title: Use of internal references for assessing CT density measurements of the pelvis as replacement for use of an external phantom.
abstract: OBJECTIVE
  The purpose of this research is to study the use of an internal reference standard for fat- and muscle as a replacement for an external reference standard with a phantom. By using a phantomless internal reference standard, Hounsfield unit (HU) measurements of various tissues can potentially be assessed in patients with a CT scan of the pelvis without an added phantom at time of CT acquisition. This paves the way for development of a tool for quantification of the change in tissue density in one patient over time and between patients. This could make every CT scan made without contrast available for research purposes.
  
  
  METHODS
  Fifty patients with unilateral metal-on-metal total hip replacements, scanned together with a calibration reference phantom used in bone mineral density measurements, were included in this study. On computed tomography scans of the pelvis without the use of intravenous iodine contrast, reference values for fat and muscle were measured in the phantom as well as within the patient's body. The conformity between the references was examined with the intra-class correlation coefficient.
  
  
  RESULTS
  The mean HU (± SD) of reference values for fat for the internal- and phantom references were -91.5 (±7.0) and -90.9 (±7.8), respectively. For muscle, the mean HU (± SD) for the internal- and phantom references were 59.2 (±6.2) and 60.0 (±7.2), respectively. The intra-class correlation coefficients for fat and muscle were 0.90 and 0.84 respectively and show excellent agreement between the phantom and internal references.
  
  
  CONCLUSIONS
  Internal references can be used with similar accuracy as references from an external phantom. There is no need to use an external phantom to asses CT density measurements of body tissue.
authors.affiliation: Department of Radiology, Isala Hospital, Zwolle, The Netherlands. m.f.boomsma@isala.nl.
authors.forename: Martijn F
authors.initials: MF
authors.lastname: Boomsma
authors.affiliation: Department of Radiology, Isala Hospital, Zwolle, The Netherlands.
authors.forename: Inge
authors.initials: I
authors.lastname: Slouwerhof
authors.affiliation: Department of Radiology, Isala Hospital, Zwolle, The Netherlands.
authors.forename: Jorn A
authors.initials: JA
authors.lastname: van Dalen
authors.affiliation: Department of Innovation and Science, Isala Hospital, Zwolle, The Netherlands.
authors.forename: Mireille A
authors.initials: MA
authors.lastname: Edens
authors.affiliation: Philips Healthcare systems, Hamburg, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Mueller
authors.affiliation: Philips Healthcare Benelux, Eindhoven, The Netherlands.
authors.forename: Julien
authors.initials: J
authors.lastname: Milles
authors.affiliation: Department of Radiology, AMC, Amsterdam, The Netherlands.
authors.forename: Mario
authors.initials: M
authors.lastname: Maas
mesh_terms: D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip; D015519:Bone Density; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010388:Pelvis / Q000000981:diagnostic imaging*; D019047:Phantoms, Imaging; D011183:Postoperative Complications / Q000000981:diagnostic imaging*; D011446:Prospective Studies; D012016:Reference Values; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed
keywords: CT; Hip; Metal-on-metal; Phantom less bone mineral density; Total hip arthroplasty