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Echocardiography

Basics
Echocardiography is a technique that uses very high frequency sound, or ultrasound (with frequency between 2 and 15 MHz, well above the human hearing limit of about 20 kHz), to visualize the details of heart anatomy. A sound wave can be transmitted in gaseous, liquid, or solid medium, but when it encounters a border, or interface, between two substances of different density, part of it will be reflected back. The probe of an echocardiography machine contains a transducer, which emits and records ultrasound. In normal operation, bursts of ultrasound emitted by the probe travel through the patient's body, hit an interface (for example, the one between the muscular tissue of the heart and the blood vessels contained within), are reflected back, and then detected by the probe. By measuring the time it takes for the return echo to reach the probe and comparing it against a calibrated time, one can measure the distance between the probe and the interface. The result is a one-dimensional display of the depth of different interfaces relative to the position of the probe. In two-dimensional echocardiography, the probe emits a sequence of pulses in different directions, and the information provided by the returning echoes is then compiled. A typical 2-D echocardiogram looks like a pie slice. The apex of the pie represents the point nearest to the probe, and the rest of the picture represents a composite of depth information from different pie-shaped sectors of tissue. It is analogous to looking at a darkened room by sweeping a flashlight back and forth in front of you to build a complete image.

Why ultrasound?
The level of detail that any imaging device provides depends on the frequency of waves used; so that higher frequency ultrasound yields greater detail. (Besides, sound with lower frequency, near audible range, is loud and damaging to the ear.) For routine examinations of adults, who have a larger heart, more deeply located within the chest, a lower frequency ultrasound, such as 3 MHz, is normally used, while for detailed examinations or for use with small children, higher frequency ultrasound is more suitable. For example, details of 1 to 2 mm can be resolved using 2.5Mhz ultrasound. You may ask, "Why not use a high frequency ultrasound, such as 15 MHz, for everything? Doesn't that give you greater detail?" One consideration is cost; higher frequency ultrasound tools require greater precision and computational power, so they are more expensive. Another point to consider is that higher frequency sounds attenuate more rapidly with distance, so that high frequency ultrasound cannot penetrate very deep into tissue.

Commonly used views
Echocardiography is performed by placing the probe on the patient's chest and aiming the ultrasound to the heart. Depending on the position of the probe, different views of the heart can be obtained.

The widely used parasternal view (pictured above left), parallel to the long axis of the left ventricle, is obtained by aiming the ultrasound through a gap in the ribcage. Also useful is the apical four-chamber view (pictured above right), obtained by pointing the probe down from below the sternum and up toward the head. For more detailed studies, high frequency miniature probes are inserted into the esophagus to image the heart from inside the body.

Doppler echocardiography
Sound is transmitted as a series of compression-rarefaction waves with a certain frequency. When a sound wave is reflected, its frequency is the same as that of the wave originally transmitted, only if the target is stationary. If the target is moving toward or away from the source, then the reflected wave, or echo, will have either a higher or lower frequency, respectively. This frequency shift, or Doppler effect, is used to calculate the velocity of moving objects detected. In echocardiography, this technique is used to detect a moving column of blood, and is graphically represented either as an orange mass moving toward the probe (which is at the apex of the fan-shaped picture) or as a blue mass moving away from the probe. Doppler echocardiography is particularly effective in detecting aberrant blood flow that may result from damaged heart valves. In the example below, which is a parasternal view, you can see a big orange mass of blood moving toward the top on the left side. That is the blood exiting the left atrium and moving into the left ventricle. Directly to the right of that, you see a weaker blue patch. That represents blood swirling around the ventricle. But notice a thin stream of orange which represents blood flowing from the aortic valve into the left ventricle, which is a classic case of aortic valve regurgitation, due to damaged aortic valves.