Figure 3. Example of a "smart" MRI probe. Ca²⁺ causes a conformational change in the molecule such that access of bulk water to the Gd³⁺ can be accessed or denied. Access of bulk water to the Gd³⁺ causes shortening in the longitudinal relaxation time of water protons and therefore signal enhancement on T₁ imaging. Chemical structures courtesy of Prof. T Meade, NorthWestern University, USA.