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What is Double track sign is seen in - and explain in detail?
On upper gastrointestinal fluoroscopy: * delayed gastric emptying * peristaltic waves (caterpillar sign) * elongated pylorus with a narrow lumen (string sign) which may appear duplicated due to puckering of the mucosa (double-track sign) * the pylorus indents the contrast-filled antrum (shoulder sign) or base of the duodenal bulb (mushroom sign) the entrance to the pylorus may be beak-shaped (beak sign)