Magnetic resonance pore imaging, a tool for porous media research

Abstract

Porous media are highly prevalent in nature and span a wide range of systems including biological tissues, chemical catalysts or rocks in oil reservoirs. Imaging of the structure of the constituent pores is therefore highly desirable for life sciences and technological applications. This thesis presents the new development and application of a nuclear magnetic resonance (NMR) technique to acquire high resolution images of closed pores. The technique is a further development of diffusive-diffraction Pulsed Gradient Spin Echo (PGSE) NMR, which has been shown to image the pore auto-correlation function averaged over all pores. Until recently it was conventional wisdom that diffusive-diffraction PGSE NMR can only measure the magnitude of the form factor, due to its similarity to diffraction techniques such as x-ray and neutron scattering. In diffraction applications the loss of phase information is commonly referred to as the "phase problem", which prevents the reconstruction of images of the pore space by inverse Fourier transform. My work is based on a recently suggested modification of the diffusive-diffraction PGSE NMR method, which creates a hybrid between Magnetic Resonance Imaging (MRI) and PGSE NMR. Therefore, we call this approach Magnetic Resonance Pore Imaging (MRPI). We provide experimental confirmation that MRPI does indeed measure the diffractive signal including its phase and thus the "phase problem" is lifted. We suggest a two-dimensional version of MRPI and obtain two-dimensional average pore images of cylindrical and triangular pores with an unprecedented resolution as compared to state of the art MRI. Utilizing a laser machined phantom sample we present images of microscopic pores with triangular shape even in the presence of wall relaxation effects. We therefore show that MRPI is able to reconstruct the pore shape without any prior knowledge or assumption about the porous system under study. Furthermore, we demonstrate i During preparation of my thesis I received help and encouragement from many people and institutions to which I would like to express my gratitude. First of all, I would like to thank my supervisor Dr. Petrik Galvosas for the guidance, stimulation and support he provided throughout the past three and a half years. His help was especially indispensable regarding the art of mastering the NMR spectrometer and the gradient hardware. Moreover, he extended his support beyond work related issues, which I experienced as exceptionally kind and which made my stay in New Zealand such an enjoyable time. I would also like to thank Professor Sir Paul Callaghan for providing such a stimulating environment and for entrusting the project for this thesis to me. He sadly passed away, but is remembered vividly in his lab. There are many people whom I would like to thank from the NMR lab at Victoria University of Wellington. Dr. Mark Hunter helped me to get going on the MRPI project and provided key simulations in the early stages of my work. My colleagues

Nuclear magnetic resonance diffusion pore imaging: Experimental phase detection by double diffusion encoding

“…For comparison, measurements using the long-narrow approach were performed [15]. In recent experiments, this method has already been demonstrated to be capable of measuring average images of cylindrical and triangular pores [18][19][20]. For this method, a long-narrow gradient profile is used, which is non-antisymmetric in time.…”

Phase incremented echo train acquisition applied to magnetic resonance pore imaging

“…The gradient pulses were applied with alternating polarity in the laboratory reference frame while they were interspersed with 180 • rf-pulses. The replacement was chosen to minimize the dephasing due to the internal gradients g int by refocusing their influence on the spin phases [7]. Fig.…”

“…the time from the end of one gradient pulse until the leading edge of the next gradient pulse. Figure 3 a) shows the rf-pulse sequence of the CPMG based MRPI experiment [7,8]. The rf-pulse phases were implemented a) Figure 3: MRPI pulse sequence with a CPMG like rf-pulse scheme (a).…”

Magnetic Resonance Pore Imaging: Overcoming the resolution limit of MRI for closed pore systems

“…Because of this distinct gradient pattern this experiment has been dubbed ''long-narrow'' PGSE NMR [8]. Experimental confirmation of the ''long-narrow'' approach has been obtained in parallel for hyperpolarised xenon gas diffusing in millimeter sized triangular capillaries [9] and for water filled cylindrical capillaries on the micrometer scale [10]. In this contribution we present our approach which we call Magnetic Resonance Pore Imaging (MRPI) because of its close resemblance of MRI phase imaging and therefore its potential inheritance of MRI pulse schemes and data processing methodology.…”