[Improved endocardium imaging using modified transthoracic echocardiography with the second harmonic frequency (tissue harmonic imaging)]

In human tissue sound propagation is not linear resulting in a modification of the ultrasound waves. Therefore the received ultrasound frequencies are not just the transmitted frequencies of the transducer, but contain additional harmonic frequencies. These frequencies were not used by standard echocardiography. Harmonic imaging processes the second harmonic frequency. Distortion of the transmitted beam by shallow surface layers or reverberations between the skin surface and ribs impair image quality in conventional 2-dimensional echocardiography. Since harmonic components develop with propagation distance, the reverberations are almost entirely made up of ultrasound energy at the fundamental frequency. The exclusive processing of the second harmonic frequency provides an impressive improvement in image quality of 2-dimensional echocardiograms. Noise and clutter artifacts are reduced and endocardial borders are enhanced. Commercially available imagers provide tissue harmonic imaging using transmit frequencies from 1.7 to 2.1 MHz and second harmonic frequencies between 3.4 and 4.2 MHz. Several clinical studies have shown that harmonic imaging was superior to standard echocardiography in the assessment of left ventricular (LV) wall motion by LV wall motion analysis. In patients with poor acoustic windows the number of myocardial segments which could be evaluated was significantly higher using harmonic imaging than standard echocardiography. Tissue harmonic imaging particularly improves the delineation of the anterior wall using the 2-chamber view and the lateral wall using the 4-chamber view.

Harmonic imaging is a clinically relevant improvement of echocardiography. We suggest that this new technology should be the method of choice in the evaluation of LV function.