Feasibility and safety of transthoracic cardiac acoustic radiation force impulse imaging methods

This work examines clinical feasibility of using noninvasive transthoracic echocardiography techniques to visualize temporal variations of stiffness through the cardiac cycle using acoustic radiation force impulse (ARFI) imaging. Custom M-mode ARFI sequences were implemented on a Verasonics Research Platform using a Philips/ATL P4-2 phased-array echocardiography transducer. The research systems robust power supply, full parallel-receive capability, and programmable interface enabled sustained excitations, rapid data acquisition, and real-time processing and display of images in the clinic. An extended radiation force pulse length of 480 μs was used to produce tissue displacements up to 12μm around a region of excitation focused at 3 cm. Quadratic motion filters were used to separate ARFI excitation-induced displacements from intrinsic cardiac and respiratory physiological motion artifacts. Acoustic intensity and face heating measurements, as well as finite element method tissue focal heating simulations, were completed. These measurements and simulations calibrated the sequences with respect to the FDA acoustic exposure limits for intensity, mechanical index (MI) and tissue heating. Tests were conducted in phantom and animal models in preparation for the clinical trial. A series of 7 healthy volunteers were scanned in accordance with an approved Duke University Medical Center Institutional Review Board (IRB) protocol. Measurements were acquired from the apical 4 chamber view of the apex, at power levels with MI's ranging from 1.9-3.0. During each M-mode ARFI acquisition, the matched ECG signal was acquired, enabling registration with cardiac cycle. The M-mode ARFI displacement images reflect the expected myocardial stiffness changes through the cardiac cycle, with greatest displacements in diastole and lowest in systole. In the 7 volunteers, the mean displacements throughout the cardiac cycle rose with increasing transmit power level. The ratio of diastolic-to-systolic displacement was examined as a possible indicator of myocardial health. In this study, the measured ratios were in range up to 3.1:1 for the 7 patients, showing agreement with previous ratios reported by an animal studies using transthoracic, intracardiac and epicardial imaging methods. These preliminary clinical results support the feasibility of real-time imaging of cardiac stiffness in vivo using transthoracic ARFI imaging.