Quantifying Skin Sodium Concentration With Novel Very Rapid Biexponential T ₂ Relaxometry and Signal Loss Compensation Methodology

Purpose To accurately measure skin tissue sodium concentration (TSC) through development of spin‐3/2 relaxometry methodology that also considers blood sodium. Methods 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 B 1 , 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 B 1 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. Results 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. Conclusion This study yields the first skin TSC measurements consistent with atomic absorption spectroscopy. The potentially large impact of blood sodium in 23 Na MRI of skin, which may also reflect other fluid‐like space, is a particular concern if single images are used for concentration measurement.