Question

A boy working on a hammer & chisel had a foreign body entering his eye. Which investigation is detrimental (contraindicated) for this boy?

• A. X-ray
• B. B scan orbit
• C. CT Scan
• D. MRI

Hint:

A magnetic field is dangerous in the presence of metallic intraocular foreign bodies.

Answer 1

The Correct Option is D

Step 1: Identify the nature of the foreign body.
A hammer and chisel are metallic tools. High-velocity metallic fragments produced during hammering on a chisel are the classic mechanism of an intraocular metallic foreign body (IOFB). The foreign body is therefore presumed to be ferromagnetic metal.

Step 2: Why is MRI contraindicated?
MRI uses a powerful magnetic field. Ferromagnetic metallic foreign bodies within the eye or orbit can be displaced, rotated, or heated by the magnetic field during an MRI scan. This can cause catastrophic intraocular injury, vitreous haemorrhage, retinal tear, or perforation. MRI is therefore absolutely contraindicated whenever a metallic foreign body is suspected.

Step 3: Why are the other investigations safe?
X-ray is the first-line investigation for suspected IOFB as metallic objects are radiopaque. CT scan is the gold standard for localising IOFBs -- it detects metallic and non-metallic foreign bodies without risk of displacement. B scan ultrasonography can also be used but must be performed carefully if globe perforation is present.

Conclusion: The correct answer is MRI -- it is the detrimental (contraindicated) investigation.