Towards detection of inflammation in adipose tissue: Microscopic field simulations to estimate water signal properties

Abstract

Inflammation of adipose tissue, particularly visceral adipose tissue, is assumed to be a causal factor for the development of type 2 diabetes, non-alcoholic fatty liver disease, and cardiovascular diseases. Invasive biopsy is currently mandatory for assessment and grading of adipose tissue inflammation. Magnetic resonance detection of the increased water content of inflamed adipose tissue is considered to be a non-invasive alternative. Additional water is mainly originating from macrophages clustering in small regions between adipocytes. This article addresses the characteristics of water signals from areas between adipocytes in terms of line width, line shape, and relaxation properties. Since water and lipids inside adipose tissue have different magnetic susceptibilities, microscopic field inhomogeneities arise depending on the geometry and orientation of the water containing confinements. Relatively pronounced microscopic field inhomogeneities in the water compartments cause a broad spectral distribution of water signals. As a consequence the water signal of adipose tissue shows special characteristics different to common parenchyma tissues, in which cell content and intercellular space consist of water. The broad and non-Lorentzian field distribution of signals emanating from the water compartments in adipose tissue results in a fast non-exponential signal decay. Therefore, short echo times are recommended for sensitive gradient-echo based imaging. A non-exponential irregular signal decay potentially leads to problems in fat/water separation using Dixon techniques. Marked microscopic field inhomogeneities in combination with diffusion related displacement of water molecules cause irreversible dephasing and therefore accelerated signal decay even for spin-echo sequences. A volume localized spectrum of porcine fat recorded at 3T by a STEAM sequence with an echo time of 5.4ms shows a broad water signal with a line width of 70Hz±4Hz, in contrast to the CH2-peak of lipids in the same spectrum with a line width of only 14.7Hz±0.7Hz. This finding is qualitatively consistent with the results of finite element modelling of the magnetic field in geometric models and experiments in phantoms with oil-filled balloons surrounded by water.