title: Cortical bone water concentration: dependence of MR imaging measures on age and pore volume fraction.
abstract: OBJECTIVE
  To quantify bulk bone water to test the hypothesis that bone water concentration (BWC) is negatively correlated with bone mineral density (BMD) and is positively correlated with age, and to propose the suppression ratio (SR) (the ratio of signal amplitude without to that with long-T2 suppression) as a potentially stronger surrogate measure of porosity, which is evaluated ex vivo and in vivo.
  
  
  METHODS
  Human subject studies were conducted in compliance with institutional review board and HIPAA regulations. Healthy men and women (n = 72; age range, 20-80 years) were examined with a hybrid radial ultrashort echo time magnetic resonance (MR) imaging sequence at 3.0 T, and BWC was determined in the tibial midshaft. In a subset of 40 female subjects, the SR was measured with a similar sequence. Cortical volumetric BMD (vBMD) was measured by means of peripheral quantitative computed tomography (CT). The method was validated against micro-CT-derived porosity in 13 donor human cortical bone specimens. Associations among parameters were evaluated by using standard statistical tools.
  
  
  RESULTS
  BWC was positively correlated with age (r = 0.52; 95% confidence interval [CI]: 0.22, 0.73; P = .002) and negatively correlated with vBMD at the same location (r = -0.57; 95% CI: -0.76, -0.29; P < .001). Data were suggestive of stronger associations with SR (r = 0.64, 95% CI: 0.39, 0.81, P < .001 for age; r = -0.67, 95% CI: -0.82, -0.43, P < .001 for vBMD; P < .001 for both), indicating that SR may be a more direct measure of porosity. This interpretation was supported by ex vivo measurements showing SR to be strongly positively correlated with micro-CT porosity (r = 0.88; 95% CI: 0.64, 0.96; P < .001) and with age (r = 0.87; 95% CI: 0.62, 0.96; P < .001).
  
  
  CONCLUSIONS
  The MR imaging-derived SR may serve as a biomarker for cortical bone porosity that is potentially superior to BWC, but corroboration in larger cohorts is indicated.
authors.affiliation: From the Laboratory for Structural NMR Imaging, Department of Radiology, University of Pennsylvania School of Medicine, 1 Founders Building, MRI Education Center, 3400 Spruce St, Philadelphia, PA 19104.
authors.forename: Cheng
authors.initials: C
authors.lastname: Li
authors.forename: Alan C
authors.initials: AC
authors.lastname: Seifert
authors.forename: Hamidreza Saligheh
authors.initials: HS
authors.lastname: Rad
authors.forename: Yusuf A
authors.initials: YA
authors.lastname: Bhagat
authors.forename: Chamith S
authors.initials: CS
authors.lastname: Rajapakse
authors.forename: Wenli
authors.initials: W
authors.lastname: Sun
authors.forename: Shing Chun Benny
authors.initials: SC
authors.lastname: Lam
authors.forename: Felix W
authors.initials: FW
authors.lastname: Wehrli
mesh_terms: D000042:Absorption; D000328:Adult; D000368:Aged; D000375:Aging / Q000378:metabolism*; D001834:Body Water / Q000737:chemistry / Q000378:metabolism*; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D016062:Porosity; D013977:Tibia / Q000737:chemistry / Q000378:metabolism*; D055815:Young Adult