Doppler Ultrasound: Pearls and Pitfalls

M. Robert DeJong, Jr, RDMS, RDCS, RVT

   ^*Radiology Technical Manager, Ultrasound, The Russell H. Morgan Department of Radiology and Radiological Sciences, The Johns Hopkins Medical Institutions, Baltimore, Maryland.
   Address correspondence to: M. Robert DeJong, Jr, RDMS, RDCS, RVT, Radiology Technical Manager, Ultrasound, The Russell H. Morgan Department of Radiology and Radiological Sciences, The Johns Hopkins Medical Institutions, Halsted Building, Room B-176A, 600 North Wolfe Street, Baltimore, MD 21287. E-mail: rdejong@jhmi.edu.

Disclosure: Mr DeJong reports having no significant financial or advisory relationships with corporate organizations related to this activity.

ABSTRACT

Most sonographic examinations today require a Doppler component to completely assess the organ or area being evaluated. However, the scanning techniques required to obtain the Doppler information can be different from the scanning techniques used to obtain the grayscale information. Color and power Doppler will visualize vessels that are not seen on the grayscale image and can add physiologic information to the anatomic grayscale image, sometimes enhancing the diagnosis. Without a clear understanding of these techniques, there is potential for misdiagnosis or referral for a more invasive test than necessary. This article discusses several important concepts in Doppler, including proper angle correction techniques, the effects of hemodynamics on the Doppler waveform, and how to optimize the image and Doppler waveform, in an effort to provide sonographers with helpful pearls and methods for avoiding potential pitfalls in diagnosis with Doppler.

Introduction
Most sonographic examinations today require a Doppler component to completely assess the organ or area being evaluated. However, the scanning techniques required to obtain the Doppler information can be different from the scanning techniques used to obtain the grayscale information. It is the lack of understanding of these different scanning techniques that can frustrate the sonographer or vascular technologist, leading to an inadequate examination.

The use of Doppler to determine the degree of arterial stenosis and to evaluate for venous thrombosis is well documented; however, Doppler can also be used to determine the direction of blood flow, differentiate tubular or cystic structures from blood vessels, and investigate blood flow to masses and organs. Color and power Doppler will visualize vessels that are not seen on the grayscale image and can add physiologic information to the anatomic grayscale image, sometimes enhancing the diagnosis. For example, when evaluating the kidney for pyelonephritis, color or power Doppler of the kidney may demonstrate an area of flow void or abnormal flow compatible with an area of focal infection or early abscess formation. To get the most out of Doppler, it is important for the sonographer to understand proper angle correction techniques, the effects of hemodynamics on the Doppler waveform, and how to optimize both the image and Doppler waveform. Having a clear understanding of these concepts can help avoid misdiagnosis or referring the patient for a more invasive test.

Doppler Principles
When evaluating red blood cells (RBCs) with Doppler, the velocity of the RBCs are determined from the Doppler equation as follows:

f_D = 2 f_t v cos θ/c

The Doppler equation is rearranged as follows to solve for the velocity of the RBCs:

v = f_D c/2 f_t cos θ

In this equation, f_D is the Doppler shift, f_t is the transmitted frequency, f_r is the received frequency, cosine (cos) θ is the angle between the sound beam and blood flow, v is the velocity of the RBCs, and c is the average velocity of sound in soft tissue (1540 m/sec).¹

The ultrasound machine knows the variables of the Doppler equation except for cos θ. This angle between the sound beam and the blood flow is calculated with the angle correction control. For correct angle correction techniques the cursor should be placed parallel to the walls of the vessel being investigated. The ultrasound machine then enters the value of cos θ into the Doppler equation to determine the velocity scale used to measure the Doppler signal. If the Doppler angle is not measured properly, it will result in erroneous measurements, which can lead to the calculation of underestimated, overestimated, or even normal values in diseased vessels or abnormal values in normal vessels.

