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.

Doppler techniques are dependent on the transducers used. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound energy while the other half is continuously receiving the reflected signals. If the transducer is being used in a pulsed wave mode, the whole transducer is used to send and then receive the returning signals.
Pulsed wave techniques allow the accurate measurement of blood flow at a specific area in the heart and the detection of both velocity and direction. Measurement is performed by timing the reception of the returning signals giving a view of flows at specific depths. The region where flow velocities are measured is called the sample volume. Errors in the accuracy of the information arise if the velocities exceed a certain speed. The highest velocity accurately measured is called the Nyquist limit.

See also Doppler Velocity Signal and Doppler Effect.

A handle or probe contains the transducer elements and is attached to the system via a relatively long coaxial cable. Handles are selected via high voltage (HV) relays. In most ultrasound machines, several different handles are available to be connected to the system, allowing the operator to select the appropriate transducer for optimal imaging.

See also Probe Cleaning, and Transducer Assembly.

The array of elements of microconvex probe is curved with a certain radius. Microconvex probes have a much smaller contact surface, which improves the coupling between the transducer and the skin surface even in complicated areas as the supraclavicular or jugular fossa. Microconvex probes, with large aperture and selection of transmission frequencies are also used in gynecological diagnostic.

See also Transvaginal Echography, Endocavitary Echography and Transrectal Ultrasonography.

The mole is the basic unit of substance.
Definition: The amount of substance that contains as many elementary units as there are atoms in 0.012 kg of carbon-12.

See also Osmole, and Part Per Million.