Page 72 - Libro vascular I

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Chapter 6
Factors that influence the Doppler spectrum
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CHAPTER CONTENTS
Introduction 63 FactorsthatinfluencetheDopplerspectrum 63
Blood flow profile 63
Nonuniform insonation of the vessel 64 Sample volume size 64
Pulse repetition frequency, high-pass filter
and gain 65
Intrinsic spectral broadening 65
Velocity measurements 66
Converting Doppler shift frequencies to velocity measurements 66
Errors in maximum velocity measurements relating to the angle of insonation 67
Errors relating to the direction of flow relative to the vessel walls 68
Errors relating to the out-of-imaging plane angle of insonation 68
Creation of a range of insonation angles by the Doppler ultrasound beam aperture 68
Optimizing the angle of insonation 69 Other potential sources of error in maximum
velocity measurements 70
Measurement of volume flow 70
Sources of error in vessel diameter measurement 70
Waveform analysis 72
Pulsatility index 72
Pourcelot’s resistance index 72 Spectral broadening 72
Pulse wave velocity 72 Subjective interpretation 73
INTRODUCTION
The shape of the Doppler spectrum can provide much useful information about the presence of dis- ease and enables the sonographer to make measure- ments to quantify the degree of vessel narrowing. However, the shape of the spectrum will also depend on other factors, such as the velocity profile of the blood flow being interrogated and how evenly the ultrasound beam insonates the vessel. Factors that relate to the equipment rather than the blood flow can also affect the shape of the waveform. It is important to understand how these factors influ- ence the waveform shape in order to be able to interpret the Doppler waveform. The sonographer should also be aware of potential errors involved in any measurements made.
FACTORS THAT INFLUENCE THE DOPPLER SPECTRUM
Blood flow profile
The Doppler spectrum displays the frequency con- tent of the signal along the vertical axis, with the relative brightness of the display representing the proportion of back-scattered power at each fre- quency, and the time along the horizontal axis. The velocity profiles seen within arteries can be quite complex and will vary over time, as discussed in Chapter 5. The frequency content displayed in the Doppler spectrum will depend on the velocities of the cells present within the blood. If we assume that the vessel is uniformly insonated by the Doppler beam, all the different velocities of blood present