Final answer:
Gd chelates are hydrophilic and thus cannot pass through the intact blood-brain barrier. They must be administered as a stable complex to prevent toxicity, and their use is contraindicated in pregnant women due to potential risks.
Step-by-step explanation:
Gadolinium (Gd) chelates do not pass the intact blood-brain barrier because they are hydrophilic. These agents, due to their water solubility and large polar surface area, cannot easily cross the lipid-rich cell membranes that constitute the blood-brain barrier. Gadolinium, a rare earth metal with seven unpaired electrons, is highly paramagnetic and is used in magnetic resonance imaging (MRI) as a contrast agent when complexed with ligands such as DTPA (diethylene triamine pentaacetic acid). However, the Gd3+ ion in its aqueous form (Gd3+ (aq)) is toxic, which necessitates its administration as a stable complex that does not dissociate in the body. This ensures it can be safely excreted by the kidneys.
Furthermore, because gadolinium can cross the placenta, it is typically contraindicated in pregnant females to avoid potential risks to the fetus. Due to the health risks associated with free gadolinium ions, great care is taken in the selection and administration of Gd-based contrast agents to prevent toxicity and ensure patient safety during medical imaging procedures.