Poly(acrylic acid)-Grafted Bi_xGd_2–xO₃ Nanoparticles as Multimodal T₁/T₂ MRI and X-ray CT Contrast Agents

Gadolinium (Gd) and bismuth (Bi) are recognized as the most effective elements for serving as contrast agents in positive ( T 1 ) magnetic resonance imaging (MRI) and X-ray computed tomography (CT), respectively. This is due to the highest pure spin of Gd 3+ (spin = 7/2) and the largest X-ray attenuation coefficient of Bi among nonradioactive elements in the periodic table. However, there have been limited studies on nanoparticles containing Gd and Bi as multimodal MRI–CT contrast agents. This study introduces ultrasmall Bi x Gd 2– x O 3 nanoparticles ( x = 0.5, 1, and 1.5) grafted with poly(acrylic acid) (PAA) (referred to as PAA–Bi x Gd 2– x O 3 ) for enhanced colloidal stability and reduced toxicity. The compositional ( x ) variations of their water proton spin relaxivities ( r 1, r 2 ) and X-ray attenuation efficiency (η) were investigated. The r 1, r 2, and η values of the PAA–Bi x Gd 2– x O 3 nanoparticles increased with higher Bi content ( x ), peaking at x ≈ 1.5. At x = 1.5, the r 1 and η values were approximately 10 (with an r 2 / r 1 ratio of 2.5) and 8 times greater than those of commercial MRI and CT contrast agents, respectively. Therefore, these nanoparticles could potentially allow for reduced Gd/Bi dosage compared to commercial contrast agents due to their considerably higher r 1, r 2, and η values. In vivo T 1 /negative ( T 2 ) MRI and CT experiments exhibited contrast enhancements postinjection, confirming the promise of PAA–Bi x Gd 2– x O 3 nanoparticles as multimodal T 1 / T 2 MRI–CT contrast agents.