Echocardiography, also known as cardiac ultrasound, is an ultrasound- based diagnostic imaging technique used for visualizing of subcutaneous body structures, including: tendons, muscles, joints, vessel and internal organs, for possible pathology or lesions. In physics, “ultrasound” applies to all sound waves with a frequency above the audible range of human hearing, about 20,000 Hz. The frequencies used in diagnostic cardiac ultrasound are typically between 2 and 18 MHz.
The choice of frequency of the cardiac ultrasound is a trade-off between spatial resolution of the image and imaging depth: lower frequencies produce less resolution but image deeper into the body. Higher frequency sound waves have a smaller wavelength and are therefore capable of reflecting or scattering from smaller structures. Higher frequency waves also have a larger reduction coefficient and are therefore more readily absorbed in tissue, limiting the depth of penetration of the sound wave into the body.
Cardiac ultrasound is most effective for imaging soft tissues of the body. Superficial structures such as muscles, tendons, testes, breast and the neonatal brain are imaged at a higher frequency, which provides better axial and lateral resolution. Deeper structures such as liver and kidney are imaged at a lower frequency with lower axial and lateral resolution, but greater penetration.
Ultrasonography uses probes containing multiple acoustic transducers to send pulses into the tissue. Whenever the wave encounters a tissue with a different density, part of the sound wave is reflected back to the probe and detected as an echo. The time it takes for the echo to travel back to the probe is measured and used to calculate the depth of the tissue interface causing the echo. The greater the difference between cardiac ultrasound acoustic impedances, the larger the echo is. If the pulse hits gases or solids, the density difference is so great that most of the acoustic energy is reflected and it becomes impossible to see deeper.
To generate a 2D-image, the cardiac ultrasonic beam is swept. A transducer may be swept mechanically by rotating or swinging. The received data is processed and used to construct the image. The cardiac ultrasound image is then a 2D representation of the slice into the body.
3D cardiac ultrasound images can be generated by acquiring a series of adjacent 2D images. Commonly, a specialized probe that mechanically scans a conventional 2D-image transducer is used.
Doppler cardiac ultrasound is used to study blood and muscle motion. The different detected speeds are represented in color for ease of interpretation, for example leaky heart valves: the leak shows up as a flash of unique color. Colors may alternatively be used to represent the amplitudes of the received echoes. Echocardiography is an essential tool in cardiology, to diagnose, for example, dilatation of parts of the heart and function of heart ventricles and valves.
The following modes of operation are mainly used in cardiac ultrasound:
- B mode: In B mode, an array of transducers scan a pane. Consequentally, a two-dimensional image of the plane is projected on the screen.
- M mode: In this mode, the motion of the boundaries of a part of the heart are drawn. This is used to determine the velocity of the parts.
- Color Doppler: Velocity information is presented as a color-coded overlay on top of B mode.
- CW Doppler: A continuous wave in Doppler mode, where information is sampled along a line through the heart and all velocities detected at each time point are presented on a time line.
- Pulsed Doppler: Doppler information is sampled from a small sample volume, and presented on a time line.
- Harmonic mode: In this mode, the fundamental (low) frequency is emitted into the heart, and the reflected harmonic overtone (second harmony) is detected. In this case, depth penetration is gained with an improved special resolution (of the double frequency echo).
The standard echocardiogram is also known as the transthoracic echocardiogram (TTE), or cardiac ultrasound. In this case, the echocardiography transducer is placed on the chest wall of the subject, and images are taken the chest wall. This is a non-invasive, highly accurate and quick assessment of the overall health of the heart.
An alternative way to perform an echocardiogram is the transesophageal echocardiogram. A specialized probe containing an ultrasound transducer at its tip is passed into the patient’s esophagus. This allows image and Doppler evaluation which can be recorded. This is known as a trasesophageal echocardiogram, or TEE. Transesophageal echocardiograms are most often utilized when transthoracic images are suboptimal and when a more clear and precise image is needed for assessment.
3D echocardiography is now possible, using an ultrasound probe with an array of transducers and an appropriate processing system. This type of cardiac ultrasound enables detailed anatomical assessment of cardiac pathology, particularly valvular defects, and cardiomyopathies. The ability to slice the virtual heart in infinite planes in an anatomically appropriate manner and to reconstruct three-dimensional images of anatomic structures make 3D cardiac ultrasound/echocardiography unique for the understanding of the congenitally malformed heart.