The ¦Ã parameter of the stretched-exponential model is influenced by internal gradients: Validation in phantoms

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• 作者：Marco Palombo^a ; ^b ; Andrea Gabrielli^a ; ^c ; Silvia De Santis^a ; ^d ; Silvia Capuani^a ; ^b ; ^{silvia.capuani@roma1.infn.it}
• 关键词：Stretched exponential model ; Anomalous Diffusion Imaging ; Diffusion ; Internal gradients ; Magnetic susceptibility differences ; NMR PGSTE ; Polystyrene micro-beads ; TiO2 micro- and nano-beads
• 刊名：Journal of Magnetic Resonance
• 出版年：2012
• 出版时间：March, 2012
• 年：2012
• 卷：216
• 期：Complete
• 页码：28-36
• 全文大小：1022 K

文摘

In this paper, we investigate the image contrast that characterizes anomalous and non-Gaussian diffusion images obtained using the stretched exponential model. This model is based on the introduction of the ¦Ã stretched parameter, which quantifies deviation from the mono-exponential decay of diffusion signal as a function of the b-value. To date, the biophysical substrate underpinning the contrast observed in ¦Ã maps, in other words, the biophysical interpretation of the ¦Ã parameter (or the fractional order derivative in space, ¦Â parameter) is still not fully understood, although it has already been applied to investigate both animal models and human brain. Due to the ability of ¦Ã maps to reflect additional microstructural information which cannot be obtained using diffusion procedures based on Gaussian diffusion, some authors propose this parameter as a measure of diffusion heterogeneity or water compartmentalization in biological tissues. Based on our recent work we suggest here that the coupling between internal and diffusion gradients provide pseudo-superdiffusion effects which are quantified by the stretching exponential parameter ¦Ã. This means that the image contrast of M¦Ã maps reflects local magnetic susceptibility differences (¦¤¦Ö_m), thus highlighting better than contrast the interface between compartments characterized by ¦¤¦Ö_m. Thanks to this characteristic, M¦Ã imaging may represent an interesting tool to develop contrast-enhanced MRI for molecular imaging.

The spectroscopic and imaging experiments (performed in controlled micro-beads dispersion) that are reported here, strongly suggest internal gradients, and as a consequence ¦¤¦Ö_m, to be an important factor in fully understanding the source of contrast in anomalous diffusion methods that are based on a stretched exponential model analysis of diffusion data obtained at varying gradient strengths g.