title: Cluster analysis of quantitative MRI T2and T1ρrelaxation times of cartilage identifies differences between healthy and ACL-injured individuals at 3T.
abstract: To identify focal lesions of elevated MRI T2and T1ρrelaxation times in articular cartilage of an ACL-injured group using a novel cluster analysis technique.
  
  
  
  Eighteen ACL-injured patients underwent 3T MRI T2and T1ρrelaxometry at baseline, 6 months and 1 year and six healthy volunteers at baseline, 1 day and 1 year. Clusters of contiguous pixels above or below T2and T1ρintensity and area thresholds were identified on a projection map of the 3D femoral cartilage surface. The total area of femoral cartilage plate covered by clusters (%CA) was split into areas above (%CA+) and below (%CA-) the thresholds and the differences in %CA(+ or -) over time in the ACL-injured group were determined using the Wilcoxon signed rank test.
  
  
  
  %CA+ was greater in the ACL-injured patients than the healthy volunteers at 6 months and 1 year with average %CA+ of 5.2 ± 4.0% (p = 0.0054) and 6.6 ± 3.7% (p = 0.0041) for T2and 6.2 ± 7.1% (p = 0.063) and 8.2 ± 6.9% (p = 0.042) for T1ρ, respectively. %CA- at 6 months and 1 year was 3.0 ± 1.8% (p > 0.1) and 5.9 ± 5.0% (p > 0.1) for T2and 4.4 ± 4.9% (p > 0.1) and 4.5 ± 4.6% (p > 0.1) for T1ρ, respectively.
  
  
  
  With the proposed cluster analysis technique, we have quantified cartilage lesion coverage and demonstrated that the ACL-injured group had greater areas of elevated T2and T1ρrelaxation times as compared to healthy volunteers.
authors.affiliation: Department of Radiology, Stanford University, Stanford, CA, USA; Department of Electrical Engineering, Stanford University, Stanford, CA, USA. Electronic address: udmonu@stanford.edu.
authors.forename: U D
authors.initials: UD
authors.lastname: Monu
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, CA, USA. Electronic address: cjordan@alumni.stanford.edu.
authors.forename: C D
authors.initials: CD
authors.lastname: Jordan
authors.affiliation: Department of Human Biology, Stanford University, Stanford, CA, USA. Electronic address: bonbon41@alumni.stanford.edu.
authors.forename: B L
authors.initials: BL
authors.lastname: Samuelson
authors.affiliation: Department of Radiology, Stanford University, Stanford, CA, USA; Department of Electrical Engineering, Stanford University, Stanford, CA, USA; Department of Bioengineering, Stanford University, Stanford, CA, USA. Electronic address: bah@stanford.edu.
authors.forename: B A
authors.initials: BA
authors.lastname: Hargreaves
authors.affiliation: Department of Radiology, Stanford University, Stanford, CA, USA; Department of Bioengineering, Stanford University, Stanford, CA, USA; Department of Orthopaedic Surgery, Stanford University, Stanford, CA, USA. Electronic address: gold@stanford.edu.
authors.forename: G E
authors.initials: GE
authors.lastname: Gold
authors.affiliation: Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK, Canada. Electronic address: emily.mcwalter@usask.ca.
authors.forename: E J
authors.initials: EJ
authors.lastname: McWalter
mesh_terms: D000328:Adult; D000070598:Anterior Cruciate Ligament Injuries / Q000000981:diagnostic imaging*; D002358:Cartilage, Articular / Q000000981:diagnostic imaging*; D016022:Case-Control Studies; D016000:Cluster Analysis; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D055815:Young Adult
keywords: ACL-injury; Cartilage; Cluster analysis; Knee; Projection maps; Quantitative MRI