Magnetic resonance microscopy of chick embryos in ovo

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Abstract
Magnetic resonance imaging (MRI) of the live 11‐day chick embryo with special radiofrequency coils and 3‐D imaging methods has produced contiguous 1.25‐mm‐thick slices with 200‐μm pixel resolution, permitting definition of cardiac chambers, cerebral ventricles, spinal cord, liver, and lungs. It was the objective of this study to image younger chick embryos in ovo with higher spatial resolution through the application of implanted radiofrequency coils. Fertilized Arbor Acre eggs were windowed at 9, 6, and 4 days. Circular coils 18 mm in diameter tuned to 85.5 MHz were suspended around the developing embryo. The eggs were sealed with tape and maintained at 37°C during the imaging procedure. MRI was performed in a 2.0‐Tesla GE system utilizing a 3‐D Fourier transform acquisition in sagittal and axial planes with a partial saturation sequence (TR = 400 ms, TE = 27 ms). Approximately 1 hour of imaging time was required to obtain 16 contiguous 600‐μm‐thick slices with 50‐μm pixel resolution. Embryos remained viable through the imaging procedure. Embryos were photographed, fixed, and cleared for correlative anatomical study. Vitelline vessels, dorsal aorta, aortic arches, cardinal veins, and cardiac chambers were identified as areas of decreased signal intensity. Cerebral ventricles and the vitreous portion of the eye have signal intensities that are less than adjacent neural, scleral, and lens tissue. Further refinements in MR instrumentation and imaging sequences promise improvements in resolution and offer the potential for sequential observations of the intact embryo.