title: Theoretical modeling of mobile target broadening in helical and axial computed tomographic imaging.
abstract: OBJECTIVE
  To investigate variations in mobile target length induced by sinusoidal motion in helical (HCT) and axial CT (ACT) imaging. A mathematical model was derived that predicts the measured broadening of the apparent lengths of mobile targets and its dependence on motion parameters, target size, and imaging couch speed in CT images.
  
  
  METHODS
  Three mobile targets of differing lengths and sizes were constructed of tissue-equivalent gel material and embedded into artificial lung phantom. Respiratory motion was mimicked with a mobile phantom that moves in one-dimension along the superior-inferior direction with sinusoidal motion patterns. A mathematical model was derived to predict quantitatively the variations of apparent lengths for mobile targets and its dependence on phantom and imaging couch motion parameters in HCT and ACT. The model predictions were verified by length measurements of the mobile phantom targets that were imaged with the different motion patterns using CT imaging.
  
  
  RESULTS
  The measured lengths of mobile targets enlarged or shrunk depending on the phantom motion parameters that include phantom speed, amplitude, frequency, phase and speed of the imaging couch. The target length variations were significant where some targets doubled lengths or shrunk to less than half of their actual length. The apparent lengths of mobile targets decreased if the target was moving in the same direction as the imaging couch motion and increased if the mobile target was moving opposed to imaging couch in both HCT and ACT. The model predicts well the variations in the mobile target apparent lengths and their dependence on the motion parameters.
  
  
  CONCLUSIONS
  The measured and model variations of apparent lengths of mobile targets are considerable and may affect the accuracy of tumor volumes obtained from HCT and ACT. This mathematical model provides a method to quantitatively assess the length variations of mobile targets and their dependence on motion parameters of the phantom and imaging system which may have potential applications in the fields of diagnostic imaging and radiotherapy.
authors.affiliation: Department of Radiation Oncology, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA.
authors.forename: Imad
authors.initials: I
authors.lastname: Ali
authors.affiliation: Department of Radiation Oncology, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA.
authors.forename: Steven
authors.initials: S
authors.lastname: Jackson
authors.affiliation: Department of Electrical and Computer Engineering, Penn State University, Behrend College, Erie, PA, USA.
authors.forename: Nesreen
authors.initials: N
authors.lastname: Alsbou
authors.affiliation: Department of Radiation Oncology, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA.
authors.forename: Salahuddin
authors.initials: S
authors.lastname: Ahmad
mesh_terms: D000465:Algorithms; D007091:Image Processing, Computer-Assisted; D008962:Models, Theoretical; D009038:Motion; D019047:Phantoms, Imaging; D014057:Tomography, X-Ray Computed / Q000295:instrumentation* / Q000379:methods*
keywords: Modeling; axial CT; elongation; helical CT; mobile target broadening; respiratory motion; shrinkage