Temperature dependent chemical shifts of pyruvate and lactate enable in vivo hyperpolarized 13C MRSI thermometry.

The chemical shift of many molecules changes with temperature, which enables non-invasive magnetic resonance imaging (MRI) thermometry. Hyperpolarization methods increase the inherently low ¹³C MR signal. The commonly-used hyperpolarized probe [1-¹³C]pyruvate, and its metabolic product [1-¹³C]lactate, exhibit temperature and concentration dependent chemical shift changes that have not previously been reported. These effects were characterized at 7 T and 11.7 T in vitro and applied for in vivo thermometry both preclinically at 7 T and to human data at 3 T. Apparent temperature values from mouse abdomen and brain were similar to rectally measured temperature. Human brain and kidney apparent temperatures from ¹³C MRSI were lower than known physiological temperatures, suggesting that additional effects may currently limit the use of this method for determining absolute temperature in humans. The temperature dependent chemical shift changes also have implications for sequence design and for in vitro studies with hyperpolarized pyruvate.