EFFECTS OF PHYSIOLOGICAL MOTION OF THE HUMAN BRAIN UPON QUANTITATIVE H-1-MRS - ANALYSIS AND CORRECTION BY RETROGATING

Signal loss and absolute quantitation errors in H-1-MRS (localized pro ton MR spectroscopy) because of physiologic brain motion are analyzed quantitatively. Cardiac and respiratory related motion lead to substan tial phase dispersion when using a standard, short echo-time STEAM seq uence. The loss in signal area varies from 6-7% with TM (middle interv al time in a STEAM sequence)= 13.7 ms, to 25-39% with TM=100 ms. The v ariation in signal area because of motion-related phase dispersion is up to 16% for TM = 100 ms. The signal phase as a function of the posit ion in the cardiac cycle is shown to be reproducible. Maximal differen ces in the signal phase are over 180 degrees for long TMs. ECG-gating reduces the phase dispersion considerably but introduces problems with variable repetition times. Using a phase calibration curve recorded w ith the water suppression turned off, it is possible to retrogate subs equent untriggered acquisitions with the water suppression activated, if the time points in the cardiac cycle are recorded for each acquisit ion. The gain in signal intensity is between 3 and 21%. For absolute q uantification via brain water, this phase analysis has the important c onsequence that reference scans must be phased individually before coa dding, otherwise metabolite concentrations may be severely overestimat ed. (C) 1998 John Wiley & Sons, Ltd.