In six subjects thermograms of the thighs and the forearms were taken before, during and after 10 min ergometer exercise at 100 W at an ambient temperature of 23 degrees C. During exercise, an intra-individually constant and reproducible skin temperature pattern with local temperature differences exceeding 3 degrees C evolved. Reactions after external local cooling or after occlusion of blood flow and measurements with a laser Doppler-flowmeter showed dispersed convective heat transport to be the source of this irregular pattern. Temperature differences of 3.6 degrees C and deviations of blood flow in the skin microcirculation of 300% within a distance of a few centimetres reduce the value of single-spot measurements of skin temperature with reference to the whole extremity.
The application of spin-echo magnetic resonance imaging sequences on non-invasive temperature imaging for temperature mapping of human limbs is investigated. In an in vitro experiment performed on a meat sample, the equilibrium magnetisation P and the spin-lattice relaxation time T1 are calculated from the values for the repetition time TR and the signal intensities obtained by a spin-echo sequence at different tissue temperatures as measured by a fibre-optic probe. T1 is linearly correlated to the tissue temperature, and P is linearly correlated to the reciprocal value of the absolute temperature. Both effects, taken together, lead to a non-linear dependency of the signal intensity on temperature. Therefore a TR leading to maximum temperature dependency of the signal intensity is calculated and used in the further experiments. In the in vivo experiments, the lower legs of two volunteers are cooled from outside. Images are acquired with a spin-echo sequence (1.5 T, TR = 1200 ms, TE = 10 ms). A rise in signal intensity in the muscle with falling skin temperature is observed, particularly in more peripheral muscle layers. This study shows that spin-echo sequences can be used to monitor temperature changes and temperature differences in living muscle tissue.
EINLEITUNG Die Kernspintomographie erlaubt die nichtinvasive, ortsaufgelöste Messung von Temperaturen in lebendem Gewebe. Dazu wurden in den letzten Jahren verschiedene Ansätze erprobt, vor allem an Phantomen, z.T. aber auch anhand von in-vivo-Studien. Eine mögliche Methode nutzt die Temperaturabhängigkeit der Spin-Gitter-Relaxationszeit T, aus [1]. Ein anderes Verfahren mißt die Phasenverschiebung, die durch die temperaturabhängige Änderung der Protonenresonanzfrequenz hervorgerufen wird [2]. Mit speziellen Sequenzen läßt sich der ebenfalls temperaturabhängige Diffusionskoeffizient bestimmen [3]. Allen diesen Verfahren ist gemeinsam, daß der temperaturabhängige Parameter aus mehreren Aufnahmen berechnet werden muß, was zu einer erhöhten Anfälligkeit gegen Bewegungsartefakte führt. Bislang wenig geprüft wurde die Möglichkeit zur Messung der Temperaturabhängigkeit der Gleichgewichtsmagnetisierung. Vorteilhaft ist die einfache Messung dieses Parameters durch Protonendichte-gewichtete Sequenzen. In der vorliegenden Studie soll dieses Verfahren sowohl invitro als auch in-vivo am Beispiel der Darstellung von Temperaturgradienten in Extremitäten erprobt werden.
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