In vivo measurements of ultrasonic backscatter from blood

Ultrasonic backscatter from whole blood has been studied extensively in vitro. It has found to be related to hematocrit, plasma protein, and flow disturbance. To validate these in vitro results, in vivo backscatter measurements have been made on whole blood flowing in the abdominal aorta and the inferior vena cava using a 10 MHz intravascular transducer in pigs. The probe was inserted into vessels of interest via a peripheral vein or artery. Backscattering coefficient (BSC) of blood was measured using a newly developed approach in which backscattered power acquired from a 6% porcine red cell suspension was used as a reference to extract the BSC from a dense scattering medium. Results from three pigs showed that BSC of blood in the vein and artery is respectively 3.7×10^-5 and 1.2×10^-5 (cm-sr)^-1 at 10 MHz and 34% hematocrit which are slightly larger than BSC from the porcine red cell suspension of the same hematocrit and frequency at 6.5×10^-6 (cm-sr)^-1. One important reason for the higher backscatter in veins is that red cell aggregation is more pronounced in veins where blood flow is less pulsatile and slower than that in arteries. Moreover, previous in vitro measurements also showed that disturbed flow may increase ultrasonic backscatter of blood. This observation was confirmed by the current in vivo measurements in which stenosis were created by ligating the artery. Backscatter of blood measured downstream of the stenosis as a function of distance from the stenosis indicates that the closer the site where the measurement was made to the stenosis, the higher the backscatter, presumably resulted from the higher degree of flow disturbance.