Nuclear Magnetic Resonance Imaging

Nuclear magnetic resonance imaging is a tomographic imaging technique that produces maps of the nuclear magnetic resonance signal in a sample. The signal from a volume element (voxel) in the sample is represented as an intensity of a picture element (pixel) in an image of the object. Nuclear magnetic resonance imaging has developed as an excellent noninvasive medical imaging technique, providing images of anatomy, pathology, circulation, and functionality that are unmatched by other medical imaging techniques. Nuclear magnetic resonance imaging is also an excellent nondestructive analytical tool for studying materials. Applications of nuclear magnetic resonance imaging in the field of materials science have also been developed, but to a lesser extent. The most probable reason why the development of materials applications of nuclear magnetic resonance imaging has lagged behind that of clinical applications is the cost-to-information ratio. The two major branches of magnetic resonance are electron paramagnetic resonance (EPR), or electron spin resonance (ESR), as it is sometimes called, and nuclear magnetic resonance (NMR). A much smaller third component of magnetic resonance is muon spin resonance. EPR and ESR measure properties of the magnetic moment of the electrons or electron orbitals, while NMR is concerned with the magnetic moment of nuclei with unpaired protons and neutrons. Electron nuclear double resonance (ENDOR) examines the interactions between the nuclear and electron moments. All of the above techniques are used to measure properties of atoms and molecules in materials. Within the fields of EPR and NMR are imaging disciplines. The term magnetic resonance imaging (MRI) should be used to describe imaging techniques based on both NMR and EPR or ESR. For historical reasons, MRI has been used for nuclear magnetic resonance imaging, the mapping of the spatial distribution of the NMR signals in matter. EPR imaging (EPRI) and ESR imaging (ESRI) have been used to describe imaging of the spatial distribution of electron and electron orbital magnetic moments in matter. This article concentrates on MRI, as previously defined, of materials. MRI is used to image both large- and small-diameter objects. When imaging small objects, <1 cm in diameter, the imaging process is typically called NMR microscopy. When imaging larger objects, the procedure is generally called MRI. In reality, the imaging procedures are the same. The distinction arises from the type of instrument used, rather than the dimensions of the material. NMR microscopy is performed on an NMR spectrometer equipped with imaging hardware. Samples that do not fit into the microscope are imaged on a small-bore imager or clinical whole-body imager. No distinction is made in this article between MRI and NMR microscopy of materials. MRI has been applied in many disciplines to study materials. MRI, as a nondestructive analytical technique, has very few competing techniques. X-ray and ultrasound imaging techniques come close to providing similar information in a nondestructive manner, but do not have the sensitivity to many of the fundamental properties of matter that MRI has. There have been two ways in which MRI has been used to study materials. One approach images the material, and the other images the absence of material. The first approach is to directly image the NMR signal from a signal-bearing substance in the sample. The second is to introduce a signal-bearing substance into voids within the material and image the signal from the signal-bearing substance. Keywords: nuclear magnetic resonance (NMR); theory; magnetic resonance imaging (MRI); practical aspects; spin-echo sequence; gradient recalled; back-projection imaging; echo-planar imaging; diffusion imaging; flow imaging; chemical shift imaging; three-dimensional imaging; solid-state imaging; sample preperation; problems; glossary of terms