TH-E-217BCD-05: Progress in Developing a Standard Reference Material for Lung CT

Abstract

Purpose: Report of progress in developing a ‘standard reference material’ for tissue density variations obtained in CT scans. This effort is driven both by the mission of NIST as a standard institute, and by the activities of the RSNA COPDGene phantom design work. Methods: We continue the development effort of one set of such standards by characterizing them in a NIST traceable standard beam quality, starting by the calibration of a small ionization chamber (0.6 cc) in the NIST standard beam qualities. The chamber then becomes the transfer detector for measurements performed on a commercial CT machine. We have selected foam blocks (cut to 5 cm × 4 cm × 1.9 cm) with 5 densities ranging from 0.06 to 0.30 g/cm3, with an HU value nominally between −950 and &,minus;695. We and performed transmission measurements on four batches foams, each containing 5 densities, and each density contains 10 blocks. We extracted the linear attenuation coefficient of the foams for each the available CT energies (90, 120, and 140 kV). A small water phantom of the same transverse dimension was scanned together with the foams to obtain the attenuation coefficient of water for the HU calculation. A full CT was then performed to obtain the HU value from the machine. Results: The HU values obtained from the three energies using the transmission measurement of the foam and water vs those obtained from the full CT agreed to within 0.3% (stdm) averaged over all samples. Conclusion: We have made progress toward developing a standard reference material for lungCT applications. Physical density measurements traceable to NIST standards are underway. The transmission measurements will also be performed on the NIST M‐beams to create a correlation between the CT spectrum and the standard spectrum for these foams.