Andersons pediatric cardiology 467

image sector Temporal Resolution To image fast-moving structures, such as valve leaflet motion, frame rate optimization is highly important The eye generally can see only 25 frames per second, providing a temporal resolution of approximately 40 ms The temporal resolution is limited by the sweep speed of the echo beam, which in turn is limited by the speed of sound because the echo from the deepest part of the image has to return before the next pulse is sent out at a different angle in the neighboring beam The speed of the sweep can be increased by reducing the number of beams, or increasing the beam width in the sector, by using the frame rate control, or by decreasing the width of the sector The first option decreases the lateral resolution, whereas the second decreases the image field Temporal resolution, therefore, cannot be increased without a trade-off, due to the physical limitations of echocardiography Newer technology such as plane wave imaging can overcome this limitation but remains an emerging technology not routinely available in the clinic Ultrasonic Imaging Artifacts There are many potential sources for artifacts in echocardiography For interpretation of the images, it is important to know and recognize them ■ Dropout of parallel structures: When structures are parallel to the ultrasonic beam, there is very little reflection caused by the structure, resulting in dropout A typical example is the atrial septum when viewed from the apex ■ Acoustic shadowing: Transmission of the ultrasonic beam through the tissue is influenced by the presence of tissue with very high density Typical examples are prosthetic valves, devices, catheters, or calcifications These structures can make it impossible to view structures behind them ■ Reverberations can occur with lateral spread of high-intensity echoes Bright echoes can have considerable width A lot of reverberations originate from the interaction between the transducer and the ribs ■ Mirror imaging This artifact appears as a display of two images, one real and one artifactual, and is due to the sound beam interacting with a strong reflector ■ Ring down or comet tail The ring down artifact comes from gas bubble in a fluid medium The comet tail originates from highly reflective structures, such as surgical clips Doppler Echocardiography The Doppler principle is the frequency shift caused in ultrasonic waves as they are reflected by a moving reflector The frequency of reflected waves increases as the target approaches the receiver and decreases as it moves away from the observer A typical example is the sound of an ambulance siren, which has a higher pitch sound when moving towards the observer, and a lower pitch sound when moving away When initially applied to echocardiography, the Doppler technique was used to measure the velocities of blood pools A Doppler shift is caused by interaction of the ultrasonic beam with a moving pool of red blood cells The Doppler shift depends on the velocity and direction of the flowing blood, the angle between the beam and the flow, and the velocity of sound within in the tissues This is expressed in the Doppler formula: where Fd is the observed shift in frequency, F0 is the transmitted frequency, V is the velocity of flow of blood, θ is the angle of intercept between the beam and the direction of blood flow, and c is the velocity of sound in human tissue, which is 1540 m/s The observed frequency shift is several kilohertz in magnitude and produces an audible signal that can be electronically processed and displayed graphically Because different velocities are present within the ultrasonic beam for any given time instance during the cardiac cycle, a range of Doppler frequencies are typically detected Thus a spectrum of Doppler shifts is measured and displayed in the spectrogram, hence the term spectral Doppler as often encountered in the literature The Doppler-shifted frequencies usually undergo fast-Fourier transformation, converting the original Doppler waveform into a spectral display with velocity on the vertical axis, time on the horizontal axis, and amplitude as shades of gray Conventionally, Doppler signals from blood moving toward the transducer are displayed above the baseline Similarly, when blood flows away from the transducer, the display is below the baseline When the Doppler shift is known, the Doppler equation outlined previously can be rearranged as: In this way, the velocity of flow can be calculated The main source of error is in the determination of the angle of intercept (cos θ) between the ultrasonic beam and the axis of flow When the angle exceeds 20 degrees, it should be measured and included in the Doppler equation In practice, it is difficult to accurately measure this angle Potential problems arising from a large angle of interrogation therefore can be overcome by obtaining the clearest audio signal with the highest velocity The ultrasonic beam can then be assumed to be nearly parallel to the direction of flow As blood velocities cause a Doppler shift in the audible range, the Doppler shift itself can be made audible to the user A high pitch, with a large Doppler shift, corresponds to a high velocity, whereas a low pitch, with a small shift, corresponds to a low velocity