Estimating the viscoelastic modulus of a thrombus using an ultrasonic shear-wave approach.

PURPOSE Measurements of the viscoelastic properties of a thrombus can be used to assess whether blood clots are likely to become occlusive or to break apart and leak into the blood circulation and block smaller vessels. An accurate method for estimating both the shear elasticity and viscosity of a blood clot in vivo is still lacking, which prompted us to use a novel shear-wave approach to measure the viscoelastic modulus of blood clots. METHODS The shear-wave dispersion ultrasound vibrometry was used to measure both the elasticity and viscosity of blood clots. The experimental system was verified by measuring the viscoelastic modulus of phantoms containing gelatin at different concentrations. Blood-clot experiments were carried out using porcine whole blood with hematocrits ranging from 3% to 40%. The measured values for both clots and gelatin phantoms were compared to those obtained using an embedded-sphere method in order to validate the accuracy of the viscoelastic modulus estimations. RESULTS The shear elastic modulus increased from 406.9 ± 15.8 (mean ± SD) Pa for 3% gelatin to 1587.2 ± 28.9 Pa for 7% gelatin, while the viscosity increased from 0.12 ± 0.02 Pa s to 0.86 ± 0.05 Pa s, respectively. The shear modulus increased from 196.8 ± 58.4 Pa for 40%-hematocrit clots to 641.4 ± 76.3 Pa for 3%-hematocrit clots, while the viscosity increased from 0.29 ± 0.02 Pa s to 0.42 ± 0.01 Pa s, respectively. CONCLUSIONS The results from the statistical analysis indicated that both the embedded-sphere and shear-wave approaches can provide accurate estimations of the shear elasticity for clots and gelatin phantoms. In contrast, the shear-wave approach as well as other methods of rheological measurements does not provide accurate viscosity estimations for blood clots. However, the measured viscosity range of 0.29-0.42 Pa s is reasonable for blood clots.