Doppler signals should be obtained at angles less than 60° for proper velocity measurements. This is because the values for cos θ change rapidly at angles greater than 60° and even small errors in adjusting this control can lead to significant measurement errors.¹ When angles of greater than 60° are obtained, the transducer or patient needs to be repositioned so that the Doppler angle is less than 60°. Sometimes, because of the patient's anatomy, this is not possible. Whenever angles greater than 60° must be used, this should be noted and care should be taken in using these velocity measurements.

The angle correction control does not affect the Doppler tracing nor will it improve poor or weak tracings. Setting this control to 60° will not cause the sound beam to be automatically adjusted to an angle of 60° to the blood flow; it only supplies the mathematical information needed to allow the machine to calculate the appropriate velocity scales. Angle correction is mandatory whenever velocity measurements are required. All sonographers and vascular technologists must employ the same angle correction techniques in the same department. Some measurements, such as the resistive index, are angle independent and therefore angle correction is not required. According to the Intersocietal Commission on the Accreditation of Vascular Laboratories (ICAVL), improper angle correction techniques are a common reason for delays in vascular accreditation.²

Hemodynamics
The sonographer and vascular technologist must understand hemodynamics because it influences the Doppler spectral signal. Remember that the signal is influenced by any changes in normal blood flow anywhere along its path, from the heart to the organ and back to the heart. Changes in signal type may not always be obvious because they can be caused by flow disturbances downstream or further upstream.

Normal arterial waveforms are described as either low-resistance or high-resistance signals. Low-resistance signals are described as having forward flow in diastole and are indicative of low pressure distally (ie, the capillaries remain open to receive blood in diastole). These types of signals can be found in the arteries of very metabolic organs, such as the internal carotid and renal arteries. High-resistance signals or waveforms demonstrate little to no flow in diastolic or a triphasic signal that has reversed flow in early diastole. A high-resistance waveform is indicative of high pressure distally and indicates that the capillaries are narrowing or closed during diastole. Normal examples of high-resistance signals include the external carotid and femoral arteries.³

Venous waveforms also have distinctive normal patterns. Venous flow is either described as continuous or pulsatile flow. Continuous flow is characterized by a continual flow in 1 direction with respiratory variations. Normal examples include flow in the femoral and portal veins. Pulsatile flow occurs in veins that are close to the heart. The Doppler spectral waveform demonstrates flow both above and below the baseline. Normal examples include flow in the subclavian and hepatic veins.³

Various diseases or pathologies can change the appearance of the normal Doppler signal. A normally low-resistive signal may become a high-resistive signal if there is a venous thrombosis, parenchymal disease, or an upstream occlusion. For example, a renal artery may become a high-resistive signal because of renal vein thrombosis or renal failure. A high-resistive signal may become a low-resistive signal because of hyperemia or small-vessel disease. For example, a femoral artery may become a low-resistive signal due to normal post-exercise changes or in small-vessel diseases, such as diabetes. Venous signals can also be affected by pathology. For example, the entire venous system can become pulsatile in right-sided heart failure, or the respiratory variations seen in continuous flow venous signals may become dampened or absent if there is an obstruction upstream or downstream. A pulsatile vein may become continuous if there is high organ parencymal pressure or an obstruction downstream. Also, the hepatic veins could exhibit continuous flow in the presence of cirrhosis or the subclavian vein can lose its pulsatility and become continuous when there is an obstruction of the brachiocephalic vein.⁴

Important Concept 1

It is very important to explain any changes from the normal expected signal type. Therefore, as the sonographer or the vascular technologist is scanning, he continuously needs to think about: (1) where the blood flow came from; (2) where the blood flow is now; and (3) where the blood flow is going. For instance, if there is an occlusive clot in the femoral vein, the flow in the iliac vein may lose its respiratory variations or phasicity (coming from). When obtaining a Doppler waveform after a high-grade arterial stenosis the waveform may be turbulent (now). The common femoral artery will have a low-resistance signal if there is a flow restricting lesion in the superficial femoral artery (going to). These examples illustrate that the disease causing the abnormal Doppler signal may not be at the location of the Doppler sample volume.⁵

