Dendrimeric calcium-sensitive MRI probes: the first low-field relaxometric study

Area of Science:

• Biomedical Imaging
• Nanotechnology
• Materials Science

Background:

• Small and nano-sized calcium-sensitive probes for magnetic resonance imaging (MRI) have been developed over the last 20 years.
• These paramagnetic agents are designed to alter MRI contrast (T1 or T2) in response to calcium ions, primarily for functional MRI.
• In vivo studies in animal models have been conducted to evaluate their efficacy.

Purpose of the Study:

• To investigate the relaxometric behavior of monomeric and dendrimer-conjugated calcium-sensitive probes at low magnetic fields.
• To compare the relaxivity of different generations of dendrimer conjugates (G0, G1, G2) with a monomeric probe.
• To assess the impact of calcium on the relaxivity and contrast enhancement of these agents.

Main Methods:

• Relaxometric measurements were performed on monomeric and dendrimer-conjugated calcium-sensitive probes (G0, G1, G2) at low magnetic fields.
• The study compared the probes' behavior in the absence and presence of calcium (Ca2+).
• T1-weighted MRI was conducted at 1 Tesla to visualize contrast changes.

Main Results:

• A relaxivity hump was observed between 10 and 100 MHz, becoming more pronounced with increasing probe size (monomer to G2 dendrimer).
• This indicates restricted rotational motion correlating with molecular size.
• Calcium significantly enhanced relaxivity, with the G2 dendrimer conjugate showing a ~310% increase compared to ~130% for the monomer at 0.5 T. This enhancement is attributed to increased hydration of Gd3+.

Conclusions:

• Dendrimer conjugation restricts rotational motion, leading to a relaxivity hump at low magnetic fields.
• Larger dendrimer conjugates exhibit significantly greater calcium-induced relaxivity enhancement.
• These findings support the potential of these calcium-sensitive probes for in vivo functional MRI applications due to their tunable contrast properties.