Integrated backscatter during harmonic and fundamental frequency imaging--effect of depth, mechanical index, and tissue anisotropy: implications for myocardial tissue characterization.

Objective: To explore the potential advantages of tissue harmonic imaging (THI) versus fundamental frequency imaging (FFI) when applied to tissue characterization.

Methods: A Philips Medical Systems Sonos 5500 echocardiograph equipped with a broadband transducer (S4) and an on-line quantitative analysis software package (Acoustic Densitometry) was used for imaging. The effect of mechanical index (MI), imaging depth, and anisotropy on relative backscatter amplitude was evaluated.

Results: This study demonstrated that imaging with tissue harmonics generated relatively greater backscatter values at clinically relevant imaging depths and instrument settings referenced to FFI. This effect was dependent on MI setting. A direct relationship between backscatter amplitude and MI was demonstrated. Additionally, tissue anisotropy had similar effects on integrated backscatter amplitude during both THI and FFI. However, relative backscatter values at each fiber orientation are greater during THI at similar instrument settings when referenced to FFI.

Conclusion: Tissue harmonic imaging may offer advantages over FFI for myocardial tissue characterization.