Theoretical modeling and performance analysis of a direct‐indirect dual‐layer flat‐panel detector for contrast‐enhanced breast imaging

BACKGROUND: Contrast-enhanced (CE) breast imaging highlights tumors with neo-angiogenesis in dual-energy (DE) images after iodinated contrast injection and has shown promise in improving diagnostic performance. Conventionally, DE images are generated by weighted subtraction of sequentially acquired low-energy (LE) and high-energy (HE) images, which are prone to motion artifacts caused by patient movement between exposures. A direct-indirect dual-layer flat-panel detector (DI-DLFPD) based imaging system, incorporating a silver (Ag) K-edge filter at the x-ray source, has recently been proposed to acquire LE and HE images simultaneously to eliminate motion artifacts. The DI-DLFPD uses a direct amorphous selenium (a-Se) front-layer (FL) detector for LE acquisition, and an indirect cesium iodide (CsI) back-layer (BL) detector for HE acquisition. PURPOSE: To develop a theoretical model to optimize a DI-DLFPD based imaging system for maximizing image quality and lesion detectability in CE breast imaging. METHODS: Established models for the x-ray spectrum and a-Se detector response are adopted, while the model for the CsI detector is extended to incorporate depth-independent optical blur variability. These optical properties are empirically characterized from measured spatial-frequency-dependent detector performance of CsI samples with varying thicknesses. The x-ray spectrum, a-Se FL, and CsI BL models are then cascaded to construct a comprehensive model of the DI-DLFPD based imaging system. Measurements from two fabricated DI-DLFPD prototypes with different CsI BL thicknesses are used to validate the model calculations. For CE breast imaging, modulation transfer function matching and weighted subtraction are incorporated. The imaging task involves detecting a cylindrical iodinated lesion (2 mm diameter and height, 1 mg/mL iodine concentration) embedded in 4 and 6 cm thick breasts with 50% glandularity. The impact of component thicknesses on energy separation, absolute iodine contrast, and detectability index (d') is systematically evaluated. RESULTS: Incorporation of depth-independent optical blur variability improved the accuracy of predicting spatial-frequency-dependent CsI detector performance across different thicknesses. DI-DLFPD model calculations agreed well with prototype measurements, confirming the validity of the developed model. Energy separation increased with both Ag K-edge filter and a-Se FL thickness. Iodine contrast was primarily determined by the relative position of the BL HE average to the iodine K-edge, rather than by energy separation alone. The main limiting factor for d' was noise in the BL HE and DE images. The optimal configuration comprises a 100 µm Ag filter, 200 µm a-Se FL, and 400 µm CsI BL. It provided d' greater than 3 for the iodinated object in both breasts at typical clinical dose levels. CONCLUSIONS: The developed dual-layer detector model accurately predicts the imaging performance of DI-DLFPD systems comprising a-Se and CsI detectors. The model reveals optimal system design parameters for a CE breast imaging task; the optimized configuration demonstrates performance comparable to current clinical mammography systems, but additionally offers spectral breast imaging that is free from motion artifacts. The modeling framework developed in this work may be extended to other CsI-based detectors and multilayer systems for a wide range of imaging applications.