Changes in backscattered ultrasonic envelope statistics as a function of thrombus age: an in vitro study.It is necessary to determine the age of thrombi in planning clinical treatment for thrombolysis.  Ultrasound imaging can potentially be used to evaluate thrombus age in real time.  The backscattered signals from thrombi may contain useful information regarding their age.  On the basis of the randomness of ultrasound backscattering, this study explored changes in backscattered US statistics as a function of thrombus age.  Porcine blood samples were used for the in vitro induction of fresh thrombi (day 0) with hematocrits ranging from 0%-40% and aged thrombi (days 0-8) with a hematocrit of 40%.  Each thrombus was imaged using a pulse-echo ultrasound scanner equipped with a 7.5-MHz linear array transducer to acquire raw backscattered signals for B-mode and Nakagami imaging, by which the backscattered statistics were visualized.  Hematoxylin and eosin staining and scanning electron microscopy were used to observe the histology of fresh and aged thrombi.  The results indicated that a decrease in the number of red blood cells in the thrombus caused by the aging effect was observed in the in vitro model, indicating that the proposed model could simulate the structural changes in the thrombus during aging.  Compared with fresh thrombi with various hematocrits, the aged thrombi exhibited a trend toward more substantial decreases in the Nakagami parameter with increasing thrombus age (the Nakagami parameter decreased from 1.1 to 0.6 as thrombus age increased from day 0 to day 8), indicating that thrombus aging causes the backscattered statistics to follow a pre-Rayleigh distribution to a high degree.  This finding may be applied to the determination of thrombus age using conventional ultrasound imaging in the future.
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