Direct detection of brown adipose tissue thermogenesis in UCP1-/- mice by hyperpolarized Xe MR thermometry.

Brown adipose tissue (BAT) is a type of fat specialized in non-shivering thermogenesis. While non-shivering thermogenesis is mediated primarily by uncoupling protein 1 (UCP1), the development of the UCP1 knockout mouse has enabled the study of possible UCP1-independent non-shivering thermogenic mechanisms, whose existence has been shown so far only indirectly in white adipose tissue and still continues to be a matter of debate in BAT. In this study, by using magnetic resonance thermometry with hyperpolarized xenon, we produce the first direct evidence of UCP1-independent BAT thermogenesis in knockout mice.

Calibration of methylene-referenced lipid-dissolved xenon frequency for absolute MR temperature measurements.

Purpose: Absolute MR temperature measurements are currently difficult because they require precalibration procedures specific for tissue types and conditions. Reference of the lipid-dissolved Xe resonance frequency to temperature-insensitive methylene protons (rLDX) has been proposed to remove the effect of macro- and microscopic susceptibility gradients to obtain absolute temperature information. The scope of this work is to evaluate the rLDX chemical shift (CS) dependence on lipid composition to estimate the precision of absolute temperature measurements in lipids.

Simple and robust referencing system enables identification of dissolved-phase xenon spectral frequencies.

Purpose: To assess the effect of macroscopic susceptibility gradients on the gas-phase referenced dissolved-phase Xe (DPXe) chemical shift (CS) and to establish the robustness of a water-based referencing system for in vivo DPXe spectra.

Methods: Frequency shifts induced by spatially varying magnetic susceptibility are calculated by finite-element analysis for the human head and chest. Their effect on traditional gas-phase referenced DPXe CS is then assessed theoretically and experimentally.