Despite being widely used in the clinical setting for sentinel lymph node detection (SLND), (99m)Tc-based colloids (e.g., (99m)Tc-human serum albumin colloids) present a set of properties that are far from ideal. Aiming to design novel compounds with improved biological properties, we describe herein the first class of fully characterized (99m)Tc(CO)₃-mannosylated dextran derivatives with adequate features for SLND. Dextran derivatives, containing the same number of pendant mannose units (13) and a variable number (n) of tridentate chelators (9, n = 1; 10, n = 4, 11, n= 12), have been synthesized and fully characterized. Radiolabeled polymers of the type fac-[(99m)Tc(CO)₃(k³-L)] (12, L = 9, 13, L = 10, 14, L = 11) have been obtained quantitatively in high radiochemical purity (≥ 98%) upon reaction of the dextran derivatives with fac-[(99m)Tc(CO)₃(H₂O)₃]+. The highly stable compounds 13 and 14 were identified by comparing their HPLC chromatograms with the ones obtained for the corresponding rhenium surrogates fac-[Re(CO)₃(k³-10)] (13a) and fac-[Re(CO)₃(k³-11)] (14a), which have been characterized both at the chemical (NMR and IR spectroscopy, and HPLC) and physical level (DLS, AFM and LDV). Compounds 13a and 14a present a positive zeta potential (+ 7.1 mV, pH 7.4) and a hydrodynamic diameter in the range 8.4-8.7 nm. Scintigraphic imaging and biodistribution studies in Wistar rats have shown good accumulation in the sentinel node at 60 min postinjection (6.71 ± 2.35%, 13; and 7.53 ± 0.69%, 14), with significant retention up to 180 min. A clear delineation of the sentinel lymph node without significant washout to other regions was observed in the scintigraphic images. The popliteal extraction of 94.47 ± 2.45% for 14 at 1 h postinjection, as compared to 61.81 ± 2.4% for 13, indicated that 14 is a very promising compound to be further explored as SLN imaging agent.