NMR IN BIOMEDICINE NMR Biomed. 2005;18:163–172 Published online 1 December 2004 in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/nbm.934 Statistical evaluation of time-dependent metabolite concentrations: estimation of post-mortem intervals based on in situ 1 H-MRS of the brain Eva Scheurer, 1 Michael Ith, 2 Daniel Dietrich, 3 Roland Kreis, 2 Ju ¨ rg Hu ¨ sler, 3 Richard Dirnhofer 1 and Chris Boesch 2 * 1 Institute of Forensic Medicine, University of Bern, Bern, Switzerland 2 Department of Clinical Research (MR Spectroscopy and Methodology), University of Bern, Bern, Switzerland 3 Department of Mathematical Statistics, University of Bern, Bern, Switzerland Received 21 April 2004; Revised 28 August 2004; Accepted 13 September 2004 ABSTRACT: Knowledge of the time interval from death (post-mortem interval, PMI) has an enormous legal, criminological and psychological impact. Aiming to find an objective method for the determination of PMIs in forensic medicine, 1 H-MR spectroscopy ( 1 H-MRS) was used in a sheep head model to follow changes in brain metabolite concentrations after death. Following the characterization of newly observed metabolites (Ith et al., Magn. Reson. Med. 2002; 5: 915–920), the full set of acquired spectra was analyzed statistically to provide a quantitative estimation of PMIs with their respective confidence limits. In a first step, analytical mathematical functions are proposed to describe the time courses of 10 metabolites in the decomposing brain up to 3 weeks post-mortem. Subsequently, the inverted functions are used to predict PMIs based on the measured metabolite concentrations. Individual PMIs calculated from five different metabolites are then pooled, being weighted by their inverse variances. The predicted PMIs from all individual examinations in the sheep model are compared with known true times. In addition, four human cases with forensically estimated PMIs are compared with predictions based on single in situ MRS measurements. Interpretation of the individual sheep examinations gave a good correlation up to 250 h post-mortem, demonstrating that the predicted PMIs are consistent with the data used to generate the model. Comparison of the estimated PMIs with the forensically determined PMIs in the four human cases shows an adequate correlation. Current PMI estimations based on forensic methods typically suffer from uncertainties in the order of days to weeks without mathematically defined confidence information. In turn, a single 1 H-MRS measurement of brain tissue in situ results in PMIs with defined and favorable confidence intervals in the range of hours, thus offering a quantitative and objective method for the determination of PMIs. Copyright # 2004 John Wiley & Sons, Ltd. KEYWORDS: 1 H-magnetic resonance spectroscopy; brain; post-mortem decomposition; time of death; virtopsy; forensic medicine; statistical analysis; time evolution INTRODUCTION Some evolutions, such as brain development in infants, aging processes, progression of tissue repair in hemorrhagic lesions or the time interval from death (post-mortem interval, PMI), lead to time-dependent metabolite concentrations or changes in other MR vari- ables. This study proposes a statistical evaluation of such data that allows for an estimation of time intervals and associated uncertainties. The importance of a reliable estimation of the post- mortem interval is obvious in criminal investigations. An inclusion or exclusion of possible suspects based on a trustworthy and early estimation of the PMI can be decisive for further measures of tracing, collection of evidence, and the direction and success of an investiga- tion. Knowing the exact time of death seems to be a very important factor in the process of mourning and the emotional management of relatives and friends of a victim. 1 In the early post-mortem interval, i.e. during the first 3 days post-mortem, the estimation of PMI in forensic medicine is mostly based on the evaluation of livor mortis, rigor mortis and the cooling of the body after Copyright # 2004 John Wiley & Sons, Ltd. NMR Biomed. 2005;18:163–172 *Correspondence to: C. Boesch, Department of Clinical Research, MR Center 1, University and Inselspital, CH-3010 Bern, Switzerland. E-mail: chris.boesch@insel.ch Contract/grant sponsor: Gebert-Ru ¨f Foundation; contract/grant number: GRS-069/99. Contract/grant sponsor: Swiss National Foundation; contract/grant number: 31-59082.99. Abbreviations used: 1 H-MRS, proton magnetic resonance spectro- scopy; ACE, acetate; ALA, alanine; BUT, butyrate; CT, computed tomography; fTMA, free trimethylammonium; GM, gray matter; NAA/NAAG, N-acetyl-aspartic acid/N-acetyl-aspartyl-glutamate; PMI, post-mortem interval; PRESS, point-resolved spectroscopy; PROP, propionate; ROI, region of interest; TE, echo time; TR, repeti- tion time; var, variance; WM, white matter.