Friday, February 27, 2009

Primates, Thumbs, and de Quervain Syndrome

The opposable thumb is said to be one of the key features that distinguishes humans from much of the animal world. Not all primates have an opposable thumb; only Old World monkeys and apes have this unique adaptation.

Photo by ucumari

The thumb is powered by numerous muscles. Two muscles contribute to the motion of radial abduction- the extensor pollicis brevis (EPB) and abductor pollicis longus (APL). The tendons for these muscles pass through the first dorsal compartment of the wrist, where they are surrounded by tendon sheaths. Inflammation of these tendon sheaths was first described by the Swiss surgeon Fritz de Quervain in 1895, who also described an inflammatory condition of the thyroid gland ("de Quervain thyroiditis")

In this case, a 36 year-old woman presented with wrist pain. Radiographs were obtained, and an erosion was noted at the base of the radial styloid, along with adjacent soft tissue calcification. There was some concern that this could represent a soft-tissue mass, and the patient was referred for an MRI. Coronal intermediate weighted image reveals the erosion (red arrow) at the base of the radial styloid:

A coronal gradient echo image again depicts the erosion (red arrow), and a fleck of soft tissue calcification (yellow arrow):

Chien et al. described focal radiographic abnormalities of the radial styloid as a manifestation of de Quervain disease (AJR 177:1383-1386, 2001). Axial images from our patient:

(A) Axial T1 image depicts erosion in distal radius. (B) Axial T2 fatsat image shows inflammation surrounding the APL and EPB tendons (green arrow).

Chein et al. noted that "in the appropriate clinical context, radiographic visualization of focal cortical erosion, sclerosis, or periosteal bone apposition of the radial styloid should suggest the diagnosis of de Quervain tenosynovitis". Although their report described the x-ray manifestations of de Quervain syndrome, the same findings can be depicted on MRI, as demonstrated in this case.

Vic David MD

Friday, February 13, 2009

Twist, Turns and Tibial Torsion

Twists and turns occur in nature, sometimes with great beauty, as in this nautilus shell:

The nautilus shell is an example of a logarithmic spiral, a spiral curve whose shape is unaltered with each successive curve. The logarithmic spiral was first described by Descartes and later extensively investigated by Jakob Bernoulli, who called it Spira mirabilis, "the marvelous spiral". Possibly as a result of this unique property, the spira mirabilis has evolved in nature, appearing in certain growing forms such as nautilus shells and sunflower heads. (Wikipedia).

Twists and turns can also occur in the human body. The cochlea of the ear is spiral-shaped. Spiral shapes can also be pathologic— spiral electrical waves have been linked to cardiac arrhythmias, and pathologic twists of the intestine can be life-threatening.

Pathologic twists are sometimes referred to as "torsion", as in intestinal torsion, testicular torsion, and ovarian torsion. The word "torsion" comes from the Latin and old French, meaning "wringing pain in the bowels, and "to twist".

In the musculoskeletal realm, one can encounter tibial torsion, defined as the degree of twisting between the proximal and distal articular surfaces of the tibia. There is a normal, physiologic amount of tibial torsion, but when it is excessive, tibial torsion is harmful.

Excessive tibial torsion is usually a disease of childhood, but on occasion one can encounter excessive tibial torsion in adulthood as well.

Tibial torsion can be measured using the cross-sectional imaging techniques of MRI and CT. Conventional radiographs are less reliable and reproducible than MRI and CT. There is no general consensus on where exactly to draw the reference lines used to calculate the tibial torsion angle, but one reasonable method is illustrated here:

(A) Transverse CT section through proximal tibia, just below proximal articular surface. A line is drawn bisecting the tibia. (B) Transverse CT section through distal tibia, just above distal articular surface. A line is drawn bisecting the tibia. (C) The angle between these two lines is calculated.

There are ethnic and gender differences in the tibial torsion angle, with reports studying Caucasians reporting values centered at 30-35 degrees, while studies of Japanese and Indian subjects reporting values centered at 22-24 degrees.

In the patient depicted above, the tibial torsion angle in the right leg was 56 degrees. The contralateral limb had a tibial torsion angle of 31 degrees (normal).

The abnormally high angle in the symptomatic limb was due to a tibial fracture which healed with rotation:

Although tibial torsion is typically measured on CT, one can obtain this angle using MRI as well, as long as the proximal and distal tibia are imaged in the axial plane on the same study.

Further information on the measurement of tibial torsion can be found in a nice paper by Mullaji et al. (IJO 43:309-313, 2008).

Vic David MD

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,

Vic David MD