High-resolution extremity cone-beam CT with a CMOS detector: task-based optimization of scintillator thickness

Abstract

Purpose CMOS x-ray detectors offer small pixel sizes and low electronic noise that may support the development of novel high-resolution imaging applications of cone-beam CT (CBCT). We investigate the effects of CsI scintillator thickness on the performance of CMOS detectors in high resolution imaging tasks, in particular in quantitative imaging of bone microstructure in extremity CBCT. Methods A scintillator thickness-dependent cascaded systems model of CMOS x-ray detectors was developed. Detectability in low-, high- and ultra-high resolution imaging tasks (Gaussian with FWHM of ~250 μm, ~80 μm and ~40 μm, respectively) was studied as a function of scintillator thickness using the theoretical model. Experimental studies were performed on a CBCT test bench equipped with DALSA Xineos3030 CMOS detectors (99 μm pixels) with CsI scintillator thicknesses of 400 μm and 700 μm, and a 0.3 FS compact rotating anode x-ray source. The evaluation involved a radiographic resolution gauge (0.6–5.0 lp/mm), a 127 μm tungsten wire for assessment of 3D resolution, a contrast phantom with tissue-mimicking inserts, and an excised fragment of human tibia for visual assessment of fine trabecular detail. Results Experimental studies show ~35% improvement in the frequency of 50% MTF modulation when using the 400 μm scintillator compared to the standard nominal CsI thickness of 700 μm. Even though the high-frequency DQE of the two detectors is comparable, theoretical studies show a 14% to 28% increase in detectability index (d′2) of high- and ultrahigh resolution tasks, respectively, for the detector with 400 μm CsI compared to 700 μm CsI. Experiments confirm the theoretical findings, showing improvements with the adoption of 400 μm panel in the visibility of the radiographic pattern (2× improvement in peak-to-through distance at 4.6 lp/mm) and a 12.5% decrease in the FWHM of the tungsten wire. Reconstructions of the tibial plateau reveal enhanced visibility of trabecular structures with the CMOS detector with 400 μm scinitllator. Conclusion Applications on CMOS detectors in high resolution CBCT imaging of trabecular bone will benefit from using a thinner scintillator than the current standard in general radiography. The results support the translation of the CMOS sensor with 400 μm CsI onto the clinical prototype of CMOS-based extremity CBCT.

Modeling and evaluation of a high‐resolution CMOS detector for cone‐beam CT of the extremities

“…The work reported below extends the simulation and experimental studies reported in Ref. [15] to provide a more detailed analysis of CMOS detector performance across a broader scope of system parameters and imaging geometries using a new, continuously tunable spectrum of imaging tasks representative of trabecular bone morphometry. The main contributions of this work include: (a) a model of a CMOS x-ray sensor that incorporates the effects of scintillator thickness; (b) a study of detectability in extremity CBCT as a function of feature size, pixel size, electronic noise, CsI:Tl thickness, focal spot size, dose, and system magnification; and (c) experimental validation in phantom and cadaver studies using two CMOS detectors, one with the current standard CsI:Tl thicknesses of 0.7 mm (denoted C700), and one (C400) custom-made with 0.4 mm thick CsI:Tl.…”

“…The values of the function Hðt CsI Þ at the CsI:Tl thicknesses of the two detectors, H 400 and H 700 , were obtained through the fit in Eq. (15). The parameters of the polynomial model h 1 and h 2 were estimated from H 400 and H 700 by an additional fitting step, resulting in the thickness-dependent Hðt CsI Þ ¼ 0:35t 2 CsI þ 0:18t CsI .…”

“…For high-frequency tasks, however, the benefits of enhanced spatial resolution may outweigh the impact of elevated noise, resulting in improved detectability. Preliminary results 15 indicate that the visibility of trabeculae can indeed be improved using a 0.4 mm scintillator compared to the standard 0.7 mm thickness.…”

“…The high spatial resolution motivates application of extremity CBCT in quantitative imaging of bone microstructure . Trabecular and cortical microarchitecture are biomarkers of bone health, with indices of bone microstructure found to improve prediction of fracture risk in osteoporosis (OP) compared to bone mineral density (BMD).…”

“…10 The high spatial resolution motivates application of extremity CBCT in quantitative imaging of bone microstructure. [11][12][13][14][15] Trabecular and cortical microarchitecture are biomarkers of bone health, with indices of bone microstructure found to improve prediction of fracture risk in osteoporosis (OP) 11,[16][17][18][19][20][21][22][23][24] compared to bone mineral density (BMD). In OA, alterations in trabecular microarchitecture of subchondral bone often precede cartilage degeneration, [25][26][27] motivating investigation of structural metrics as an early biomarker of disease.…”