Multimodal non-invasive imaging of placental structure and function

Objectives The goal of this study was to evaluate the use of small-animal magnetic resonance imaging (MRI), positron emission tomography (PET), and kinetic modeling of the resulting imaging data for non-invasive assessment of placental function in pregnant mice.

Methods Female CD-1 timed pregnant mice (E15.5) were anesthetized in the imaging laboratory using a Plexiglas chamber flowing with 1½-2% Isoflurane. For dynamic images, the tail vein was catheterized using 0.025” O.D. x 0.012” I.D. micro-renathane tubing attached to a heparinized .300 cc, 29 Gauge x ½” insulin syringe filled with saline. Coincidental with the start of the PET scan, the mice were injected with 0.6-0.7 mCi 18FDG via the tail-vein catheter. PET data were collected for 60 minutes, followed by the injection of 500 µl Omnipaque (Iohexol, 240 mg/ml) through the tail-vein catheter coincident with the initiation of a computed tomography (CT) scan. After 6 minutes of CT data acquisition, mice were returned to their cages to recover. Imaging data were acquired with either Inveon PET/CT or Focus 220 (Siemens Medical Solutions) and reconstructed with the maximum aposteriory probability (MAP) algorithm. The data were analyzed with Inveon Research Workstation software.

Results As expected, the fluorinated tracer efficiently entered the circulation and distributed throughout the body. The PET/CT coregistered images demonstrated excellent resolution and allowed accurate elucidation of regions of interest corresponding to individual placentas and fetuses. Time-activity curves were measured, yielding distinct tracer-uptake curves for placental and fetal tissues.

Conclusions These results indicate that 18FDG PET/CT can be utilized as an efficient and informative tool for monitoring placenta - fetus nutrient exchange in pregnant mice, laying the groundwork for studies in mouse models of intra-uterine growth restriction (IUGR) and placental insufficiency