Chapter 11

Echocardiography has become the major imaging and diagnostic modality of cardiac disease, so all cardiac specialists, cardiologists or surgeons, need to understand basic principles, approaches, indications, physiologic determinants, pitfalls, and expected results in order to make appropriate interpretations. Beyond the physical principles affecting imaging results, Doppler-echocardiography is, like all complex testing modalities, subject to operator dependence and Bayesian interpretation of results. Hence, quality of imaging and congruence of reported results should be critically challenged by clinician “consumers.” The present summary of Doppler-echocardiographic principles provides basic to advanced knowledge for major cardiac conditions mostly acquired, to ensure proper functioning of the team echocardiographer-cardiac surgeon.

Echocardiography uses high-frequency ultrasound (2.0 to 7.5 MHz) to image the heart and measure blood flow velocity. It is important to understand some principles of ultrasound for proper interpretation. Imaging is produced by reflection of ultrasound (produced by crystals contained in the transducer) on cardiac walls. To achieve reflection, ultrasound first has to penetrate body tissues. Penetration of ultrasonic energy is excellent in water (blood), mediocre in fat, and minimal through air and bones because they are such strong reflectors that no energy is left to progress within the cardiac tissues. Therefore, echocardiographers use windows (between the ribs, sternum, and lungs) such as parasternal, apical, subcostal, suprasternal, transesophageal, or intracardiac to ensure good penetration of ultrasound. Ultrasonic imaging is not photography, and images of cardiac structures (eg, aortic walls) are generated by a “scan-converter” counting the time between emission of ultrasound and return of the reflected wave to the same crystal and calculating the distance from transducer to reflective structure assuming a constant speed of ultrasonic progression in blood. This mostly true assumption ensures excellent measurement of depth, but lateral resolution is less precise because strong reflectors tend to diffract ultrasounds, which rebound not only toward the original crystal, but also adjoining ones. Thus, a point is well defined in depth but appears fattened laterally (lateral lobes). Clinically, this lateral “thickening” of echoes tends to minimize apparent size of cavities. Strong reflectors (eg, calcified walls) also reflect so much ultrasonic energy that travels back to the transducer, to the wall again, and transducer again that artifact at double depth may be generated. Single crystal imaging is M-mode echo, whereas two-dimensional (2D) echo is produced by a series of crystals. 2D echo is a tomography that slices the heart and requires comprehensive imaging to mentally reconstruct the heart. Newly developed three-dimensional (3D) imaging produces an ultrasonic cone reflecting the true 3D heart structure, but diffraction of ultrasound waves and computational issues degrade imaging definition. Hopefully future development will provide high resolution and 3D imaging.

Intracardiac velocity is measured based on the Doppler effect; that is, a moving target changes the wavelength of the reflected sound. The frequency change (Doppler shift) magnitude is proportional to the velocity of the moving target and its direction indicates the direction of the moving target (toward or away from the transducer). Velocity measurement is ...

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