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Why does water-rich tissue appear bright in many MRI images?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

Water-rich tissue appears bright in magnetic resonance imaging (MRI) because the technique primarily detects the oscillations of hydrogen atoms, which are highly abundant in the body in the form of water.12

During an MRI scan, a powerful background magnetic field aligns these hydrogen atoms. When a second, oscillating magnetic field is applied at specific pulse rates, the hydrogen atoms resonate and align with that field. Once the second field is turned off, the atoms swing back to align with the background field, emitting a signal in the process. Because water-rich tissues contain a high concentration of these hydrogen atoms, they produce a stronger signal, which is translated into a brighter appearance on the resulting image. Conversely, tissues with little to no hydrogen, such as bone, appear black.1