For example, a normal common carotid artery signal should have diastolic flow. This is because the blood that is going to the brain through the internal carotid artery will also influence the common carotid artery signal by giving it some diastolic flow. However, if there is no diastolic flow, then the signal only reflects the external carotid high-resistance flow. With this type of signal, the sonographer should suspect an internal carotid artery occlusion because the internal carotid artery component of the common carotid artery is missing. If there was an internal carotid artery stenosis, there should still be diastolic flow in the common carotid artery. This abnormal signal should alert the sonographer that there is a possible occlusion downstream. If the extracranial portion of the internal carotid artery is seen and also displays the same high-resistance flow pattern, then an intracranial occlusion should be suspected. Remember, we do not always have to see the actual disease but should be suspicious that something is wrong when we encounter abnormal Doppler signals.⁵

It is also important to compare flow states in each limb. If 1 venous limb signal is different from the other, this might imply that there is a thrombus on that side even if it is not visible by ultrasound. If both sides have abnormal Doppler signals, this is indicative of a central venous obstruction. The subclavian veins illustrate these examples. If the right subclavian vein is pulsatile but the left subclavian vein is continuous, then there is a high suspicion of a clot in the left innominate vein. Every attempt should be made to try to see the clot, although this is not always possible. However, if both the right and left subclavian veins display a continuous pattern, then a clot in the superior vena cava would be suspected. If there were not a contralateral signal obtained for comparison then it would be hard to differentiate between a unilateral obstruction in the innominate vein versus a central obstruction in the superior vena cava. Failure to obtain the contralateral venous signal in the lower and upper extremities is another common cause of a delay in vascular accreditation according to the ICAVL.²

Important Concept 2

While scanning, it is important to remember to think about where the blood flow came from. If an abnormal signal is obtained, think about whether there is disease before this point. If so, is it documented? It might be a thrombosis or a stenosis. Continue to think about where the flow is now and where the Doppler signal is being obtained. Is there any evidence of disease in the same image, for example, parenchymal disease, a clot, or a high-grade stenosis? Finally, consider where the flow is headed, especially if the vessel itself, so far appears normal. Where the flow is headed greatly influences the signal. Is there a stenosis, occlusion, or thrombosis upstream? If so, can it be documented? To obtain a diagnostic examination, the sonographer needs to constantly consider the effects of hemodynamics on the signal. Ignoring the abnormal signal is doing a disservice to the patient and may cause them to undergo more invasive testing. Again, it may be as simple as changing the transducer type to see more of the vessel or thinking about potential causes for this abnormal signal and discussing them with the interpreting physician.

Doppler Controls
The sonographer must have a good understanding of the various controls used in obtaining the Doppler information. These controls include but are not limited to: color gain, color priority, color velocity scale, color wall filters, Doppler spectral gain, Doppler wall filters, scale, baseline, and output power. The sonographer should also know how to remove the color information from the grayscale image, change color maps, adjust focal zones, activate measurements, and properly measure signals. Controls and their names may vary with different manufacturers of ultrasound machines but their function and how they affect the image are the same.

The best way to learn all these controls is to turn the control to its maximum value and observe any change in the quality and strength of the Doppler signal. Next, the control should be stepped down in small increments to its minimal value, observing the effects on the Doppler signal with each change. This process should be repeated for various frequencies and transducer types, because different transducer types as well as different frequencies can affect the ease at which the signal is obtained and the quality of the Doppler signal. For example, a 5-MHz curved linear transducer may have deeper Doppler sensitivity than a 5-MHz linear array. It is also important to understand the interaction between controls. For example, decreasing the color velocity scale may cause an increase in the amount of displayed color signal, causing the image to appear to be overgained. The next few sections will provide a more in-depth discussion of some of these controls.

Color Velocity Scale and Color Gain
Vessel fill-in and color sensitivity can be improved by properly using the color velocity scale. The color velocity scale control affects the pulse repetition frequency (PRF). This control acts like a filter, concentrating on the range of velocities selected. If this control is set too low, color aliasing will occur and will make the determination of flow direction difficult. If this control is set too high, color may not be displayed in a vessel, thus leading to an incorrect diagnosis of vessel occlusion (Figures 1–3).

