Quantifying Skin Sodium Concentration With Novel Very Rapid Biexponential T₂ Relaxometry and Signal Loss Compensation...

To accurately measure skin tissue sodium concentration (TSC) through development of spin-3/2 relaxometry methodology that also considers blood sodium.

M_xy generation during RF excitation is substantially reduced by very rapid T_2f. Compensation for TSC measurement requires knowledge of both T_2f and B₁, but the effects of these two parameters are correlated together with a blood sodium contribution that does not exhibit very rapid T_2f. The new spin-3/2 relaxometry methodology uses 7 scans with 90° RF pulse durations ranging from 70 to 900 μs (constant TE = 2 ms), 2 scans with 60° and 120° flip-angles, and 2 scans with TE = 6 and 10 ms to disentangle and simultaneously fit skin T_2f, T_2s, relative B₁ and blood sodium fraction. Simulated M_xy from the fitted parameters was used for image compensation to attain skin sodium concentration. A gellan gum phantom with known sodium concentration was used for validation. Calf skin of 20 healthy volunteers was scanned with a surface-coil at 4.7T, measuring T_2f, T_2s, blood sodium fraction and TSC.

The new methodology measured skin T_2f = 0.10 ± 0.03 ms, T_2s = 13.7 ± 1.5 ms, blood sodium fraction = 0.08 ± 0.05, and ultimately TSC = 90 ± 8 mM, which was not correlated with age or sex. However, blood sodium fraction in this model positively correlated with age.

This study yields the first skin TSC measurements consistent with atomic absorption spectroscopy. The potentially large impact of blood sodium in ²³Na MRI of skin, which may also reflect other fluid-like space, is a particular concern if single images are used for concentration measurement.