MR-VISIBLE WATER-CONTENT IN HUMAN BRAIN - A PROTON MRS STUDY

In vivo measurement of metabolite concentrations in the human brain by means of proton-MRS contributes significantly to the clinical evaluat ion of patients with diseases of the brain. The fully relaxed water si gnal has been proposed as an internal standard for calibration of the MRS measurements. The major drawbacks are the necessity to make the as sumptions that the water concentration in the brain and that all tissu e water is MR-visible. A number of in vivo measurements were carried o ut to estimate the concentration of MR-visible water in the brain of h ealthy volunteers divided into four age groups: newborn (0-23 days), a dolescents (10-15 yr), adults (22-28 yr), and elderly people (60-74 yr ). The examinations were carried out using a Siemens Helicon SP 63/84 MR-scanner operating at 1.5 T. Except for the newborn, four regions we re studied in each subject using stimulated echo (STEAM) sequences wit hout water suppression. In vitro measurements on a standard phantom we re used for calibration. The calculated water concentrations ranged be tween 35.8 and and 39.6 (mean 36.9) mol(.)[kg wet weight](-1) in the t hree groups, whereas it was 51.5 mol.[kg wet weight](-1) in the newbor n, p<.01. The observed water concentration of neither the four regions nor of the three oldest age groups were significantly different. Comp arisons between the water concentrations measured and those expected b ased on estimation of the content of grey and white matter in the regi on of interest from T-1-weighted images and biochemical data published , suggest that only a small fraction (<5%) of the tissue water may be MR-invisible. The study of healthy volunteers thus shows that errors i ntroduced by using the unsaturated water signal for calibration are le ss than 10%, which is comparable to expected errors when other calibra tion procedures are used under similar measurement conditions.