In vivo radiofrequency ultrasound analysis of normal human heart structures

Twenty young subjects were studied with a microprocessor system for quantitative analysis of backscattered radiofrequency (RF) signals from normal heart structures. This system allows an "on line" quantitative evaluation of the amplitude of the RF "native" signal, before the chain of processing and display, with the acquisition gate displayed on a conventional M-mode machine. Septum, posterior wall, and anterior mitral leaflet were analyzed. The gate length was kept at 3 microseconds (2.35 mm) for the ventricular walls (excluding endo- and epicardial reflections), and at 1 microsecond (0.8 mm) for the mitral valve. Integrated backscatter index (IBI) was calculated as the time integral of [u(t)[, where u(t) = i(t) X s(t); is the time sequence of backscattered echoes and s(t) is the time gate delimiting the thickness of the insonated tissue. The IBI was expressed in percent, normalized for the pericardial interface (the strongest reflection was assumed to be 100%). The percent IBI for the septum was found to be 22 +/- 4%; for the posterior wall it was 17 +/- 3%; for the anterior mitral leaflet it was 5 +/- 2%. A second reading of the same structures was performed by the previous observer and by a new one. Good intraobserver (r = 0.92) and interobserver (r = 0.88) correlations were obtained. In conclusion, a regional variation in echo amplitude from different heart structures can be observed in man. This set of values can be used as normal values for future studies in pathologic conditions.