What Is Your Diagnosis?

Andrew T. Parry Department of Veterinary Clinical Sciences, The Royal Veterinary College, University of London, North Mymms, Hertfordshire, AL9 7TA, England.

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Andrew Tanner Department of Veterinary Clinical Sciences, The Royal Veterinary College, University of London, North Mymms, Hertfordshire, AL9 7TA, England.

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Kate Chandler Department of Veterinary Clinical Sciences, The Royal Veterinary College, University of London, North Mymms, Hertfordshire, AL9 7TA, England.

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Christopher R. Lamb Department of Veterinary Clinical Sciences, The Royal Veterinary College, University of London, North Mymms, Hertfordshire, AL9 7TA, England.

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History

A 4-year-old neutered male Border Collie with a 6-week history of signs of lumbar pain was referred for magnetic resonance (MR) imaging. No neurologic deficits were evident on physical examination. Magnetic resonance images of the lumbar portion of the vertebral column were obtained (Figure 1).

Figure 1—
Figure 1—

Sagittal (A) and dorsal (B) magnetic resonance images of the lumbar portion of the vertebral column of a 4-year-old neutered male Border Collie with a 6-week history of signs of lumbar pain.

Citation: Journal of the American Veterinary Medical Association 237, 4; 10.2460/javma.237.4.359

Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page

Imaging Findings and Interpretation

A lobed black featureless region is centered on the left dorsal aspect of the abdomen. As a result, most of the lumbar portion of the vertebral column and abdomen is not visible. The black featureless region represents a lack of MR signal (ie, a signal void). The most likely cause for this artifact is distortion of the magnetic field by a metal object in the abdomen. Abdominal radiographs were obtained and revealed (Figure 2) a metal object (ie, nail) in the descending colon.

Figure 2—
Figure 2—

Lateral (A) and ventrodorsal (B) radiographic views of the abdomen of the same dog as in Figure 1. Notice the metal object that appears to be a nail in the descending colon.

Citation: Journal of the American Veterinary Medical Association 237, 4; 10.2460/javma.237.4.359

Comments

Metal objects within the bore of an MR scanner can cause problems because they affect the magnetic field and because they are affected by the magnetic field. Any material has a property called magnetic susceptibility, which is the degree to which it becomes magnetized when exposed to a magnetic field. Materials can be either paramagnetic (positive magnetic susceptibility, which strengthens the magnetic field), diamagnetic (negative magnetic susceptibility, which weakens the magnetic field), or ferromagnetic (strongly positive magnetic susceptibility, which strengthens and distorts the magnetic field).1 Ferromagnetic objects are particularly prone to causing susceptibility artifacts, which usually appear as areas of distorted anatomy associated with a signal void. The effect is greatest for iron- and nickel-containing objects (including surgical stainless steel) and less for titanium.1

Magnetic susceptibility artifacts on MR images may be seen after insertion of metal implants or because of the existence of tiny metal fragments from drills, burrs, and suction devices.2,3 Magnetic resonance imaging should be done with caution in patients known to have undergone surgical procedures involving metal implants.

In addition to the ferromagnetic material having an effect on the magnetic field, the magnetic field will affect the ferromagnetic material, with the potential for loosening and migration of hemostatic clips or orthopedic implants and heating of metal and surrounding tissue.3 During MR imaging, burns can occur involving metal implants or tattoos (which may contain ferromagnetic particles) or from wires and leads lying on the skin. Less common injuries are caused by malfunction of pacemakers. The immensely strong attraction of the magnet can turn a metal object into a projectile, with the potential for serious injury to the patient or damage to the scanner. It is important to remember that the magnet is always turned on. To switch the magnet off (ie, quench) suddenly in an emergency situation can release cryogenic gases, which can severely chill and deplete oxygen in an enclosed area.4

An MR scanner represents a serious hazard, and comprehensive guidelines for safe practice have been issued.4 The use of MR imaging should be done with caution in patients known to have metal implants or suspected of having a foreign body. The dog of this report had undergone abdominal radiography several days previously by the referring veterinarian, and the nail was not evident at that time. In any patient, there is the potential for an unexpected metal inclusion.4 Prior to performing MR scans, owners should be questioned about their pet's history with regard to risk factors such as previous surgery or possibility of ingestion, and if a metal inclusion is considered likely, radiographs should be made prior to MR imaging. In the dog reported here, the artifact was recognized as a metal object in the initial series of MR images and the dog was removed immediately from the MR scanner. Fortunately, the nail passed the following day without signs of injury to the dog.

  • 1.

    Lee MJ, Kim S, Lee SA, et al. Overcoming artifacts from metallic orthopedic implants at high-field-strength MR imaging and multi-detector CT. Radiographics 2007;27:791803.

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  • 2.

    Freer SR, Scrivani PV. Postoperative susceptibility artifact during magnetic resonance imaging of the vertebral column in two dogs and a cat. Vet Radiol Ultrasound 2008;49:3034.

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  • 3.

    Kumar R, Lerski R, Gandy S, et al. Safety of orthopedic implants in magnetic resonance imaging: an experimental verification. J Orthop Res 2006;24:17991802.

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  • 4.

    Kanal E, Barkovich AJ, Bell C, et al. ACR guidance document for safe MR practices: 2007. AJR Am J Roentgenol 2007;188:14471474.

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