Gadolinium based contrast agents brighten enhancing lesions on T1-weighted MRI even though gadolinium ions emit no RF signal themselves. The mechanism by which they produce this increased signal is:

• A Shortening the T1 relaxation time of nearby water protons ✓
• B Lengthening the T2 relaxation time of nearby water protons
• C Increasing the net magnetisation by raising local proton density
• D Generating their own detectable radiofrequency signal after excitation

Explanation

Gadolinium has seven unpaired electrons and strong paramagnetism. It does not emit signal directly; instead it fluctuates at frequencies matching the Larmor frequency of surrounding water protons, dramatically shortening their T1 relaxation time. Tissues that take up gadolinium therefore recover longitudinal magnetisation faster and appear bright on T1-weighted images. Gadolinium also shortens T2 slightly, which matters mainly at high concentrations where it causes signal loss rather than brightening. Proton density is essentially unchanged.

Reference: Bushberg, Seibert, Leidholdt and Boone, The Essential Physics of Medical Imaging, 4th ed.

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Written and medically reviewed by the StethoPrep medical team.