Quantitative Magnetic Resonance Imaging Study Based On Water Diffusion

Magnetic resonance imaging(MRI)technology is an irreplaceable imaging tool in clinicalapplications.The signals for producing the MR images are mainly from the hydrogen protons.In biological tissues,water molecules are the important imaging substances rich in hydrogen protons.A large number of water molecules diffuse in the tissue microstructure,that is,in the intracellular or extracellular microenvironment,and also between the intracellular and extracellular microenvironment.The latter is the water exchange across the cell membrane.Therefore,the MRI methods based on water diffusion and water exchange across the cell membrane will provide important functional information about tissue microstructures in vivo.Diffusion weighted imaging(DWI)technology is sensitive to water diffusion.A key characteristic of DWI is quantification by the apparent diffusion coefficient(ADC).When using DWI technology for clinical diagnosis,the pathological processes usually accompany with the changes in the ADC parameters.Some potential hypotheses have been proposed to explain the decrease in ADC,including the change in membrane permeability,intracellular volume fraction(IVF),tortuosity of extracellular spaces,and intracellular diffusivity.However,no experimental study has been conducted to quantitatively assess the effects of these microstructural properties on the ADC measurements.In view of this situation,we designed a series of physical phantoms with adjustable parameters to conduct an experimental study.Three phantom parameters(i.e.,fiber permeability,volume fraction,and polyvinylpyrrolidone(PVP)concentration of the intra-fiber spaces)were used to characterize three cellular tissue parameters(i.e.,membrane permeability,intracellular volume fraction,and diffusivity),respectively.At given phantom parameters,the ADC values range from 0.986μm~2/ms to 2.097μm~2/ms.Decreased ADC with the decreased fiber permeability(the mean pore diameter decreases from 5 nm to 2 nm),increased volume fraction of the intra-fiber spaces(from 0.297 to 0.356),and increased PVP concentration of the intra-fiber spaces(up to 40%(w/w))were observed,and the percentage decrease were up to 5.1%,11.3%,and 30.0%,respectively.Experimental results demonstrate that the effect of the volume fraction of the intra-fiber spaces on the ADC measurements depends on the PVP concentration of the intra-fiber spaces,and vice versa.These results suggest that the combined effect of intracellular volume fraction and diffusivity on the ADC measurements cannot be ignored.Based on traditional DWI technology,we designed and implemented the filter exchange imaging(FEXI)method for imaging water exchange across the cell membrane.The parameter to quantify the water exchange effect is the apparent exchange rate(AXR).Furthermore,we presented to add a double-refocused spin echo(TRSE)gradient scheme to the FEXI sequence to compensate the eddy current effect.The FEXI method designed in this subject realizes the accurate measurement of the water exchange rate.After adding the eddy current compensation scheme,the measurement accuracy is further improved.