Researchers from MIT, Boston Children’s Hospital, and Massachusetts General Hospital have joined forces in an ambitious new project to use magnetic resonance imaging (MRI) to evaluate the health of fetuses.
Typically, fetal development is monitored with ultrasound imaging, which is cheap and portable and can gauge blood flow through the placenta, the organ in the uterus that delivers nutrients to the fetus.
But MRI could potentially measure the concentration of different chemicals in the placenta and in fetal organs, which may have more diagnostic value.
Earlier this year, in a project led by Boston Children’s Hospital (BCH), the research team presented a paper showing that MRI measurements of oxygen absorption rates in the placenta can indicate placental disorders that might endanger the fetus.
An MRI image might consist of hundreds of two-dimensional cross sections of an anatomical region, stitched into a three-dimensional whole.
Measuring chemical changes over time requires analyzing sequences of such three-dimensional representations — about 300, in the case of the new paper.
The researchers refer to each MRI image in a series as a “frame,” analogous to frames of video.
The first step in localizing chemical changes to particular organs, of course, is identifying the organs. That’s where the researchers’ new algorithm comes in.
With MRI images of brain activity, it’s comparatively easy to determine which anatomical features in one frame correspond to which features in the next.
The subject’s head is immobilized, and brain regions don’t change shape or location over the course of a scan.
Algorithmically, the standard method for coordinating frames is to identify a region in the first frame and then map it separately onto each of the frames that follow.
With fetal MRIs, that won’t work, because the fetus may have moved dramatically between, say, frame one and frame 200. So the researchers took a different approach.
Their algorithm begins by finding a mathematical function that maps the pixels of the first frame onto those of the second; then it maps the mathematically transformed version of the first frame onto the third, and so on.
The end result is a series of mathematical operations that describes the evolution of the scan as a whole.
Next, a human expert draws very precise boundaries around the elements of interest in the first frame — in this case, not just the placenta but the brain and liver as well.
Those elements’ movements or deformations from frame to frame can then be calculated using the previously determined mathematical operations.
Hand-drawing organ boundaries — or “segmenting” an MRI scan — is a time-consuming process. But performing it only once is much less onerous than performing it 300 times.
In order to evaluate the accuracy of their algorithm, the researchers hand-segmented an additional five frames. The algorithm’s segmentations accorded very well with those performed by hand.
The researchers suggest that modeling and incorporating the deformation estimation in MR acquisition is a big challenge. Many people have been working on it, and the current study is a great step forward.
News source: Liao R, et al. (2016). Temporal Registration in In-Utero Volumetric MRI Time Series. Paper presented at 19th International Conference on Medical Image Computing & Computer Assisted Intervention. DOI: http://hdl.handle.net/1721.1/105124.
Figure legend: This Knowridge.com image is credited to MIT.