Spectral Doppler Ultrasound: For Precise Recording and Analysis of Blood Flow.
©Walter Rasmussen, R.D.C.S.
Spectral Doppler ultrasound pertains to Pulsed-Wave Doppler and Continuous-Wave Doppler. Spectral Doppler is a method of graphically displaying the velocity of blood flow through the analysis of frequency and phase shift of the reflected ultrasound.
The spectral Doppler graph consists of a horizontal baseline, which represents zero velocity/zero frequency shift with vertical velocity marks. The spectral Doppler scale and baseline can be adjusted as needed for each site under interrogation in order to fit the recorded velocities within the confines of the graph.
Blood flow moving toward the transducer is displayed above the baseline, while blood flow moving away from the transducer is placed below the baseline. The spectral Doppler recording demonstrates acceleration rate, peak velocity, and deceleration rate of blood flow. The strength of the returning signal is conveyed through the brightness of the displayed velocities and depends upon beam penetration, signal frequency and the quantity of red blood cells within the sampled area.
With repeated manual increase of PW Doppler scale, the machine will at some point automatically shift to high pulse repetition frequency mode in order to achieve the desired range. (On some machines, High PRF has to be enabled in the setup menus). High PRF Doppler allows higher velocities to be recorded by sampling more than one site along the ultrasound beam, thus combining blood flow from multiple areas into the spectral Doppler graph. High PRF Doppler introduces some uncertainty as to the exact location of the blood flow being sampled and so, the sonographer needs to try to eliminate ambiguity by avoiding the placement of the cursor in line with areas where higher velocities are likely to be present.
Pulmonic Valve (low scale) Pulmonic Valve (high scale)
For the highest accuracy, adjust the velocity profile so that it is not difficult to interpret and measure.
For Doppler and M-Mode recordings the standard horizontal sweep speed is 100mm/sec however, the sweep speed can be adjusted faster to record events that happen quickly and slower to record events that happen slowly. A patient with a heart rate of 45 beats per minute might require a sweep speed of 50mm/sec, while one with tachycardia will record better at 150mm/sec. A chart speed sufficient to include 2 full cardiac cycles on a frozen image assures those interpreting the images that the recorded profile is representative of the baseline physiologic state.
Effects of Doppler Gain, Compression, and Dynamic Range
Doppler Gain, Compression, and Dynamic Range are the primary adjustments used to enhance the appearance of the spectral Doppler display. Too much Gain makes the spectral Doppler too bright and muddies the background. The use of more or less Dynamic Range affects the weak signals, which changes the grayscale, and if over-adjusted, can cause velocities to be overestimated because the border becomes less distinct. While recording spectral Doppler, adjust the dynamic range so that there is a distinct border on the edges of the tracing and still faint speckling in the background. The speckling represents noise (low intensity ultrasound), and assures the reader of the study that important Doppler data has not been omitted.
Ultrasound frequency affects the strength of the returning signal. For adult echocardiography, the spectral Doppler frequency should be at or near 1.9 MHz. Not all ultrasound machines allow direct spectral Doppler frequency adjustment however, the sonographer should check the setup menus to see if it is possible.
Some ultrasound machines offer other Compression and Smoothing adjustments to spectral Doppler, which change contrast and smoothness of the edge of the spectral Doppler recording. Care should be taken not to over-adjust this setting as it can make interpretation and measurement difficult.
As started in an earlier section, colorization of the spectral Doppler recording (shown above), can greatly enhance the visual perception of the image. This adjustment can allow the eye to perceive finer details within the recording that are not visible with basic gray scale.