Financial support to K.Z. by the Austrian Science Foundation (FWF) (Project No. P24742) is gratefully acknowledged. E.S. thanks the Austrian Academy of Sciences for a DOCfFORTE

fellowship. “
“Although magnetic resonance Transmembrane Transporters modulator imaging (MRI) of the gas phase is possible without the use of hyperpolarized (hp) spin states [1], the density of gases at ambient pressure and temperature is typically reduced by about three orders of magnitude compared to the respective condensed phase. This significantly lowers Nuclear Magnetic Resonance (NMR) signal intensities and limits magnetic resonance imaging (MRI) resolution as the MRI experiments require gases with high gyromagnetic ratio, γ, high spin concentrations, and shorter longitudinal (T1) relaxation times (to allow for rapid signal averaging). Hp spin states, on the other hand, can enhance the NMR signals by many orders of magnitude compared to thermally polarized states and enable gas phase MRI of both dilute spin systems and nuclei with low gyromagnetic ratios. Since the hyperpolarization is almost always produced outside the MRI detection region, the hp gas typically requires some form of transport from the hyperpolarizer to the detection zone and sufficiently long relaxation times are needed to sustain the generated hyperpolarized state until NMR signal detection has

occurred. There is no disadvantage from slow T1 relaxation in hyperpolarized MRI because signal averaging is not based on relaxation recovery but on renewed delivery of hyperpolarized species for every scan. Unfortunately, most molecules PLX3397 nmr experience fast relaxation in the gas phase due to spin–rotation interactions. A noticeable exception is the group of mono-atomic noble gases where spin–rotation relaxation only occurs during short-lived interaction with other atoms [2]. Therefore T1 times of many hours and even days can be possible unless additional relaxation

mechanisms are present [2], [3], [4] and [5]. To date, the most widespread and successful MRI applications of hp noble gases utilize the isotope mafosfamide 3He (spin I = ½, NMR frequency 75.905 MHz at 2.35 T) for preclinical and clinical studies of pulmonary pathophysiology. A review of the successful applications with hp 3He MRI would exceed the purpose of this paper and is therefore best left to the specialists in this field (see for instance [6], [7] and [8] for previous reviews). Furthermore, the main supply source for 3He is tritium decay in nuclear (fusion) warheads with no viable current alternative in sight. The very high demand for this isotope for many types of applications has therefore led to a 3He supply crisis as evidenced by US congressional hearings [9]. The best remedies to this problem for the MR community may be rigorous 3He recycling whenever possible and the exploration of alternative techniques.