Synonyms

Duplex/Doppler ultrasound (US); Vascular ultrasound

Definition

A blood flow study is a noninvasive imaging technique which is used to measure blood flow and pressure through arteries and veins, as well as chambers and valves of the heart. Doppler ultrasound (US) may be used to diagnose vascular conditions such as cerebral vasospasm, cerebral or limb thrombosis, vascular stenosis, valvular heart disease, and peripheral vascular and aneurysmal disease. It may also be used to evaluate the condition of bypass grafts and blood flow to transplanted organs.

Current Knowledge

Blood flow studies are used most often to study blood flow particularly in the legs, neck, and brain. Blood flow studies such as Doppler US uses a transducer that sends high-frequency sound waves which bounce off of solid objects including red blood cells. The sound waves are reflected back to the transducer. Moving objects, such as the red blood cells, cause a change in pitch of the sound waves (also known as the “Doppler effect”). These reflected waves are sent to and processed by a computer which translates the waves into pictures or graphs. The images are representative of the flow of blood through the vessel.

There are different types of Doppler US studies currently being utilized by physicians. Continuous wave Doppler is typically used at the bedside and only produces sound from the transducer which the practitioner uses to listen for blockage or stenosis of the vessel – usually a superficial one. Duplex Doppler produces both a picture of the blood vessel and a graph representing the speed and direction of blood flow (hence the name “duplex”). Color Doppler uses a computer to convert the Doppler sounds into colors and overlay those colors on an image of the blood vessel. Power Doppler is more sensitive than color Doppler in detecting blood flow. It combines the results given by color Doppler with those of duplex Doppler. It is commonly used to evaluate the flow of blood through vessels within solid organs.

Transcranial Doppler (TCD) is used to measure blood flow through the brain’s blood vessels. It is becoming more widely used to evaluate for emboli, stenosis, vasospasm, and the risk of stroke.

Limitations to studies include obesity, cardiac arrhythmias, heart disease, and smoking within an hour of study. Studies are done in the inpatient and outpatient settings monitoring.

Variations in blood flow can affect microvascular and macrovascular beds for a wide range of clinical situations. There are new expanding techniques to study blood flow but ultrasound remains the commonly used techniques in hospital an outpatient clinical settings.

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