Cardiovascular Imaging

Echocardiography

cardiac imaging technique based upon the velocity of sound travelling through and reflected from acoustic interfaces in cardiovascular structures. It has progressively evolved from M-mode echocardiography to the current multifaceted capabilities including transthoracic and transoesophageal echocardiography, three-dimensional echocardiography, Doppler velocity measurement, colour flow mapping and intravascular imaging. Echocardiography has become the most frequently performed diagnostic study for cardiac diseases.

Types of echocardiographic studies

M-mode echocardiography provides a one-dimensional (distance from the transducer versus time) view of cardiac structures. Cardiac motion is displayed as a change in position of cardiac structures; i.e. mitral leaflet motion, over the cardiac cycle. The distance between and changes in distance between various cardiac structures is displayed on one-dimensional echocardiograms. The M-mode method provides interrogation of moving cardiac structures with a sampling rate of nearly 1000 cycles/sec. The M-mode echocardiogram also depicts abnormal patterns or velocity of motion in cardiac structures such as the mitral leaflet in flail mitral valve and mitral stenosis, respectively.

Two-dimensional echocardiography (2DE) uses rapid movement of a one-dimensional ultrasonic beam across the heart to provide real-time cross-sectional images. It is the standard ultrasound imaging method for the heart. There are two major types of two-dimensional imaging devices, mechanically driven large crystals and electronically driven phased crystal arrays. The electronically driven systems are now dominant.

Doppler echocardiography allows the measurement of intracardiac and intravascular flow velocities by detecting changes in the frequency of reflected ultrasound emitted by and then returned to the transducer. After emitted ultrasound strikes moving red blood cells, the frequency of the ultrasound is shifted in proportion to the velocity of the cells. This velocity difference of the ultrasound is displayed as a function of time and direction of the flow in relation to the transducer. There are two types of Doppler modalities: pulse wave Doppler and continuous wave Doppler. Pulse wave Doppler is capable only of measuring velocities accurately in the lower range due to aliasing. Flow mapping or colour Doppler is a special form of pulsed wave Doppler. Colour Doppler is used to screen the heart for flow disturbances such as valvular regurgitation and stenosis. Continuous wave Doppler obviates aliasing and can be used to accurately measure high velocity flows such as those associated with stenoses (Fig.1).

Transoesophageal echocardiography (TEE) involves the placement of the ultrasound transducer into the oesophagus in proximity to the heart (Fig.2). TEE can be done using monoplane, biplane or multiplane transducers. Because it is semi-invasive compared with standard transthoracic echocardiography (TTE) it is utilized for specific indications rather than as a replacement for TTE. A major use of TEE is imaging of the heart during and after cardiac surgery in the operating theatre. Stress echocardiography involves the evaluation of regional wall motion or thickening in the basal state and after some form of stress in order to elicit wall motion abnormalities as a surrogate of inadequate myocardial perfusion. While exercise was initially employed, current approaches use pharmacological stress such as dobutamine and dipyridamole.

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Fig.2

Transoesophageal echocardiogram in the four chambers view demonstrates a myxoma (MYX) attached to the atrial septum. LA = left atrium; LV = left ventricle; RA = right atrium; RV = right ventricle. Reproduced from Himelman RB: Echocardiography in Acquired Heart Disease. In: Higgins CB: Essentials of Cardiac Radiology and Imaging. JB Lippincott, Co., Phila., PA, 1992.
Echocardiography, Fig.1
Echocardiography, Fig.2