BOLD mapping in the placenta using multi-echo MRI.

BOLD mapping in the placenta using multi-echo MRI.

Blood flow in the maternal and fetal regions within the placenta permit the transfer of both oxygen and nutrients to the fetus and the removal of waste products. One of the basic MRI measures of placental integrity that has been proposed for placenta evaluation has been the blood oxygenation level. This can be evaluated using quantitative MRI techniques. BOLD MRI can assess tissue oxygenation non-invasively and is based on the magnetic properties of hemoglobin. Several animal studies have been performed on BOLD-MRI in pregnancy and these techniques allow for detection of abnormal oxygenation or perfusion. Decrease in the BOLD SI in the placenta has been shown with maternal hypoxia. Previous studies in humans have used short BOLD sensitive acquisitions to examine the effect of maternal oxygen challenges in the presence of tissue motion. In particular, techniques such as R2* mapping provide a marker of BOLD that is robust to tissue motion and spin history effect. However, unlike in the brain, in the placenta there is a lack of baseline normative values for R2* based measurements and how they change with age. In this study we used long duration multi-slice dual echo imaging (3 to 5 minutes) to estimate average values for R2* in regions of the placenta using a set of carefully placed manually defined regions of interest in the fetal and maternal regions of the placenta. These acquisitions did not make use of breath-hold imaging, but instead used retrospective marking of tissues regions within the placenta that were carefully adjusted to account for frame to frame and slice to slice movements allowing the collection of a much longer time sequence of data for signal averaging.