Is it safe to undergo MRI when a retained metal fragment is present in soft tissue, and how can I determine if the fragment is MRI‑compatible?

MRI Safety with Retained Metal Fragments in Soft Tissue

MRI can be performed safely in most patients with retained metal fragments in soft tissue, provided the fragment is not ferromagnetic and is not located near vital structures such as the brain, eyes, or major vessels. 1, 2

Determining MRI Compatibility

Step 1: Obtain Plain Radiographs First

• Always obtain anteroposterior and lateral radiographs to characterize the fragment's location, size, and relationship to surrounding structures before considering MRI. 3
• Radiographs help identify the number of fragments and their proximity to vital organs. 2

Step 2: Assess Fragment Composition Using Imaging Characteristics

The radiographic and CT appearance of ballistic fragments reliably predicts their ferromagnetic properties, which determines MRI safety. 4

Indicators of Ferromagnetic (MRI-Unsafe) Fragments:

• Presence of a debris trail on radiographs or CT 4
• Deformation or fragmentation of the primary projectile 4
• Steel-containing shrapnel or military fragments (particularly intracranial) 5

Indicators of Non-Ferromagnetic (MRI-Safe) Fragments:

• Intact projectile without debris trail 4
• Copper-jacketed or lead bullets without steel components 5
• Shotgun pellets (typically non-ferromagnetic) 5

Step 3: Evaluate Fragment Location

Fragments near vital structures (brain, eyes, spinal cord, major vessels) require extreme caution regardless of composition. 1, 5

• Subcutaneous fragments distant from vital organs pose minimal risk even if mildly ferromagnetic. 2
• One study of 17 patients with warfare-related subcutaneous fragments (1-10 mm, retained 2-39 years) showed only one superficial migration of a 10-mm fragment at 1.5T, with no other adverse events. 2

Step 4: Consider CT for Definitive Assessment

If radiographs suggest possible ferromagnetic properties or fragment location is uncertain, obtain CT with metal artifact reduction techniques to precisely localize the fragment and assess surrounding anatomy. 6, 3

• CT provides superior visualization of the bone-metal interface and anatomical relationships compared to radiographs alone. 6, 3

MRI Safety Protocol

When MRI is Safe to Proceed:

• Non-ferromagnetic fragments in subcutaneous soft tissue 2, 4
• Fragments showing no debris trail or deformation on imaging 4
• Fragments located away from the brain, eyes, spinal cord, and major vessels 1, 5

When MRI is Contraindicated:

• Ferromagnetic fragments (showing debris trail or deformation) near vital structures 5, 4
• Steel-containing shrapnel, particularly intracranial 5
• Any fragment where composition cannot be determined and location is near critical anatomy 1

Field Strength Considerations:

• 1.5T MRI is safer than 3T for patients with retained metal fragments, as magnetic forces increase with field strength. 7, 2
• All safety studies demonstrating successful imaging of warfare fragments used 1.5T scanners. 2
• Field strength (1T to 3T) does not significantly affect image quality for non-ferromagnetic projectiles, but ferromagnetic materials show increased artifact and torque at higher field strengths. 7

Expected Imaging Quality

Non-Ferromagnetic Fragments:

• Produce only mild to moderate metal artifact on both spin-echo and gradient-echo sequences 5
• Image quality varies from very good to moderate depending on specific bullet composition 7
• Metal artifact reduction sequences effectively mitigate limitations 6

Ferromagnetic Fragments:

• Create severe artifacts and image distortion, rendering diagnostic quality poor 7, 5
• May exhibit rotational movement in response to magnetic torque 7, 5

Critical Pitfalls to Avoid

• Never proceed with MRI without first obtaining plain radiographs to assess fragment characteristics and location. 3, 4
• Do not assume all bullets are ferromagnetic—approximately 17 of 21 tested projectiles (81%) were non-ferromagnetic and safe for MRI. 5
• Avoid MRI if steel-containing military shrapnel is suspected, particularly if intracranial. 5
• Do not rely solely on patient history of the injury—imaging characteristics are more reliable for determining composition. 4
• Exercise extreme caution with fragments near the eyes, brain, or major vessels regardless of presumed composition. 1, 5

Alternative Imaging if MRI is Contraindicated

If extensive metal artifact precludes diagnostic MRI or ferromagnetic fragments are present, CT with metal artifact reduction techniques or labeled leukocyte scan with sulfur colloid marrow scan are appropriate alternatives. 6

• CT can visualize the bone-metal interface even when MRI cannot. 6
• CT is also susceptible to metal artifact but may provide diagnostic information when MRI is contraindicated. 6