MR relaxometry: Introduction and applications in the adult brain and placenta

Though MRI is most often used clinically to perform structural
imaging, aspects of MR signals are sensitive to
physiology. Longitudinal (T1) and transverse (T2) relaxation times
depend on many aspects of tissue microstructure, such as extracellular
water volume (T1), blood oxygen saturation (T1 and T2) and vascular
structure (T2 and T2*). Though relaxometry could inform clinical
decision making, there are many challenges to acquiring reliable
values for T1/T2/T2* in a clinical setting. For example, several
strategies have been proposed for isolating a pure blood signal for T2
analysis with the intention of determining blood oxygen saturation. I
will present one of these techniques, called QUIXOTIC, to demonstrate
how relaxometry can be used to quantify brain oxygen consumption and
to illustrate some of the complications related to measuring the
relaxivity of tissues. I will then describe ongoing efforts to use
magnetic resonance fingerprinting, a new approach to quantitative MRI,
to determine the baseline physiology of the placenta.