title: Measuring water content using T2 relaxation at 3T: Phantom validations and simulations.
abstract: OBJECTIVE
  In vivo measurement of water content would be very useful for evaluating microstructural tissue changes, such as edema, that occur in neurological diseases. Careful assessment of the T2 relaxation decay curve can provide simultaneous measurements of total water content (TWC) and myelin water fraction, a marker for myelin which is also relevant in brain pathology. This work validates a T2 relaxation based method for TWC measurement at 3T using phantoms and simulations.
  
  
  METHODS
  A phantom consisting of tubes with known water concentrations was scanned using 3T MRI. T2 relaxation data was collected with both gradient echo spin-echo (GRASE) and spin echo sequences, while an inversion recovery experiment provided T1 relaxation data. Voxel-wise T2 distributions were calculated by fitting the T2 relaxation data with a non-negative least squares algorithm that incorporated a correction for errors in flip angle due to B1(+) inhomogeneity. TWC was calculated as the sum of the signal in the T2 distribution, corrected for T1 relaxation, relative to that of a tube containing 100% water. TWC from GRASE was compared to that of spin echo in order to test if the accuracy of the TWC measurement was impacted by using additional gradient echoes to fill k-space. Simulations were performed to determine theoretical errors in TWC.
  
  
  RESULTS
  Measured TWC strongly correlated to actual TWC (R=0.997, p=9×10(-8), mean discrepancy=1.8%). Accuracy of GRASE and spin echo TWC measurements did not significantly differ. Simulations indicated a mean systematic TWC error of 0.07% and random error of 0.8%, and revealed that the technique performs well in the presence of B1(+) inhomogeneity.
  
  
  CONCLUSIONS
  This work demonstrates that, using the T2 relaxation decay curve, TWC can be measured to within 3% accuracy at 3T. Given that T2 relaxation can provide accurate estimates of both TWC and myelin water fraction, multi-echo T2 measurement should be considered a multifaceted approach for assessing pathology and evaluating therapy of central nervous system diseases.
authors.affiliation: Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC, V6T 1Z1, Canada. Electronic address: smeyers@ualberta.ca.
authors.forename: Sandra M
authors.initials: SM
authors.lastname: Meyers
authors.affiliation: Medicine, University of British Columbia, 2211 Wesbrook Mall, Vancouver, BC, V6T 2B5, Canada. Electronic address: shannon.kolind@ubc.ca.
authors.forename: Shannon H
authors.initials: SH
authors.lastname: Kolind
authors.affiliation: Radiology, University of British Columbia, 2211 Wesbrook Mall, Vancouver, BC, V6T 2B5, Canada; Pathology & Laboratory Medicine, University of British Columbia, 2211 Wesbrook Mall, Vancouver, BC, V6T 2B5, Canada; International Collaboration on Repair Discoveries, University of British Columbia, 818 West 10th Avenue, Vancouver, BC, V5Z 1M9, Canada. Electronic address: corree@physics.ubc.ca.
authors.forename: Cornelia
authors.initials: C
authors.lastname: Laule
authors.affiliation: Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC, V6T 1Z1, Canada; Radiology, University of British Columbia, 2211 Wesbrook Mall, Vancouver, BC, V6T 2B5, Canada. Electronic address: mackay@physics.ubc.ca.
authors.forename: Alex L
authors.initials: AL
authors.lastname: MacKay
mesh_terms: D000465:Algorithms; D001921:Brain / Q000000981:diagnostic imaging* / Q000473:pathology; D002493:Central Nervous System Diseases / Q000000981:diagnostic imaging; D003198:Computer Simulation; D004487:Edema / Q000473:pathology; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D016018:Least-Squares Analysis; D008279:Magnetic Resonance Imaging; D009186:Myelin Sheath / Q000737:chemistry*; D016011:Normal Distribution; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D059629:Signal-To-Noise Ratio; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water
keywords: Magnetic resonance; Myelin water fraction; Proton density; T(2) relaxation; Validation; Water content