Magnetic Resonance Imaging of Patients With Retained Ballistic Projectiles: A Review of Ferromagnetism, Ammunition Composition, and Safety Protocols

Retained ballistic projectiles are a common consequence of firearm injuries in the United States. As the role of magnetic resonance imaging (MRI) in medicine continues to expand, clinicians increasingly encounter patients with retained bullets who would benefit from an MRI. However, the interaction between the strong magnetic field of an MRI and ferromagnetic implants can cause dangerous projectile movement and significant susceptibility artifacts, creating both safety and diagnostic concerns. Most bullets encountered in the United States are composed of copper alloy jackets and lead cores, which are diamagnetic and generally safe for MRI. Yet, some bullets are manufactured with steel components and are ferromagnetic. Radiologists have identified several approaches to determine the MRI compatibility of retained bullets, but few definitive guidelines exist to decide which of these patients can safely undergo MRI. In this article, we review the interactions of ferromagnetic bullets with the magnetic field of an MRI, list the material construction of various ammunition, discuss various proposed methods for determining the MR safety of retained projectiles, and synthesize literature recommendations into simplified algorithms that can be utilized to effectively triage patients with retained ballistic projectiles. Standardizing this screening process enables clinicians to stratify patient risk and avoid unnecessary MRI exclusions. EVIDENCE LEVEL: 5. TECHNICAL EFFICACY: Stage 5-Improvements in Patient Care.