title: CT reconstruction techniques for improved accuracy of lung CT airway measurement.
abstract: OBJECTIVE
  To determine the impact of constrained reconstruction techniques on quantitative CT (qCT) of the lung parenchyma and airways for low x-ray radiation dose.
  
  
  METHODS
  Measurement of small airways with qCT remains a challenge, especially for low x-ray dose protocols. Images of the COPDGene quality assurance phantom (CTP698, The Phantom Laboratory, Salem, NY) were obtained using a GE discovery CT750 HD scanner for helical scans at x-ray radiation dose-equivalents ranging from 1 to 4.12 mSv (12-100 mA s current-time product). Other parameters were 40 mm collimation, 0.984 pitch, 0.5 s rotation, and 0.625 mm thickness. The phantom was sandwiched between 7.5 cm thick water attenuating phantoms for a total length of 20 cm to better simulate the scatter conditions of patient scans. Image data sets were reconstructed using STANDARD (STD), DETAIL, BONE, and EDGE algorithms for filtered back projection (FBP), 100% adaptive statistical iterative reconstruction (ASIR), and Veo reconstructions. Reduced (half) display field of view (DFOV) was used to increase sampling across airway phantom structures. Inner diameter (ID), wall area percent (WA%), and wall thickness (WT) measurements of eight airway mimicking tubes in the phantom, including a 2.5 mm ID (42.6 WA%, 0.4 mm WT), 3 mm ID (49.0 WA%, 0.6 mm WT), and 6 mm ID (49.0 WA%, 1.2 mm WT) were performed with Airway Inspector (Surgical Planning Laboratory, Brigham and Women's Hospital, Boston, MA) using the phase congruency edge detection method. The average of individual measures at five central slices of the phantom was taken to reduce measurement error.
  
  
  RESULTS
  WA% measures were greatly overestimated while IDs were underestimated for the smaller airways, especially for reconstructions at full DFOV (36 cm) using the STD kernel, due to poor sampling and spatial resolution (0.7 mm pixel size). Despite low radiation dose, the ID of the 6 mm ID airway was consistently measured accurately for all methods other than STD FBP. Veo reconstructions showed slight improvement over STD FBP reconstructions (4%-9% increase in accuracy). The most improved ID and WA% measures were for the smaller airways, especially for low dose scans reconstructed at half DFOV (18 cm) with the EDGE algorithm in combination with 100% ASIR to mitigate noise. Using the BONE + ASIR at half BONE technique, measures improved by a factor of 2 over STD FBP even at a quarter of the x-ray dose.
  
  
  CONCLUSIONS
  The flexibility of ASIR in combination with higher frequency algorithms, such as BONE, provided the greatest accuracy for conventional and low x-ray dose relative to FBP. Veo provided more modest improvement in qCT measures, likely due to its compatibility only with the smoother STD kernel.
authors.affiliation: Department of Medical Physics, University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin 53705.
authors.forename: A
authors.initials: A
authors.lastname: Rodriguez
authors.affiliation: Department of Medical Physics, University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin 53705 and Department of Radiology, University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin 53792.
authors.forename: F N
authors.initials: FN
authors.lastname: Ranallo
authors.affiliation: Brigham and Women's Hospital, Boston, Massachusetts 02115.
authors.forename: P F
authors.initials: PF
authors.lastname: Judy
authors.affiliation: Department of Radiology, Washington University, St. Louis, Missouri 63110.
authors.forename: D S
authors.initials: DS
authors.lastname: Gierada
authors.affiliation: Department of Medical Physics, University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin 53705; Department of Radiology, University of Wisconsin School of Medicine and Public Health, Madison, Wisconsin 53792; and Department of Biomedical Engineering,University of Wisconsin School of Engineering, Madison, Wisconsin 53706.
authors.forename: S B
authors.initials: SB
authors.lastname: Fain
mesh_terms: D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008168:Lung / Q000000981:diagnostic imaging*; D011829:Radiation Dosage; D059629:Signal-To-Noise Ratio; D014057:Tomography, X-Ray Computed