title: Influence of rigid coregistration of PET and CT data on metabolic volumetry: a user's perspective.
abstract: BACKGROUND
  While non-rigid fusion is by definition expected to alter the information of positron emission tomography (PET) data, we assessed whether rigid transformation also influences metabolic tumor volume (MTV) determination.
  
  
  METHODS
  The PET/computed tomography (CT) data of 28 solid pulmonary lesions of 20 tumor patients examined with 18 F-Fluordeoxyglucose (FDG) was retrospectively analyzed. The original (OR) hardware-coregistered PET images were fused with contrast-enhanced diagnostic CT (CT1, 1 mm slices) and low dose CT (CT5, 5 mm slices). After automatic rigid transformation (Mirada Fusion7D) using two algorithms (rigid fast (RF), rigid slow (RS)), MTV and maximal standardized uptake value (SUVmax) were determined applying four different segmentation methods with either fixed or background-adapted thresholding and compared to OR-PET data.
  
  
  RESULTS
  Relative differences in SUVmax compared to OR data revealed no significant differences for RF (median, -0.1%; interquartile range (IQR), -1.1% to 0.9%; p = 0.75) and RS (median, 0.5%; IQR, -0.6% to 1.3%; p = 0.19) in CT1, whereas in CT5 significant deviations were observed for RF (median, -9.0%; IQR, -10.9 to -6.1; p < 0.001) and RS (median, -8.4%; IQR, -11.1 to -5.6; p < 0.001). Relative MTV differences were 0.7% (IQR, -3.0% to 2.7%; p = 0.76) for RF and -1.3% (IQR, -3.6% to 0.9%; p = 0.12) for RS in CT1. Coregistration led to significant MTV differences in RF (median, 10.4%; IQR, 7.4% to 16.7%; p < 0.001) and RS (median, 10.6%; IQR, 5.4% to 17.7%; p < 0.001) in CT5.
  
  
  CONCLUSIONS
  Rigid coregistration of PET data allows a quantitative evaluation with reasonable accuracy in most cases. However, in some cases, it can result in substantial deviations of MTV and SUVmax. Therefore, it is recommended to perform quantitative evaluation in the original PET data rather than in coregistered PET data.
authors.affiliation: Klinik für Radiologie und Nuklearmedizin, Universitätsklinikum Magdeburg A,ö,R, Leipziger Strasse 44, Magdeburg 39120, Germany. ingo.steffen@med.ovgu.de.
authors.forename: Ingo G
authors.initials: IG
authors.lastname: Steffen
authors.forename: Frank
authors.initials: F
authors.lastname: Hofheinz
authors.forename: Julian Mm
authors.initials: JM
authors.lastname: Rogasch
authors.forename: Christian
authors.initials: C
authors.lastname: Furth
authors.forename: Holger
authors.initials: H
authors.lastname: Amthauer
authors.forename: Juri
authors.initials: J
authors.lastname: Ruf