Energy Calibration of the Pixels of Spectral X-ray Detectors

Abstract

The energy information acquired using spectral X-ray detectors allows noninvasive identification and characterization of chemical components of a material. To achieve this, it is important that the energy response of the detector is calibrated. The established techniques for energy calibration are not practical for routine use in pre-clinical or clinical research environment. This is due to the requirements of using monochromatic radiation sources such as synchrotron, radio-isotopes, and prohibitively long time needed to set up the equipment and make measurements. To address these limitations, we have developed an automated technique for calibrating the energy response of the pixels in a spectral X-ray detector that runs with minimal user intervention. This technique uses the X-ray tube voltage (kVp) as a reference energy, which is stepped through an energy range of interest. This technique locates the energy threshold where a pixel transitions from not-counting (off) to counting (on). Similarly, we have developed a technique for calibrating the energy response of individual pixels using X-ray fluorescence generated by metallic targets directly irradiated with polychromatic X-rays, and additionally γ-rays from (241)Am. This technique was used to measure the energy response of individual pixels in CdTe-Medipix3RX by characterizing noise performance, threshold dispersion, gain variation and spectral resolution. The comparison of these two techniques shows the energy difference of 1 keV at 59.5 keV which is less than the spectral resolution of the detector (full-width at half-maximum of 8 keV at 59.5 keV). Both techniques can be used as quality control tools in a pre-clinical multi-energy CT scanner using spectral X-ray detectors.

Measuring Identification and Quantification Errors in Spectral CT Material Decomposition

“…These events appear to be only one thus resulting in an information loss. It also leads to events in low energy ranges being detected which exceeds the maximum possible photon energy which also complicates the process of energy calibration and results in a loss of energy resolution [31]. Low X-ray flux was used with the brass filter protocol (protocol-1) to reduce the probability of the pileup effect whilst providing adequate statistics in a reasonable exposure time as shown in our previous studies [32,62].…”

“…Threshold equalization involves per-pixel calibration to reduce the intrinsic variations in the threshold levels associated with each counter and provides a consistent individual pixel response to a uniform X-ray flux [45]. The energy response of the detector was calibrated against the reference energies such as X-ray fluorescence (XRF) emitted from metallic foils (Mo and Pb) and γ-rays emitted from an Am-241 radioisotope [31]. …”

Quantitative imaging of excised osteoarthritic cartilage using spectral CT

“…In this study the detector was operated in this chargesumming mode to minimize charge sharing effects. Energy calibration of the detector was performed with kVp technique [17] using a range of x-ray tube potentials.…”

Energy Calibration of a CdTe Photon Counting Spectral Detector with Consideration of its Non-Convergent Behavior

“…The energy calibration has been accomplished using monochromatic radiation sources of a known photon energy such as a synchrotron x-ray source [ 8 ], radio-isotopes [ 9 , 10 , 11 ], or x-ray fluorescent materials [ 12 , 13 , 14 ]. However, use of a monochromatic x-ray source for the energy calibration of every detector in mass production of x-ray imaging systems is not practical since most x-ray imaging systems are not easily configured to adopt monochromatic x-ray sources in them and it has a limitation of long measurement time [ 14 , 15 ].…”

“…Recently, a practical and efficient energy calibration method has been introduced in which the x-ray tube voltage is used as a reference for the energy calibration [ 15 , 16 ]. In this method, a global threshold of the comparators in the detector is found that makes 50% of the detector elements turned off at a given tube voltage, and then, the global threshold is supposed to be corresponding to the maximum photon energy determined by the peak tube voltage, that is, kVp.…”