title: Repeatability of diffusion-weighted imaging in rectal cancer.
abstract: OBJECTIVE
  Serial diffusion-weighted MRI (DW-MRI) measurements of the apparent diffusion coefficient (ADC) of rectal tumors are used for rectal cancer response evaluation after neo-adjuvant treatment. In this study, we determined the repeatability of DW-MRI to distinguish therapy-related response from measurement variations.
  
  
  METHODS
  In 18 patients with rectal cancer on five consecutive days, 1.5 Tesla (T) MR imaging was performed including two identical DW-MRI sequences. The repeatability of the tumor ADC measurements and the intraobserver ADC variation were depicted in a Bland-Altman plot. The repeatability coefficient was calculated as the range of ADC values of two identical DWI measurements for 95% of subjects. It was expressed as percentage of the mean ADC value.
  
  
  RESULTS
  Three females and 15 males were included. The mean tumor ADC value was 1.15 × 10(-3) mm(2)/s (SD 0.07 × 10(-3) mm(2)). The repeatability coefficient of the ADC value was 9.8% and for the intraobserver repeatability 4.7%.
  
  
  CONCLUSIONS
  In serial DW-MRI for rectal cancer treatment response evaluation, a repeatability coefficient of 9.8% has to be considered to account for measurement variations in rectal tumor ADC. These variations represent observer judgement and patient and MR spectrometer induced variations.
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Martijn
authors.initials: M
authors.lastname: Intven
authors.forename: Onne
authors.initials: O
authors.lastname: Reerink
authors.forename: Marielle E P
authors.initials: ME
authors.lastname: Philippens
mesh_terms: D000328:Adult; D000368:Aged; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D012004:Rectal Neoplasms / Q000473:pathology* / Q000628:therapy*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016896:Treatment Outcome
keywords: apparent diffusion coefficient; diffusion-weighted MRI; rectal cancer; repeatability