title: High spatiotemporal resolution measurement of regional lung air volumes from 2D phase contrast x-ray images.
abstract: OBJECTIVE
  Described herein is a new technique for measuring regional lung air volumes from two-dimensional propagation-based phase contrast x-ray (PBI) images at very high spatial and temporal resolution. Phase contrast dramatically increases lung visibility and the outlined volumetric reconstruction technique quantifies dynamic changes in respiratory function. These methods can be used for assessing pulmonary disease and injury and for optimizing mechanical ventilation techniques for preterm infants using animal models.
  
  
  METHODS
  The volumetric reconstruction combines the algorithms of temporal subtraction and single image phase retrieval (SIPR) to isolate the image of the lungs from the thoracic cage in order to measure regional lung air volumes. The SIPR algorithm was used to recover the change in projected thickness of the lungs on a pixel-by-pixel basis (pixel dimensions ≈ 16.2 μm). The technique has been validated using numerical simulation and compared results of measuring regional lung air volumes with and without the use of temporal subtraction for removing the thoracic cage. To test this approach, a series of PBI images of newborn rabbit pups mechanically ventilated at different frequencies was employed.
  
  
  RESULTS
  Regional lung air volumes measured from PBI images of newborn rabbit pups showed on average an improvement of at least 20% in 16% of pixels within the lungs in comparison to that measured without the use of temporal subtraction. The majority of pixels that showed an improvement was found to be in regions occupied by bone. Applying the volumetric technique to sequences of PBI images of newborn rabbit pups, it is shown that lung aeration at birth can be highly heterogeneous.
  
  
  CONCLUSIONS
  This paper presents an image segmentation technique based on temporal subtraction that has successfully been used to isolate the lungs from PBI chest images, allowing the change in lung air volume to be measured over regions as small as the pixel size. Using this technique, it is possible to measure changes in regional lung volume at high spatial and temporal resolution during breathing at much lower x-ray dose than would be required using computed tomography.
authors.affiliation: School of Physics, Monash University, Victoria 3800, Australia. andrew.leong@monash.edu
authors.forename: Andrew F T
authors.initials: AF
authors.lastname: Leong
authors.forename: Andreas
authors.initials: A
authors.lastname: Fouras
authors.forename: M Sirajul
authors.initials: MS
authors.lastname: Islam
authors.forename: Megan J
authors.initials: MJ
authors.lastname: Wallace
authors.forename: Stuart B
authors.initials: SB
authors.lastname: Hooper
authors.forename: Marcus J
authors.initials: MJ
authors.lastname: Kitchen
mesh_terms: D000465:Algorithms; D000818:Animals; D008168:Lung / Q000000981:diagnostic imaging* / Q000502:physiology*; D008176:Lung Volume Measurements / Q000379:methods*; D011817:Rabbits; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*; D014961:X-Ray Diffraction / Q000379:methods*