Evaluation of a frequency-domain ultrasonic imaging attenuation compensation technique.

Abstract

Ultrasound attenuation is typically compensated for in clinical scanners by using time gain compensation (TGC). However, TGC operates in a frequency-independent fashion and therefore the spatial resolution of the echographic images degrades as the examination depth increases. In the current study, the capability of a multi-band attenuation compensation (MBAC) TGC technique to recover both magnitude and spatial resolution in lossy media was evaluated. Simulations were performed using a 5-MHz transducer for imaging point targets embedded in a medium with attenuation coefficient slope (ACS) of 0.5 dB/(cm.MHz). For performance assessment, the magnitude and spatial resolution of the reflected point spread functions (PSFs) were compared to the ones obtained from point targets embedded in a lossless medium. The results showed a complete recovery of the spectral content when using MBAC for all depths when compared to the lossless case. Both the magnitude and spatial resolution of the compensated PSFs were in agreement with the lossless result (i.e., less than 1 dB and 3 % difference in PSF magnitude and spatial resolution, respectively). The MBAC was then applied to in vivo liver imaging using a scanner equipped with a 5-MHz linear array. Attenuation compensation was performed using ACSs reported in the literature for skin, fat and muscle, and experimentally estimated ACS using the spectral log difference technique for the liver. The lateral and axial extent of the autocorrelation function was estimated in the liver tissue. The experimental MBAC image exhibited only 6 % and 11 % variation in speckle magnitude and lateral autocorrelation length for depths between 2.5 and 4 cm. These results suggest that MBAC technique may enhance speckle uniformity in homogeneous tissue regions.