We describe a radiological sign, "inside-outside sign," observed during the cannulation of an expandable contrast-filled tubular structure in the human body. In this optical illusion, a catheter or guidewire appears to be outside the lumen when it is inside the lumen in reality. Knowing this rare optical illusion is essential to avoid mistaking it for a catheter or guidewire outside the lumen.
Keywords: Illusions in radiology; Interventional radiology; Percutaneous transhepatic biliary drainage; Portogram; Transjugular intrahepatic portosystemic shunt.
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