On the relation between aggregation, packing and the backscattered ultrasound signal for whole blood

Previous studies have shown that the backscattered ultrasound (US) power fr om blood depends on the manner in which red blood cells (RBCs) are packed a nd, in particular, on spatial variations in the red blood cell number densi ty (i.e., the RBC concentration variance). Experimental measurements have a lso shown that the backscattered US power depends on the degree of RBC aggr egation, and it has been hypothesized that this is primarily due to the eff ect of RBC aggregation on the concentration variance. An initial simulation study of the relationship between RBC aggregation and packing statistics i s presented, in which the effects of hematocrit, aggregate size, shape and size distribution on concentration variance are investigated. Both two-dime nsional (2-D) and 3-D samples of aggregated and disaggregated RBCs were sim ulated; these enabled the concentration variance to be calculated. In agree ment with theoretical predictions and experimental US results, the concentr ation variance for disaggregated RBCs is shown to be lowest at low and high hematocrits, and to peak at intermediate hematocrits, The concentration va riance is shown to be particularly sensitive to changes in aggregate size a nd size distribution, and less sensitive to the shape of small aggregates. The results of this study provide a foundation for relating the state of ag gregation in a blood sample to the manner in which RBCs are packed and, the refore, to the backscattered US power. (C) 1999 World Federation for Ultras ound in Medicine & Biology.