A good starting point for the color velocity scale control is 10 to 20 cm/sec for venous flow and 20 to 30 cm/sec for arterial flow. Values need to be adjusted according to the patient and his cardiac output. For example, a patient with a strong cardiac output may need to have his color velocity set to values greater than 20 cm/sec to avoid color aliasing in the venous system whereas a patient with a poor cardiac ejection fraction may need the color velocity scale levels lowered to less than 20 cm/sec in the artery so that flow is seen in the vessel lumen. This control should be set to its lowest values, typically less than 10 cm/sec, when looking for trickle flow or in slow flow states. However, it is important to know your system because when the color velocity scale control is set at its lowest setting, the machine may only display very slow states. Again, this may lead to a misdiagnosis of venous thrombosis. If no flow is detected, then the control should be increased 1 step at a time until a value greater than 10 cm/sec is reached. Increasing the color velocity scale control to higher values (eg, >60–70 cm/sec) can be helpful to accentuate high velocities, such as an area of stenosis, by eliminating venous and normal flow, therefore only displaying the high-velocity stenotic flow. Increasing the color velocity scale control to purposely suppress venous flow may be helpful in some instances (eg, to obtain a better color Doppler image of a renal artery).

Important Concept 3

The color velocity scale control can greatly affect the image, and it is important to adjust it as needed during the Doppler examination. For example, a portal vein thrombosis should not be diagnosed without images obtained from the same location with different color velocity scale or PRF settings, including 1 image obtained at a value less than 10 cm/sec and 1 obtained from the lowest setting. It is also important to understand that as the color velocity scale is adjusted, the overall color gain also needs to be adjusted. When the PRF is lowered, it may be necessary to decrease the color gain. More importantly, when the PRF is raised, it is important to ensure that the color gain is increased as needed and set properly so that a misdiagnosis of an occlusion or thrombus does not occur. In the author's opinion, this is the most dangerous control on an ultrasound unit because it can easily lead the interpreter to the wrong conclusion, with the second most dangerous control being the angle correction control. When these controls are not used properly, it can lead to false-positive interpretation of the examination.

One overlooked control is the color gain. Just because the grayscale gain has been properly adjusted does not mean that the color gain will be properly set. When activating the color control, if there is not enough color in the vessels, the color gain should be increased until color noise, color background speckles, appear. Then turn the gain down until they go away. If there is still not enough color fill-in, then the color velocity scale should be lowered. Note that there is a relationship between the color gain and color velocity scale. Typically, if the color velocity scale is lowered, then the color gain may also need to be reduced. If the color velocity scale is increased, the color gain usually needs to be increased.

Color Wall Filter
Initial scanning should be performed with the color wall filter at its lowest setting because it will affect the color displayed inside the vessel. Increasing the setting of the color wall filter will filter out or eliminate low-velocity echoes from the lumen, starting with the slower velocities next to the vessel wall. This can lead to the misdiagnosis of a thrombosis or plaque inside the vessel. This can be a real problem in the carotid bulb, where an increased color wall filter can cause the normal reversed flow component of the bulb not to be displayed. This can lead to the misdiagnosis of a plaque in the carotid bulb because this is an area of frequent plaque formation. If the wall filter is set too high, it can eliminate flow altogether inside the vessel, suggesting vessel thrombosis or occlusion. Low-level artifactual echoes from tissue vibration or vessel wall pulsation may be eliminated from the image by increasing the color wall filter. Again, caution should be used when using this control because it does cause real information not to be displayed. Therefore, before a small clot or thrombus is diagnosed, the color wall filter setting should be decreased and the affect on the image evaluated. Most machines increase the amount of black between the 2 color bars to help the sonographer easily identify whether there is filtering applied to the color Doppler image.

Presets and Other Color Doppler Controls
It is important to have a good understanding of these 3 color Doppler controls: color gain, color velocity scale, and color wall filter. Not having these controls set properly may affect the image and lead to a misdiagnosis. Presets on the ultrasound machine are helpful in quickly having a variety of controls adjusted to the average flow state of the organ selected. However, presets are not perfect and further adjusting of these controls are needed to optimize the ultrasound machine for the flow state of the patient or the flow state caused by any pathology. Using the correct preset will also optimize the machine for the type of flow and may result in not only a better image but an image that is quicker to obtain, as there are fewer, if any, controls that may need to be adjusted.

