Synopsis

Keywords: Contrast mechanisms: Contrast agents, Contrast mechanisms: Molecular imaging

MR images are generated based on the magnetic properties of water protons, particularly their relaxation rates, in the body. Paramagnetic substances are often used to alter the relaxation rate of protons in tissues or around biomolecules of interest, creating enhanced MR signal contrast over surrounding tissues for anatomical or molecular imaging. This presentation discusses the basic principles, including relaxivities, contrast enhancement, structure and chelation stability, and molecular imaging of relaxation-based contrast agents. Approaches for designing contrast agents with high relaxivities and clinically translatable targeted contrast agents for Magnetic Resonance Molecular Imaging (MRMI) will also be discussed.

Relaxation-based contrast agents, primarily those based on gadolinium (GBCAs), are routinely used in clinical practice to enhance the imaging signals of tissues or organs of interest for accurate diagnostic imaging. Understanding the basic principles of these contrast agents is valuable for their safe use in clinical practice and for the design and development of safer and more efficacious contrast agents to meet unmet clinical needs. The presentation will discuss the following aspects of the contrast agents.
1) Relaxivity and contrast enhancement. Relaxivity is a measurement of the ability of a contrast agent to alter T1or T2 relaxation rate of water protons. The clinically used contrast agents are mostly T1-weighted agents. Generally, high relaxivity results in high signal enhancement.
2) Structure and relaxivity. The relaxivity of a contrast agent largely depends on its structure features, such as coordination number, second-sphere relaxation, and size.
3) Chelation stability and safety. The clinical contrast agents are the chelates of paramagnetic metal ions, including Mn2+ and Gd3+. Chelation stability, including both thermodynamic stability and kinetic stability, is critical for the safety of the agents.
4) Targeted contrast agents for MR molecular imaging. MR molecular imaging (MRMI) provides three-dimensional visualization of molecular targets of interest in high spatial resolution. The low sensitivity of MRI for molecular imaging can be overcome by designing targeted contrast agents that bind to abundant molecular targets, such as extracellular matrix proteins.

Acknowledgements

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References

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