The performance of digital x‐ray imaging systems in detection of subtle lung nodules

Abstract

Subtle lung nodules (SLNs) are lesions that are commonly missed in radiologic examinations of the chest. This thesis explores three factors that influence the detection of these lesions: contrast‐diameter characteristics of SLNs, physical performance of the imaging system, and structured noise in the chest radiographs. The contrast‐diameter characteristics of SLNs were determined from radiographs containing confirmed lesions. The nodules were found to have similar subject contrast profiles with diverse variability in peak contrast (0.02–0.13) and diameter (3–16 mm). The physical performance of a digital chest radiographic system was assessed by measuring its Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), and Detective Quantum Efficiency (DQE) responses. The results showed that the system only minimally blurs the appearance of SLNs, and that for a typical chest exposure, the quantum mottle is low enough not to obscure the visibility of even the smallest SLNs. The relative influence of the structured and quantum noise was explored in a Receiver Operating Characteristic (ROC) study in which simulated tissue‐equivalent nodules were superimposed on images of either structured or quantum noise backgrounds. The detection threshold =0.8) was achieved for nodules 4.5 mm in diameter against structured backgrounds, and for nodules 1.0 mm in diameter against quantum noise backgrounds. The results indicate that anatomical structured noise is the main factor limiting the detection of SLNs in chest radiographs.