Reinvestigating hyperpolarized 129Xe longitudinal relaxation time in the rat brain with noise considerations

The longitudinal relaxation time of hyperpolarized (HP) 129Xe in the brain is a critical parameter for developing HP 129Xe brain imaging and spectroscopy and optimizing the pulse sequences, especially in the case of cerebral blood flow measurements. Various studies have produced widely varying estimates of HP 129Xe T1 in the rat brain. To make improved measurements of HP 129Xe T1 in the rat brain and investigate how low signal‐to‐noise ratio (SNR) contributes to these discrepancies, we developed a multi‐pulse protocol during the washout of 129Xe from the brain. Afterwards, we applied an SNR threshold theory to both the multi‐pulse protocol and an existing two‐pulse protocol. The two protocols yielded mean ± SD HP 129Xe T1 values in the rat brain of 15.3 ± 1.2 and 16.2 ± 0.9 s, suggesting that the low SNR might be a key reason for the wide range of T1 values published in the literature, a problem that might be easily alleviated by taking SNR levels into account. Copyright © 2007 John Wiley & Sons, Ltd.