BS EN 61262-5:1995 Characteristics of electro-optical X-ray image intensifiers for medical electrical equipment. - Part 5 . Determination of the detective quantum efficiency
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This International Standard applies to electro-optical X-ray image intensifier for medical use, as components of diagnostic X-ray equipment. This International Standard describes a method of determining the detective quantum efficiency (DQE) of X-ray image intensifiers by analysis of the single gamma-ray photon scintillation pulse-height spectrum. The method of this standard applies only to electro-optical X-ray image intensifiers having output radiance decay rates approximately equal to or faster than that of a P-20 phosphor.
Detective quantum efficiency (DQE) is a measure of the imaging quality of a system based on a comparison of the signal-to-noise ratio (SNR) at its output to the SNR at its input. For linear imaging systems, SNR and DQE can be conveniently analysed in terms of sinusoidally varying signals. This standard gives detailed specifications for the determination of the DQE of electro-optical X-ray image intensifiers near zero frequency for both spatial and temporal frequencies. The method employed is scintillation spectrum analysis (SSA).
The source of input radiation is the radionuclide 241Am, which is preferred over an X-ray source because of the radionuclide’s freedom from drift and periodic fluctuations in its output while providing gamma-rays in a radiologically interesting energy range. The signal at the output phosphor of the electro-optical X-ray image intensifier is integrated over an area that is large compared to the image of the source.
Additionally, the SSA method requires integration of essentially all of the optical photon energy at the electro-optical X-ray image intensifier output attributable to the absorption of a single gamma-ray photon. These characteristics result in a measurement near zero for both spatial and temporal frequencies.
This standard specifies the measurement of DQE only near the centre of the entrance field.
Also, the SSA method is not recommended for an electro-optical X-ray image intensifier with output phosphor radiance decay that is considerably slower than that for a P-20 phosphor. Generally, upon absorption of a single gamma-ray photon, the resulting light pulse intensity at 1 ms after the onset of the pulse should be less than 10 % of the peak intensity. This assumes that the time interval between pulse onset and peak intensity is much shorter than 1 ms. Since the SSA method entails integrating the radiance due to individual gamma-ray photons, phosphors having appreciably slower decay would necessitate the use of very low gamma-ray photon count rates which can become comparable to background count rates.
It is recognized that other methods of DQE measurement are available, e.g., pulse-burst analysis, r.m.s. noise analysis, and estimation of quantum absorption from physical characteristics of the electro-optical X-ray image intensifier (see Annex D — Bibliography). Methods that provide measurements of DQE that agree within measurement accuracy with measurements obtained by the method of this standard are equally acceptable.
BS EN 61262-5:1995 Characteristics of electro-optical X-ray image intensifiers for medical electrical equipment. - Part 5 . Determination of the detective quantum efficiency
