Page 39 - Feline diagnostic imaging

32 3 Ultrasound: Physical Principles of Ultrasound Imaging
improved, and there is better border definition and reduced velocities than pulsed wave Doppler and is therefore
artifact [5, 8, 9]. needed when evaluating high‐velocity flow such as would
occur across with valvular insufficiency jets or stenotic
valves during echocardiography.
3.6 Doppler Principles The Doppler spectral tracing is generated with both
pulsed wave and continuous wave Doppler techniques.
The Doppler effect is a change in frequency caused by the The spectral display presents flow information as a func-
motion of a sound source, receiver, or reflector [2, 5, 6]. In tion of time (Figure 3.6a,b) [5, 10]. Flow directed above the
medical imaging, the Doppler effect is most often used to baseline is toward the probe, and flow directed below the
allow detection of the direction and velocity of blood flow. baseline is away from the probe.
There is a shift in frequency as sound waves are reflected by Color Doppler (Figure 3.6c) is also a pulsed Doppler
moving targets (blood cells). If the motion is toward the technique [5]. The Doppler shift will be color coded rather
transducer, the frequency of the returning echoes will be than being displayed on a spectral tracing. The clinician
higher than the transmitted frequency. If the motion is chooses the location of the Doppler evaluation via an
away from the transducer, the frequency of the returning adjustable color box. The choice of colors is operator
echoes will be less than the transmitted frequency. This is dependent and chosen by using a color map. One of the
referred to as the Doppler shift. The greater the velocity, the most commonly used color maps is known as a “BART”
greater the Doppler shift. The maximum Doppler shift map, meaning that flow away from the transducer will be
occurs when the sound waves are parallel to the direction blue and flow toward the transducer will be red (Blue Away
of flow but most vessels will not be parallel to the ultra- Red Toward). The velocity of flow is indicated by the color
sound beam. The Doppler angle is the angle between the hue or intensity of the color [3, 5]. As in pulsed wave
direction of blood flow and the ultrasound beam. The Doppler, the maximum velocity is limited and flow perpen-
Doppler shift will be underestimated if this angle is not dicular to the ultrasound beam cannot be recorded [4, 5].
accounted for and cannot be measured if the Doppler angle Power Doppler is sometimes referred to as energy
is at 90°. Ideally, the Doppler angle should be between 30° Doppler (Figure 3.6d). This is a signal processing method
and 60° to be considered accurate [5]. that relies on the strength of the Doppler shift/signal rather
than the directional information. Power Doppler is there-
fore dependent on the amplitude of all Doppler signals
3.7 Doppler Modes regardless of the frequency shift [4, 5]. Power Doppler
improves the sensitivity to flow at the expense of direc-
The most common Doppler modes utilized are pulsed wave tional and quantitative information. Just like color flow
Doppler, continuous wave Doppler, color Doppler, and Doppler, there is a color map and the intensity of the color
power Doppler (Figure 3.6). is related to the signal strength. The advantage of power
With pulsed wave Doppler (Figure 3.6a), sound is trans- Doppler is that it is very accurate at recording very subtle or
mitted in pulses similar to real‐time scanning [5, 6]. Pulsed slow blood flow that could be missed with color Doppler
wave Doppler is used in conjunction with B‐mode imaging. evaluation [4, 5].
Using the B‐mode image, a vessel is selected and a pulse
gate is shown over the plane of the vessel [4]. The gate is
the only location where the Doppler shift will be recorded. 3.8 Artifacts
In addition to the pulse gate, a separate line superimposed
over the gate will be visible. This line is used to estimate the Artifacts are common in diagnostic imaging and ultrasound
Doppler angle so more accurate Doppler measurements is no exception. An imaging artifact is any misrepresenta-
can be obtained. Pulsed wave Doppler is dependent on the tion of the imaging anatomy. Many of these artifacts can
PRF. As a result, flow rates are typically limited to less than result in misleading information. A basic understanding of
1–1.5 m/s [5]. these artifacts is needed if ultrasound is utilized. Some of
Continuous wave Doppler (Figure 3.6b) utilizes a probe the more common artifacts include acoustic shadowing,
with two separate sets of crystals allowing sound to be distal enhancement, reverberation artifact, mirror image
simultaneously transmitted and received [5, 6]. Similar to artifact, slice thickness artifact, and edge shadow artifacts.
pulsed wave, the B‐mode image is used to select the loca- Acoustic shadowing (Figure 3.7a) occurs when the ultra-
tion of the continuous wave interrogation. Unlike pulsed sound beam encounters a highly attenuating reflector such
wave Doppler, a gate is not present and the Doppler shift as gas or mineral that absorbs or reflects the sound [3, 5, 11].
will be recorded across the entire line of interrogation [4]. The amplitude of the echoes beneath this reflector is reduced.
Continuous wave Doppler can measure much higher flow This artifact is useful in that it gives some indication as to the