  SQUID-based MRI (magnetic resonance imaging) at microtesla fields has
developed significantly over the past few years. Here we describe application
of this method for magnetic relaxation measurements in the living human brain.
We report values of the longitudinal relaxation time T1 for brain tissues,
measured in vivo for the first time at microtesla fields. The experiments were
performed at 46 microtesla field using a seven-channel SQUID system designed
for microtesla MRI and MEG. Values of T1, measured for different tissues at
this field, are found to be close (within 5%) to the corresponding values of
the transverse relaxation time T2 at the same field. Implications of this
result for imaging contrast in microtesla MRI are discussed.
