Introduction In this paper, we introduce a fluorescent magnetic hybrid nanoprobe (5011), based on gadolinium fluoride nanoparticles functionalized with luminescent conjugated polythiophenes moieties (Fig 1). The aim of this study was to investigate its potential for molecular imaging in experimental stroke. Specifically, we aimed to test the hypothesis that 5011 may cross the disrupted blood-brain barrier (BBB) in the ischemic lesion. Methods Transient middle cerebral artery occlusion (tMCAO) was performed in 7 rats using the intraluminal model. In this model, mild BBB disruption was consistently shown at 3 days post-surgery (1). Therefore, dynamic MRI of the brain was performed 3 days post-tMCAO, before, during and after bolus injection of 5011 (N=6) or Gd-DOTA (N=1) both at 150 µmol Gd/kg (7T, TE/TR: 10.5/800-ms, flip angle 30°, 40 repetitions, one-hour acquisition). We used a model of focal BBB opening by heat injury (N=6) to ascertain that the MR sequence used was sensitive enough to detect contrast enhancement due to 5011 leakage through a severely disrupted BBB. Brains and livers of tMCAO rats were prepared either for biphotonic microscopy (N=3) or inductively coupled plasma-mass spectroscopy (ICP-MS, N=3). Results As expected in tMCAO rats, Gd-DOTA injection resulted in a significant enhancement of the ischemic lesion (22% compared to pre-contrast signal). In contrast, there was no signal enhancement difference between the ischemic lesion (-3±1%) and the contralateral region (-2±1%) in rats injected with 5011 (Fig 2A). Biphotonic microscopy showed sparse agglomerates of 5011 in the parenchyma of the ischemic lesion (Fig 2, insert). In the model of focal BBB opening, 5011 injection resulted in a significant enhancement of the lesion (13±3%) (Fig 2B). ICP-MS analyses are in progress. Conclusions Transport of 5011 across the disrupted BBB could be detected in tMCAO rats using a sensitive two-photon microscope but its accumulation was too low to be detected with MRI. Based on these results, we aim to functionalize the hybrid nanoprobe to target vascular adhesion molecules that are highly expressed after experimental stroke, rather than intraparenchymal (extravascular) targets.