Real-time strain rate imaging: Validation of peak compression and expansion rates by a tissue-mimicking phantom

Strain rate imaging is a new modality in echocardiography intended for analysis of left ventricular function. This modality extends ultrasonographic techniques for analysis of tissue velocities by providing information about rates of local myocardial compression and expansion. Cyclic cardiac deformation is a complex process. Precision and accuracy of real-time strain rate (rtSR) measurements have not been studied under controlled laboratory conditions. Using a cyclically compressed tissue-mimicking gelatin phantom, we compared rtSR values to corresponding strain rate values calculated off line from local tissue velocities measured by Doppler echocardiography. We tested a clinically relevant range of strain rates (0.5- 3.5 sec^-1) and different angles of insonation. Initial tests showed high precision (r ≥ 0.973, P < 0.001), but the assessment of accuracy (bias ≤ 0.559 sec^-1) suggested a trend toward systematic underestimation of the reference values. We suspected a confounding influence of a clutter filter and repeated the tests with the filter inactive. The resulting accuracy improved tenfold (bias ≤ 0.045 sec^-1), and the systematic underestimation was no longer present. We conclude that the rtSR is precise and accurate for a range of the tested values.