Prospects for in vivo estimation of photon linear attenuation coefficients using postprocessing dual-energy CT imaging on a commercial scanner: Comparison of analytic and polyenergetic statistical reconstruction algorithms

• Published in: Medical physics (Lancaster) 2013-12, Vol.40 (12), p.121914-n/a
• Main Authors: Evans, Joshua D., Whiting, Bruce R., O’Sullivan, Joseph A., Politte, David G., Klahr, Paul H., Yu, Yaduo, Williamson, Jeffrey F.
• Format: Article
• Language: English
• Subjects: ACCURACY; ALGORITHMS; alternating minimization; ATTENUATION; Biological material, e.g. blood, urine; Haemocytometers; BRACHYTHERAPY; Calibration; Computed tomography; Computerised tomographs; computerised tomography; COMPUTERIZED TOMOGRAPHY; Digital computing or data processing equipment or methods, specially adapted for specific applications; Dose‐volume analysis; dosimetry; dual‐energy computed tomography; ERRORS; filtered backprojection; Image data processing or generation, in general; IMAGE PROCESSING; Image Processing, Computer-Assisted - methods; image reconstruction; KEV RANGE; Medical image noise; medical image processing; Medical image reconstruction; MEV RANGE; minimisation; PHANTOMS; photon attenuation coefficient estimation; PHOTONS; Radiation Imaging Physics; RADIATION PROTECTION AND DOSIMETRY; RADIOLOGY AND NUCLEAR MEDICINE; Reconstruction; Scintigraphy; statistical analysis; tin; Tissues; Tomography, X-Ray Computed - instrumentation; Uncertainty; X‐ray spectra
• ISSN: 0094-2405; 2473-4209; 0094-2405
• DOI: 10.1118/1.4828787

Purpose: Accurate patient-specific photon cross-section information is needed to support more accurate model-based dose calculation for low energy photon-emitting modalities in medicine such as brachytherapy and kilovoltage x-ray imaging procedures. A postprocessing dual-energy CT (pDECT) technique for noninvasivein vivo estimation of photon linear attenuation coefficients has been experimentally implemented on a commercial CT scanner and its accuracy assessed in idealized phantom geometries. Methods: Eight test materials of known composition and density were used to compare pDECT-estimated linear attenuation coefficients to NIST reference values over an energy range from 10 keV to 1 MeV. As statistical image reconstruction (SIR) has been shown to reconstruct images with less random and systematic error than conventional filtered backprojection (FBP), the pDECT technique was implemented with both an in-house polyenergetic SIR algorithm, alternating minimization (AM), as well as a conventional FBP reconstruction algorithm. Improvement from increased spectral separation was also investigated by filtering the high-energy beam with an additional 0.5 mm of tin. The law of propagated uncertainty was employed to assess the sensitivity of the pDECT process to errors in reconstructed images. Results: Mean pDECT-estimated linear attenuation coefficients for the eight test materials agreed within 1% of NIST reference values for energies from 1 MeV down to 30 keV, with mean errors rising to between 3% and 6% at 10 keV, indicating that the method is unbiased when measurement and calibration phantom geometries are matched. Reconstruction with FBP and AM algorithms conferred similar mean pDECT accuracy. However, single-voxel pDECT estimates reconstructed on a 1 × 1 × 3 mm3 grid are shown to be highly sensitive to reconstructed image uncertainty; in some cases pDECT attenuation coefficient estimates exhibited standard deviations on the order of 20% around the mean. Reconstruction with the statistical AM algorithm led to standard deviations roughly 40% to 60% less than FBP reconstruction. Additional tin filtration of the high energy beam exhibits similar pDECT estimation accuracy as the unfiltered beam, even when scanning with only 25% of the dose. Using the law of propagated uncertainty, low Z materials are found to be more sensitive to image reconstruction errors than high Z materials. Furthermore, it is estimated that reconstructed CT image uncertainty must be limited t