MO-D-332-01: Noise Evaluation of a New Solid-State X-Ray Image Intensifier (SSXII)

Abstract

Purpose: To measure the impact of thermal and multiplication‐excess noise and the instrumentation‐noise equivalent exposure (INEE) of a new solid‐state x‐ray image intensifier (SSXII) based on electron‐multiplying CCDs (EMCCDs). Method and Materials: The SSXII consists of a 350μm thick CsI(Tl) phosphor grown on a fiber‐optic plate optically coupled to a Peltier cooled EMCCD camera with a fiber‐optic input window via a 4:1 minifying fiber‐optic taper. The EMCCD provides signal multiplication gain prior to the read‐out electronics, thereby eliminating this primary noise source, effectively allowing for instrumentation‐noise free operation even at very low x‐ray exposures. Two additional potential noise sources, thermal and multiplication‐excess noise, were investigated. Thermal noise was determined using a linear fit of dark‐signal variance versus integration time at a constant readout rate. Multiplication‐excess noise was determined using signal variance normalized to gain versus signal plots. The gradient of a linear fit to this data at 1x gain as compared to higher gain values (up to 14x) was used to determine the excess noise factor (ENF). Overall noise‐performance was then determined using the INEE. Results:Thermal noise was measured to be < 2% of the maximal signal (12‐bits) with an EMCCD gain of 275x and an integration time of 30 ms, and is considered an upper bound under typical clinical operating conditions, indicating thermal noise has a negligible impact on overall image quality. The ENF was unity for the range of gains tested, suggesting the inherent excess noise of the electronic‐multiplication process has little effect when imaging secondary quanta from an x‐ray converting phosphor. An INEE of < 0.2μR further demonstrates the SSXII's capability to operate instrumentation‐noise free at very low x‐ray exposures. Conclusion: The SSXII provides quantum‐noise‐limited performance well below typical fluoroscopic exposure levels with negligible thermal and multiplication‐excess‐noise. (Support: NIH Grants R01‐NS43924, R01‐EB002873, Toshiba Medical Systems Corporation).