Demonstration of the temperature dependence of T₂^eff of water performed using two different commercial nuclear magnetic resonance system

In this study we demonstrate a method to relate T₂^eff data from two commercial nuclear magnetic resonance systems: A horizontal bore Bruker 2.35 T small animal scanner and a 0.25 T NMR MOUSE® unilateral permanent magnet system. A simple mathematical transform is performed to the governing theoretical equations to facilitate interpretation of the data. Water samples have been used in this work as it represents a common material, which is well understood in terms of chemistry but poorly quantified in terms of T₂^eff temperature dependence. The temperature range selected for this series of experiments is between the freezing and boiling point of water at atmospheric pressure.

It is show that in the system with a homogeneous field gradient, such as the Bruker 2.35 T small animal scanner, the effect of temperature upon T₂^eff provided a linear fitting (Fig 1(a)). This would be expected in a highly homogenous field where dephasing due to the spins diffusing within the field gradient should be minimal. In contrast it was observed for a system with an extremely large field gradient, the NMR MOUSE® (Fig 1(b)), that T₂^eff experienced an exponential dependency upon temperature. This effect was shown to be a result of the MOUSE's® extremely large gradient in the direction of the polarizing field (B₀) coupled with the thin sensitive volume, meaning that diffusion within the gradient made a significant contribution to the spin dephasing.

This work demonstrated a methodology for the collection of meaningful T₂^eff data in a system with a high magnetic field gradient (the NMR MOUSE®), with the effect of diffusion across this gradient leading to a an exponential variation in T₂^eff with temperature. It is further demonstrate how a simple mathematical treatment can be used to negate this effect and provide a technique for the modeling of T₂^eff as a function of temperature in other systems when the magnetic field gradient is known.

Fig 1. T₂^eff as a function of temperature. a: displays data from the Bruker system and NMR MOUSE®, note the linear dependency of these data sets. b: Insert showing an expanded image of the unmodified NMR MOUSE® data.