L. Allard et al., EFFECT OF THE INSONIFICATION ANGLE ON THE DOPPLER BACKSCATTERED POWERUNDER RED-BLOOD-CELL AGGREGATION CONDITIONS, IEEE transactions on ultrasonics, ferroelectrics, and frequency control, 43(2), 1996, pp. 211-219
It has been reported that power color Doppler ultrasound has important
advantages over conventional color Doppler flow imaging, Some of thes
e advantages are the aliasing free capability, the increased sensitivi
ty to flow, and the angular independence. This last characteristic of
power Doppler ultrasound was evaluated to verify if it was still valid
in some well-defined flow conditions where porcine whole blood, calf
red cells suspended in saline solution, and carbon fibers suspended in
a water-glycerol mixture were used as scattering particles, Experimen
ts were conducted under steady flow conditions (mean shear rates acros
s the tube between 8.5 and 102 s(-1)) for insonification angles betwee
n 40 and 80 degrees. Different hematocrit values (5, 10, 20, and 40%)
were specifically tested for porcine whole blood. Results indicated no
angular dependence for the saline suspension of calf red cells while
a strong anisotropy was observed for the carbon fibers, In this last s
ituation, the Doppler power in decibels increased linearly with the in
sonification angle, The maximum found at 80 degrees suggests that the
fibers were aligned with the direction of the flow, For porcine whole
blood, an angular dependence was observed at some specific shear rate
conditions, At 40% hematocrit, the anisotropy was about 5 dB for shear
rates between 17 and 51 s(-1), while for a lower (8.5 s(-1)) or highe
r shear rate (102 s(-1)), the anisotropy was reduced to approximately
2 dB, In all of these situations, the maximum Doppler power was observ
ed for an insonification angle between 45 and 60 degrees. For hematocr
it values of 5, 10, and 20%, the anisotropy was respectively on the or
der of 2, 3, and 4 dB or less, depending on the shear rate conditions,
Among the possible mechanisms that may explain the anisotropic effect
observed in the present study, the structure of the red cell aggregat
es is believed to he the determinant factor. A hypothesis concerning t
he structure of the aggregates under flowing conditions in large diame
ter tubes is proposed.