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

The longitudinal relaxation time of hyperpolarized (HP) Xe-129 in the brain is a critical parameter for developing HP Xe-129 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 Xe-129 T-1 in the rat brain. To make improved measurements of HP Xe-129 T-1 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 Xe-129 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 Xe-129 T-1 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 T-1 values published in the literature, a problem that might be easily alleviated by taking SNR levels into account. Copyright (c) 2007 John Wiley & Sons, Ltd.