The degree that a transducer is finely tuned to specific narrow frequency range. For example: A low Q-value means wide bandwidth and high Q-value means narrow bandwidth.

See also Pulse Inversion Doppler, Narrow Bandwidth, Dead Zone, Ultrasound Phantom.

QB-mode (Quadratic Brightness-mode) images are gray scale images from the quadratic component. QB-mode achieves higher contrast and increased dynamic range than the standard B-mode ultrasound images, without loss in spatial resolution.

Quadrature detection is used in Doppler ultrasound as well as in magnetic resonance imaging and is also called quadrature demodulation or phase quadrature technique. Quadrature detection is the acquisition of Mx and My simultaneously as a function of time by using two separate detector channels. This signal processing method is used for directional Doppler in which the signal reference frequency for the two channels has a phase shift of 1/4 period. The output Doppler signal phase for both channels also depends on the Doppler shift whether positive or negative.
The fast Fourier transform analyzer performs spectral Doppler analysis in ultrasound machines and displays different quadrature Doppler frequencies, when a sample volume cursor is used along time.

Quantison, consisting of air-filled microbubbles with stiff and rigid human serum albumin shells, is an investigational ultrasound contrast agent for the assessment of coronary artery disease.
The stiff shell inhibits the bubbles from oscillating and decrease non-linear scattering. Quantison is capable of long lasting cavity contrast and myocardial opacification using intermittent imaging.

From Sonus Pharmaceuticals
QW7437 (SonoGen) is a perfluorocarbon-based ultrasound contrast agent under development. This anionically charge-stabilized emulsion, similar to EchoGen, has been designed for echocardiographic examinations with greater safety and efficacy.