Science commentary: Echocardiography
Authors: A. Berger
Journal: BMJ (2000)

Echocardiography uses very high frequency sound waves (ultrasound), which are inaudible to human ears. Sound waves are forms of pressure wave; they can travel through any medium. In echocardiography these waves are generated and detected by means of a piezoelectric crystal contained within a transducer.1 A simple mechanical scanner comprises a single crystal that is oscillated by a small motor, while the newer (“phased array”) systems comprise multiple crystal elements that are pulsed in a rapid and controlled sequence. In cardiac imaging the ultrasound beam arises from a small source and is directed, in a pyramidal sector, across the heart.

In the body, echoes are generated when ultrasound waves reach tissue, where the acoustic properties (densities) change, and are then reflected back. This usually happens at organ boundaries, tissue interfaces, and cellular boundaries. The transducer detects echoes before being converted into electrical signals that are represented on an oscilloscopic display. Dense structures, such as the pericardium and calcified valves, appear bright (white), whereas blood filled cavities (atria, ventricles) are almost echo free (black). Multiple reflections occur when the ultrasound beam reaches an object with greatly differing transmission characteristics from those of the surrounding tissues. For example, prosthetic valves may produce multiple parallel echoes and associated “ghost” images.

Ultrasound waves are sinusoidal fluctuations in pressure; the size of each pressure wave is termed the amplitude, the distance between the two waves is the wavelength, and the number of waves per second is the frequency. For medical applications, the frequency of ultrasound tends to lie within the range of 2-20 million cycles per second (MHz), and echocardiography sits at the lower end of this spectrum (2.5-10 MHz). During transmission through tissues, energy is lost due to absorption and scattering, and thus the reflected energy of the echo is invariably lower than the original ultrasound. Higher frequency waves may provide better image quality, although tissue penetration is not as good.

Complete echocardiographic examination entails the use of three different imaging modes: two dimensional, M mode (a graph of motion against time), and Doppler studies. Two dimensional imaging enables the operator to make a subjective assessment of the size of the cardiac chamber and ventricular function and allows detailed assessment (morphology/mobility) of valvar structures. M mode is useful for making accurate measures of chamber size and wall thickness at specific points in the cardiac cycle. Doppler techniques are divided into three types: pulsed wave, continuous wave, and colour flow. Pulsed wave and continuous wave Dopplers are used to detect the direction and velocity of blood flow across heart valves (this allows calculation of valve gradients). Colour flow imaging provides a pictorial coloured representation of blood flow in the heart and across valves. This is a useful screening tool as the direction of blood flow and the presence of turbulence can be identified. It is particularly useful for detecting evidence of regurgitation across valves or abnormal patterns of blood flow (for example, in a ventricular septal defect).