What is tissue harmonic imaging in echocardiography?

If the transmit and receive frequency are the same in echocardiography, the image quality may be poorer because of higher levels interference and artifacts. In harmonic imaging, reception is on the second harmonic frequency, like 5MHz for a 2.5 MHz transmitted signal, then interference and artifacts are lesser, resulting in better image quality. Harmonic imaging is now the norm, compared to the fundamental frequency imaging which was used initially.