FIG. 5.

Single-echo underestimation of $\mathrm{\Delta }{R}_2^{*}$ time courses. A three-interleaf, three-echo PERMEATE acquisition was performed on a normal volunteer. PERMEATE multiecho perfusion calculations were performed using direct $R_2^{*}$ calculation. Each echo was additionally treated as a separate, single-echo acquisition. $\mathrm{\Delta }{R}_2^{*}$ time courses are shown using Eq. [2] PERMEATE data, or Eq. [3] in pseudo-single-echo sets. The third-echo-only time course mimics a standard GRE-EPI DSC experiment. a: The AIF determined for each dataset is shown. Later echoes underestimate the size of the AIF due to noise bias as the signal saturates during the bolus maximum. b: The $R_2^{*}$ tissue time courses in GM and WM are plotted. The increased concentration of the tracer in GM shows underestimation by the later, single-echo datasets, while WM is estimated well by all methods. After bolus passage, T₁ shortening underestimates the tracer concentration in the single-echo datasets due to the short TR used to maintain high temporal resolution. This effect is most notable in the early, single echo, which has less $T_2^{*}$ weighting but equal T₁ weighting compared to the later echoes.