In vivo transthoracic measurements of acoustic radiation force induced displacements in the heart over the cardiac cycle
Myocardial elasticity is an important indicator of cardiac function and is affected in many disorders associated with heart failure. Ultrasound based interrogation of cardiac stiffness has been extensively studied in ex-vivo, open chest and intracardiac imaging environments. The ability to make these measurements robustly through non-invasive means such as transthoracic imaging would make them more clinically viable and widely applicable. However, transthoracic imaging is a challenging environment for displacement estimation due to poor SNR, acoustic clutter and complex cardiac motion. This work aims to address some of those challenges on a clinical ultrasound system. Sequences to make M-mode measurements of acoustic radiation force induced displacements in the heart over the entire cardiac cycle were implemented on the Siemens SC2000 and a cardiac phased array probe. Pulse inversion harmonic tracking was employed on the tracking beams to suppress the effect of stationary clutter on displacement estimation. Two families of motion filters, high pass filters and polynomial fit filters were analyzed for their performance in being able to remove the background cardiac motion and isolate the radiation force induced tissue response. Clinical data was acquired on 4 subjects and analyzed for repeatability of diastolic vs. systolic displacements. A high pass filter with a cutoff of 100 Hz and a 2nd order polynomial fit filter were found to be equally effective in suppressing intrinsic motion. Diastolic-to-systolic displacement ratios measured in the interventricular septum ranged from 1.3 to 2.2 across subjects but were found to be fairly consistent between the parasternal long axis and the parasternal short axis views for each subject.
