State of being separated in phase by 90°. Also known as phase quadrature.
A signal derived from the original signal by applying a Hilbert transform or a 90° phase shift. Quadrature signals are used in Doppler processing to extract directional flow information. For example, the quadrature signal of a cosine wave is a sine wave.
A measure of the sharpness of resonance of a system, being 2π times the ratio of the maximum stored energy to the energy dissipated during one cycle. Note: Historically, the letter Q was an arbitrarily chosen symbol to designate the ratio of reactance to resistance of a circuit element. The name “quality factor” was introduced later.
The translation of a range of analog values into discrete fixed levels. For example, values between 0 and 1 into 0.0, 0.25, 0.50, 0.75, and 1.0.
The errors introduced by converting an analog signal to a digital value. These errors can be cumulative: for example, in computing a beam profile at a specific location, such as the geometric focal length, small digitization errors can increase side lobe levels over what they would be using the original analog values.
Related terms: beam profile, digitization (quantization) noise, geometric focal length, side lobe
The addition of a layer in front of a transducer with the thickness of the layer equaling one-fourth of the acoustic wavelength within it. This layer is intended to result in the maximum transfer of acoustic power to and from the transducer, thus providing the greatest sensitivity. For example, the impedance of a single matching layer is the geometric mean of the transducer and tissue impedances for maximum power transfer. In more sophisticated transducers, two or more such layers are sometimes used for improved performance.
Related term: matching layer
A naturally occurring crystal with piezoelectric properties formerly used in the construction of ultrasonic transducers but still used for frequency control purposes.
An adjective meaning somewhat, to some degree, in some manner, resembling, or having a likeness to.
A description of a condition in which change is so slow that for practical purposes it can be considered stationary or constant in value.
A method of elastography that generates an elastogram by applying slow, external mechanical compression to tissue, measuring strain response to assess stiffness differences. Quasistatic elastography is used to evaluate abnormalities such as tumors and fibrosis.
Related terms: elastography, (tissue) sonoelasticity imaging, sonoelastography, transient elastography