A crossed (or cross-hatch) grid consists of crossed stripes, also available with rhombic angles. This type of grids is not suitable for the use of a tube angle.

(DQA) This procedure is used by system operators to verify an x-ray system, gamma camera, or CT scanner operation based on relevant image quality parameters like e.g., geometric distortion, x-ray yield, signal to noise ratio. The quality assurance should carry out according to instructions of the manufacturer.

See also Acceptance Checking, Calibration and Calibration Factor.

(dB) A customary logarithmic measure most commonly used (in various ways) for measuring sound. Decibel is a way to express the ratio of two sound intensities: dB=10log10I1/I2 being I1 the reference. If one sound is 1 bel (10 decibel) 'louder' than another, this means the louder sound is 10 times louder than the fainter one. A difference of 20 decibel corresponds to an increase of 10 x 10 or 100 times in intensity.
The intensity of ultrasound is measured in db/cm and decreases during the propagation.
For sound pressure (the pressure exerted by the sound waves) 0 decibel equals 20 micropascal (µPa), and for ultrasonic power 0 decibel sometimes equals 1 picowatt.
See also Phon, Pascal, Logarithms, Proportionality Constant, Watt, Diagnostic Imaging.

Another term for detector, by analogy to broadcast radio receivers. Other terms used for devices of this type include modulators, phase sensitive detector and mixers.

A differential equation expresses a relationship between functions and their derivatives. In MRI, the Bloch equation for example is based on this mathematical function.