Friday, February 6, 2009

Mummies and Metal Artifact

Is it a mummy or a Mummy? Sometimes your occupation determines your reaction to a word. For example, here is an artifact:

Photo by Shain Etin

To an archeologist, an artifact such as this is a boon, an insight into a past age. To a radiologist, the word means something very different. An artifact is an annoyance, a pox on what could have been a beautiful image.

Yi Zhao at the University of Chicago summarizes how the "artifact" can mean very different things:

An artifact (artefact in British spelling; from Latin arte, ablative of ars, art, and factum, neutral past participle of facere, to make), according to the Oxford English Dictionary, is “[i]n technical and medical use, a product or effect that is not present in the natural state (of an organism, etc.) but occurs during or as a result of investigation or is brought about by some extraneous agency.” This meaning of the term artifact has been in use since 1908, and contrasts with the first meaning: “Anything made by human art and workmanship; an artificial product. In Arch├Žol. applied to the rude products of aboriginal workmanship as distinguished from natural remains,” which appeared in the early 19th century. In all meanings of the word, it signifies the presence of an artificial object or element, in contrast with a natural medium.

The word artifact has come into popular use mainly due to wide developments in imaging technologies, starting with the telescope, but especially since the invention of photography, and most recently with the proliferation of digital technologies. Although the applications of such technologies were at first primarily technical and scientific, they quickly became parts of people's daily lives (the telescope has always been a curiosity for the public, and digital photography has recently become a regular practice for most households). Therefore the current definitions of the word, not totally acknowledged by the Oxford English Dictionary, depend heavily on context, and the contexts in turn depend on technologies with which the artifact is associated.


Thus, with the advent of scientific technology and digital imaging in particular, the archeologist artifact has been transmuted to a word with negative connotations. In MRI, a common cause of artifact is metallic hardware. The degree of metal artifact is variable, and depends on many factors. Lee et al. have written a nice summary on overcoming metal artifact from metallic orthopedic implants (Radiographics 27:791-803, 2007). They point out that the degree of artifact will depend on the composition of the metal hardware, with stainless steel causing much more artifact than titanium.

Unfortunately, it can be hard to predict how much artifact a particular piece of hardware will cause. As long as the hardware is not directly in the area of interest, however, in many cases it is possible to obtain diagnostic information. For example, here is a CT topogram image from a patient with shoulder pain. She has a humeral rod in place, due to a fracture:

The presence of this much metal might lead you to think that an MRI would be markedly degraded by metal artifact, and be nondiagnostic. I am sure that many MRI scans are not ordered in this setting because the clinician thinks that the study will be nondiagnostic.

In fact, despite the presence of the rod, and resulting metal artifact, the shoulder MRI is able to clearly depict a full thickness tear of the supraspinatus tendon:

Thus, MRI may remain a reasonable option in patients with indwelling metal hardware, particularly if the hardware is titanium, and is not directly in the area to be imaged. Some modifications can be made to pulse sequences to decrease metal artifact, but these improvements are typically not dramatic. In addition, this is one setting where low-field strength MRI can be a good option, as metal artifact will be less pronounced at low field strength.

Finally, for questions about metal hardware and MRI safety, it's worth visiting Dr. Frank Shellock's outstanding website, MRIsafety.com.



Vic David MD
Orthoradiology.com


2 comments:

Craig said...

Very nice blog once again. I am a msk in western co doing general. NO doubt based on that plain film I would have done a ct arthrogram. Strong work!

Vic David MD said...

Thx for your kind comment.

It's nice to be able to share one's experiences with other medical professionals, and this blog is a good way to do that.