Afterglow DC-offset calibration in a nuclear imaging system
Method and system are provided for calibrating a nuclear medical imaging apparatus for DC shift caused by gamma event afterglow pulses. Detector responses to weak and to high count rate radiation sources are compared with each other, and a zero correction value is incremented until the detector response is the same for the weak source and the high count rate source. The zero correction value is then stored as a static zero correction value, which multiplies a dynamic zero correction value obtained just prior to the occurrence of a gamma event, in order to remove the effects of DC shifts from the output energy signal Esum.
1. A method of calibrating a nuclear medical imaging apparatus for effects of scintillation pulse afterglow, comprising the steps of:
a) determining a minimum value of an Esum output signal of said apparatus in response to a relatively low count rate radiation source;
b) determining a minimum value of an Esum output signal of said apparatus in response to a relatively high count rate radiation source;
c) comparing the minimum value of the high count rate Esum output signal with the minimum value of the low count rate Esum output signal;
d) incrementing an afterglow correction value used to correct said Esum output signal if the minimum value of the high count rate Esum output signal is greater than the minimum value of the low count rate Esum output signal, and repeating steps a)-c) using the incremented afterglow correction value; and
e) storing a present value of said afterglow correction value as a,calibrated correction value if the minimum value of the high count rate Esum output signal is not greater than the minimum value of the low count rate Esum signal.
2. The method of claim 1 , further comprising the step of processing an Esum output signal using a digital filter, to generate an average minimum value of said Esum output signal.
3. The method of claim 1 , further comprising the step of digitally filtering indicated increments to said afterglow correction value to reduce any statistical noise introduced by the calibration process.
4. The method of claim 1 , wherein said afterglow correction value is used to subtract a DC offset from said Esum output signal.