Method and apparatus for stabilizing the measurement of CT numbers

A method of stabilizing the measurement of CT numbers by a CT system, the CT system including an x-ray source for generating x-rays in response to a voltage provided by a voltage source, the method comprising:

• a) establishing a reference level for said voltage by adjusting the voltage to a value for which calculation by said CT system of the CT number of a sample having a known CT number value yields said known CT number value;
• b) using a principal detector to detect x-rays generated by said x-ray source so as to generate a first intensity magnitude;
• c) using an auxiliary detector to detect x-rays generated by said x-ray source so as to generate a second intensity magnitude, wherein said auxiliary detector includes an absorber that preferentially absorbs x-ray photons having a relatively low energy;
• d) generating a voltage control signal, and providing said voltage control signal to said voltage source, so as to maintain said voltage substantially constant at said reference level, thereby substantially reducing in said CT system a variation in the calculated values of CT numbers;
• wherein said voltage control signal is proportional to a known function of said first and second intensity magnitudes.

A CT system, comprising:

• a) an x-ray source for generating x-rays in response to a voltage provided by a voltage source;
• b) a detection system for detecting x-rays generated by said x-ray source and transmitted through a target object;
• c) a kV meter for measuring an energy spectrum of x-rays generated by said x-ray source;
• d) a processor for calculating the CT numbers of said target object;
• and
• (e) a feedback controller for providing to the voltage source a voltage control signal;
• wherein the energy spectrum measured by said kV meter is used to adjust said voltage control signal so as to maintain said voltage substantially constant at a reference level established during calibration, and
• wherein said reference level is the voltage level at which calculation by said CT system of the CT number of a sample having a known CT number value yields the correct known CT number value.

A CT system for performing stabilized CT number measurements, said CT system comprising:

• a) an x-ray source for generating x-rays in response to a voltage provided by a voltage source;
• b) a detection system for detecting x-rays emitted by said x-ray source and transmitted through a target object;
• c) a kV meter, comprising:
• i) a principal detector for detecting x-rays generated by said x-ray source so as to generate a first intensity magnitude;
• ii) an auxiliary detector for detecting x-rays generated by said x-ray source so as to generate a second intensity magnitude, said auxiliary detector including an absorber that preferentially absorbs x-ray photons having a relatively low energy;
• d) a processor for calculating CT numbers of said target object so as to reconstruct a CT image of said target object;
• and
• e) a feedback controller for providing to the voltage source a voltage control signal;
• wherein said voltage control signal is adjusted as a function of a ratio of said first and second intensity magnitudes so as to maintain said voltage substantially constant at a reference level established during calibration, thereby substantially reducing in said CT system a variation in the measured values of CT numbers;
• and
• wherein said reference level is the magnitude of the voltage at which calculation by said CT system of a sample having a known CT number value yields the correct known CT number value.

An apparatus for stabilizing CT number calculations in a CT system having an x-ray source for generating x-rays in response to a voltage provided by a voltage source, the apparatus comprising:

• a) a principal detector for detecting x-rays generated by said x-ray source so as to generate a first intensity magnitude;
• b) an auxiliary detector having an absorber that preferentially absorbs x-rays photons having a relatively low energy, the auxiliary detector being adapted to detect x-rays generated by said x-ray source so as to generate a second intensity magnitude;
• and
• c) a feedback controller for providing to the voltage source a voltage control signal;
• wherein said voltage control signal is adaptively adjusted, based on a predetermined function of said first intensity magnitude and said second intensity magnitude, so as to maintain the voltage provided to the x-ray source constant at a reference level for which the calculation by said CT system of the CT number of a sample having a known CT number value yields the correct known CT number value.