Abstract
Sodium is crucial for the maintenance of cell physiology, and its regulation of the sodium‐potassium pump has implications for various neurological conditions. The distribution of sodium concentrations in tissue can be quantitatively evaluated by means of sodium MRI (23Na‐MRI). Despite its usefulness in diagnosing particular disease conditions, tissue sodium concentration (TSC) estimated from 23Na‐MRI can be strongly biased by partial volume effects (PVEs) that are induced by broad point spread functions (PSFs) as well as tissue fraction effects. In this work, we aimed to propose a robust voxel‐wise partial volume correction (PVC) method for 23Na‐MRI. The method is based on a linear regression (LR) approach to correct for tissue fraction effects, but it utilizes a 3D kernel combined with a modified least trimmed square (3D‐mLTS) method in order to minimize regression‐induced inherent smoothing effects. We acquired 23Na‐MRI data with conventional Cartesian sampling at 7 T, and spill‐over effects due to the PSF were considered prior to correcting for tissue fraction effects using 3D‐mLTS. In the simulation, we found that the TSCs of gray matter (GM) and white matter (WM) were underestimated by 20% and 11% respectively without correcting tissue fraction effects, but the differences between ground truth and PVE‐corrected data after the PVC using the 3D‐mLTS method were only approximately 0.6% and 0.4% for GM and WM, respectively. The capability of the 3D‐mLTS method was further demonstrated with in vivo 23Na‐MRI data, showing significantly lower regression errors (ie root mean squared error) as compared with conventional LR methods (p < 0.001). The results of simulation and in vivo experiments revealed that 3D‐mLTS is superior for determining under‐ or overestimated TSCs while preserving anatomical details. This suggests that the 3D‐mLTS method is well suited for the accurate determination of TSC, especially in small focal lesions associated with pathological conditions.
Highlights
With recent advances in MRI hardware, sodium MRI (23Na-MRI) has received increasing attention as it provides direct biochemical information on tissue viability such as cell membrane integrity.[1,2] Sodium is a crucial component that helps to maintain cell physiology via the sodium-potassium pump (Na+/K+-ATPase), which is a high energy consumer and dependent upon adenosine triphosphate production.[3]
This is due to low tissue sodium concentrations (TSCs) in the brain and biexponential T2 relaxation behavior with a very short T2.12 TSC is defined as a weighted average of intracellular sodium and extracellular sodium concentrations,[9] which leads to a TSC of approximately 40 mM in brain tissue
We have demonstrated a robust voxel-wise partial volume correction (PVC) method in 23Na-MRI with an attempt to reduce the inherent spatial blurring for the accurate determination of TSC values in the brain
Summary
With recent advances in MRI hardware (eg high-field magnets and strong gradient capabilities), sodium MRI (23Na-MRI) has received increasing attention as it provides direct biochemical information on tissue viability such as cell membrane integrity.[1,2] Sodium is a crucial component that helps to maintain cell physiology via the sodium-potassium pump (Na+/K+-ATPase), which is a high energy consumer and dependent upon adenosine triphosphate production.[3]. A major obstacle in the clinical application of 23Na-MRI is its relatively low MR sensitivity when compared with conventional proton MRI (1H-MRI), despite yielding the second strongest NMR signal among all nuclei present in biological tissue. This is due to low tissue sodium concentrations (TSCs) (around 40 mM) in the brain and biexponential T2 relaxation behavior with a very short T2.12 TSC is defined as a weighted average of intracellular sodium (approximately 10-15 mM) and extracellular sodium (approximately 140 mM) concentrations,[9] which leads to a TSC of approximately 40 mM in brain tissue. It is necessary to correct for PVEs to avoid under- or overestimation of TSC in the brain
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