Purpose: To quantify the contrast‐detail detectability limits of soft‐tissue structures in cone‐beam CT (CBCT) and to investigate the influence of the plane of visualization (axial/sagittal) and level of prior knowledge on observer performance. Method and Materials: Custom‐built cylindrical phantoms containing spherical lesions of varying size and contrast were imaged on a CBCT bench across a broad range of dose. Two‐alternative forced choice (2AFC) tests were conducted under controlled conditions using 7 observers (physicists and radiation therapists). For each 2AFC test, the proportion of correct responses, Pcorr, was analyzed as a function of lesion size (1.6 – 12.7mm) and contrast (20 – 165HU), dose (2.1 – 6.4mGy), plane of visualization (axial/sagittal), apodization filter (smooth Hanning to sharp Ram‐Lak), and degree of prior knowledge provided to the observer (ranging from Signal‐Known‐Exactly (SKE) to Signal‐Unknown (SUK)). Results: 2AFC analysis provided valuable quantitation of contrast‐detail detectability limits. For example, the lowest contrast lesion (20 HU) was detected at Pcorr>70% for diameters down to ∼6mm at doses >2mGy, but smaller 20 HU lesions (<3.2mm) were barely detectable (Pcorr<60%) at any dose. Detectability was significantly improved in axial versus sagittal planes, and the effect was amplified by sharper apodization filters in a manner consistent with 3D noise‐power spectrum asymmetry. Prior knowledge had a marked influence on detectability — e.g., a ∼6mm (20 HU) sphere was detected at Pcorr∼70–85% for SKE conditions, compared to Pcorr∼55–65% under SUK conditions across the same range of dose. Conclusion: Comprehensive human observer tests provide valuable quantitation of soft‐tissue detectability limits in CBCT and help to define low‐dose techniques for specific imaging tasks. Two factors in particular — plane of visualization and prior knowledge — hold significant practical implications: axial planes typically offer improved detectability, and performance is maintained at significantly lower dose under SKE conditions (e.g., lesion‐known image guidance) than in SUK conditions (lesion‐unknown diagnostic imaging).
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging