Utilizing T1- and T2-Specific Contrast Agents as “Two Colors” MRI Correlation

Magnetic resonance imaging (MRI) is widely used as a medical imaging technique due to its non-invasive nature, high spatial contrast, and virtually unlimited depth of penetration. Different modalities can be used for contrast in MRI, including <i>T</i>₁ (spin-lattice) and <i>T</i>₂ or <i>T</i>₂ * (spin-spin) proton relaxation times, and specific contrast agents (CAs) have been developed that locally enhance the contrasts in MRI images. We present a method combining <i>T</i>₁- and <i>T</i>₂-specific CAs in a single imaging technique, referred to as correlation MRI. This technique allows different CAs to be used simultaneously to visualize contrast between multiple types of tissue in the same image when applied as targeted CAs. An obstacle for the quantitative use of correlation MRI is that <i>T</i>₁ and <i>T</i>₂ relaxivity changes generated by CAs are not independent of each other. Here, we measured relaxivities in mixtures with various concentrations of Cas, including Magnevist (Gd³⁺-based, primarily a T₁ CA) and Feridex (Fe²⁺- and Fe³⁺-based, primarily a T₂ CA), and compared them to theoretically predicted values. It was found that, at clinically relevant concentrations, relaxivities of the mixtures deviate from linearly added values. We finally propose a three-dimensional calibration curve to quantitatively determine the concentration in mixtures of CAs, based on the measured relaxivities.