1H magnetic resonance imaging of freezing and thawing in freeze-tolerant frogs.

Proton magnetic resonance imaging (MRI) of the processes of freezing and thawing in the wood frog Rana sylvatica provided noninvasive and real-time analysis of the mode of ice propagation through the body of a freeze-tolerant vertebrate. MRI revealed a directional movement of ice from the exterior inward that required several hours to reach completion. Freezing in core organs such as liver, which produces and exports cryoprotectant, and heart, which circulates it, was delayed and occurred well after the organs were surrounded by extraorgan ice. Natural thawing was a very different process; thawing began uniformly throughout the body, but core organs melted more rapidly than peripheral ones, an adaptation that may be key to the early restoration of heartbeat and breathing. The images presented demonstrate the sensitivity and power of MRI and its potential to become a critical monitoring technology in the development of cryopreservation techniques for mammalian organ explants.