大血管

• The edge of a group of blood is visible and easy to track . there is little distortion of the shape of a group of blood between two frames . there are many methods to measure blood velocity such as optical flow techniques [ 40 , 44 , 45 ] , using fluorescent images [ 42 ] , template matching method [ 17 ] and so on [ 43 ]
  大血管的情况复杂一些，第一类血管的血液和周围的组织都很复杂（见图3 . 1 ） ，这类血管有些血液和组织还很相似，特征是流速比较低，细胞团块边缘清晰，在一定时间内的形变较小，本文用模板匹配法，较好的测量出这类血液的流速。