Tissue Harmonic Imaging

- Experimental Analysis of the Mechanism of Image Improvement -

Abstract

Tissue harmonic scanning visually improves echocardiographic image quality. The aim of the present study was to objectively assess the improvement in harmonic image quality under controlled laboratory conditions. A tissue-mimicking phantom that contained 8-mm-diameter cystic lesions at depths ranging from 2 to 12 cm was used. Harmonic scans (1.7 MHz transmit, 3.4 MHz receive) of the phantom were obtained and lesion detectability was compared to that in scans acquired with 2 fundamental frequencies (2.0 and 3.3 MHz). A 2 cm-thick ethanol layer was also used to simulate the nonlinear effect of human fat. Cyst detectability was quantified by measurement of the contrast-to-speckle ratio (CSR). The results indicated no significant difference in the CSR between harmonic and fundamental images obtained without the ethanol layer. With images obtained with the ethanol layer, a relative increase of the CSR during harmonic imaging was observed with respect to fundamental imaging (p<0.05). In conclusion, a fat layer, here simulated by ethanol, plays a significant role in determining the resulting image quality. Without this layer, the contribution of the second harmonic mode was not significant. Thus, in a slim patient, the harmonic mode may not be as beneficial to image improvement as in an obese patient.