The placenta is a fundamental organ that plays an important role in the nutrition supply and respiration of the fetus, supporting normal fetal development throughout the pregnancy [1]. Placental flow and oxygen supply to the tissue increases across gestational age as does blood flow in the arteries and intervillous space [[2], [3], [4]]. The endometrial blood supply of non-pregnant women changes from a few millimeters (mm) per minute to 500 mm per minute in pregnant women at term [5]. The continuous distension of the uteroplacental arteries during the 2nd and 3rd trimester causes an increase of blood supply favorable to the growth of the placenta and the fetus [6]. To date, 3D/4D ultrasound (US) and spatiotemporal image correlation have been used clinically to evaluate the fetal vasculature [[7], [8], [9]]. However, US is limited by the small field of view (FOV), cross sectional resolution, the presence of bony structures, abnormal fetal position, and maternal obesity.

Magnetic resonance angiography (MRA) is a powerful tool to evaluate tissue vasculature and, in the case of imaging the placental vasculature, could be a valuable accessory to US imaging. Some studies have evaluated the use of MR imaging in ex-vivo studies of the placenta using a contrast agent [[10], [11], [12]]. Our previous study showed that in vivo imaging of the placenta without a contrast agent can be accomplished using a conventional MRA approach based on 2D time-of-flight (TOF) imaging to generate both placental and fetal vasculature [13]. That study was done using a 3 T scanner (Verio, Siemens, Erlangen, Germany), and all the experiments were performed in the late 3rd trimester. Although currently more centers are trying to use 3 T for imaging the fetus, the majority of studies still take place at 1.5 T during both the 2nd and 3rd trimesters. Therefore, our goal was to investigate the feasibility of using 2D TOF MRA in the placenta and the fetus at both 1.5 T and 3 T in the 2nd and the 3rd trimesters.