Advanced temperature compensation and control circuit for single photon counters

Abstract

A PET scanner includes a ring of detector modules encircling an imaging region. Each of the detector modules includes one or more sensor avalanche photodiodes (APDs) that are biased in a breakdown region in a Geiger mode. The sensor APDs output pulses in response to light from a scintillator corresponding to incident photons. A reference APD also biased in a breakdown region in a Geiger mode is optically shielded from light and outputs a voltage that is measured by an analog to digital converter. Based on the measurement, a bias control feedback loop directs a variable voltage generator to adjust a bias voltage applied to the APDs such that a difference between a voltage of a breakdown pulse and a preselected logic voltage level is minimized.

Having thus described the preferred embodiments, the invention is now claimed to be: 
     
         1 . A method of maintaining a signal to noise ratio substantially constant in an imaging detector, the method comprising:
 measuring a breakdown voltage of one or more photodiodes in the detector;   comparing the breakdown voltage to a preselected voltage level;   determining a difference between the preselected voltage level and the measured breakdown voltage;   adjusting a temperature in the detector based on the difference.   
     
     
         2 . A radiation detector for use in imaging comprising:
 a plurality of avalanche photodiodes;   a biasing circuit configured to bias the photodiodes to operate in a Geiger mode in which the photodiodes breakdown in response to receiving radiation generating an output pulse and the biasing circuit being configured to bias each photodiode back to the Geiger mode after each breakdown;   a first cooling element thermally coupled to the photodiodes and configured to remove heat from the photodiodes;   a control circuit configured to measure the breakdowns and control the first cooling element in accordance with the measured breakdowns.   
     
     
         3 . The detector module as set forth in  claim 2 , wherein at least one of the diodes is a reference photodiode which is shielded from light, the control circuit measuring the breakdowns of the at least one reference photodiode. 
     
     
         4 . The detector module as set forth in  claim 3 , wherein the control circuit includes a counter configured to count the output pulses generated by the at least one reference photodiode, the control circuit controlling the first cooling element in accordance with the count. 
     
     
         5 . The detector module as set forth in  claim 4 , wherein the control circuit is configured to control the first cooling element in accordance with a rate at which the output pulses are counted by the counter. 
     
     
         6 . The detector module as set forth in  claim 4 , wherein the biasing circuit applies a recharge pulse to bias each photodiode back to the Geiger mode after each breakdown; and
 wherein the counter is configured to count the recharge pulses.   
     
     
         7 . The detector module as set forth in  claim 3 , wherein the control circuit is further configured to measure a breakdown voltage across the at least one reference photodiode and adjust the bias voltage of the photodiodes to a predetermined characteristic logic voltage level. 
     
     
         8 . The detector module as set forth in  claim 2 , wherein the first cooling element includes a Peltier cooling element which is electrically controlled by the controller and further including:
 a second cooling element which transfers heat from the Peltier cooling element to ambient surroundings.   
     
     
         9 . The detector module as set forth in  claim 2 , wherein the control circuit is configured to control the first cooling element in accordance with a rate of the measured breakdowns. 
     
     
         10 . The detector module as set forth in  claim 2 , wherein the control circuit is configured to measure a voltage of the breakdowns and control the first cooling element in accordance with the measured breakdown voltages. 
     
     
         11 . An imaging apparatus comprising:
 a gantry defining an imaging region;   a subject support configured to support a subject in the imaging region;   a detector array that includes a plurality of detector modules as set forth in  claim 2 ;   an event verification processor that analyzes detected radiation to determine whether the detected radiation originated from valid events;   a reconstruction processor configured to reconstruct the valid events into an image representation.   
     
     
         12 . A method of controlling a detector array for use imaging, the method comprising:
 biasing a plurality of avalanche photodiodes to operate in a Geiger mode in which the photodiodes breakdown in response to receiving radiation generating an output pulse;   after each breakdown, biasing each photodiode back to the Geiger mode;   cooling the photodiodes;   measuring the breakdowns and controlling the cooling in accordance with the measured breakdowns.   
     
     
         13 . The method as set forth in  claim 12 , further including shielding at least one of the photodiodes from light; and
 wherein measuring the breakdowns measures the breakdowns of the at least one shielded photodiode.   
     
     
         14 . The method as set forth in  claim 13 , wherein measuring the breakdowns includes counting the output pulses generated by the at least one shielded photodiode and controlling the cooling in accordance with the count. 
     
     
         15 . The method as set forth in  claim 14 , wherein the cooling is controlled in accordance with a rate at which the output pulses from the at least one shielded photodiode are counted. 
     
     
         16 . The method as set forth in  claim 14 , wherein biasing the photodiodes back to the Geiger mode after each breakdown includes applying a recharge pulse and wherein the counting includes counting the recharge pulses. 
     
     
         17 . The method as set forth in  claim 12 , wherein the temperature is controlled in accordance with a rate of the measured breakdowns. 
     
     
         18 . The method as set forth in  claim 12 , further including:
 measuring a voltage of the breakdowns and wherein the cooling is adjusted in accordance with the measured breakdown voltage.   
     
     
         19 . The method as set forth in  claim 13 , further including:
 measuring a breakdown voltage across the shielded photodiode and adjusting the biasing of the photodiodes to a predetermined characteristic logic voltage level.   
     
     
         20 . The method as set forth in  claim 12 , wherein the cooling includes electrically controlling a Peltier cooling element to adjust the temperature, and further including:
 transferring heat from the Peltier cooling element to ambient surroundings.