Approaches to Dead Time Correction

The simplest method for dead time correction involves constructing a look-up table of dead time correction factors derived from decaying source measurements. However, this approach does not account for spatial variations in source distribution that may alter the relative count-rate load in the different sub-systems within the scanner. In practice more accurate dead time correction schemes are constructed in which, where possible, the "live time" (= acquisition time x [1-fractional dead time]) is measured for each subsystem. For those sub-systems where it is impractical to measure the live time, an analytic model incorporating knowledge of the system architecture is constructed and fitted to data obtained from decaying source experiments. The decay correction scheme then consists of applying a series of measured and modelled correction factors to the acquired data.

The live time in a sub-system may be measured in a variety of ways. One possibility is to implement a second circuit parallel to the measurement circuit for which the live time estimate is to be made. Regular

Figure 5.13. Count-rate curves as a function of radioactivity concentration in a phantom. The curves demonstrate the effect of (a) dead time on count-rate linearity and (b) shortening the signal integration time. Note the divergence of the measured true coincidence rate from the ideal trues rate at high radioactivity concentrations and that dead time is reduced when the integration time is shortened. Also note the discontinuity in the count-rate curves at approximately 55 kBq/ml which is caused by the bandwidth limit of the coincidence processing electronics.

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