Imaging of ballistic wounds may predict retained bullet composition and MRI safety
CAPTION
Scout (A), T1-weighted spin-echo (SE) (B), T2-weighted SE (C), T2-weighted gradient-recalled echo (GRE) (TR/TE, 500/10; D), and T2-weighted GRE (TR/TE, 700/30; E) MR images show jacket hollow point .45 automatic Colt pistol bullet (Corbon) (1), solid lead .45 Long Colt bullet (Winchester) (2), full metal jacket (FMJ) automatic Colt pistol bullet (Winchester) (3), 5.56-mm FMJ bullet (Federal Ammunition) (4), #7 lead shotgun pellet (Winchester) (5), and 5-mm lead air gun pellet (Sheridan) (6). On all sequences, metallic artifact is minimal. Although metallic artifact increases or blooms with increased TR/TE in GRE images (D and E), amount of surrounding distortion is still minimal.
CREDIT
American Roentgen Ray Society (ARRS), American Journal of Roentgenology (AJR)
Atlanta, GA: Magntic Resonance Imaging is frequently denied to patients with ballistic embedded fragments because without shell casing it is not possible to determine the bullet composition. Now, a recent study in the American Journal of Roentgenology has found that CT and radiography can be used for the identification of nonferromagnetic projectiles that are safe for MRI. The researchers also presented an algorithm for the determination of the triage of patients with retained bullets.
The purpose of this article by Arthur J. Fountain, Emory University, Atlanta, GA, and colleagues aimed to determined whether the radiographic and CT appearance of ballistic projectiles predicts their composition. They also characterized the translational, rotational, and temperature effects of a 1.5-T MRI magnetic field on representative bullets.
For the purpose, commercially available handgun and shotgun ammunition representing projectiles commonly encountered in a clinical setting was fired into ballistic gelatin as a surrogate for human tissue. Radiographs and CT images of these gelatin blocks were obtained. Using T1- and T2-weighted sequences, MR images of unfired bullets suspended in gelatin blocks were also obtained. Magnetic attractive force, rotational torque, and heating effects of unfired bullets were assessed at 1.5 T.
Key findings of the study include:
- Fired bullets were separated into ferromagnetic and nonferromagnetic groups based on the presence of a debris trail and deformation of the primary projectile in the gelatin blocks.
- Whereas ferromagnetic bullets showed mild torque forces and marked imaging artifacts at 1.5 T, nonferromagnetic bullets did not have these effects.
- Heating above the Food and Drug Administration limit of 2°C was not observed in any of the projectiles tested.
"We found that radiography and CT can be used to identify nonferromagnetic projectiles that are safe for MRI," concluded the authors. "We also present an algorithm for determining the triage of patients with retained bullets."
"Nonferromagnetic ballistic projectiles do not undergo movement or heating during MRI, and the imaging modality can be performed when medically necessary without undue risk and with limited artifact susceptibility on the resulting images, even when the projectile is in or near a vital structure," the authors concluded.
The study, "Imaging Appearance of Ballistic Wounds Predicts Bullet Composition: Implications for MRI Safety," is published in the American Journal of Roentgenology.
DOI: https://www.ajronline.org/doi/abs/10.2214/AJR.20.23648
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