Introduction

Magnetization transfer (MT) asymmetry is routinely calculated to measure pH sensitive amide proton transfer (APT) effect. ^{1, 2} However, MT asymmetry is susceptible to concomitant contributions, including semisolid MT and nuclear overhauser effect (NOE). ³ The aim of this study was to quantify multi-pool contribution from NOE, MT, CEST at 2 ppm and APT during the acute stroke ⁴ in order to elucidate the conventional MTR_asym effect in the characterization of acute stroke ischemia.

Materials and methods

Adult male Wistar rats (n=8) underwent MRI 60 min after unilateral middle cerebral artery occlusion on a 4.7 T scanner. Multi-slice echo planar imaging (5 slices, slice thickness/gap=1.8/0.2 mm, FOV=20×20 mm², matrix size=48×48) was acquired, including diffusion MRI ( b=250 and 1000 s/mm², TR/TE=3250/42 ms, 16 averages, scan time=1.5 min), arterial spin labeling MRI (TR/TE=5400/20 ms, time of saturation (TS)=3400 ms, 32 averages, B₁=4.7 μT, labeling distance of 15 mm, and scan time=2 min) and APT MRI (frequency offsets between ±6 ppm with intervals of 0.25 ppm, B₁=1.5 μT, TR/ TS/TE = 6500/3000/10 ms). In addition, WASSR map was collected with B₁=0.5 μT (frequency offsets between ±0.5 ppm with intervals of 0.05 ppm) for field inhomogeneity correction. Perfusion and diffusion lesions were manually segmented. Z-spectrum was flipped as 1- S_z/S₀ and fitted with a sum of five Lorentzian functions corresponding to NOE, MT, direct water saturation, CEST and APT effects at -3.2, -1.5, 0, 2.0 and 3.5 ppm, respectively ⁴, and multi-pool contribution was quantified in ADC lesion, CBF lesion and their mismatch, and their corresponding contralateral normal regions. One-way ANOVA with Bonferroni correction was conducted and P<0.05 was considered statistically significant.

Results

Fig. 1 shows manually segmented perfusion and diffusion lesions. Flipped Z-spectra from lesion and respective control areas were fitted with a sum of five Lorentzian functions, from which individual CEST effects of the five contributors to Z-spectra were resolved (Fig. 2). Figure 3 shows maps of the conventional MTR_asym and resolved NOE, MT, CEST at 2 ppm and APT effect maps. In all the three lesion regions, MT effect increased significantly while CEST effects of NOE, CEST at 2 ppm, APT and routinely measured MTR_asym decreased significantly (Fig. 4). Absolute change of CEST effects between ischemia and control regions were listed in Table 1. The change in MT and NOE has no significant difference among the three lesion regions, and furthermore, their comparably opposite alteration trend led to negligible contributions (<0.2%) to the routine MTR_asym. Meanwhile, CEST effect at 2 ppm and APT presented significantly greater alterations in ADC and CBF lesions compared to ADC/CBF lesion mismatch. Importantly, APT reduction was shown to approximately two-fold of that at 2 ppm in all the three lesion regions, demonstrating the dominant contribution of APT effect to commonly observed pH-sensitive MTR_asym change during acute stroke.

Discussion and conclusion

APT imaging has been shown capable of dividing perfusion/diffusion mismatch into regions of benign oligemia and metabolic penumbra, refining ischemic tissue delineation. We quantified multi-pool contribution to elucidate the contrast mechanism of MTR_asym in acute ischemia. Results confirmed the changes in MT and NOE offset each other, and pH-sensitive amide proton transfer effect dominates the commonly used MTR_asym effect.

Acknowledgements

Financial support from National Basic Research Program of China (2015CB755500), NSFC (81571668), SSTP (JCYJ20140610151856743 and GJHZ20160229200622417).

References

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