title: Feasibility and accuracy of relative electron density determined by virtual monochromatic CT value subtraction at two different energies using the gemstone spectral imaging.
abstract: BACKGROUND
  Recent work by Saito (2012) has demonstrated a simple conversion from energy-subtracted computed tomography (CT) values (ΔHU) obtained using dual-energy CT to relative electron density (RED) via a single linear relationship. The purpose of this study was to investigate the feasibility of this method to obtain RED from virtual monochromatic CT images obtained by the gemstone spectral imaging (GSI) mode with fast-kVp switching.
  
  
  METHODS
  A tissue characterization phantom with 13 inserts made of different materials was scanned using the GSI mode on a Discovery CT750 HD. Four sets of virtual monochromatic CT images (60, 77, 100 and 140 keV) were obtained from a single GSI acquisition. When we define Δ HU in terms of the weighting factor for the subtraction α, Δ HU ≡ (1 + α)H - αL (H and L represent the CT values for high and low energy respectively), the relationship between Δ HU and RED is approximated as a linear function, a × Δ HU/1000 + b (a, b = unity). We evaluated the agreement between the determined and nominal RED. We also have investigated reproducibility over short and long time periods.
  
  
  RESULTS
  For the 13 insert materials, the RED determined by monochromatic CT images agreed with the nominal values within 1.1% and the coefficient of determination for this calculation formula was greater than 0.999. The observed reproducibility (1 standard deviation) of calculation error was within 0.5% for all materials.
  
  
  CONCLUSIONS
  These findings indicate that virtual monochromatic CT scans at two different energies using GSI mode can provide an accurate method for estimating RED.
authors.affiliation: Department of Radiology, Osaka University Hospital, 2-15 Yamadaoka, Suita, Osaka, Japan. ogata@k-mcc.net
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Ogata
authors.forename: Takashi
authors.initials: T
authors.lastname: Ueguchi
authors.forename: Masashi
authors.initials: M
authors.lastname: Yagi
authors.forename: Sachiko
authors.initials: S
authors.lastname: Yamada
authors.forename: Chikako
authors.initials: C
authors.lastname: Tanaka
authors.forename: Ryota
authors.initials: R
authors.lastname: Ogihara
authors.forename: Fumiaki
authors.initials: F
authors.lastname: Isohashi
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Yoshioka
authors.forename: Noriyuki
authors.initials: N
authors.lastname: Tomiyama
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Ogawa
authors.forename: Masahiko
authors.initials: M
authors.lastname: Koizumi
mesh_terms: D004583:Electrons; D005240:Feasibility Studies; D019047:Phantoms, Imaging; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*