Definition/Introduction

Color flow imaging is a vascular technology used to assess the vascular anatomy and flow within blood vessels. It relies on ultrasonographic technology to determine the flow direction, volume, and turbulence through the vessels.[1] It provides a color Doppler imaging of the relevant vasculature examined. The operator typically utilizes an ultrasound probe composed of an acoustic lens coupled with a piezoelectric transducer; this probe receives sound waves and transduces them to produce a two-dimensional image.[2] 

This modality is augmented with Doppler technology, which tracks the changes in soundwaves of particles passing the probe, which yields a flow pattern when transduced to an image. The technique enhances the information obtained using color to highlight the direction of blood flow. Typically, red and blue colors are used to highlight the blood flow in one direction or the other regarding the probe's position. The speed of the blood flow is shown with a color scale. Usually, blood flow away from the probe is shown in blue, while blood flow toward the probe is red.

Issues of Concern

Doppler flow imaging relies on ultrasound, and its interpretation can be greatly user-dependent; this is a potential limitation of the study. Modern ultrasounds come equipped with tools to identify flow velocities and volumes. This technique can be incredibly beneficial to the clinician in clinical correlations for conditions such as aortic stenosis, in which ultrasound findings and flow diagnostics are critical to the staging of the disease. Personalized adjustment of ultrasound settings to optimize the image is a skill that is essential to every user of ultrasound and color flow imaging studies.

Clinical Significance

Vascular color flow imaging is heavily utilized in cardiovascular assessments such as echocardiography (see Video. Severe Aortic Stenosis, Doppler Color Flow Imaging). By way of color flow imaging, a clinician can evaluate the patient for valvular dysfunction, including stenosis and regurgitation. Aortic stenosis is a condition that relies heavily on ultrasonographic findings and color flow imaging for diagnosis and staging.[3] Aortic stenosis is described as severe when the aortic valve area is 1.0 squared cm or less, the peak pressure gradient is 40mmHg or greater, and the peak aortic jet velocity is 4.0 m/s or greater.[3][4][5] 

The determination of these factors relies on ultrasound visualization of the valve and Doppler-guided assessment of valvular jet velocity. Vascular color flow imaging can also be utilized in acute aortic syndromes such as Stanford Type A aortic dissections (see Video. Severe Aortic Insufficiency in Stanford Type-A Aortic Dissection, Transthoracic Echocardiography With Doppler Color Flow Imaging). These syndromes can often present with severe aortic insufficiency and significantly turbulent flow across intimal flaps and valves; this can be visualized by changing color patterns on Doppler imaging during echocardiography.[6] Other valvular abnormalities that can be diagnosed with Doppler color flow imaging include mitral regurgitation and tricuspid regurgitation.

Vascular color flow imaging can also provide diagnostic benefit in several noncardiac syndromes. These include reduction of flow in renal arteries seen in renal artery stenosis and backflow of venous blood in the deep veins of the lower extremities due to deep vein thromboses. The advancement of ultrasound technologies has allowed for even better tissue attenuation of neurological tissue. Transcranial Doppler ultrasound flow imaging is now being utilized in neuro-critical care settings to visualize brain flow abnormalities and hematomas.[7] Vascular color flow imaging is used to evaluate cervical carotid arteries for stenosis, occlusion, or reduced flow at the carotid bifurcation and the internal carotid artery. Color flow imaging is also used frequently for obstetric evaluation of maternal-fetal circulation. Doppler color flow imaging has also been utilized to assess vasculitic disorders. Duranoglu et al. demonstrated the use of color flow imaging in visualizing increased intravascular resistance in orbital vessels in ocular Behçet's disease.[8]

The colors become apparent on Doppler imaging when the soundwaves are reflected off passing red blood cells, which are subsequently accepted by the ultrasound probe. The color can appear in varying shades, which reflects the frequency of the soundwaves reflected. In general, a lighter shade signifies a higher frequency.[9] Movement through a unidirectional and uniform blood flow will yield a uniform color pattern through the vessel on ultrasound. Alternatively, an obstructed or disturbed flow due to an intravascular pathology will yield differentials in frequencies captured by the ultrasound and result in varying shades of color through the vessel, reflecting the intravascular pathology.[9]

Flow acquisition in Doppler imaging relies on a shift in wavelength of sound produced by particles in motion. The particles that reflect these sound waves are the erythrocytes. The probe emits a sound wave at a frequency designated as the transmitted frequency, and the probe also accepts sound waves reflected by erythrocytes, designated as the received frequency. These two factors are factored into the Doppler frequency shift, the formula which is F = 2fvcos(a)/c, where F is the Doppler frequency shift, f is the transmitted frequency, v is the flow velocity of blood, a is the insonation angle between the ultrasound beam and the direction of blood flow, and c is the speed of sound. It can be inferred from the equation that the Doppler frequency shift is proportional to the blood's flow velocity. This supports the finding that higher frequencies, evidenced by lighter hues, are indicative of faster flow.[10]

Doppler color flow imaging relies on the acquisition of a good signal via the probe. There are two components of the Doppler signal; the amplitude and phase. An increased amplitude is seen with stronger ultrasonic waves and increased reflectivity of red blood cells, as more sound waves are delivered and received by the probe.[9] Modulation of the pulse repetition frequency is a technique to achieve increased flow sensitivity.[9] The pulse repetition frequency specifies how frequently the equipment emits a pulse and listens for the return signals from the moving red blood cells. By decreasing the frequency, the clinician can optimize the image for slower vascular motion. However, it is important to note that without setting a baseline range of frequency shifts, color aliasing may occur, which may obscure findings on the Doppler examination and display the incorrect velocity of the blood. Images are typically obtained at a frame rate of 18 frames per second for depths up to 4 cm and nine frames per second for depths up to 9 cm, which provides optimal real-time vascular flow assessment.[9] 

Selecting an appropriate probe for visualizing vascular structures and flow is also crucial to obtaining an ideal image. Typical probes used to perform Doppler color flow vary from 5.0 to 7.5 MHz, with higher frequency probes providing greater attenuation of the signal and a sharper image.[9] Higher frequency probes provide better resolution at a short depth, while lower frequency probes produce greater tissue penetration.

Nursing, Allied Health, and Interprofessional Team Interventions

Although vascular Doppler color flow imaging is generally a noninvasive imaging modality, an interdisciplinary team should share the responsibility of minimizing risks of infection, not only to the patient but also to other staff members. The ultrasound probes and machines used in color flow imaging should be sanitized and cleaned after every use to minimize this risk. Modern ultrasound technologies have allowed operators to introduce the probes into the body for tests such as transesophageal echocardiography. The ultrasound probe is introduced into the esophagus and is oriented toward the heart to obtain a closer and more accurate image. While this technique offers a better view of the heart and its internal structures, it is associated with risks that include esophageal damage, oxygen desaturation, and induction of tachyarrhythmias.[11] Team members should be constantly aware of these complications and be prepared to address them should they occur.