An anatomic clutter phantom for lung-ventilation-imaging studies

Abstract

Xenon-enhanced, dual-energy x-ray radiography has been proposed for imaging of lung ventilation. It is important to assess the ability of dual-energy subtraction to suppress anatomic noise associated with lung parenchyma. Anatomic noise in thoracic radiography obeys an inverse power law and there exist imaging phantoms that mimic this power law. Such phantoms are based on a random, tight packing of solid acrylic spheres and are not suitable for lung ventilation studies. We developed a phantom based on randomly-packed, hollow acrylic cylinders with inner diameters of 1.59 cm, wall thicknesses of 0.16 cm and lengths of 1.59, 1.27, 0.95, 0.64 or 0.32 cm. The number of segments of each length was chosen to approximately match the volume of space occupied by each set of segments. Measurements of the effective density of the packed cylinders yielded ~0.26 g cm-3. A randomly-packed-sphere phantom was also constructed as a reference. Both phantoms were imaged using a flat-panel detector at tube voltages of 50 kV to 150 kV. A power-law model (NPS ∝ κ/|u|β) was fit to the anatomic noise power spectra. The β-value of the cylinder phantom was within 1/5 of that of the sphere phantom, although both phantoms yielded power-law parameters ranging from 2.0 to 2.4, which is lower than that reported in the literature. The κ-value of the cylinder phantom was ~1.1 times that of the sphere phantom. We conclude that the cylinder-based clutter phantom, with some modifications, can be used to simulate the anatomic noise power spectrum in thoracic radiography.