Fig. 2A illustrates the relative uncertainty in the estimation of contrast agent concentration (ɛrel) as a function of the concentration (Ct) for blood, gray matter, white matter and CSF. The relationship assumes FSPGR sequence parameters as described in the MRI Scanning section with a flip angle αb=12°, a constant SNR=8 for all tissues and T10/T20⁎ values of 1441/290 ms for blood, 1000/49 ms for gray matter, 750/68 ms for white matter and 3000/1500 ms for CSF [19] and [31]. The figure clearly demonstrates that the concentration estimation error greatly
increases for concentrations typical of those measured in this study, i.e., Ct<0.2 mM. The largest error occurs in white matter, where for typical concentrations of 0.01 mM, ɛrel=681%. Fig. 2B demonstrates AZD8055 that the flip angle used in this study is well optimized for low concentration measurements in white matter, as increasing the flip angle leads to increased ɛrel at lower concentrations, albeit with slightly reduced error at high concentrations. Increasing the flip angle results in errors of 782% at 16°, 911% at 20° and 1053% at 24°, compared to 681% at 12° for Ct=0.01 mM in white matter.
Reducing the flip angle does slightly improve the measurement error at low concentration; a flip angle of 8° reduces the error at 0.01 mM from 681% to 663%, BI 6727 chemical structure but at the expense of a considerably poorer performance at high concentrations. Fig. 2C demonstrates that a considerable reduction in ɛrel can be achieved by increasing the number of post-contrast measurements (equivalent to increasing the SNR of the experiment); however, around 10,000 measurements are required to reduce ɛrel to a reasonably acceptable 7%, if it is assumed that the SNR increases in proportion to √N. Finally, Fig. 2D demonstrates that modest reductions in ɛrel can also be obtained by increasing
the number of baseline pre-contrast measurements, reducing ɛrel from 681% for Nb=1, 587% for Nb=2 and 500% Adenylyl cyclase for Nb=10, provided that scanning time constraints and patient compliance allow. Fig. 1 and Table 1 indicate that post-contrast signal enhancement measured in mild stroke patients is small, ranging from less than 2% in white matter, 8% in gray matter, to 16% in CSF. When comparing measurements between the high- and low Fazekas-rated patients, relatively large differences were observed by imaging study standards, i.e., as high as 24% in CSF for Etave and Ctave, so it is somewhat disappointing that these differences did not reach statistical significance. The reason for this is due to the small absolute enhancement relative to noise, resulting in a large variance in the measurements, as illustrated in Table 1. Percentage coefficients of variation (100×S.D./mean) averaged over all tissues were 13% for T1, 49% for Etave and 56% for Ctave, indicating that the T10 measurement is reasonably precise, while those of Etave and Ctave are considerably less so.
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