title: A new two-step accurate CT-MRI fusion technique for post-implant prostate cancer.
abstract: OBJECTIVE
  To develop an accurate method of fusing computed tomography (CT) with magnetic resonance imaging (MRI) for post-implant dosimetry after prostate seed implant brachytherapy.
  
  
  METHODS
  Prostate cancer patients were scheduled to undergo CT and MRI after brachytherapy. We obtained the three MRI sequences on fat-suppressed T1-weighted imaging (FST1-WI), T2-weighted imaging (T2-WI), and T2*-weighted imaging (T2*-WI) in each patient. We compared the lengths and widths of 450 seed source images in the 10 study patients on CT, FST1-WI, T2-WI, and T2*-WI. After CT-MRI fusion using source positions by the least-squares method, we decided the center of each seed source and measured the distance of these centers between CT and MRI to estimate the fusion accuracy.
  
  
  RESULTS
  The measured length and width of the seeds were 6.1 ± 0.5 mm (mean ± standard deviation) and 3.2 ± 0.2 mm on CT, 5.9 ± 0.4 mm, and 2.4 ± 0.2 mm on FST1-WI, 5.5 ± 0.5 mm and 1.8 ± 0.2 mm on T2-WI, and 7.8 ± 1.0 mm and 4.1 ± 0.7 mm on T2*-WI, respectively. The measured source location shifts on CT/FST1-WI and CT/T2-WI after image fusion in the 10 study patients were 0.9 ± 0.4 mm and 1.4 ± 0.2 mm, respectively. The shift on CT/FST1-WI was less than on CT/T2-WI (p = 0.005).
  
  
  CONCLUSIONS
  For post-implant dosimetry after prostate seed implant brachytherapy, more accurate fusion of CT and T2-WI is achieved if CT and FST1-WI are fused first using the least-squares method and the center position of each source, followed by fusion of the FST1-WI and T2-WI images. This method is more accurate than direct image fusion.
authors.affiliation: Department of Radiation Oncology, Juntendo University, Tokyo, Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Kunogi
authors.affiliation: Department of Radiation Oncology, Juntendo University, Tokyo, Japan.
authors.forename: Hidehiro
authors.initials: H
authors.lastname: Hojo
authors.affiliation: Department of Urology, Juntendo University, Tokyo, Japan.
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Wakumoto
authors.affiliation: Department of Radiation Oncology, Juntendo University, Tokyo, Japan.
authors.forename: Anneyuko I
authors.initials: AI
authors.lastname: Saito
authors.affiliation: Department of Radiation Oncology, Juntendo University, Tokyo, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Ishikura
authors.affiliation: Department of Radiology, Juntendo University, Tokyo, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Yamashiro
authors.affiliation: Department of Radiology, Juntendo University, Tokyo, Japan.
authors.forename: Ryouhei
authors.initials: R
authors.lastname: Kuwatsuru
authors.affiliation: Department of Radiation Oncology, Juntendo University, Tokyo, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Sasai
keywords: brachytherapy; fusion; magnetic resonance imaging; post-implant; prostate cancer