QUANTITATIVE IN-VIVO TISSUE SODIUM CONCENTRATION MAPS - THE EFFECTS OF BIEXPONENTIAL RELAXATION

The biexponential relaxation behavior of the sodium nucleus affects th e accuracy of quantitative measurement of in vivo tissue sodium concen tration (TSC). Theoretical analysis and in vivo experimental results a re used to demonstrate the extent of the large bias in the measured TS C that arises when the relaxation behavior in vivo differs significant ly from that of the calibration standards which is when a significant fraction of the total sodium signal decays with a relaxation time much shorter than the echo time (TE) used for imaging. This bias can be as large as 20% for measurements of TSC in a normal rat brain with TE = 2 ms. Our findings indicate that shortening the echo time (TE < 0.5 ms ) by projection imaging is a reliable means of obtaining accurate in v ivo estimates for TSC using MR.