  The analysis of the ultrasonic frequency-dependent backscatter coefficient of
aggregating red blood cells reveals information about blood structural
properties. The difficulty to apply this technique in vivo is due to the
frequency-dependent attenuation caused by intervening tissue layers that
distorts the spectral content of backscattering properties from blood
microstructures. An optimization method is proposed to simultaneously estimate
tissue attenuation and blood structure factor. In an in vitro experiment, the
method gave satisfactory estimates with relative errors below 22% for
attenuations between 0.101 and 0.317 dB/cm/MHz, signal-to-noise ratios>28 dB
and kR<2.7 (k being the wave number and R the aggregate radius).
