Accuracy and reproducibility of effective atomic number and electron density measurements from sequential dual energy CT

Abstract

This study assesses the accuracy of effective atomic number (Z eff ) and electron density measurements acquired from dual energy CT and characterizes the response to clinically relevant variables representative of challenges in patient imaging, including: phantom size, material position within the phantom, variation over time, off-center positioning, and large cone beam angle. Methods: The Gammex Multi-Energy CT head and body phantoms were used to measure Z eff and electron density from 35 rod inserts that mimic tissues and varying concentrations of iodine and calcium. Scans were performed on a Canon Aquilion ONE Genesis CT scanner over a period of 6 months using default dual energy protocols appropriate for each phantom size. Theoretical Z eff and electron density values were calculated using data provided by the phantom manufacturer and compared to the measurements. Sources of variance were separated and quantified to identify the influences of random photon statistics, ROI placement, and variation over time. A subset of measurements were repeated with the phantom shifted in the vertical and horizontal directions, and over all slices in the volumetric scan. Results: All measurements showed strong correlation (r > 0.98) with their corresponding theoretical values; however, the system did demonstrate a bias of −0.58 atomic units in the body phantom and 0.28 atomic units in the head phantom for Z eff measurements. The mean absolute percent error (MAPE) was 6.3% for the body phantom and 3.2% for the head phantom. Electron density measurements of the body and head phantoms gave MAPE values of 4.6% and 1.0%, respectively. Z eff and electron density measurements significantly varied within the solid water background, showing a positional dependence within the phantom that dominated the total standard deviation in measurements. Z eff values dropped by 0.2 atomic units when the phantom was off-center; electron density measurements were less affected by phantom position. Along the z-axis, the accuracy drops off markedly at more than 50-60 mm from the central slice. Conclusion:The Canon dual energy system offers an accurate way of measuring the Z eff and electron density of clinically relevant materials. Accuracy could be improved further by calibration to remove bias, careful attention to centering within the FOV, and avoiding measurements at the edges of the cone beam.

Improving the Accuracy of the Effective Atomic Number (EAN) and Relative Electron Density (RED) with Stoichiometric Calibration on PCD-CT Images

“…The EAN of various tissue-equivalent materials was calculated as 2.94 in the study. Schaeffer et al [ 19 ] reported that it is most appropriate for EAN calculations because it produces the smallest matching interval in the head and body phantom when the EAN value is set as 2.94. The diameter of the phantom used in this study is 20 cm, which is close to the circumference of the head.…”

Image synthesis of effective atomic number images using a deep convolutional neural network-based generative adversarial network

“…Schaeffer evaluated the accuracy of the Z eff between the theoretical and measurement Z eff from DECT. The MAPE was 6.3% for the body phantom and 3.2% for the head phantom [ 19 ]. The current study showed that the MAPE of the Z eff was 1.16% ± 0.14 % with the GAN method.…”

Dual-Layer Spectral–Computed Tomography Enhances the Separability of Calcium-Based Implant Material from Bone: An Ex Vivo Quantitative Imaging Study

“…Despite such limitations, we aimed at calculating a theoretical Zeff of our materials of interest, namely bone, bone marrow, and the calcium‐based implant AGN1, by using two formulas. One formula followed Mayneord (as in formula 1 in Schaeffer and colleagues ( 18 ) ) and a second formula was adapted from Landry and colleagues ( 15 ) (see Appendix S2). We calculated theoretical values for our materials and compared them to data reported in the literature, obtained either from experimental Zeff measurements or from known compositions; eg, as provided by the bone density calibration (BDC)‐phantom manufacturer.…”

“…( 13‐17 ) The actual algorithm used in a CT device is proprietary but Zeff and ED measurements from projection space have been reported to be more accurate than in image space. ( 18 )…”