Nonlinear effects in magnetic resonance localized spectroscopy and images

The nonlinear effects associated with intermolecular multiple-quantum coherences (iMQCs) that are present in magnetic resonance imaging (MRI), localized spectroscopy (MRS), and spatially resolved thermometry of biological tissues are reviewed. These nonlinear effects occur especially for samples with a high concentration of resonant nuclei, at ultra-high magnetic fields or under hyperpolarization conditions. The classical Bloch equations and approaches based on quantum mechanical density operator evolution were employed for description of nonlinear effects on the spin system response in the presence of distant (long-range) dipolar field in samples containing high molecular mobility like liquids. The multiple spin echoes that appear in the presence of dipolar demagnetization fields in the presence of homogenous and heterogenous spin interactions and their applications are also discussed. One emphasis of the review is on the excitation, evolution, and detection of intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) in the presence of correlated field gradients that represent the basis for CRAZED pulse sequences (Warren et al. Science 262 (1993) 2005-2009). The physics behind these methods employed for magnetically equivalent and non-equivalent spins, J-coupled spin, in homonuclear and heteronuclear systems is discussed. The principles of magnetic resonance localized spectroscopy and imaging applications for brain investigations to reduce the effect of inhomogeneous magnetic fields and to increase the image resolution is reviewed. The physics related to the used of CRAZED methods to produce fundamentally different contrast than does conventional imaging is also addressed. Collective effects in the presence of strong nuclear magnetization that can affect MRI and MRS results such as spectral clustering and spin turbulence are summarized.