Retrospective correction for T1-weighting bias in T2 values obtained with various spectroscopic spin-echo acquisition schemes

Localized tissue transverse relaxation time (T2) is obtained by fitting a decaying exponential to the signals from several spin-echo experiments at different echo times (TE). Unfortunately, time constraints in magnetic resonance spectroscopy (MRS) often mandate in vivo acquisition schemes at short repetition times (TR), that is, comparable with the longitudinal relaxation constant (T1). This leads to different T1-weighting of the signals at each TE. Unaccounted for, this varying weighting causes systematic underestimation of the T2's, sometimes by as mush as 30%. In this article, we (i) analyze the phenomenon for common MRS spin-echo T2 acquisition schemes; (ii) propose a general post hoc T1-bias correction for any (TR, TE) combination; (iii) show that approximate knowledge of T1 is sufficient, since a 20% uncertainty in T1 leads to under 3% bias in T2; and consequently, (iv) efficient, precision-optimized short TR spin-echo T2 measurement protocols can be designed and used without risk of accuracy loss. Tables of correction for single-refocusing (conventional) spin-echo and double refocusing, such as, PRESS acquisitions, are provided.