There are other controls on the ultrasound unit that can affect the color Doppler image and these may vary among the different manufacturers. Although these controls may not affect the image as drastically as the color controls discussed earlier in this article, it is important to learn how these controls can also affect the color Doppler and to know when they need to be adjusted. For example, if the machine has a color priority control, it may be helpful to increase the value to allow the color pixel to be displayed as opposed to the grayscale pixel. This can overwrite "noise" in a vessel lumen, improving the color fill-in. However, it could also cause color to overwrite a soft thrombus causing the thrombus to potentially be missed. This may be important in a vessel, such as the portal vein, where compression maneuvers cannot be performed. Lowering the level to give priority to the grayscale pixel may help to see a catheter in a vessel or a small plaque that was hidden by the color bleeding into the grayscale. Other controls include color map selection, color baseline, and how to remove the color overlay. The last can be very helpful to show the pathology with and without the color Doppler information.

To summarize, if there is poor color fill-in of a vessel, the first control to look at is the color velocity scale setting. Is it within a range for that type of blood flow and the patient's cardiac output? If the setting looks to be fine, then increase the color gain. If in order to have good fill-in there is a lot of color speckle or noise in the background, then the output power will need to be increased. Once the output power is at a proper setting then the color velocity scale should be re-evaluated and the color gain correctly set.

Power Doppler is an adjunct to color Doppler and can be very helpful for detecting slow flow states, as well as for evaluating flow at near perpendicular angles. Power Doppler differs from color Doppler in that the ultrasound system evaluates the intensity of the returning echoes that have a Doppler shift to them. Therefore, if there is no Doppler shift of the returning echoes then the processing component does not process them. The advantages of power Doppler are that it is more sensitive then color Doppler, not as angle dependent, and has increased spatial resolution. The disadvantages include the inability to determine flow direction, although newer versions do have that capability, and it is more prone to color artifacts.

Spectral Doppler Controls
Just as with color Doppler, activating the spectral Doppler does not ensure that the Doppler gain is set at an appropriate level. If the signal can be heard audibly, but is not seen on the screen, start by increasing the Doppler gain. If the signal is still not well displayed, the output power may need to be increased because the signal strength from the Doppler signals is much lower than the signal strength from the grayscale echoes. Remember, in Doppler it is very important to use your ears more than your eyes to listen for signals because you will hear the signal or a change in its frequency before you see it on the monitor. Understanding this is important when interrogating a very high-grade stenosis because it may be that both systole and diastole are aliasing and the spectral display looks like noise. However, with careful listening, you can hear the high-pitched signal. When this occurs, it may be necessary to try 1 of the following techniques to display some aspect of the signal for documentation purposes:
   1. Use continuous wave or high PRF Doppler;
   2. Decrease the transducer frequency; or
   3. Increase the Doppler angle so that it is closer to 90°, thus a lower Doppler shift is calculated.

Typically in this setting of very high-grade stenosis, it is not important to have the absolute velocity number because the true value will exceed the number associated with that category. For example, the report may read that both the systolic and diastolic velocities in the internal carotid artery exceed 125 cm/sec, which is compatible with a hemodynamically significant stenosis of greater than x%, depending on the chart used. In these cases, it is not so much the systolic peak that needs to be seen, but the end diastolic flow so that the interpreter can see that a signal is present and that the display is not just noise. If the interpreter is available, it may be helpful to have him come into the room and hear the signal, thus he can better understand the spectral Doppler signal. Please note, that in these instances the Doppler scale has been set to the maximum scale, the baseline is set at the bottom, and diastolic aliasing is still occurring.

