The halo sign during a percutaneous nephrolithotomy puncture

When attempting to gain pelvicalyceal system (PCS) access for a percutaneous nephrolithotomy (PCNL), the initial puncture is the most important step. Free efflux of urine through the needle or on aspiration is an absolute sign of the needle tip being in the PCS. The trouble occurs when the needle tip appears to be in the system, but no urine can be aspirated. Attempts to pass a guidewire are unsuccessful. These are moments of anxiety all urologists have experienced. This is where the halo sign is a useful guide to the location of the needle tip