Ultra-short TE NMR One-dimensional Spatially Resolved T2 Mapping Method for Porous Media

Short T2 relaxation components are typical characteristics of porous media such as soil and rock. The ultra-short TE NMR method based on the CPMG sequence has created conditions for the overall measurement of such samples. In the evaluation of sample heterogeneity and the porous flow, in addition to the overall T2 spectrum, the 1D or multi-dimensional T2 mapping is more needed. Traditional MRI technology based on pulsed gradient field is limited by the gradient spatial encoding time and gradient eddy currents, resulting in relatively long TE, with the shortest TE being about 1.5ms, which is significantly different from the ultra-short TE required for the porous media. 1D spatially resolved T2 mapping based on constant gradient fields is a passive effect brought by the gradient magnetic field of NMR logging tools. The gradient value is not adjustable, making it difficult to regulate the layer thickness.

It is feasible to use traditional MRI gradient system combined with cooling modules to achieve 1D spatially resolved T2 mapping under constant gradient field conditions. The challenge lies in the contradiction between the Larmor frequency range in the sample selection layer direction and the -3dB bandwidth of the radio frequency coil. To address this issue, we used high Q and narrowband radio frequency coils to ensure the SNR. During 1D spatial layer selection excitation, the resonance point is tuned to match different excitation positions of the sample, or the sample position is moved to the fixed excitation frequency. Therefore, there are two technical branches, one is the radio frequency coil tuning method and the other is the sample movement method.

Since porous media have internal gradient fields, excessively high magnetic field strength will introduce internal diffusion relaxation effects. Therefore, this abstract focuses on the non-correction method which is suitable for porous media. It combines actual cases to analyze the technical features and comparison of the two branches of the non-correction method. The advantages of the tuning method are that the sample position is fixed, which is suitable for measurement conditions with peripheral accessories on the sample. The disadvantages are that the sample test length is limited, and the radio frequency hardware is complex. The advantages of the sample movement method are that the test sample length is not limited, and the radio frequency hardware is simple. The disadvantage is that sample movement is not suitable for measurement conditions with peripheral accessories. In applications, the appropriate method can be chosen according to actual needs.