2.A. Phantoms

In order to assess the accuracy of various spectral results as a function of patient size and radiation dose, a custom spectral CT phantom was used (Tissue Equivalent Materials & CTIodine®, QRM GmbH, Moehrendorf, Germany). The phantom, shown in Fig. ​Fig.1,1, contains a 10 cm diameter insert that can hold up to eight different tissue‐mimicking or iodine density rods at a time. The insert and 1 cm diameter material rods are all 10 cm in length. The insert is made of a solid water equivalent plastic with X‐ray attenuation properties similar to those of (liquid) water. Due to the importance of iodine in spectral CT imaging, we included four iodine rods at different concentrations: 0.5, 2, 5, and 10 mg/ml. The remaining rods were made of liver, adipose, 100 mg/ml hydroxyapatite (HA) and 400 mg/ml HA tissue‐mimicking materials (HA simulates bone).

70 keV virtual mono‐energetic images results of the spectral CT pediatric phantom used to evaluate spectral accuracy as a function of patient size and radiation dose. (a) Photograph of the scanner and phantom, with both 15 and 20 cm extension rings, which were used in our study. The 10 cm diameter insert (b) contains eight different tissue‐mimicking and iodine density rods. The insert was scanned within two extension rings that have outer diameters of 15 cm (c) and 20 cm (d) to evaluate the size dependency of various spectral results. Window Level/Window Width = 50/500.

Reported waist circumference measurements in infants and children ²⁴ , ²⁵ were converted into approximate body diameters (through division by π) to determine median patient sizes of newborns, 1.5‐2 yr‐old, and 9‐yr‐old patients and select the phantom sizes (10–20 cm diameter) utilized in this study. That said, the large variability in waist circumferences at these early ages imply that, for example, a 20 cm diameter phantom size corresponds to the 85th percentile of 5‐yr‐old female patients as well as the 10th percentile of 13‐yr‐old male patients such that imaging of the selected phantom sizes are relevant for some older patients. In order to evaluate the size dependency of various spectral results, the insert was scanned at three different configurations: alone, within a single 3D‐printed extension ring, and within two 3D‐printed extension rings [Figs. ​[Figs.11(b), 1(c)]. The extension rings, 10 cm in length and 15 or 20 cm in diameter, were fabricated from a PLA (polylactate acid) filament by using a 3D printer (F400 printer by Fusion3, Greensboro, NC, USA) with a 0.4 mm nozzle. The infill setting was set to 80%, with densely rendered outer shells. The attenuation of the interior of the rings varied between −5 and −25 HU, as measured on a conventional 120 kVp CT image, providing a reasonable approximation for a fat and soft tissue mixture. Polymeric foam layers of different thicknesses raise the insert and the 15 cm extension ring in order to achieve similar positioning of the material rods between the different phantom configurations (Fig. ​(Fig.11).