Repeatable and consistent velocity measurements require that the data be generated in the same manner at each site and time. Any variations from the standard protocol must always be documented for future reference and for repeatability in follow-up studies.
Because the criteria for the disease state are based on velocity spectral waveforms, it is of the utmost importance that the protocol be adhered to as closely as possible.
6.2 Angle:
A constant angle measurement should be maintained between the ultrasound beam and the axis of the vessel for each signal taken for analysis. The axis of the vessel is defined as an imaginary line drawn parallel to the vessel walls. The angle cursor should be placed in this position before recording a spectral waveform. The classifications for disease used in this protocol were validated using a constant angle of 60 degrees to the vessel axis. If a 60-degree angle is not possible, as in a tortuous vessel, then an angle of less than 60 degrees should be used and noted in the patient's record. Caution must be practiced when sampling velocities in a curve, as it is difficult to define the axis of the vessel. Angles above 60 degrees should never be used for any data analysis, as the cosine value changes very rapidly, and small errors in angle measurement cause large errors in the peak velocity calculations. For follow-up studies, the same direction to flow, i.e., the Doppler beam placed cephalad or caudad, in addition to the same angle should be used to ensure consistent and repeatable data.
6.3 Velocity or frequency:
The data can be recorded as either frequency or calculated velocity. The angle should be 60 degrees in either case in order to apply the criteria for classifying disease in the internal carotid artery. A 5 MHz Doppler frequency was used to create the frequency data.
6.4 Sample volume placement:
The sample volume of the pulsed Doppler is placed in the center of the vessel or the flow channel in order to detect the highest velocity and to minimize the spectral broadening at the walls of the artery caused by the normal slowing of blood flow.
6.5 Sample volume size:
The sample volume should be kept as small as possible to detect discrete changes within the blood flow. In addition, a small sample volume will decrease the amount of spectral broadening in the waveform secondary to the normal slowing of velocity at the walls. The sample volume size may be increased when searching for a small jet or a total occlusion.
6.6 Scan planes:
The examination should include both a longitudinal and a transverse scan of the vessels.
Some technologists prefer to begin with the transverse scan for orientation and to identify the appropriate anatomy in the area of the bifurcation. Any measurement of vessel
diameter or plaque assessment should be done in the transverse plane to obtain angles perpendicular to the vessel walls. Doppler velocity waveforms should be generated from the longitudinal plane, which provides the most favorable angles between the Doppler ultrasound beam and the axis of the vessels.
6.7 Documenting stenosis:
The classifications of disease are based on locating the highest peak systolic and end-diastolic velocities or the most disturbed signal. Therefore, it is essential that the sample volume be moved through the area of the stenosis. Color imaging is helpful in pinpointing areas of interest but should not be solely relied on to identify the area of highest velocity. A profile of the stenotic segment should always be documented with spectral waveforms to differentiate a true stenosis absolutely from the apparent increase in the frequency shift caused by an acute Doppler angle measurement as in a tortuosity. The stenosis profile should consist of:
6.7.1 A pre-stenotic waveform, which may or may not have a disturbance in the flow signal, depending on the shape of the lesion. It may also be dampened.
6.7.2 A stenotic waveform taken at the point of maximum velocity or most disturbed flow, from which the classification of the disease will be made.
6.7.3 A post-stenotic waveform taken from the area just distal to the stenosis. This waveform reflects the chaotic nature of the turbulence. It usually displays bidirectional spectral broadening with very decreased velocities. This waveform should be identified with every high-grade stenosis to verify a true velocity increase attributable to stenosis and also to document the distal end of the stenosis. Post-stenotic turbulence should also be seen in the color display as a region of mottled colors distal to the stenosis.
6.7.4 B-Flow mode, a newer technique similar to digital subtraction is available in some newer ultrasound scanners. It is especially useful in locating and detecting very slow moving blood flow and it is not affected by angle of insonation and other common limitations of regular colorflow and pulsed wave Doppler techniques.
B-Mode ("brightness" mode) is an ultrasound technique where structures are displayed as dots. The brightness of these dots depends on the echo intensity at the interface. Each single amplitude corresponds to a brightness, e.g. from 0 to 100, on a scale ranging from white to black.
The direction and delay of the impulse as well as the echo intensity and the localization of the activated crystals of the transducer are registered. In the so-called "real-time
technique" a cross sectional image is generated immediately. Consequently, B-Mode is nothing more than the brightness allocations of the echo reflections.
B-Mode is a proven technology for imaging vascular disease, in particular large vessel dissections and hematomas with poor echo signal. With ultrasound these disease states can be detected and displayed with ease.
Note: Both B-mode imaging and Doppler information should be used to identify major vessels and to evaluate the arterial wall for the presence of any
irregularities (e.g., wall thickness, plaque, aneurysm). Although scan planes are discussed above, the additional importance of combining the transverse and longitudinal views is to enhance the evaluation of the nature, location, extent, and severity of any wall abnormality or plaque in the extracranial carotid system.
7 PROCEDURE: TEST PROTOCOL