Enhanced echocardiographic techniques for imaging tissue.

T are more technologic advances in ultrasonic imaging today than at any time in the past 30–40 years. Among these many developments is enhanced ultrasonic imaging of tissue. Table I shows some of the new tissue imaging techniques that have become commercially available. The first of the major advances is the introduction of digital ultrasonic instruments with broadband transducer technology. These transducers transmit and receive multiple frequencies. Thus, they enhance the penetration and resolution of the ultrasonic examination. In recent years, there has been great interest in using Doppler to image tissue in addition to its usual visualization of blood. A modification of the Doppler technique utilizes a movingtarget indicator approach to highlight those objects that move and suppress those that do not. Harmonic imaging of tissue has become an extremely important tool for enhancing myocardial visualization. Finally, parallel processing, whereby 1 ultrasonic signal is sampled multiple times, has become a principle means of increasing the rate of image acquisition. Digital broadband transducers have become standard with all new ultrasonic echocardiographic equipment. This development now permits the transthoracic visualization of cardiac structures using much higher-frequency transducers. For example, it is possible to visualize coronary arteries transthoracically using 8–12 MHz transducers.1 The multiple frequencies permit imaging at deeper depths and with finer definition. Doppler echocardiography is a very sensitive technique for detecting moving red blood cells. The principle advantages of this approach are its sensitivity in detecting small objects such as red cells and its ability to record the direction and velocity of moving targets. By filtering out the higher velocity blood, one can record the Doppler signal from the slower moving tissue. This approach provides an opportunity to image myocardium with greater sensitivity and to quantitate its motion. There are multiple ways in which Doppler tissue imaging can be displayed. Doppler tissue imaging can be shown as a color 2-dimensional or M-mode recording with the color indicating the direction of motion. One can also obtain a pulsed Doppler tracing of a specific segment of the heart. Applications for this technique are discussed elsewhere in this supplement. There is great enthusiasm for the possibility that Doppler tissue imaging can provide quantitation of global and regional left-ventricular function.2–7 Clinical applications have already been described using this approach for both diastolic and systolic assessment of left-ventricular performance.8–15 The ability to identify motion provides an opportunity to enhance those signals coming from moving targets and suppress those that are stationary. Movingtarget indicator technology essentially uses the Doppler principle. Instead of the usual velocity recording, this approach uses the power or amplitude component of the myocardial Doppler signal. The advantage of using power Doppler is that it is more sensitive and less angle dependent than is velocity Doppler. The end result is that this technique basically enhances the signal-to-noise ratio when the desired target moves and the undesired signals do not. Because the heart moves with cardiac and respiratory motion, cardiac echoes are enhanced. Most of the nonmoving echoes come from the chest wall or electronic noise and are suppressed. With improved signal-to-noise ratios, weak signals can be amplified without increasing noise.16 Probably the most important application that has been accepted almost immediately and universally is harmonic imaging of tissue. Although harmonic imaging was initially developed for recording contrast bubbles, it turns out that this approach is also useful in recording myocardial tissue. The mechanism by which harmonic signals are generated from tissue is still being debated; however, it is clearly different from that which involves oscillating bubbles. It seems that as the ultrasonic beam propagates through tissue, higher frequencies are created.17 By sampling those faster moving sound waves at their harmonic frequency, one can identify those signals that are transmitted through tissue. Echoes originating from artifact and noise do not generate harmonic frequencies. Turning up the gain permits one to record these weak tissue harmonic signals without increasing the noise. It is important to note that when using tissue harmonic imaging, one increases the mechanical index or gain, whereas with harmonic imaging of contrast bubbles one uses low gain settings to distinguish blood-containing bubbles from tissue. The data showing the clinical efficacy of harmonic imaging of tissue are now voluminous and very consistent.18–23 Investigators using several different inFrom the Indiana University School of Medicine, Indiana University; and the Krannert Institute of Cardiology, Indianapolis, Indiana, USA. Address for reprints: Harvey Feigenbaum, MD, Indiana University Hospital, 550 North University Boulevard, Room 5420, Indianapolis, Indiana 46202-5250. TABLE I New Tissue Imaging Techniques