CT energy weighting in the presence of scatter and limited energy resolution

• Published in: Medical physics (Lancaster) 2010-03, Vol.37 (3), p.1056-1067
• Main Author: Schmidt, Taly Gilat
• Format: Article
• Language: English
• Subjects: Algorithms; Computed tomography; Computer Simulation; computerised tomography; COMPUTERIZED TOMOGRAPHY; cone-beam CT; ENERGY RESOLUTION; ENERGY SPECTRA; Energy Transfer; energy weighting; energy-resolved CT; IMAGE PROCESSING; image reconstruction; Image sensors; IODINE; Medical image artifacts; Medical image noise; medical image processing; Medical image reconstruction; Medical imaging; Models, Theoretical; MONTE CARLO METHOD; Monte Carlo methods; NOISE; photon counting; Photon scattering; PHOTONS; Radiographic Image Enhancement - methods; Radiographic Image Interpretation, Computer-Assisted - methods; RADIOLOGY AND NUCLEAR MEDICINE; Reconstruction; Reproducibility of Results; RESPONSE FUNCTIONS; scatter; Scattering, Radiation; Sensitivity and Specificity; SIMULATION; spectrum tailing; Tomography, X-Ray Computed - methods; X-RAY DETECTION; X-Rays; X‐ray imaging; X‐ray scattering
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.3301615

Purpose: Energy-resolved CT has the potential to improve the contrast-to-noise ratio (CNR) through optimal weighting of photons detected in energy bins. In general, optimal weighting gives higher weight to the lower energy photons that contain the most contrast information. However, low-energy photons are generally most corrupted by scatter and spectrum tailing, an effect caused by the limited energy resolution of the detector. This article first quantifies the effects of spectrum tailing on energy-resolved data, which may also be beneficial for material decomposition applications. Subsequently, the combined effects of energy weighting, spectrum tailing, and scatter are investigated through simulations. Methods: The study first investigated the effects of spectrum tailing on the estimated attenuation coefficients of homogeneous slab objects. Next, the study compared the CNR and artifact performance of images simulated with varying levels of scatter and spectrum tailing effects, and reconstructed with energy integrating, photon-counting, and two optimal linear weighting methods: Projection-based and image-based weighting. Realistic detector energy-response functions were simulated based on a previously proposed model. The energy-response functions represent the probability that a photon incident on the detector at a particular energy will be detected at a different energy. Realistic scatter was simulated with Monte Carlo methods. Results: Spectrum tailing resulted in a negative shift in the estimated attenuation coefficient of slab objects compared to an ideal detector. The magnitude of the shift varied with material composition, increased with material thickness, and decreased with photon energy. Spectrum tailing caused cupping artifacts and CT number inaccuracies in images reconstructed with optimal energy weighting, and did not impact images reconstructed with photon counting weighting. Spectrum tailing did not significantly impact the CNR in reconstructed images. Scatter reduced the CNR for all energy-weighting methods; however, the effect was greater for optimal energy weighting. For example, optimal energy weighting improved the CNR of iodine and water compared to energy-integrating weighting by a factor of ∼ 1.45 in the absence of scatter and by a factor of ∼ 1.1 in the presence of scatter (8.9° cone angle, SPR 0.5). Without scatter correction, the difference in CNR resulting from photon-counting and optimal energy weighting was negligible ( < 15 % )