A research team led by Associate Professor Yasuhiko Terada of the Faculty of Pure and Applied Sciences at the University of Tsukuba has improved a magnetic resonance (MR) microscope that can magnify the interior of small objects to achieve a spatial resolution of 0.01 millimeter. Featuring the world's highest spatial resolution to date, this microscope can depict the minute structures of the cranial nerves and organs in a human embryo with several dozen times higher resolution than previously achieved. Hardware and imaging methods were improved, and techniques were applied to efficiently collect and reconstruct data. This achievement is expected to contribute to research on human embryology and was published online on August 30, 2023 in the Journal of Magnetic Resonance.
Magnetic resonance imaging (MRI) machines visualize hydrogen atoms in the body and are widely used for clinical diagnosis and research. The interior of the objects measuring 1−2 centimeters can be visualized with high resolution. Conventional MR microscopes have a maximum spatial resolution of approximately 4/100ths of a millimeter (40 micrometers) and cannot produce high-quality images of structures as fine as 1/100th of a millimeter. MRIs with spatial resolutions of approximately 10 micrometers are needed to understand the growth process during the embryonic period. However, three-dimensional morphological models constructed from conventional MR microscopy data mainly cover only the outer surface, leaving the growth process of ultrastructures such as the central nervous system unclear.
In this study, the research group miniaturized the radiofrequency coil to increase its sensitivity and improved the pulse sequence (i.e., the timing and parameters of a series of radiofrequency pulse signals and gradient magnetic field signals required to generate MRI images) to maximize contrast between the organs. Furthermore, a compressed sensing technology was adopted for efficient data collection and reconstruction.
The group succeeded in reducing the imaging time from several months or more (which was previously required) to approximately two weeks. In addition, the stability of the console, which served as the control and signal-acquisition system, was improved, thereby enabling imaging for extended periods of time. These improvements enabled high-resolution human-embryo imaging with a spatial resolution of 10 micrometers. Line profile, signal-to-noise ratio, and histogram analyses using phantom (i.e., an artificial structure used to evaluate MRI system performance and image quality) images demonstrated that the pixel size and resolution of the instrument were identical.
This MR microscope clearly depicted previously obscure structures of human embryonic specimens at a resolution of 10 micrometers, achieving an image quality comparable with that of an optical microscope.
Terada commented, "The MRI equipment commonly used in hospitals usually has a resolution of about one millimeter, but the MRI we have developed this time achieves an ultra-high resolution of 1/100th of a millimeter, thereby enabling the detailed observation of the inside of a human embryo for the first time worldwide. A large amount of effort was required to improve the resolution of the MR microscope, but we managed to achieve this goal. In the future, I hope to contribute to research that clarifies the developmental process of the brain."
Publication: Journal of Magnetic Resonance
Title: High-resolution MRI for human embryos with isotropic 10 µm resolution at 9.4 T
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