Noise equivalent count rate

The image noise is characterized by a parameter called the noise equivalent count rate (NECR) which is given by 

Another type of image noise arises from nonrandom or systematic addition of counts due to imaging devices or procedural artifacts. For example, bladder uptake of 18F-FDG may obscure the lesions in the pelvic area. Various streak type artifacts introduced during reconstruction may be present as noise in the image. 

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Peak noise equivalent count rate (keps) (3D) 94 110 100 165 170 (TrueV) 78 76... 

This simple analysis is appropriate at low count rates when random events are negligible, but at higher rates the advantage must be defined in terms of improvement in statistical noise as embodied in the parameter noise-equivalent counts (NEC). It is found that there is an NEC gain with the 3D mode over the whole range of count rates. For brain studies this varies from about a factor of 5 at low rates (for example, in receptor binding studies with relabelled compounds) to about 3 at high rates (such as are encountered in blood flow studies with i- O-labelled water). 

Thus the standard error of is larger than V(Tj, j) and increases with the rate of random events. Any procedure that reduces randoms, such as better shielding of extraneous radiation or reduction in the coincidence time window (Eqn [4]), will lower this uncertainty. The resultant standard error is conventionally described by the noise equivalent count (NEC) which is defined as follows. The fractional error of is V(T,ot + 2R)/T,<,r If a number (NEC) of hypothetical counts are collected (free of background) the fractional error is (VNEC)/NEC = 1/VNEC. Equating this to the fractional error of gives... 

In the ideal case of a noiseless detector of 100% absorption efficiency the DQE= 1. For a noiseless detector but with an entrance window which attenuates the input and/or with a fractional absorption efficiency, then the DQE decreases from 1 to e where 0< <1. In this case, the time needed to make a measurement to a desired accuracy, q, for a fixed count rate onto the specimen, increases as 1. If, in addition, the detector contributes a noise equivalent to the signal given by tj photons, then the DQE is further reduced according to the relation ... 

Because of the electronic noise characteristics of semiconductor detectors, the maximum count rate of an ED system is usually no more than 30 000-50 000 counts per second, compared with in excess of 10 counts for a WD spectrometer, acting as a single channel analyzer. The reason is the relatively long count time constant that must be selected for ED preamplifier circuits (typically 6 ps), compared with < 1 ps for counter preamplifiers used on WD spectrometers. As a consequence, equivalent analytical precision can only be achieved by extending ED spectrum count times to a significant extent compared to WD. 

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