Positive contrast MR-lymphography using inversion recovery with ON-resonant water suppression (IRON)

Purpose: To investigate the utility of inversion recovery with ON-resonant water suppression (IRON) to create positive signal in normal lymph nodes after injection of superparamagnetic nanoparticles. Materials and Methods: Experiments were conducted on six rabbits, which received a single bolus injection of 80 μmol Fe/kg monocrystalline iron oxide nanoparticle (MION-47). Magnetic resonance imaging (MRI) was performed at baseline, 1 day, and 3 days after MION-47 injection using conventional T₁- and T₂*- weighted sequences and IRON. Contrast-to-noise ratios (CNR) were measured in blood and in paraaortic lymph nodes. Results: On T₂*-weighted images, as expected, signal attenuation was observed in areas of paraaortic lymph nodes after MION-47 injection. However, using IRON the paraaortic lymph nodes exhibited very high contrast enhancement, which remained 3 days after injection. CNR with IRON was 2.2 ± 0.8 at baseline, increased markedly 1 day after injection (23.5 ± 5.4, P < 0.01 vs. baseline), and remained high after 3 days (21.8 ± 5.7, *P < 0.01 vs. baseline), CNR was also high in blood 1 day after injection (42.7 ± 7.2 vs. 1.8 ± 0.7 at baseline, P < 0.01) but approached baseline after 3 days (1.9 ± 1.4, P = NS vs. baseline). Conclusion: IRON in conjunction with superparamagnetic nanoparticles can be used to perform 'positive contrast' MR-lymphography, particularly 3 days after injection of the contrast agent, when signal is no longer visible within blood vessels. The proposed method may have potential as an adjunct for nodal staging in cancer screening.