Validation of Thickness Measurements of Skin Layers Using a Mobile NMR Sensor

Kurzfassung

In microgravity 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 of local fluid volume regulation, new techniques are required for in vivo measurements of changes in thickness and fluid content of different skin layers. In this study the NMR-MOUSE® (Nuclear Magnetic Resonance Mobile Universal Surface Explorer) was evaluated for its capability of measuring different skin layers. An instrument with 0.47 Tesla was used that recorded 1H NMR signals at a resonance frequency of 20 MHz in a planar slice of 50 µm thickness at 5500 µm distance from the magnet surface. This slice was oriented parallel to skin surface. By moving the NMR-MOUSE in equidistant 50 µm steps the layer structure of the skin was analysed within 15 min. For 10 male subjects (34±13.2 years old, BMI of 24.2±2.75) the skin at the inner side of the right lower arm was examined. On each subject five measurements were performed before and during venous occlusion (VO). Each test was done on a different day. During VO the NMR measurement began 5 min after VO had started. In addition skin structure at the same place on the lower arm was analysed using an ultrasound imaging instrument with a 10 MHz linear scanner head in B mode. In the NMR profiles one can identify two skin layers which represent the dermis including the epidermis with rather homogeneous signal intensity and the subcutis with a more variable 1.5 to 2 times higher intensity. However, the borders between the two layers were not sharp but represented zones of very variable thickness. The general shape of the skin profiles varied between subjects and in some subjects large variation was even observed within the five repetitions. For impartial determination of the thickness of dermis and subcutis the profiles were curve fitted and analysed for turning points representing the bordermarks between layers. For all subjects mean thickness (±SD) were 1230 ±180 µm for the dermis, 1740 ±840 µm for subcutis, and 2970 ± 830 µm for the whole skin thickness, respectively. The differences in thickness between normal state and VO were +100±130 µm for dermis, +30 ±200 µm for subcutis, and +130 ±260 µm for the whole skin, respectively. All mean changes were not significant. Only in one subject the increase by VO was significant. In all other cases variation in profiles within repetitions were larger than the effect of VO. Thickness values measured with ultrasound imaging were 1440±176 µm for the dermis, 1390±291 µm for the subcutis and 2760±313 µm for the whole skin thickness. Ultrasound measurements visualise that in the skin of lower arm layers of dermis and subcutis were not plain and not constant in thickness. This likely explains the variability in skin profiles received from the NMR-MOUSE. The NMR-MOUSE detects signals from plain slices of about 1 cm2, which was therefore not geometrically adequate for an analysis of skin structure. Furthermore, ultrasound imaging was by far faster and more reliable than the detection of NMR-MOUSE profiles. This NMR technique may still be of interest if it could provide information about e.g. water content, water biding of protein, or fat content. However, information by NMR variables like T2-spinrelaxion is rather unspecific in its physical origin and spectroscopy using a NMR-MOUSE shows poor resolution and requires too long acquisition time.