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Vascular Technology Color Flow Imaging

Editor: Orlando De Jesus Updated: 10/31/2022 8:19:22 PM

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 2-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 highlight the blood flow in 1 direction or another 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

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Issues of Concern

Doppler flow imaging relies on ultrasound, and its interpretation can be greatly user-dependent; this is a potential study limitation. 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). Through 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 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 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 diagnosed with Doppler color flow imaging include mitral regurgitation and tricuspid regurgitation.

Vascular color flow imaging can also provide diagnostic benefits in several noncardiac syndromes. These include reduced 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 utilized in neuro-critical care settings to visualize brain flow abnormalities and hematomas.[7] Vascular color flow imaging evaluates 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 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, reflecting the frequency of the sound waves. In general, a lighter shade signifies a higher frequency.[9] Movement through a unidirectional and uniform blood flow yields a uniform color pattern through the vessel on ultrasound. Alternatively, an obstructed or disturbed flow due to an intravascular pathology yields differentials in frequencies captured by the ultrasound and results in varying shades of color through the vessel, reflecting the intravascular pathology.[9]

Flow acquisition in Doppler imaging relies on a shift in the 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 2 factors are factored into the Doppler frequency shift. This formula 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, indicate faster flow.[10] Doppler color flow imaging relies on acquiring a good signal via the probe. There are 2 components of the Doppler signal: 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] Modulating 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. Decreasing the frequency allows the clinician to optimize the image for slower vascular motion. However, it is important to note that color aliasing may occur without setting a baseline range of frequency shifts, 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 9 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 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. To minimize this risk, the ultrasound probes and machines used in color flow imaging should be sanitized and cleaned after every use. Modern ultrasound technologies have allowed operators to introduce 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, including 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.

Media


(Click Video to Play)

Severe Aortic Stenosis, Doppler Color Flow Imaging. Severe aortic stenosis is seen on Doppler color flow imaging in echocardiography.

Contributed by M Odak, MD


(Click Video to Play)

Severe Aortic Insufficiency in Stanford Type-A Aortic Dissection, Transthoracic Echocardiography With Doppler Color Flow Imaging. The varied directionality of flow is indicated by the red and blue colors.

Contributed by M Odak, MD

References


[1]

Merritt CR. Doppler color flow imaging. Journal of clinical ultrasound : JCU. 1987 Nov-Dec:15(9):591-7     [PubMed PMID: 2445785]


[2]

Zhao T, Su L, Xia W. Optical Ultrasound Generation and Detection for Intravascular Imaging: A Review. Journal of healthcare engineering. 2018:2018():3182483. doi: 10.1155/2018/3182483. Epub 2018 Apr 30     [PubMed PMID: 29854358]


[3]

Abbas AE, Pibarot P. Hemodynamic characterization of aortic stenosis states. Catheterization and cardiovascular interventions : official journal of the Society for Cardiac Angiography & Interventions. 2019 Apr 1:93(5):1002-1023. doi: 10.1002/ccd.28146. Epub 2019 Feb 21     [PubMed PMID: 30790429]


[4]

Schwartzenberg S, Sagie A, Shapira Y, Monakier D, Yedidya I, Ofek H, Kazum S, Kornowski R, Vaturi M. Echocardiographic Assessment of Aortic Stenosis under Sedation Underestimates Stenosis Severity. Journal of the American Society of Echocardiography : official publication of the American Society of Echocardiography. 2019 Sep:32(9):1051-1057. doi: 10.1016/j.echo.2019.04.422. Epub 2019 Jun 21     [PubMed PMID: 31230781]


[5]

Marquis-Gravel G, Redfors B, Leon MB, Généreux P. Medical Treatment of Aortic Stenosis. Circulation. 2016 Nov 29:134(22):1766-1784     [PubMed PMID: 27895025]


[6]

Ciccone MM, Dentamaro I, Masi F, Carbonara S, Ricci G. Advances in the diagnosis of acute aortic syndromes: Role of imaging techniques. Vascular medicine (London, England). 2016 Jun:21(3):239-50. doi: 10.1177/1358863X16631419. Epub 2016 Mar 8     [PubMed PMID: 26957573]

Level 3 (low-level) evidence

[7]

Blanco P, Abdo-Cuza A. Transcranial Doppler ultrasound in neurocritical care. Journal of ultrasound. 2018 Mar:21(1):1-16. doi: 10.1007/s40477-018-0282-9. Epub 2018 Feb 10     [PubMed PMID: 29429015]


[8]

Duranoğlu Y, Apaydin C, Karaali K, Yücel I, Apaydin A. Color Doppler imaging of the orbital vessels in Behçet's disease. Ophthalmologica. Journal international d'ophtalmologie. International journal of ophthalmology. Zeitschrift fur Augenheilkunde. 2001 Jan-Feb:215(1):8-15     [PubMed PMID: 11125263]


[9]

Foley WD, Erickson SJ. Color Doppler flow imaging. AJR. American journal of roentgenology. 1991 Jan:156(1):3-13     [PubMed PMID: 1898567]


[10]

Terslev L, Diamantopoulos AP, Døhn UM, Schmidt WA, Torp-Pedersen S. Settings and artefacts relevant for Doppler ultrasound in large vessel vasculitis. Arthritis research & therapy. 2017 Jul 20:19(1):167. doi: 10.1186/s13075-017-1374-1. Epub 2017 Jul 20     [PubMed PMID: 28728567]


[11]

Mathur SK, Singh P. Transoesophageal echocardiography related complications. Indian journal of anaesthesia. 2009 Oct:53(5):567-74     [PubMed PMID: 20640107]