Efficacy of 1.5 Harmonic Imaging for Intravenous Myocardial Contrast Echocardiography

Background. Although second harmonic (2H) imaging is utilized to minimize the tissue signals in myocardial contrast echocardiography (MCE), the native tissue harmonic signal is sufficiently high enough to cause a misreading of myocardial opacification when high mechanical index (MI) is used.Purpose. The backscattered signal from microbubbles at high MI is broadband, while it is narrowband at low MI. Therefore, the bubble/tissue signal ratio is expected to be high between the fundamental signal and 2H frequencies at high MI. Our aim was to elucidate the efficacy of a new imaging method using an in-between frequency, denoted as 1.5 harmonic (1.5H) imaging.Methods. Six open-chest dogs were examined using Toshiba APLIO with 1.5H and conventional 2H system. The transmitting/receiving frequencies were 2.3/3.3 MHz for 1.5H and 1.4/2.8MHz for 2H imaging, respectively. Intermittent MCE was performed at a MI of 1.6 after injection of Levovist®, and baseline intensity and peak intensity of opacification of the ventricular septum were measured.Results. While baseline intensity of the septum was significantly lower using 1.5H than 2H (26±4 vs. 43±14 gray level, p<0.001), the opacification after Levovist was significantly higher using 1.5H than 2H (91±32 vs. 51±38, p<0.001). Due to the combination of these effects, visual recognition of myocardial opacification was quite superior with 1.5H imaging.Conclusion. The 1.5H imaging system provides superb contrast opacification when using high MI.