[3] This is particularly useful in cardiovascular studies (sonography of the vascular system and heart) and essential in many areas such as determining reverse blood flow in the liver vasculature in portal hypertension.

Any sudden changes in direction of blood flow produces audible sounds on the ultrasound machine.

[3] Upstream and downstream stenosis refers to the location of the stenotic site relative to the ultrasound probe.

Still other laboratories display the Doppler color map in accordance with published data, with red shift representing longer wavelengths (scattered) from blood flowing away from the transducer and blue representing the shorter wavelengths from blood flowing toward the transducer.

Because of this confusion and lack of standards, the sonographer must understand the underlying physics of color Doppler and the physiology of normal and abnormal blood flow in the human body (see Red shift).

These modes of medical imaging conduct a spectral analysis of the acoustic signals they receive and can therefore be classified as methods of active acoustocerebrography.

They are used as tests to help diagnose emboli, stenosis, vasospasm from a subarachnoid hemorrhage (bleeding from a ruptured aneurysm), and other problems.

[7] The equipment used for these tests is becoming increasingly portable, making it possible for a clinician to travel to a hospital, to a doctor's office, or to a nursing home for both inpatient and outpatient studies.

It can evaluate central (abdominal) and peripheral arteries and veins, it helps determine the amount of vascular stenosis (narrowing) or occlusion (complete blockage) within an artery, it assists in ruling out aneurysmal disease, and it is the main aid to rule out thrombotic events.

[citation needed] An alternative to Doppler to visualize vessels is B-flow,[9] which digitally highlights weak flow reflectors (mainly red blood cells) while suppressing the signals from the surrounding stationary tissue.

[11] It is also used to confirm patency of dorsalis pedis arteries when edema or other conditions make manual palpation impractical.

Renal vessels are easily depicted by the color Doppler technique in order to evaluate perfusion.

Velocity measurements allow assessment of cardiac valve areas and function, any abnormal communications between the left and right side of the heart, any leaking of blood through the valves (valvular regurgitation), calculation of the cardiac output and calculation of E/A ratio[14] (a measure of diastolic dysfunction).

Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements.

[citation needed] Doppler ultrasonography can help distinguishing benign from malignant soft tissue lumps.