A mobile NMR sensor for local examinations of fluid shifts in the skin

Kurzfassung

In microgravity and the resulting absence of a hydrostatic component of local blood pressure the skin at the head shows an oedema whereas the skin at the legs undergoes dehydration. As a prerequisite to study the mechanisms behind this phenomena in more detail, new techniques are required for in vivo measurements of changes in volume and fluid content of different skin layers. In this study the technology of the NMR-MOUSE® (Nuclear Magnetic Resonance-Mobile Universal Surface Explorer) was evaluated with regard to its capability to detect local differences in water content measured as relative changes in 1H signal intensity. In our study an instrument with 0.4 Tesla was used that recorded 1H spins at a resonance frequency of 17 MHz in a planar slice of 50 µm thickness at 4 mm distance from the magnet surface. This slice was oriented parallel to the plane of the skin. By moving the NMR-MOUSE in 80 equidistant 50 µm steps under computer control, the layer structure of the skin could be profiled up to a depth of 4 mm. The total measuring time for a profile was about 20 min. For 7 male subjects the skin at the outer side of the right lower arm was examined. The measurements were performed before and during venous occlusion (VO). During VO the NMR measurement started 5 min after induction of the occlusion. NMR spin density profiles allow the identification of two skin layers which represent the dermis with rather homogeneous signal intensity and an average thickness of 1400 ± 250 µm; 1150 to 1900 µm (mean ± SD; range) and the subcutis with a more variable 1.5 to 2 times higher intensity and a thickness of 1500 ± 600 µm; 500 to 2150 µm, respectively. In the dermis of all subjects, the application of VO causes a significant increase in thickness by 180 ± 160 µm; 50 to 450 µm (P < 0.01). The thickness of subcutis increased only in 4 of 7 subjects with 100 ± 270 µm; -100 to 700 µm. The dermis signal intensity did not change at VO. In the subcutis a distinct drop in signal intensity was found in those subjects who also showed an increase in thickness. VO seemed to have different effects on dermis and subcutis in lean and fat skin. There were trends that a) subjects with a thin skin showed smaller changes in dermis with VO; b) in subjects with thin subcutis the thickness of subcutis increased during VO, whereas in subjects with thick subcutis a decrease or no change were observed; c) the water in lean skin probably shows a lower 1H signal intensity than the lipids stored in fat cells. However, all trends observed in this pilot study need further verification by examination of a larger number of subjects. The NMR-MOUSE has the potential to become a very useful tool studying local fluid volume regulation in human skin.