CPMG measurements and ultrafast imaging in human lungs with hyperpolarized helium-3 at low field (0.1 T)

This work reports the use of single-shot spin echo sequences to achieve in vivo diffusion gas measurements and ultrafast imaging of human lungs, in vivo, with hyperpolarized (3)He at 0.1 T. The observed transverse relaxation time of (3)He lasted up to 10 s, which made it possible to use long Carr-Purcell-Meiboom-Gill echo trains. Preliminary NMR studies showed that the resolution of lung images acquired with hyperpolarized (3)He and single-shot sequences is limited to about 6 mm because of the diffusion of the gas in applied field gradients. Ultrafast images of human lungs in normal subjects, achieved in less than 0.4 s with the equivalent of only 130 micromol of fully polarized (3)He, are presented. Comparison with other studies shows that there is no SNR penalty by using low fields in the hyperpolarized case. Advantage was taken of the self diffusion-weighting of the rapid acquisition with relaxation enhancement (RARE) sequence to acquire apparent diffusion coefficient (ADC) images of the lungs. Time scales of seconds could be explored for the first time because there is no hindrance from T(*)(2) as with the usual approaches. At 0.1 T, 180 degrees RF pulses can be repeated every 10 ms without exceeding specific absorption rate limits, which would not be the case for higher fields. Moreover, at low field, susceptibility-induced phenomena are expected to be milder. This supports the idea that low-field imagers can be used for hyperpolarized noble gas MRI of lungs and may be preferred for ADC measurements.