Quantitative chemical exchange saturation transfer (CEST) MRI of glioma using Image Downsampling Expedited Adaptive Least-squares (IDEAL) fitting

Iris Yuwen Zhou,Giulia Fulci,Enfeng Wang,Xiaoan Zhang,Jerry S Cheung,Phillip Zhe Sun

doi:10.1038/s41598-017-00167-y

Abstract

Chemical Exchange Saturation Transfer (CEST) MRI is sensitive to dilute metabolites with exchangeable protons, allowing tissue characterization in diseases such as acute stroke and tumor. CEST quantification using multi-pool Lorentzian fitting is challenging due to its strong dependence on image signal-to-noise ratio (SNR), initial values and boundaries. Herein we proposed an Image Downsampling Expedited Adaptive Least-squares (IDEAL) fitting algorithm that quantifies CEST images based on initial values from multi-pool Lorentzian fitting of iteratively less downsampled images until the original resolution. The IDEAL fitting in phantom data with superimposed noise provided smaller coefficient of variation and higher contrast-to-noise ratio at a faster fitting speed compared to conventional fitting. We further applied the IDEAL fitting to quantify CEST MRI in rat gliomas and confirmed its advantage for in vivo CEST quantification. In addition to significant changes in amide proton transfer and semisolid macromolecular magnetization transfer effects, the IDEAL fitting revealed pronounced negative contrasts of tumors in the fitted CEST maps at 2 ppm and −1.6 ppm, likely arising from changes in creatine level and nuclear overhauser effects, which were not found using conventional method. It is anticipated that the proposed method can be generalized to quantify MRI data where SNR is suboptimal.

Highlights

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Significant amount of noise appeared in the amplitude maps obtained from the conventional fitting after Gaussian noise superposition, indicating it's more susceptible to signal-to-noise ratio (SNR) degradation than the proposed Image Downsampling Expedited Adaptive Least-squares (IDEAL) fitting algorithm
We proposed a new fitting algorithm that harnesses the high SNR of downsampled images for iterative fitting and achieved reliable Chemical Exchange Saturation Transfer (CEST) MRI quantification

Summary

Introduction

The Harvard community has made this article openly available. Please share how this access benefits you. CEST quantification using multi-pool Lorentzian fitting is challenging due to its strong dependence on image signal-to-noise ratio (SNR), initial values and boundaries. In addition to significant changes in amide proton transfer and semisolid macromolecular magnetization transfer effects, the IDEAL fitting revealed pronounced negative contrasts of tumors in the fitted CEST maps at 2 ppm and −1.6 ppm, likely arising from changes in creatine level and nuclear overhauser effects, which were not found using conventional method. Chemical Exchange Saturation Transfer (CEST) MRI is a sensitive imaging technique for detecting compounds containing exchangeable protons[1,2,3]. In vivo CEST quantification remains challenging due to concomitant effects such as RF spillover (direct water saturation), semisolid macromolecular magnetization transfer (MT) and nuclear overhauser effects (NOE). To avoid asymmetric MT effect, a three-offset approach that subtracts the label image from the average of two boundary images that have an equal offset shift from the label image www.nature.com/scientificreports/

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