In an arterial signal, it may be important to evaluate the signal for a clear systolic window to determine whether there is turbulent flow. Decreasing the Doppler gain to eliminate any background noise in the tracing may be necessary to evaluate for this. It is also important to make sure that the sample volume is the appropriate size and in the center of the vessel. If the sample volume is too large, it will bring in the slower flow near the vessel wall causing the spectral waveform to fill in. Remember that small vessels do not have a systolic window because the sample volume will cover the entire width of the vessel. Sometimes, to see a spectral signal from an especially weak signal, it may be necessary to increase the Doppler gain and display a "noisy" spectral signal. If the Doppler spectrum is weak or noisy, freeze the 2-dimensional image. This will improve the appearance of the spectrum by allowing the machine to concentrate on obtaining the Doppler information and may clean up the systolic window and sharpen the signal.

Important Concept 4

Optimizing the signal-to-noise ratio is important for good Doppler signals. This is accomplished by using the correct transducer frequency, the proper output power, proper gain settings for all modalities, and the correct placement of the focal zone.

The Doppler signals returning to the transducer are being attenuated at the same rate as the grayscale echoes. However, the intensity of the reflected signals from the RBCs is much weaker in intensity and may not have enough energy to return to the transducer. Increasing the output power of the sound beam will result in stronger returning echoes by increasing the intensity of the initial sound beam. However, this will also increase the acoustic exposure to the patient, and in some cases, guidelines vary depending on the patient. For obstetrical patients, it is important to stay within the US Food and Drug Administration (FDA) guidelines of acoustical exposure. Choosing the proper preset on the ultrasound machine will keep the acoustic intensities within these FDA guidelines. However, when using transcranial Doppler (TCD), the presets will elevate the acoustic output past the recommended power levels set by the FDA for all examinations except TCD examinations. This is because additional energy is needed to penetrate the skull. It is important to read and follow the warning that is displayed when activating the TCD preset. This preset should only be used for TCD scanning through intact bone; it should not to be used with transorbital windows or through surgical openings. It is important that with this high level of acoustic energy the sound beam passes through the bone to decrease the acoustical energy that reaches the brain. Remember that sound energy is used to shatter kidney stones and heel spurs, as well as heat tissue in therapeutic settings; therefore, it can be destructive in the wrong setting and with high enough output levels.

Doppler Tips and Techniques
Technically, the most difficult aspect of the Doppler aspect of the examination is overcoming the scanning techniques used in grayscale imaging. A good Doppler signal will not always accompany an aesthetically appealing grayscale image. For resolution purposes of the grayscale image, it is necessary to use perpendicular scan planes and high-frequency transducers. However, when obtaining the Doppler aspect of the examination, scan planes of less then 60° to blood flow are required to obtain the best Doppler signal. A lower frequency transducer may be required to counteract the effects of attenuation on the weak intensity Doppler signals to improve the Doppler signals. This may require that 2 sets of images be obtained—1 for the grayscale image and 1 for the Doppler waveform.

As mentioned earlier, because of attenuation, it may be necessary to image first and then change to a lower frequency transducer to obtain the Doppler signals. This is especially true in pediatric patients, in whom a 5-MHz transducer is routinely used for acquiring the grayscale image, but it may not easily elicit Doppler signals. This may require the sonographer to change to a lower frequency transducer to obtain Doppler signals, resulting in a good Doppler tracing but a poor resolution image. Fortunately, current transducer technology takes advantage of the wide bandwidth emitted by the piezoelectric crystal of the transducer. This allows the same transducer to image at the center frequency and Doppler at a lower frequency. For example, a 5-MHz transducer will automatically use the lower frequencies of its bandwidth, whenever Doppler or color Doppler is activated, to help improve the Doppler signal. Some machines will have a control that needs to be activated to downshift the transducer into its lower frequency domain. Other machines may do this automatically. Sometimes it may still be necessary to change to a lower frequency transducer to elicit good Doppler signals.

