Since the pioneering work by Reid et al. on measuring ultrasonic scatt
ering in blood, this phenomenon has been extensively studied both theo
retically and experimentally. The knowledge on ultrasonic scattering p
roperties of blood is needed for the design of ultrasonic methods for
measuring blood flow, and a better interpretation of ultrasonic images
. The development of high frequency intravascular or intracardiac imag
ing devices raises the possibility of measuring blood properties, e.g.
, erythrocyte aggregation and fibrinogen concentration, in situ. A num
ber of theoretical approaches have been developed to analyze this phen
omenon where in general ultrasound wavelength is much greater than the
erythrocytes. These results show that the backscattering coefficient
of blood, defined as power backscattered by a unit volume of blood per
steradian per unit incident intensity, is proportional to variance of
the erythrocyte number fluctuation and backscattering cross-section o
f a single erythrocyte. In this paper, we will show that similar resul
ts can also be obtained by taking a continuum approach.