An industrial worker was admitted to the hospital after an eye injury sustained while cutting stone with a hammer and iron chisel. A foreign body was suspected to be impacted in his eye. Which investigation can be detrimental to this patient?

Correct Answer: C. MRI

MRI is absolutely contraindicated in suspected intraocular metallic foreign bodies because the strong magnetic field (1.5–3 Tesla in clinical practice) will exert a powerful Lorentz force on ferromagnetic materials like iron, steel, and nickel. In this case, the worker was using an iron chisel—a classic source of ferromagnetic debris. When exposed to the MRI field, a metallic foreign body will experience rapid acceleration and rotation within the globe, causing catastrophic intraocular hemorrhage, retinal detachment, corneal perforation, and permanent blindness. This is one of the few absolute contraindications to MRI in emergency ophthalmology. The mechanism of injury (stone-cutting with iron chisel) makes ferromagnetic contamination highly likely. Even small fragments (1–2 mm) can cause severe damage. Indian guidelines and standard ophthalmology practice (as per AIIMS and other tertiary centers) mandate exclusion of metallic foreign bodies before any MRI is performed. The patient's history alone warrants imaging with non-magnetic modalities first.

Why the other options are wrong

A. CT — CT is safe and highly sensitive for detecting both metallic and non-metallic intraocular foreign bodies. It uses ionizing radiation (X-rays) but poses no magnetic risk. CT is the gold standard for characterizing the size, location, and composition of foreign bodies in ocular trauma and is routinely used in Indian emergency departments before any MRI consideration. B. X-ray — Plain X-ray (skull or orbital views) is a safe, non-magnetic imaging modality that can detect radiopaque metallic foreign bodies, particularly ferrous materials like iron. While less sensitive than CT, it is often the first-line screening tool in resource-limited settings and carries no risk of magnetic displacement. It is commonly used in Indian primary care hospitals. D. B mode scan — B-mode ultrasonography is a non-magnetic, non-ionizing imaging technique that is safe even in the presence of metallic foreign bodies. It can detect both metallic and non-metallic foreign bodies and assess posterior segment involvement. Ultrasound is widely available in Indian hospitals and is often used as a first-line investigation in ocular trauma when CT is unavailable.

High-Yield Facts

Mnemonics

MRI METAL TRAP MRI is Magnetic — Explosive with True Aferromagnetic Load. Never use MRI if metallic foreign body is suspected; use CT, X-ray, or ultrasound instead. SAFE imaging in ocular trauma Sonography (B-mode), Angiography (CT), Fluoroscopy (X-ray), Exclude MRI. These are the safe modalities for suspected intraocular foreign bodies.

NBE Trap

NBE pairs industrial trauma with imaging modalities to test whether students reflexively choose the \"best\" imaging (MRI) without considering the specific contraindication posed by ferromagnetic materials in ocular trauma. The trap is assuming MRI's superior soft-tissue resolution overrides safety concerns.

Clinical Pearl

In Indian emergency departments, any patient with stone-cutting, welding, or grinding injuries presenting with eye trauma must be screened for metallic foreign bodies using CT or X-ray before any advanced imaging is ordered. A single missed ferromagnetic fragment can result in irreversible blindness within minutes of MRI exposure—a preventable catastrophe that has occurred in clinical practice.

_Reference: Bailey & Love Ch. 41 (Ophthalmology); Harrison Ch. 428 (Ocular Emergencies); Robbins Ch. 29 (Eye pathology)_