J Korean Soc Magn Reson Med.  1999 Jun;3(2):159-166.

The Effects of Rotational Correlation Time of Paramagnetic Contrast Agents on Relaxation Enhancement: Partial Binding to Macromolecules

• Chang Y

Abstract

PURPOSE
To evaluate the effect of rotational correlation time ((tau)R)) and the possible related changes of other parameters, (tau)M, (tau)s, and (tau)v of gadolinium (Gd) chelate on T1 relaxation enhancement in two pool model.
MATERIALS AND METHODS
The NMRD (Nuclear Magnetic Relaxation Dispersion) profiles were simulated from 0.02 MHz to 800 MHz proton Larmor frequency for different values of rotational correlation times based on Solomon-Bloembergen equation for inner-sphere relaxation enhancement. To include both unbound pool (pool A) and bound pool (pool B), the relaxivity was divided by contribution from unbound pool and bound pool. The rotational correlation time for pool A was fixed at the value of 0.1 ns, which is a typical value for low molecular weight complexes such as Gd-DTPA in solution and (tau)R for pool B was changed from 0.1 ns to 20 ns to allow the slower rotation by binding to macromolecule. The fractional factor f was also adjusted from 0 to 1.0 to simulate different binding ratios to macromolecule. Since the binding of Gd-chelate to macromolecule can alter the electronic environment of Gd ion and also the degree of bulk water access to hydration site of Gd-chelate, the effects of these parameters were also included.
RESULTS
The result shows that low field profiles, ranged from 0.02 to 40 MHz, are dominated by contribution from bound pool, which is bound to macromolecule regardless of binding ratios. In addition, as more Gd-chelate bound to macromolecule, sharp increase of relaxivity at higher field occurs. The NMRD profiles for different values of (tau)s show the enormous increase of low field profile whereas relaxivity at high field is not affected by (tau)s. On the other hand, the change in (tau)v does not affect low field profile but strongly influences on both inflection field and the maximum relaxivity value. The results shows a parabolic dependence of relaxivity on (tau)M.
CONCLUSION
Binding of Gd-chelate to a macromolecule causes slower rotational tumbling of Gd-chelate and would result in relaxation enhancement, especially in clinical imaging field. However, binding to macromolecule can change water enchange rate ((tau)M) and electronic relaxation time (T1e) via structural deformation of electron environment and the access of bulk water to hydration site of metal-chelate. The clinical utilities of Gd-chelate bound to macromolecule are the less dose requirement, the tissue specificity, and the better perfusion and intravascular agents.

MeSH Terms

Contrast Media*
Gadolinium
Gadolinium DTPA
Hand
Molecular Weight
Organ Specificity
Perfusion
Protons
Relaxation*
Water
Contrast Media
Gadolinium
Gadolinium DTPA
Protons
Water