Final answer:
An X-ray is the first choice for imaging when a foreign body like a pin is suspected due to its effectiveness at detecting metallic objects and its widespread availability.
Step-by-step explanation:
The first line imaging test for suspected retained foreign body of a pin is an X-ray. X-rays are capable of producing images of bones and can detect metallic objects such as a pin with high precision.
Although other imaging techniques like CT scans MRIs and ultrasounds can be used for detailed images of the soft tissues and organs, for the initial assessment of a suspected foreign body, especially metal an X-ray is the most straightforward and widely available option.