The placenta's job is oxygen exchange. MRI measures a quantity called T2 that decreases when hemoglobin is deoxygenated — a signal that should track how well the placenta is working. Turnbull, Hutchinson, and colleagues measured both T2 and magnetic susceptibility in placentas across pregnancy and found that the obvious interpretation is wrong.
T2 drops over gestation, but not primarily because the placenta extracts more oxygen as the fetus grows. The dominant contribution to the T2 change comes from local magnetic field inhomogeneities — microscopic variations in tissue structure that shorten the signal independent of oxygen status. The bulk oxygenation change matters, but the structural change dominates. A measurement intended to track physiology is dominated by anatomy.
Susceptibility mapping disentangles the two. Unlike T2, susceptibility responds to the paramagnetic properties of deoxyhemoglobin directly and is less sensitive to structural inhomogeneity. In compromised placentas — those with growth restriction or preeclampsia — susceptibility variation increases while T2 falls. The two metrics together separate the oxygen signal from the structural noise that T2* alone cannot distinguish.
The spatial pattern adds a second finding. The central region of the placenta maintains more consistent oxygenation than the periphery. The fetal side perfuses less effectively than the maternal side. The placenta is not uniformly functional — it has a geography of competence, with the center holding while the edges degrade.
The single number that was supposed to measure placental health turns out to measure two things at once. Separating them requires a second measurement that responds to one but not the other.