Calibration method and photon flux influences tiled flat-panel photon counting detector image uniformity in computed tomography

Abstract

The aim of this study is to compare how different calibration methods influence the image quality of photon-counting detector computed tomography (PCD-CT) at high and low photon fluxes. We investigate the performance of flat-field correction, signal-to-equivalent thickness calibration (STC), and polynomial correction (PC) methods using polymethyl methacrylate (PMMA) and iron as calibration materials. Two different cylindrical imaging phantoms containing contrast targets were scanned: an agar phantom and a phantom consisting of titanium hip implant embedded in agar. The scans were acquired using 120 kVp, and the energy thresholds of the PCD were set at 10 keV and 60 keV to obtain low energy (10–60 keV), high energy (60–120 keV) and total energy images (10–120 keV). Additionally, virtual monochromatic images (VMIs) with energies between 60–180 keV with 20 keV increments were generated from PC data. The reconstructions were made using filtered back projection, and image quality was assessed by evaluating image noise, contrast-to-noise ratio (CNR), and image uniformity. Overall, STC with PMMA as calibration material yielded the best image quality in terms of CNR and uniformity. Flat-field correction produced uniform reconstruction at low photon flux, but the performance degraded substantially at high flux. STC with iron as calibration material did not improve the reconstructions of the titanium hip implant. The beam hardening effects arising from metal were reduced when the VMI energy was increased while the CNR evaluated from agar phantom decreased with increasing energy of the VMI. Over the methods investigated, STC with PMMA was the most optimal calibration method for PCD-CT, yielding excellent image uniformity with both photon flux conditions.

Experimental optimization of single‐exposure dual‐energy angiography with photon‐counting x‐ray detectors

“…This effect was observed in our system as well. Inkinen et al 43 . also reported that these issues are exacerbated at high‐flux conditions.…”

“…This effect was observed in our system as well. Inkinen et al 43 also reported that these issues are exacerbated at high-flux condi-tions. It is unclear whether or not these are limitations of current PCD technology that may be overcome with further technological development, or if they are fundamental limitations that will persist into the next generation of PCD technology.…”

“…Ahmad et al 42 also reported that the image correction for PCDs is dependent on tube voltage, energy threshold, and beam filtration. Inkinen et al 43 reported that the varying energy response between tiles that make up large field-of -view PCDs makes it difficult to apply flat-field corrections to images. In particular, the tile-borders can introduce artifacts in images.…”

Virtual monochromatic imaging reduces beam hardening artefacts in cardiac interior photon counting computed tomography: a phantom study with cadaveric specimens

“…In addition, high-resolution photon counting detector (PCD) CT improved the coronary stent lumen visibility compared to dual-energy CT (DECT), and VMIs enabled estimation of iodine and gadolinium concentration estimation from the stent region (Symons et al 2018). In PCD-CT, the beam hardening artefacts are more prominent in the low energy images (Inkinen et al 2020), and in cardiac imaging, the surrounding bony structures i.e. ribs and spine cause beam hardening in low energy PCD-CT images if not corrected.…”

“…ribs and spine cause beam hardening in low energy PCD-CT images if not corrected. One approach to reduce beam-hardening artefacts is to use VMIs (Kuchenbecker et al 2015, Inkinen et al 2020.…”