Background
The use of echocardiography as an imaging modality has increased substantially over the past decade. Cardiologists perform most echocardiography studies, with internists being the next most common providers of these studies (15% of all Medicare charges submitted for the procedure).
The objective of this article is to provide clinicians with a brief review of basic principles and instrumentation of echocardiography.
Neither bone nor air is a good transmission medium for ultrasound waves; accordingly, specific windows (eg, apical, parasternal, subcostal, and suprasternal) are used to image the heart.
Two-dimensional (2-D) and 3-D echocardiography provide real-time imaging of heart structures throughout the cardiac cycle. Doppler echocardiography provides information on blood movement inside cardiac structures and on hemodynamics.
Tissue Doppler imaging (TDI) provides information about movement of cardiac structures. The relation between the dynamics of cardiac structures and the hemodynamics of the blood inside these structures provides information about cardiac diastolic and systolic function. Echocardiography is continuously evolving and constantly being augmented by newer modalities, such as tissue harmonics, speckle tracking, tissue Doppler strain, and tissue characterization.
The advent of myocardial perfusion echocardiography (MCE) has allowed functional evaluation of the coronary microcirculation, including quantitative coronary blood flow and fractional flow reserve.
This has helped to improve “the definition of ischemic burden and the relative contribution of collaterals in noncritical coronary stenosis.” Moreover, MCE identifies no-reflow within myocardial infarctions (MIs) and low-flow around MIs, predicts potential functional recovery of stunned myocardium with appropriate interventions, and appears to have a diagnostic performance comparable to positron emission tomography (PET) scanning in microvascular reserve/dysfunction in angina patients.
Dedicated training for competent performance and interpretation of echocardiography is essential. The American College of Cardiology (ACC) and the American Heart Association (AHA) have recommended a set of minimum knowledge and training requirements for the performance and interpretation of echocardiography, including a minimum number of 150 performed and 300 interpreted examinations for basic level II (for level III: 300 performed/750 interpreted) in interpreting echocardiography.
Similar guidelines have been developed in Canada.
Basic principles of echocardiography
Humans can hear sound waves with frequencies ranging from 20 to 20,000 cycles per second—that is, from 20 Hertz (Hz) to 20 kHz. Frequencies higher than 20 kHz are referred to as ultrasound. Diagnostic medical ultrasonography usually uses transducers with frequencies of 1-20 MHz. Ultrasound waves are described in terms of the following features:
Frequency (Hz)
Wavelength (mm)
Amplitude, or loudness (decibels [dB])
Velocity of propagation (m/sec) – This varies according to the type of tissue medium carrying the wave (it is approximately 1540 m/sec in blood)
These phenomena are the basis for many of the clinical applications and calculations of echocardiography.
Interaction of ultrasound waves with tissue results in reflection, scattering, refraction, and attenuation of the waves. 2-D echocardiographic imaging is based on reflection of transmitted ultrasound waves. Doppler analysis is based on the scattering of ultrasound waves from moving blood cells, with a resulting change in the frequency of the waves received at the transducer. Attenuation limits the depth of ultrasound penetration. Refraction, a change in the direction of the ultrasound wave, results in imaging artifacts.
Because wavelength (λ) times frequency (ƒ) equals the propagation velocity (с), or λ × ƒ = с, and the propagation velocity in the heart is 1540 m/sec, the wavelength for any transducer frequency can be calculated as follows:
λ (mm) = 1.54/ƒ (MHz)
Wavelength has clinical importance, in that image resolution cannot be greater than 1-2 wavelengths. That is to say, the higher the frequency, the shorter the wavelength and the better the image resolution.
The depth to which ultrasound waves penetrate the body is directly related to wavelength (ie, shorter wavelengths penetrate a shorter distance).
The depth of penetration for adequate imaging tends to be limited to about 200 wavelengths. Thus, an obvious tradeoff exists between image resolution and depth of penetration. Thus, a 1-MHz transducer has a penetration depth of 30 cm and resolution of 1.5 mm, whereas a 5-MHz transducer has a lesser penetration depth, only 6 cm, but a higher resolution, approximately 0.3 mm.
2-D imaging depends on reflection of ultrasound waves. These waves are reflected at tissue boundaries and interfaces, with the quantity dependent on the relative change in acoustic impedance (ie, density) between the two tissues. Optimal return of reflected ultrasound waves occurs at a perpendicular angle (90°).
Doppler flow imaging depends on the scattering phenomenon, and its optimal flow angle is the opposite of that for 2-D imaging (ie, parallel to the flow of interest, rather than perpendicular). The Doppler effect may be simply stated as follows: A person moving toward a sound source will hear a tone with higher frequency than the emitted wave frequency, whereas a person moving away from the source of sound will hear the tone with a lower frequency than the emitted wave frequency.
Additional common terms and concepts employed in echocardiography include the following:
Duty factor – The fraction of time during which a transducer is sending an ultrasound impulse
Intensity – Power divided by area, expressed as watts (W)/cm2
Attenuation – Reduction in intensity with travel, resulting from the effects of absorption plus those of scattering; in tissues, attenuation can be calculated by means of the following equation: 0.5 dB/cm-MHz × depth (cm) × transducer frequency (MHz)
Round-trip travel of an ultrasound beam in tissue – 13 μsec/cm
Variables affecting pulse repetition frequency (PRF) – PRF is limited by the following equation: Depth (cm) × number of foci × lines/frame × frames/sec < 77,000