echocardiography

echocardiography is the clinical technique, developed with related ultrasound techniques in the 1960s, of producing images of the functioning heart by reflecting pulses of ultrasound from the structures of the heart and then recording the reflected (‘echoed’) sound signals. It is a close relative of the methods used to produce live images of the fetus developing in the womb. With this technique, physicians have a further powerful ‘non-invasive’ method to aid both diagnosis and treatment of many heart complaints. The ‘echo’ images can reveal the size of the heart's chambers, the width of the major veins and arteries entering and leaving it, and the status of the valves that define the direction of blood flow into and out of the chambers. Powerful computing techniques mean that the echocardiography can be done in ‘real time’, with the patient fully conscious, enabling the dynamic features of heart function to be observed over the course of many heartbeats.

The method involves two basic elements, a transmitter and a receiver. The transmitter emits an electrical signal to a piezo-electric crystal each time an oscillation is required. The electrical signal is converted by the crystal to an ultrasound wave. The ultrasound beam is directed through the chest and the sound wave is transmitted and reflected as internal structures of the chest are encountered. The reflected waves are collected by the receiver. Currently, most echocardiographers use sector scanners to produce images of the heart. With these instruments the ultrasound beam is swept in an arc across the heart from a single point on the chest wall. The solid parts of the heart walls and even the very thin valves produce strong reflections, whereas blood-filled spaces produce little reflection and so are reported as ‘empty’ by ultrasound. The received pattern of sound waves is often displayed in a video format, but a permanent record can be produced by using photography or chart recorder technology. Since the time between the emission and reception of an ultrasound signal can be accurately measured, it is possible to display the depth at which a structure lies within the chest using a simple formula: D = V × T or, distance = (velocity of the ultrasound signal) × (time between emission and reception of the signal). In this way, an image of a ‘section’ of the heart can be built up over a number of cycles. Cross-sectional images can be produced by electronic sector scanners which have small multi-element transducers cut from the same piezo-electric crystals.

The narrowing (stenosis) of heart valves is a typical clinical problem that can be studied by echocardiography. In association with valve narrowing, the speed of blood flow and chamber dimensions are increased. By applying the Bernoulli principle it is possible to estimate the pressure differences across a damaged valve. Cardiac output can be assessed by measuring the velocity of blood flow leaving the heart and multiplying this by the echocardiographically-derived cross-sectional area of the aorta.

Doppler imaging, usually performed at the same time as echocardiography, utilizes the principal of the Doppler shift of ultrasound to measure velocity of blood flow in the chambers of the heart and in the aorta. In the ‘continuous wave Doppler’ technique, two piezo-electric crystals are utilized; one continually sends and another continually receives the ultrasound wave. Information is thus received from the entire length of the beam and range resolution is not possible. Its great advantage, however, is that high maximal velocities of blood flow, such as occur when blood is being ejected from the ventricles, can be reliably measured. In the pulsed Doppler system, the transmitter is turned off after sending the pulse to the crystal, while the receiver is immediately turned on. The receiver waits for the pressure wave-front to return to the crystal and amplifies it to allow frequency analysis. This technique is useful for localizing high velocity flow, as commonly associated with defective ventricular exit (aortic and pulmonary) valves, rather than for measuring an absolute velocity.

The current uses of cross-sectional and Doppler echocardiography are to record the dimensions of heart structures, cardiac output, jet velocities, and flow disturbances. Abnormal flow patterns can also be recorded and used, for example, to map jets of blood leaking from damaged valves. Recent technology has allowed colour coding of blood flow direction using a combination of cross-sectional and Doppler echocardiography.

David J. Miller, and Niall G. MacFarlane

See also heart; imaging techniques.