motion ▁and ▁de formation ▁parameters . ▁The ▁motion ▁of ▁a ▁single ▁kernel ▁can ▁be ▁resolved ▁into ▁disp lacement ▁curves , ▁and ▁the ▁distance ▁between ▁two ▁k ern els ▁into ▁stra in ▁( de formation ). ▁St rain ▁rate ▁will ▁then ▁be ▁time ▁derivative ▁of ▁stra in . ▁In ▁some ▁commercial ▁applications , ▁the ▁ac oust ic ▁markers ▁are ▁track ed ▁more ▁individually , ▁calculating ▁the ▁velocity ▁from ▁the ▁motion ▁and ▁the ▁sampling ▁interval ▁( in verse ▁of ▁frame ▁rate ) ▁generating ▁a ▁velocity ▁field . ▁Un like ▁t issue ▁Do pp ler