Illustration of intravesical drug delivery via a transurethral catheter in female mice. Combined instillation of paramagnetic, gadolinium chelate, and superparamagnetic iron oxide nanoparticle with a polyglucose sorbitol carboxymethylether coating (ferumoxytol) by a catheter darkens the lumen but brightens the bladder wall. This occurs because the large molecular weight of ferumoxytol (731 kD, analogous to onabotulilinumtoxinA) exclusively spoils the gadolinium signal in the bladder lumen but not in the bladder wall and  the larger molecular size of ferumoxytol restricts its diffusion to the bladder wall, whereas smaller sized gadolinium chelate (analogous to oxybutynin) can easily diffuse paracellularly into bladder wall. T2-weighted images of mouse bladder acquired pre- and post-instillation of 0.05 mL (compared to 50 mL for human bladder) of contrast mixture illustrate the dramatic improvement in image contrast with a darkened lumen and a brightened bladder wall. The parameters for Turbo spin echo T2-weighted multi-slice imaging in coronal orientation at 7T were: repetition time 3000 ms, echo time 40 ms, echo train length = 8; field of view 4 × 4 cm2, acquisition matrix 256 × 256, slice thickness 1 mm and interslice gap of 1 mm in 15 slices, number of excitations = 4, and pixel size of 0.156 mm. LUTS lower urinary tract symptoms