Fast spectral approach for delay correction in heterogeneous media

Ultrasound imaging makes fundamental assumptions about the interrogated medium, including the propagation speed of the waves. For many clinical applications, assuming a constant speed of sound (SoS, typically 1540 m/s) is sufficiently accurate to yield diagnostically useful images. However, when the medium is appreciably heterogeneous (e.g., trans-skull, or through deep layers of subcutaneous fat), this approximation fails and the distortion of the wavefronts yields a degraded image (i.e., there is aberration). Correcting these distortions for a known SoS distribution is typically intensive computationally. Here, we demonstrate the utility of the heterogeneous angular spectrum method (HASM), a spectral method developed for passive cavitation imaging, to recover efficiently the appropriate apodization and delays in transmit and receive toward corrected images. Compared with a straight ray computation (i.e., no refraction) with mean 5% SoS variation, HASM was roughly tenfold faster, and computed delays matched to within (−84 ± 89) ns [compared with (−660 ± 312) ns error for 1540 m/s assumption]. Furthermore, transmit corrections for focused or plane-wave sequences may be obtained similarly with the method for a specified depth. HASM, thus, has promise toward real-time image enhancement and improved diagnostic utility. [Work Supported by the ASA FV Hunt Postdoctoral Fellowship.]