# Basic Concepts of Doppler Frequency Spectrum Analysis and Ultrasound Blood Flow Imaging

3 Basic Concepts of Doppler Frequency Spectrum Analysis and Ultrasound Blood Flow Imaging

# Spectrum Analysis

## The Doppler Spectrum

The word spectrum, as derived from Latin, means image. You may think of the Doppler spectrum as an image of the Doppler frequencies generated by moving blood.18 In fact, this image is a graph showing the mixture of Doppler frequencies present in a specified sample of a vessel over a short period of time.13 The key elements of the Doppler spectrum are time, frequency, velocity, and Doppler signal power. These elements are best described in pictorial form; therefore, this information is provided in Figure 3-1, rather than in the text. Please review this figure now, directing particular attention to the four key elements cited previously.

## The Power Spectrum

The Doppler frequency spectrum that you have just reviewed in Figure 3-1 is sometimes called a power spectrum,13 because the power, or strength, of each frequency is shown by the brightness of the pixels. The power of a given frequency shift, in turn, is proportionate to the number of red blood cells producing that frequency shift. If a large number of blood cells are moving at a certain velocity, the corresponding Doppler frequency shift is powerful, and the pixels assigned to that frequency are bright. Conversely, if only a small number of cells are causing a certain frequency shift, the pixels assigned to that frequency are dim. The power spectrum concept is important for understanding power Doppler flow imaging, which is discussed later in this chapter. The concept of the power Doppler spectrum is nicely illustrated in Figure 2-29.

## Frequency Versus Velocity

The echoes that are reflected back to the transducer from moving cells in a sampled blood vessel contain only Doppler frequency shift information; yet the Doppler spectrum often displays both velocity (cm/sec or m/sec) and frequency (kHz) information. How does the instrument convert the Doppler frequency shift to velocity? This conversion occurs when the sonographer “informs” the duplex instrument of the Doppler angle,1,2,9 which is shown in Figure 3-2. If the instrument “knows” the Doppler angle, it can then compute the blood flow velocity via the Doppler formula (see Chapter 2). You may note in this formula that the frequency shift is proportional to the cosine of the Doppler angle, theta. When the operator informs the ultrasound machine of this angle, the frequency shift is proportional to blood flow velocity. Voila! The frequency spectrum becomes a velocity spectrum. A Doppler angle of 60 degrees or less is required to derive accurate frequency and velocity measurements. If the angle is greater than 60°, velocity measurements are unreliable. Although there is greater error in measurements obtained at higher angles, some applications (e.g., carotid examinations) are more easily performed at angles closer to 60°. It is generally recommended that the Doppler angle should be less than or equal to, but not greater than 60° for greatest accuracy.

In spite of potential measurement inaccuracy described in the previous paragraphs, it is desirable to operate the duplex instrument in the velocity mode rather than the frequency mode for two reasons.1,2,9 First, velocity measurements compensate for variations in vessel alignment relative to the skin surface. For instance, the Doppler frequency shift observed in a tortuous internal carotid artery might be radically different from one point to another, but angle-corrected velocity measurements will be similar throughout the vessel, in spite of dramatic changes in vessel orientation relative to the skin. Second, the Doppler frequency shift is inherently linked to the output frequency of the transducer, but velocity measurement is independent of the transducer frequency. For instance, if the output frequency goes from 5 to 10 MHz, the frequency shift is doubled. Imagine the clinical consequences of such frequency changes. If transducers with different frequencies were used to determine stenosis severity, different diagnostic parameters would be needed for each ultrasound transducer (e.g., 3.5, 5, or 7.5 MHz). This problem is eliminated when the instrument converts the “raw” frequency information to velocity data.

# Auditory Spectrum Analysis

## Acceleration

Acceleration is another important flow feature evident in Doppler spectral waveforms.24,25 In most normal situations, flow velocity in an artery accelerates very rapidly in systole, and the peak velocity is reached within a few hundredths of a second after ventricular contraction begins. Rapid flow acceleration produces an almost vertical deflection of the Doppler waveform at the start of systole (Figure 3-5, A). If, however, severe arterial obstruction is present proximal (upstream) to the point of Doppler examination, systolic flow acceleration may be slowed substantially, as shown in Figure 3-5, B and C. Quantitative measurement of acceleration is achieved by measuring the acceleration time and the acceleration rate (index), as illustrated in Figure 3-6. These measurements are used, for example, in evaluating renal artery stenosis.

## Vessel Identity

As you may have already surmised, vessels can be identified by their waveform pulsatility features.1,2,14,2123,26 For example, Doppler waveforms readily differentiate between lower extremity arteries, which are distinctly pulsatile, and veins, which have gently undulant flow features. Doppler waveforms are particularly helpful in distinguishing the internal and external carotid arteries, which have low and moderate pulsatility, respectively. Pulsatility is also of value within the liver for differentiating among the portal veins, hepatic veins, and hepatic arteries, as discussed in Chapter 30.

## Laminar and Disturbed Flow

Blood generally flows through arteries in an orderly way, with blood in the center of the vessel moving faster than the blood at the periphery. This flow pattern is described as laminar, because the movement of blood is in parallel lines.1,2,4,14,15 When flow is laminar, the great majority of blood cells are moving at a uniform speed, and the Doppler spectrum shows a thin line that outlines a clear space called the spectral window (Figure 3-7).*

In disturbed flow, the movement of blood cells is less uniform and orderly than in laminar flow. Disturbed flow is manifested as spectral broadening or filling in of the spectral window.1,2,4,1519 The degree of spectral broadening is proportionate to the severity of the flow disturbance, as illustrated in Figure 3-8. Although disturbed blood flow often indicates vascular disease, it must be recognized that flow disturbances also occur in normal vessels. Kinks, curves, and arterial branching may produce flow disturbances, as illustrated quite vividly in the carotid bulb, where a prominent area of reversed flow is a normal occurrence11,20,21 (Figure 3-9). In addition, a spurious disturbed flow appearance may be created in normal arteries through the use of a large sample volume that encompasses both the slow-flow area near the vessel wall and more rapid flow at the vessel center.1619 The Doppler spectrum, in such cases, appears broadened because both the high-velocity flow at the vessel center and the slow flow at the periphery of the vessel are encompassed by the wide sample volume.

## Volume Flow

Modern duplex instruments are capable of measuring the volume of blood flowing through a vessel (volume flow).1,2,3032 This is done by measuring the average flow velocity across the entire lumen (slow peripheral flow and high central flow) through several cardiac cycles while simultaneously measuring the vessel diameter, which is converted mathematically into cross-sectional area. Knowing the average velocity and the vessel area, it is an easy matter for the Doppler instrument to calculate the blood flow (in mL/min), and this is done automatically by the ultrasound instrument. Although the ability to calculate volume flow has been available on duplex instruments for more than 20 years and measurement accuracy appears satisfactory, issues of reproducibility have kept volume flow measurements from routine use in a clinical setting.

Mar 5, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on Basic Concepts of Doppler Frequency Spectrum Analysis and Ultrasound Blood Flow Imaging
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