The divergence is an ultrasound beam characteristic of the far field. The beam divergence angle q, depends on the transducer frequency and diameter according to the following approximation:
sin q 1.22 ld
where l is the wavelength of the ultrasound in the medium of propagation and d is the diameter of the transducer element.

Dobutamine stress echocardiography is a variation of exercise echocardiography to unmask coronary artery blockages. During this test, the heart is stressed by infusing dobutamine. Dobutamine is commonly used to treat patients with low blood pressure, to improve circulation and as a diagnostic testing agent. Dobutamine increases the heart rate and blood pressure and the degree to which the heart contracts. It therefore mimics the effects of exercise for patients who are unable to exercise to a sufficient degree.
The examination starts with the resting transthoracic echocardiogram. After that, dobutamine is infused at a low dose. Following, the dose is increased every three minutes to increase the heart rate and cardiac contractions. At each of these three-minute stages, the heart is scanned using transthoracic echocardiography. An electrocardiogram has to be continuously recorded and blood pressure automatically checked every three minutes.
Disconcerting and stress-related sensations of the patient is to expect during infusion of dobutamine and does not represent any harmful effect. As soon as the infusion is completed, it is discontinued and usually within five to ten minutes the heart rate and blood pressure will return to the levels before starting the exam. Only qualified staff with specific training should perform this test. As for all types of cardiovascular stress testing, all of the necessary equipment and medicines to treat complications must be available in the stress testing room.

The Doppler angle (theta) is the angle of incidence between the ultrasound beam and the estimated flow direction (parallel to the long axis of the vessel). If the beam is parallel to the flowing blood, the Doppler angle is zero, and the determination of flow is most accurate.

See also Beam Vessel Angle, Doppler Effect and Doppler Ultrasound.

Christian Johann Doppler first described the effect of motion of sound sources and the frequency change of the sound to the observer.
Doppler ultrasound uses this effect to detect and measure blood flow, and the major reflector is the red blood cell. Doppler ultrasound depends on the fact that if the reflecting surface is moving in relation to the transducer (blood flowing in a vessel) the frequency of the received ultrasound wave will be different from that of the transmitted wave. If blood cells are moving towards the transducer, they increase the frequency of the returning signal. As cells move away from the transducer, the frequency of the returning signal decreases.

See also Quadrature Detection and Doppler Techniques.

Doppler fluximetry is a method to study the hemodynamic of fetus and placenta. Fluximetry is based on Doppler ultrasound. The resistance of blood flow in the vessels can be determined by quantitative or qualitative analyzing of sound waves depending on the pulsating blood flow.
Doppler fluximetry can be theoretically applied to vessels in every area of the body, but in practice insufficient size of some vessels is a limitation.

See also Doppler Effect.