AJR 2002; 178:1092-1094
© American Roentgen Ray Society
Spontaneous Discharge of a Firearm in an MR Imaging Environment
Anton Oscar Beitia1,
Steven P. Meyers1,
Emanuel Kanal2 and
William Bartell3
1 Department of Radiology, University of Rochester Medical Center, 601 Elmwood
Ave., Box 648, Rochester, NY 14642.
2 Department of Radiology (D-132), University of Pittsburgh Medical Center, 200
Lothrop St., Pittsburgh, PA 15213.
3 Rochester Police Department, Rochester, NY 14624.
Received September 4, 2001;
accepted after revision November 6, 2001.
Address correspondence to A. O. Beitia.
Introduction
An incident recently occurred at an outpatient imaging center in western
New York State, in which a firearm spontaneously discharged in a 1.5-T MR
imaging environment with active shielding. To our knowledge, this is the first
documented case of such an occurrence. The event confirms previously reported
theoretic risks of a firearm discharging in an MR imaging environment
[1]. In this report, we examine
the incident in detail from the official police and ballistic reports.
Case Report
An off-duty police officer went to an outpatient imaging center (not
affiliated with our institution) in western New York State to have an MR
imaging examination. The facility housed a 1.5-T MR unit (Signa; General
Electric Medical Systems, Milwaukee, WI) with active shielding. The officer
was carrying a model 1991 A-1 compact.45 caliber semiautomatic pistol (Colt's
Manufacturing, Hartford, CT).
The officer notified the technologist that he was carrying the weapon
before entering the MR dressing room. The technologist told the officer to
take the gun with him. The technologist intended to meet the officer in the MR
patient waiting area before the examination and secure the weapon in that
room, where he felt it would be safe. However, the officer apparently
misunderstood and took the gun into the MR suite. The technologist was
entering the officer's personal data into the computer and did not see him
entering the MR suite.
Once the officer was inside the MR suite, the gun was pulled from his hand
as he attempted to place the gun on top of a cabinet 3 ft (0.9 m) away from
the magnet bore. The gun was immediately pulled into the bore, where it struck
the left side and spontaneously discharged a round into the wall of the room
at the rear of the magnet. Fortunately, no one was injured. Although the gun
struck the magnet bore, only minimal cosmetic damage occurred to the magnet
itself. The MR unit had full functional capability immediately after the gun
discharged. The weapon's thumb safety was reportedly engaged when the gun
discharged.
An unsuccessful attempt to remove the gun from the magnet resulted in the
gun being pulled to the right side of the magnet
(Fig. 1). The decision was then
made to power down the magnet to remove the gun.
Examination of the weapon by a ballistics laboratory concluded that the
force of the magnetic field was responsible for the firearm's discharge. To
understand how the gun discharged requires a brief discussion of the firing
mechanics of the Colt 1991 A-1.45 caliber pistol and the weapon's safety
mechanisms [2]. When the weapon
is normally fired, the trigger is pulled, which releases the sear. The sear,
in turn, releases the hammer. The hammer then moves forward to strike the
firing pin, which moves forward to strike the primer of the chambered
round.
The Colt 1991 A-1 pistol has three safety mechanisms (Fig.
2A,2B,2C,2D),
including a thumb safety, grip safety, and firing pin block. The thumb safety
locks the sear in place and prevents the hammer from moving forward when the
trigger is pulled. The thumb safety also locks the slide in place. The thumb
safety is the weapon's only active safety mechanism; it must be turned on in
order to work. The grip safety is located at the back of the gun handle and
prevents the trigger from being depressed. The grip safety is a passive
mechanism; it is always on unless deactivated. To deactivate it, the grip
safety must be depressed at the same time the trigger is depressed; otherwise,
the trigger cannot be pulled. The firing pin block is a small metal block,
approximately the size of a pencil eraser, that sits in the firing pin channel
and prevents the firing pin from moving forward. The firing pin block is held
in place by a small spring. When the trigger is pulled, a series of levers cam
the firing pin block up into its own well within the slide to allow the firing
pin to move freely within its channel.
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Fig. 2C. —Photographs of 1991 A-1.45 caliber semiautomatic pistol
(Colt's Manufacturing, Hartford, CT). Hammer, thumb safety, and grip safety of
gun are shown. Gun is in cocked and locked position with hammer cocked and
thumb safety turned on to prevent hammer from striking firing pin. This is
condition in which gun was recovered from magnet.
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At the time the weapon discharged, it was reportedly in a cocked and locked
position; that is, the hammer was cocked and the thumb safety was engaged to
prevent the hammer from striking the firing pin. A live round was in the
chamber. (Many people who choose this weapon for personal protection will
carry it in this manner because it allows them to quickly fire the weapon if
needed.)
When the firearm was removed from the magnet, the gun was still in a cocked
and locked position. An empty cartridge was found in the chamber. The presence
of an empty cartridge in the chamber is highly unusual. If the thumb safety
were not engaged and the weapon fired normally by depressing the trigger, the
normal backward recoil of the slide should have automatically ejected the
empty cartridge, and a new live round should have automatically been
chambered. As discussed earlier, the thumb safety performs two functions: it
prevents the sear from releasing the hammer, thereby preventing the hammer
from striking the firing pin; it also locks the slide in place, preventing
retrograde motion of the slide and automatic ejection of the empty cartridge.
Thus, the presence of an empty cartridge in the chamber confirms that the
thumb safety was engaged at the time the gun was fired. Given that the thumb
safety was engaged when the gun discharged, it is also likely that the normal
trigger and hammer mechanism of firing the gun was bypassed because the thumb
safety would have also prevented release of the hammer.
The gun likely discharged as a result of the effect of the magnetic field
on the firing pin block. The firing pin block was probably drawn into its
uppermost position by force of the magnetic field. The firing pin block has to
overcome only light pressure from a relatively small spring to release the
firing pin. The pistol was likely drawn into the magnetic field so that the
muzzle struck the magnet's bore first. With the firing pin allowed to move
freely in its channel, the force of the impact on the muzzle end was
sufficient to cause the firing pin to overcome its spring pressure and move
forward to strike the primer of the chambered round.
This account explains how the weapon discharged when the thumb safety was
engaged.
The presence of an empty cartridge in the chamber explains why the gun did
not discharge a second time when it was moved from the left to the right side
of the bore. Even if the identical forces were repeated, an empty cartridge,
not a live round, was in the chamber at this time.
Discussion
In this incident, the gun discharged despite the thumb safety being
engaged. This has important implications in that it shows that the weapon
poses a risk for discharging in an MR imaging environment even with the thumb
safety engaged.
One can look at the sequence of events preceding the discharge of the
weapon and see several points at which the incident could have been prevented.
When the officer came in with the gun, it should have been immediately secured
in a safe location, even before the officer changed for the examination. The
technologist, knowing the officer had a firearm, should have instructed him
that under no circumstances could he bring the weapon into the MR suite. Also,
the technologist should have been monitoring the officer more closely to make
sure he did not enter the MR suite with the weapon. Signs should have been
posted at that site, if they were not already there, warning the public of the
dangers of approaching the magnetic field of the MR imager with implants,
metallic devices, or objects such as firearms.
In light of this incident, all radiologists should reexamine our own site's
screening methods to ensure that steps are implemented to prevent such a
situation from ever recurring.
References
-
Kanal E, Shaibani A. Firearm safety in an MR imaging environment.
Radiology
1994;193:875
-876[Abstract/Free Full Text]
-
Sweeney P. Gunsmithing: pistols and
revolvers. Iola, WI: Krause, 1998:30
-34
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Introduction
Case Report
