Item

Abstract

Purpose/Introduction: Dual frequency chemical exchange saturation transfer (dualCEST) enables determination of intramolecular magnetization transfer pathways (‘‘saturation crosstalk’’) between CEST effects at two different frequency offsets Dx and DxC [1]. With this novel approach, it is possible to selectively detect the inhomogeneous component of the semi-solid magnetization transfer (ssMT). Imaging of the inhomogeneous component (ihMT) was recently introduced as a contrast with considerable specificity for myelinated structures in white matter (WM) [2, 3]. In contrast to previous studies, we propose an unbiased approach free of contributions due to direct water saturation (DS) and T1 relaxation. Subjects and Methods: dualCEST detects the water signal MW(Dx,DxC) after alternating RF irradiation at two different frequency offsets Dx and DxC. The saturation crosstalk, T(Dx,DxC), between these two frequency offsets can be calculated according to [4]: with the normalized water signal Z(Dx,DxC) = MW(Dx,DxC)/M0. For reference scans, the frequency of the off-resonant irradiation was set to Dxref = ±150 ppm. Pre-saturation was obtained by a total of 90 Gaussian-shaped pulses (mean B1 = 1.5 lT, tp = 100 ms, duty cycle DC = 37.5 ) leading to a saturation duration of 12 s. For comparison, conventional CEST effects were calculated using the unbiased inverse Z-spectrum analysis [5]: AREX = R1/DC (1/Z - 1/ZDS), where ZDS is the contribution of DS approximated by a Lorentzian fit. As model solutions 10 (w/v) heat-shocked bovine serum albumin (hs-BSA) and WM lysate extracted from pig brain, each at pH 7, were measured on a 14.1 T spectrometer (Bruker). Results: Regarding signal strength and spectral distribution, conventional AREX spectra of hs-BSA and WM lysate exhibit similar ssMT (Fig. 1A). In contrast, dualCEST at DxC = -10 ppm enables differentiation between the two samples with a four-fold difference in signal strength at frequencies Dx[0 ppm (Fig. 1B). Discussion/Conclusion: The dualCEST approach enables imaging of the ihMT without perturbations due to DS and T1 relaxation. Unbiased calculation of the saturation crosstalk was achieved by extension of the inverse Z-spectrum analysis (AREX) to dual frequency irradiation. Assignment of the dualCEST contrast to ihMT is motivated by (i) the saturation crosstalk only in WM observed here and (ii) the recently published observation of a large inhomogeneous component in WM [2, 3]. Contributions of selective CEST effects in the range between ±7 ppm can be excluded by evaluation of the saturation crosstalk at ±10 ppm.