Here we present a protocol for measuring fetal blood flow rapidly with MRI and retrospectively performing motion correction and cardiac gating.
Fetal MRI faces several challenges. This protocol addresses the issues of fetal motion, high, spatial and temporal resolution requirements, and lack of external gating methods. This technique makes use of accelerated imaging through compressed sensing which reduces the imaging time, retrospectively corrects for fetal motion, and allows extraction of the fetal heart rate using metric optimized gating.
Currently, this technique is only for research, however it has the potential for the monitoring and therapy guidance for fetal pathologies, such as congenital heart disease and intrauterine growth restriction. After assisting the mother in an appropriate comfortable position on the MRI table, set the instrument to run a localizer exam to locate the fetal body at a 0.9 by 0.9 by 10 cubic centimeter resolution, an echo time of five milliseconds, a repetition time of 15 milliseconds, a field of view of 450 by 450 square millimeter and six slices. Set the parameters for running a refined localizer exam to locate the fetal vasculature with the slice group centered on the fetal heart at 1.1 by 1.1 by six cubic millimeter resolution, echo time of 2.69 milliseconds, repetition time of 1335.4 milliseconds, a field of view of 350 by 350 square millimeters, 10 slices and an axial to fetus orientation.
Then repeat the refined localizers with sagittal and coronal orientations to obtain a clearer view of the fetal vessels. To measure the fetal blood flow, after locating the fetal vessels identify the vessels of interest. For example, the descending aorta is a long straight vessel near the spine in the sagittal plains, the ascending aorta and main pulmonary arteries can be identified as vessels leaving the left and right ventricles respectively.
The ductus arteriosus can be tracked as a downstream segment of the main pulmonary artery proximal to the descending aorta. The superior vena cava can be identified from axial plains near the base of the fetal heart as the vessel adjacent to the ascending aorta. Prescribe a slice perpendicular to the axis of the fetal vessel of interest and rotate and move the slice guideline on the MRI console such that the slice intersects the target vessel perpendicularly.
Set the scan parameters to a radial phase contrast MRI acquisition, a 1.3 by 1.3 by five cubic millimeter resolution, echo time of 3.25 milliseconds, resolution time of 5.75 milliseconds, field of view of 240 by 240 square millimeters, one slice, 100 to 150 centimeter per second velocity and coating according to the vessel of interest, through the plane, radio views velocity and coating direction, and 1500 per end code free breathing. After running the scan, verify the prescription based on the initial time averaged reconstruction performed and displayed on the MRI console. Repeat for each target blood vessel and if the target vessel is absent or identifiable.
After reconstructing fetal flow CINEs, load the reconstructed data files into an appropriate flow analysis software program, and draw a region of interest encompassing the lumen of the blood vessel of interest in the anatomical and velocity sensitive images. Propagate the region of interest to all the cardiac phases and correct for changes in the vessel's diameter. Then record the flow measurements within each region of interest.
In this representative analysis, the extracted motion parameters for fetus one and fetus two depict the motion of the descending aorta over the duration of the scan. Here, the shared mutual information of each real time frame with all of the other co registered frames for fetus one and fetus two can be observed. The second real time reconstructions used to derive the cardiac gating information took 10 minutes per slice, and the fetal heartbeat intervals were derived by metric optimized gating using a multi parameter model as demonstrated.
Final CINE reconstructions. Using the retrospectively motion corrected and gated data took three minutes per slice, and allowed the generation of anatomical and velocity reconstructions for the fetuses, at peak systole. Reconstructions with motion correction show vessels with sharper walls.
Without motion correction, the descending aorta is blurrier and less conspicuous. The measured flow curves from each fetus revealed higher peak and mean flows in the reconstructions without motion correction than those with motion correction. The technique is being used to study the blood distribution in fetal pathologies.
An extension of this method has allowed multi-dimensional fetal flow visualization and measurement.