Spectral, color, and power Doppler signals can be obtained in longitudinal, coronal, oblique, or transverse planes. To obtain the best Doppler signals, it is necessary to optimize the beam to vessel angle. To optimize this angle it may be necessary to change the position of the patient and/or transducer. In vessels where flow is perpendicular to the sound beam, it is necessary to create good Doppler angles by using "heel-toe" scanning techniques. This is accomplished by angling the transducer in a superior or inferior manner, causing the vessel to go from a parallel course across the screen to an oblique course, creating a 45° to 60° angle with the sound beam. With linear array transducers, the Doppler beam can be automatically steered by the ultrasound machine into less than 60° angles. With curved linear array transducers, it may be necessary to obtain the Doppler spectral waveform from the edge of the image. When scanning in a transverse plane, remember to angle the probe to be less than 60°, either toward the head or toward the feet. For example, if a patient has massive abdominal lymph adenopathy, one would use this technique to demonstrate the relationship of the inferior vena cava and aorta to the nodes. However, it is not recommended to obtain spectral Doppler signals from a transverse view that needs their waveforms measured, unless the transverse plan is the long axis of the vessel, as is the case with the renal vessels. If one needed to verify a vessel in a transverse plan (eg, to differentiate between the internal carotid artery and the external carotid artery), then it is acceptable to obtain a spectral Doppler signal just for vessel identification. Remember, before labeling a vessel, it should be identified by its Doppler signal characteristics. Anatomical variations do exist because not everyone's anatomy is where it should be (ie, not every internal carotid artery is the lateral vessel or more posterior one) and labeling vessels incorrectly can lead to confusion later in the examination.

For abdominal Doppler examinations, the patient will need to hold his breath during the acquisition of the Doppler signals. Having the patient hold his breath while adjusting all the imaging and Doppler controls may tire the patient out quickly, making it difficult for the patient to hold his breath for the actual examination. Thus, it is helpful to set all the controls with the patient breathing normally. If a patient has difficulty holding his breath or is on a respirator, remember that 1 or 2 beats are just as diagnostic as a full tracing of 4 or more beats. Only 1 full beat and the start of systole of the next beat is needed to determine flow presence, signal type, direction of flow, and perform any measurements needed. Do not frustrate yourself or, just as important, your patient, trying to obtain a full strip of Doppler signals.

Making the Pieces of the Puzzle Fit
The relationship between the various modalities used to create the duplex image need to support each other—that is, the grayscale image, the color or power Doppler information, and the spectral Doppler signal all need to lead to the same answer. If 1 of these 3 does not support the diagnosis suggested by the other 2 modalities, then the reason needs to be explained. A good example is when an increased velocity is obtained in the internal carotid artery. Taken by itself it may indicate a moderate stenosis. However, if the grayscale and color/power Doppler images only demonstrate minimal stenosis, the conflict needs to be resolved. This increased velocity of the internal carotid artery can be explained by noting the high velocity of the common carotid artery. When an internal carotid artery/common carotid artery ratio is calculated, the ratio agrees with the grayscale and color/power Doppler findings. More problematic is the patient with a low flow state. In these patients, one needs to evaluate the whole picture because the patient's systolic velocities may never exceed normal velocities despite the fact that there is a high-grade stenosis. In these cases, it is important to note the amount of plaque, the amount of the remaining vessel lumen, and the presence of post-stenotic turbulence.⁴

Conclusions
Doppler examinations can be very rewarding, as it provides a closer assessment of an organ or area. With a clear understanding of the techniques used to obtain Doppler information, sonographers can minimize the risk of misdiagnosis or unnecessary invasive testing. Numerous textbooks and articles teach the diagnostic information and criteria of various Doppler examinations; the information in this article is meant to compliment those textbooks by providing technical information to obtain diagnostic Doppler examinations. As with any aspect of ultrasound, it requires a dedication to learning and refining these skills.

References
1. Taylor G, Burns P, Wells PNT. Clinical Applications of Doppler Ultrasound. 2nd ed. New York, NY: Raven Press; 1995.

2. The Intersocietal Commission for the Accreditation of Vascular Laboratories Web site. Available at: http://www.icavl.org/. Accessed December 2, 2008.

3. Rumack CM, Wilson SR, Charboneau JW. Diagnostic Ultrasound. 3rd ed. St. Louis, Mo: Mosby Yearbook, Inc.; 2004.

4. Zwiebel WJ, Pellerito J, ed. Introduction to Vascular Sonography. 5th ed. Philadelphia, PA: W.B. Saunders; 2004.

5. Polak JF. Peripheral Vascular Sonography: A Practical Guide. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004.