Second harmonic imaging: a new tune for an old fiddle?

As with so many important discoveries in science, the significant improvements in echocardiographic image quality generated by second harmonic imaging were discovered almost by chance. Sound signals, including ultrasound transmitted through tissue, usually contain harmonics. These are additional frequencies, at multiples of the main frequency (called the fundamental). Therefore, second harmonic signals have been present within transmitted and reflected ultrasound data since echocardiography began. However, our ultrasound scanners have been “tuned in” to receive only the main or fundamental frequency and the second harmonic has been ignored. The paper by Franke and colleagues in this issue of Heart is one of several studies that have appeared over the past year or so indicating the improved endocardial definition and therefore diagnostic accuracy obtained by using harmonic imaging. This improvement is especially important during stress echocardiography studies. Second harmonic imaging was originally developed as a technique to increase the sensitivity of detection of ultrasound contrast agents, particularly when evaluating organ perfusion or to delineate cardiac chambers when assessing left ventricular function in patients with poor echocardiography windows. Ultrasound contrast agents usually consist of very small gas microspheres. They are strong reflectors of ultrasound and resonate when placed in an ultrasound field. As they resonate, they generate harmonics and these harmonic echoes are larger in amplitude than those from tissue. This makes it easier to detect and image the contrast agent within tissues and the cardiac chambers. However, it was soon appreciated that quality of the echocardiography image and endocardial definition was improved when second harmonic imaging was used alone, before the injection of any contrast agent. There appear to be several factors that explain why second harmonic imaging improves echocardiography image quality and these rely on the fact that second harmonic frequencies are generated as the ultrasound pulse travels through tissue. Strictly speaking, a sound wave travelling through tissue should only contain the transmitted (fundamental) frequencies. However, a sound wave is rather like an ocean wave travelling towards the shore. As the wave approaches the shore it changes shape because the peak of the wave travels a little faster than the trough. When the