Measurement and correction of the bulk magnetic susceptibility effects of fat: application in venous oxygen saturation imaging.

Abstract

To develop a correction method for the effects of the magnetic susceptibility of fat (χFat ) on the calculation of venous oxygen saturation (SvO2 ). The magnetic field shifts associated with the magnetic susceptibility of deoxyhemoglobin can be used to estimate SvO2 , a measure of oxygen extraction and metabolism. However, the distinct magnetic susceptibility of fat surrounding targeted veins will give rise to magnetic field perturbations that will extend into the vein and surrounding tissues, potentially confounding the calculation of SvO2 . Multi-echo modified Dixon fat-water separated imaging was used to quantify fat-water distributions around the superficial femoral vein (venous return from the lower leg). Fat fraction images were used to generate χFat images, to calculate and remove the associated fat-susceptibility-induced magnetic field shifts before the estimation of SvO2 . This approach was evaluated at rest and with plantar flexion exercise to evaluate calf muscle oxygen extraction in 10 healthy subjects. The presence of fat around the vein resulted in complex magnetic field shifts and errors in estimated SvO2 . Corrected resting SvO2 values were significantly larger than those measured with conventional methods, at rest (72.6 ± 11.0% vs. 65.2 ± 12.2%, P < 0.05) and post-exercise (37.4 ± 12.3% vs. 31.7 ± 12.7%, P < 0.05), with larger errors in individuals and/or regions with increased fat volumes. Estimation and removal of the field-effects from χFat enabled the use of fat tissues for the measurement and removal of the background magnetic field. The magnetic susceptibility effects of fat can confound SvO2 estimation, but the susceptibility field effects can estimated and removed with the use of modified Dixon fat-water separated imaging.