A Comparison of Magnetic Resonance Methods for Spatially Resolved T₂ Distribution Measurements in Porous Media

Naturally occurring porous media are usually characterized by a distribution of pore sizes. If the material is fluid saturated, the ¹H magnetic resonance (MR) signal of the wetting fluid depends on the pore size, the surface relaxivity and the fluid itself. Measurement of the transverse relaxation time T₂ is a well-established method to characterize material samples by means of MR.T₂ distribution measurements are the most basic measurement employed to determine the fluid-matrix properties in MR core analysis [1].
Three methods for T₂ distribution mapping, namely spin-echo SPI (SE-SPI) [2], DANTE-Z CPMG [3] and adiabatic inversion CPMG [4] are compared in terms of spatial resolution, minimum observable T₂ and sensitivity [5]. SE-SPI is an imaging method, which measures spatially resolved T₂s. A variant is introduced in this work that employs pre-equalized magnetic field gradient waveforms [6] and is therefore able to measure shorter T₂s than previously reported. The DANTE-Z pulse train and adiabatic inversion pulse are compared in terms of T₁ or T₂ relaxation time effects during the RF pulse application, minimum pulse duration, requisite RF pulse power, and inversion profile quality. In addition to experimental comparisons, simulation results are presented .
For a Berea sandstone with a typical sandstone T₂ distribution, employing 1.58 ms duration selective RF pulses, the T₂ distribution measurement results are similar for SE-SPI and slice selective CPMG methods as shown in the figure. One may replace SE-SPI measurements by slice selective CPMG at the expense of losing high spatial resolution, but the T₂ distribution measurements will feature higher sensitivity.
For slice selective CPMG methods, two inversion schemes, a DANTE-Z pulse train and an adiabatic inversion pulse were tested. The adiabatic inversion pulse may be applied in as short a time as 0.3 ms, which is very beneficial when measuring the T₂ distribution for samples including short lifetime T₁ and T₂ components (< 5 ms).
Our conclusion is that adiabatic inversion (insensitive to B₁ inhomogeneities) followed by a CPMG pulse train is the preferred way to measure local T₂ distributions provided a regional measurement of coarse spatial resolution is sufficient.

(a, c and e) Contour plot and (b, d and f) waterfall plot of the T₂ distributions measured with SE-SPI, DANTE-Z CPMG and adiabatic inversion CPMG at different positions along the partly de-saturated Berea sandstone core.