Several of the newer imaging techniques generate better pictures of soft tissues. Among these newer techniques is ultrasound imaging. Ultrasound imaging produces pictures of the anatomical structures by “illuminating” the body with sound waves of high frequency, far above the threshold of hearing. The sound waves used by medical ultrasound equipment have frequencies between 10⁶ Hz and 10⁷ Hz and wavelengths (in soft issue) between 1.5 mm and 0.15 mm. In contrast to X-rays, which produce a picture by penetrating through the body and shadowing the organs, the ultrasonic waves produce a picture by reflecting at the walls of organs.

Ultrasound imaging exploits an echo-detection scheme similar to that of sonar and radar. The sonar equipment in a ship detects an underwater target by sending a short pulse of sound waves into the water and listening for the echo returned by the target. The time delay for the return of the echo indicates the distance to the target. Likewise, medical ultrasound equipment detects the anatomical structures within the body by sending a short pulse of sound waves into the body and listening for the echo. The speed of sound in soft body tissues is about the same as that in water, v= 1500m/s . If the time delay for the return of the echo is Δt, then the round-trip distance traveled by the pulse is vΔt, and the actual distance to the point of reflection at the wall of the heart or some other organ is vΔt/2. For instance, if Δt = 10-4 s, then the distance is v Δt/2 = 1500m/s ×10-4 s/2 = 0.075m. Since the travel times for ultrasound echoes are quite short, the imaging equipment must be able to measure time with high precision.

The pulses of ultrasound are generated by a transducer, which converts a high-frequency electric signal into a sound signal. In principle, such a transducer is similar to the loudspeaker in radio, which also converts an electric signal into a sound wave. For this purpose, the loudspeaker uses a small electromagnet to push a diaphragm back and forth, thereby communicating periodic pushes to the air in front of the diaphragm. The ultrasound transducer uses a piezoelectric material, such as PZT, which contracts when an electric signal is applied to it. A layer of such material can easily vibrate at frequencies of 107 Hz. The ultrasound transducer is mounted at the tip of a probe which is placed in contact with the surface of the body. For good contact between probe and skin, the end of the probe is smeared with a schematic diagram of a piezoelectric transducer. The layer of piezoelectric material in the transducer has a curved shape to concentrate and focus the ultrasound in a narrow beam.

To build up a picture of the interior of the body, the beam of ultrasound must be swept through the body, either by moving the probe back and forth over the body or by swinging it from side to side, through some angle. For each position or direction of the probe, the time delays of the echoes must be recorded. Such a sweeping search for echoes is called a scan. The time-delay information for a complete scan is accumulated in the memory of a computer, which later processes this information and displays a picture on a video monitor. 

Ultrasound is now routinely used to examine pregnant women, to detect fetal abnormalities at an early stage. X-rays are unsuitable for this purpose because they tend to produce genetic damage, to which the fetus is especially susceptible.

In the ultrasound probe is used for producing such a picture of the heart, several transducers are placed side by side, so the individual waves form a single wavefront by superposition. The advantage of this arrangement, called a phased array, is that the wavefront can be settled to the left or the right; such a delay tilts the wavefront and alters its direction of advance. A probe with a phased array can automatically sweep the direction of the ultrasound beam across the heart, and we do not need to move the probe itself. To perform a scan, the operator merely holds the probe steady against the body, and the electronic circuitry quickly sweeps the beam of ultrasound.

Fast scanning is especially valuable in echocardiography because it is desirable to sweep the beam across the entire heart in a time so short that the heart hardly moves at all. Such a fast scan produces a snapshot of the heart and by repeating these snapshots in quick succession, we can make a movie that shows the motion of the heart in real-time.