Image Quality for Digital Detectors

The maximum spatial resolution on an image is defined
by detector element size and spacing, i.e., pixel pitch.

A system with a 200 µm detector element cannot adequately
record spatial frequencies above 2.5 cycles/mm.
The Nyquist frequency (1/2a, where a is the pixel pitch) characterizes this resolution limit for a given system.

Ferrania FPD's Nyquist frequency is 3.5 cycles/mm (143 µm pixel pitch), well above the spatial frequency of interest for general radiography imaging (between 0.5 and 3.0 cycles/mm).

Spatial resolution and Modulation Transfer Function (MTF)
cannot stand alone as gauges of a given system's image quality.
Instead, a new standard has to be used: the DETECTIVE QUANTUM EFFICIENCY (DQE).

DQE describes the signal-to-noise-ratio (SNR) transfer property of the detector at each spatial frequency.
Higher DQE translates into better image quality for the same radiation dose, or to lower irradiation doses for a given image quality.

DQE is the only parameter to be used to evaluate image quality for digital detectors since it includes the noise effect (intrinsic to all digital detectors), which is ignored by MTF.

FPDs show much better MTFs than other digital systems.

The sampling theory states that a digital detector is only capable of accurately representing frequency components below the Nyquist frequency (f_ny).
fny is equal to 1/2a, where a is the pixel pitch of the detector. Frequency components above the Nyquist frequency will be overturned as spurious signals/artifacts (aliasing, known as Moirè effect) below the f_ny.

The higher the MTF above the f_ny, the higher the spurious signals.

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