An improved freeze-thaw durability wet cast cementitious composition is provided that uses in-situ production of gas by gas generating additives and optionally polymeric microspheres that are blended directly into the mixture. , wherein at least one first heat exchanger cools the RF coil or the resonator of the at least one detection channel, and at least one further heat exchanger cools the RF coil or the resonator of the at least one transmitting or decoupling channel, and wherein the at least one first heat exchanger is separated from the at least one further heat exchanger. The transparent element is arranged in the first region in a region of the recess.