Calcified lesions of the brain occasionally appear bright on T1-weighted MR images. This report shows that particulate calcium can reduce T1 relaxation times by a surface relaxation mechanism. Calcium particles with greater surface area show greater T1 relaxivity. Reduced proton density and reduced T2 tend to diminish signal intensity, but reduced T1 increases signal intensity. Thus, for concentrations of calcium particulate of up to 30% by weight, the signal intensity on standard T1-weighted images increases but subsequently decreases.