Dilution system identification methods for contrast ultrasound ejection fraction assessment

Abstract

Standard left ventricle (LV) ejection fraction (EF) measurements by image analysis techniques are typically time- consuming. Indicator dilution methods allow a fast EF mea- surement, but their application is limited due to the need for cardiac catheterization. The injection of an ultrasound contrast agent bolus permits a non-invasive indicator dilution curve (IDC) detection, however, a ventricular contrast injection remains necessary. We have recently proposed the use of LV dilution system identification methods for a minimally-invasive EF measurement, in which the contrast bolus is injected in an arm vein. In this paper, we present, validate, and compare two least-mean-squares (LMS) algorithms for the LV impulse response estimation and the LV EF assessment. The left atrium and LV IDC represent the input and output of the LV dilution system. In the first algorithm, the LV dilution impulse response is determined by the solution of the Wiener-Hopf equation and fitted by a mono-compartment model for the EF assessment. In the second algorithm, the LV impulse response is parameterized by the model before the application of the LMS algorithm. The algorithms are validated by 50 measurements in patients with EF between 10% and 80%. The measurements are compared with echocardiographic bi-plane estimates after contrast opacification. The correlation coefficients are 0.68 and 0.88 and the standard deviations are 11.8% and 8.1% for the first and second algorithm, respectively. We may conclude that EF measurements by contrast echocardiography are feasible. Adding a priori knowledge in the system identification algo- rithm leads to an increased accuracy of the measurement.