How spectroscopic x-ray imaging benefits from inter-pixel communication

Abstract

Spectroscopic x-ray imaging based on pixellated semiconductor detectors can be sensitive to charge sharing and K-fluorescence, depending on the sensor material used, its thickness and the pixel pitch employed. As a consequence, spectroscopic resolution is partially lost. In this paper, we study a new detector ASIC, the Medipix3RX, that offers a novel feature called charge summing, which is established by making adjacent pixels communicate with each other. Consequently, single photon interactions resulting in multiple hits are almost completely avoided. We investigate this charge summing mode with respect to those of its imaging properties that are of interest in medical physics and benchmark them against the case without charge summing. In particular, we review its influence on spectroscopic resolution and find that the low energy bias normally present when recording energy spectra is dramatically reduced. Furthermore, we show that charge summing provides a modulation transfer function which is almost independent of the energy threshold setting, which is in contrast to approaches common so far. We demonstrate that this property is directly linked to the detective quantum efficiency, which is found to increase by a factor of three or more when the energy threshold approaches the photon energy and when using charge summing. As a consequence, the contrast-to-noise ratio is found to double at elevated threshold levels and the dynamic range increases for a given counter depth. All these effects are shown to lead to an improved ability to perform material discrimination in spectroscopic CT, using iodine and gadolinium contrast agents. Hence, when compared to conventional photon counting detectors, these benefits carry the potential of substantially reducing the imaging dose a patient is exposed to during diagnostic CT examinations.

Evaluation of conventional imaging performance in a research whole-body CT system with a photon-counting detector array

“…This result is different from what is reported in (Koenig et al 2014). According to (Koenig et al 2014), when charge sharing and K-escape occur, a charge cloud dedicated to one pixel may be detected by neighboring pixels, which results in loss of spatial resolution. However, this was not the case for the PCD subsystem, because the smallest readout unit was one macro pixel, which consisted of 4-by-4 sub-pixels.…”

“…However, this was not the case for the PCD subsystem, because the smallest readout unit was one macro pixel, which consisted of 4-by-4 sub-pixels. Note that the size of a macro pixel is 900 μ m by 900 μ m, which is much larger than the 55 μ m-by-55 μ m pixel size used in (Koenig et al 2014). Although a charge cloud dedicated to one sub-pixel within a macro pixel may still be detected by neighboring sub-pixels, loss of spatial resolution would not occur as long as the measured charge cloud did not exceed the corresponding macro pixel size.…”

“…Note that, the binning process of the macro pixel is performed after analog signal processing of the front-end chip, and it does not compensate for K-escape or charge sharing effects occurring between sub-pixels. The binning process of the macro pixel is not related to the previously described charge summing approach (Koenig et al 2014). …”

Spectral performance of a whole-body research photon counting detector CT: quantitative accuracy in derived image sets

“…In multi-energy CT, material decomposition capability depends on the spectral separation. Although in theory PCCT could enable perfect spectral separation, the real spectra are usually associated with considerable overlaps due to non-ideal effects such as charge sharing and k-escape (Taguchi and Iwanczyk, 2013, Gutjahr et al, 2016, Koenig et al, 2014). One technique to reduce charge sharing and improve spectral resolution is to use charge sharing correction circuit, which is not available in the system investigated in this study (Koenig et al, 2014).…”

A Ring Artifact Correction Method: Validation by Micro-CT Imaging with Flat-Panel Detectors and a 2D Photon-Counting Detector

“…Spectral images taken with a Si PCXD [7] and a CdTe PCXD [2] have recently been reported. Recent developments of photon-counting readout chips for CdTe sensors, which can detect X-ray photons with energy higher than 100 keV, aim to perform 3D spectral imaging for human studies in the near future [8,9,10]. In X-ray imaging with a PCXD, low-intensity X-ray beams are used to avoid output pulse pileups [11,12].…”