Noise-equivalent quanta

If there are other sources of noise, SNR, will decrease below the value predirted by the number of interacting quanta so that DQE will fall below tj. From the measurement of SNR it will appear that fewer X-rays have been used to form die image than has actually been die case and DQE is a measure of that apparent lack of efiiciency. In fact, the quantity, SNR which, in the absence of additional noise sources is just the number of X-rays detected, is known as the number of noise-equivalent quanta or NEQ. 

In Chap. 1, the basic concepts of noise in X-ray imaging were introduced and the dependence of one source, referred to as X-ray quantum noise, on the number of X-rays used to form the image, was discussed. In addition, several quantities used to describe imaging performance, namely the signal-to-noise ratio (SNR), the signal-difference-to-noise ratio (SDNR), the detective quantum efficiency (DQE), and the noise equivalent quanta (NEQ) were reviewed. 

In this section, we will review some of the objective measures that are useful for understanding camera system performance. These include the modulation transfer function (MTF, signal to noise ratio (SNR), and an advanced Fourier metric, noise equivalent quanta (NEQ). 

From the point of view of this work the main parameters are specific detectivity and the detector bandwidth. It is especially convenient to use a synthetic parameter obtained by multiplying the specific detectivity with the bandwidth. Specific detectivity unites sensitivity or quantum efficiency with noise equivalent power. Thus, the D f product describes all fundamental limitations of a detector, taking into account both its behavior at high operating frequencies and at low signal intensities. 

LC devices vary between 1 and 0.07 p.p.b. of quinine sulphate as equivalent to the noise. Quinine sulphate is one of the most sensitive compounds for fluorimetric detection (high quantum yield) for a compound with a high sensitivity ir UV photometric detectioh (molar absorptivity 50,000, molar weight 300), current noise -4... 

Figure 2(a) depicts a typical photodiode detector circuit, in this case ac coupled to an amplifier. The equivalent circuit, in Figure 2(b), shows a current source proportional to incident optical power, a parallel dark current, //), a noise current source which represents the shot or photon noise associated with these currents, and a resistance and capacitance attributable to both the detector and the electrical amplifier. For the vacuum or semiconductor diode, the responsivity, DI, determined for a given wavelength, X, and quantum efficiency, rj, is... 

The fact that the Brownian and quantum diffusions are indistinguishable by inteimined measurements perfomied in the configuration space (and this is just the equivalent of experimental techniques by means of which the periodic chemical reactions are investigated) justifies the direct comparison of empirical diffusion coefficients with the Furth s value Dg =h/2M. Assuming that the noise sources behind classical and quantum stochastic behavior are independent the obvious formula for diffusion coefficient may be written as D = DsDq /(Ds +... 

The sensitivity is limited by the smallest change AQ of the photodiode charge which can be still measured, and is influenced by charge leakage from the surface of the target due to thermal dark current, and also by the amplifier noise. The lower limit for detectable signals is about 2000 photoelectrons per video count (which means per photodiode) which is equivalent to about 2500 photons per diode at a quantum efficiency of about 80%. 

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