Subtraction magnetic resonance image visualizing tracer immediately below the cribriform plate. The illustration constitutes of a combination of T1-GRE images obtained at different time points: a summation of pre-contrast and 48 h’ post-contrast time points (with no or small amounts of CSF tracer) were subtracted from the 3- and 6-h’ time points (with high concentrations of tracer in CSF) to visualize the distribution of tracer. In this example, narrow streaks of tracer are depicted immediately below the cribriform plate (arrow). No tracer enhancement in the nasal mucosa can be readily seen and was confirmed by the ROI-analysis through all time points at group level.