A pH-responsive T 1 -T 2 dual-modal MRI contrast agent for cancer imaging.

Magnetic resonance imaging (MRI) is a non-invasive imaging technology to diagnose health conditions, showing the weakness of low sensitivity. Herein, we synthesize a contrast agent, SPIO@SiO 2 @MnO 2 , which shows decreased T 1 and T 2 contrast intensity in normal physiological conditions. In the acid environment of tumor or inflamed tissue, the manganese dioxide (MnO 2 ) layer decomposes into magnetically active Mn 2+ (T 1 -weighted), and the T 1 and T 2 signals are sequentially recovered. In addition, both constrast quenching-activation degrees of T 1 and T 2 images can be accurately regulated by the silicon dioxide (SiO 2 ) intermediate layer between superparamagnetic iron oxide (SPIO) and MnO 2 . Through the "dual-contrast enhanced subtraction" imaging processing technique, the contrast sensitivity of this MRI contrast agent is enhanced to a 12.3-time difference between diseased and normal tissue. Consequently, SPIO@SiO 2 @MnO 2 is successfully applied to trace the tiny liver metastases of approximately 0.5 mm and monitor tissue inflammation.