A 3D Numerical Model of Ultrasonic Transthoracic Propagation for Cardiac Focused Ultrasound Therapy.

Objective: Non-invasive focused ultrasound therapies of abdominal organs, including the heart and the liver, have emerged in the last decades. Transthoracic focusing of ultrasound poses challenges such as pressure loss and aberrations. Numerical models of ultrasonic propagation have been developed to study the focalization in heterogeneous tissues, particularly for transcranial applications. However, ribcage models were less studied than skull models, and no experimental validation of ribcage models has been performed so far.

Methods: Both linear and nonlinear k-space simulations were used to model the ultrasonic propagation from a clinical system dedicated to transthoracic cardiac therapy. Tissue acoustic properties were determined from computed tomography scans. Experimental model validation was performed with hydrophone measurements of pressure fields through in vitro human ribs and in vitro porcine flail chest.

Results: An excellent agreement of pressure distribution between the acquired and simulated pressure fields was found for the linear propagation model with a mean correlation coefficient between the measured and simulated pressure fields of R2 = 0.89±0.07. For the nonlinear propagation, the mean correlation coefficient was R2 = 0.91±0.06. The feasibility of the simulations through the human thorax was demonstrated on 9 patients who underwent non-invasive therapy of the aortic valve. The global attenuation estimated numerically was correlated withthe amplitude at the focus necessary to nucleate cavitation (R2 = 0.64).

Conclusion: The numerical model of transthoracic ultrasound propagation was validated and used on a human patient's thorax.

Significance: With further development, this model could be used as a treatment planning tool for non-invasive ultrasonic cardiac therapy.