title: MRI assessment of percutaneous ablation of liver tumors: value of subtraction images.
abstract: OBJECTIVE
  To evaluate the value of subtraction images when using MRI to assess liver tumors treated with percutaneous ablation.
  
  
  METHODS
  Following percutaneous ablation of 35 liver tumors, two abdominal radiologists, blinded to outcomes, independently reviewed follow-up MRI examinations for tumoral enhancement suggestive of residual/recurrent tumor and rated their confidence level. After one year, the readers reviewed the same examinations with added subtraction images. Accuracy of the detection of residual/recurrent tumor and contrast-to-noise ratios (CNR; for tumoral enhancement-to-liver, tumoral enhancement-to-ablation zone, and ablation zone-to-liver) were calculated with and without subtraction images and compared using Wilcoxon signed rank test. Interobserver variability was computed using Kappa (κ) statistics.
  
  
  RESULTS
  Residual/recurrent tumor was present in 8 (23.5%) of 34 tumors. Accuracy of detecting residual/recurrent tumor with subtraction images and interobserver agreement (κ = 0.72, good) were better than accuracy of detecting residual/recurrent tumor and interobserver agreement (κ = 0.57, moderate) of enhanced MR images without subtraction. Mean CNR of subtraction images was significantly higher than that of enhanced MR images for tumoral enhancement-to-liver (0.2 ± 5 versus 11.6 ± 14.4, P = 0.03), tumoral enhancement-to-ablation zone (10.1 ± 12.5 versus 34.4 ± 29.4, P = 0.02), and ablation zone-to-liver (11.8 ± 13.3 versus 102.5 ± 238.4, P = 0.03).
  
  
  CONCLUSIONS
  When using MRI, subtraction images help both detect and exclude residual/recurrent tumor following percutaneous liver ablations.
authors.affiliation: Division of Abdominal Imaging and Intervention, Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA. statli@partners.org
authors.forename: Servet
authors.initials: S
authors.lastname: Tatli
authors.forename: Murat
authors.initials: M
authors.lastname: Acar
authors.forename: Kemal
authors.initials: K
authors.lastname: Tuncali
authors.forename: Cheryl A
authors.initials: CA
authors.lastname: Sadow
authors.forename: Paul R
authors.initials: PR
authors.lastname: Morrison
authors.forename: Stuart G
authors.initials: SG
authors.lastname: Silverman
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003452:Cryosurgery / Q000379:methods*; D005260:Female; D006801:Humans; D008113:Liver Neoplasms / Q000473:pathology* / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D009364:Neoplasm Recurrence, Local / Q000473:pathology* / Q000517:prevention & control*; D018365:Neoplasm, Residual / Q000473:pathology / Q000517:prevention & control; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D016037:Single-Blind Method; D013382:Subtraction Technique; D016896:Treatment Outcome