Wednesday, October 29, 2008

Gadolinium and MR Arthrography

We do a great deal of MR arthrography in our practice. In this test the radiologist first instills contrast into the joint, typically under imaging guidance. Immediately thereafter, the patient has an MR examination. The contrast that is typically used for this examination is dilute gadolinium, mixed in iodinated contrast, saline, or a mixture of the two. There have been excellent articles published about what concentration of gadolinium to use; for example, see this article by Montgomery et al.

I have seen some unusual contrast properties on various MRI pulse sequences, sometimes varying with the brand of iodinated contrast that is used in the cocktail. I finally decided to try to gather some additional data, to see how various mixtures performed under the pulse sequence parameters that we use.

The following graph summarizes the results:

Saline = normal saline
Iodine = iodinated contrast (240mg iodine/ml)
Gad = Magnevist
The graph plots the signal intensity for several different mixtures, using sequences routinely used for MRI arthrography at 1.5 Tesla.

There are a number of interesting conclusions that can be drawn from this data. Most importantly, the highest signal intensity for all pulse sequences is obtained by using a 1:1 mixture of iodinated contrast and normal saline, with a gadolinium concentration of 1.25 mmol/L. This is the mixture we currently use for MR arthrography. Next, signal intensity for T2 weighted sequences is lower for all mixtures when one goes from TE=36 ms to a TE=70 ms. This is not surprising, but emphasizes that one should use TE values that are at the lower end of T2 values that give T2 weighting.

MR arthrography, properly done, is an excellent test that can give a great deal of valuable information about the joint and its environs.
Some of this information can be difficult to see or inapparent using conventional (noncontrast) MRI. This data helps validate the techniques that we use to perform MR arthrography.

Vic David MD

Friday, October 24, 2008

Ankle Sprains and the ATFL

Ankle sprains are extremely common injuries, and occur in a wide variety of sports:

The anterior talofibular ligament (ATFL) is a major component of the lateral collateral ligament complex of the ankle joint and plays an important role in stabilizing the ankle. The ATFL is the most frequently torn ligament in ankle sprains.

The ATFL is usually thought of as a single structure, but the ATFL can have more than one band (Milner and Soames;J. Anat 191, 457-458, 1997). These investigators carefully dissected 26 cadaver ankles. In 38% of ankles, the ATFL had a single band; in 50% of ankles, the ATFL had two bands, and in 12% of ankles, the ATFL had three bands. The overall width of the ATFL did not vary greatly, irrespective of the number of bands present.

This variation in structure can be seen on MRI as well. In the following patient, the ATFL has a single band:

In this ankle, the ATFL has two bands:

In the final example, we see an ATFL composed of three bands:

The individual bands of the ATFL are best perceived on coronal images. Thus, do not confuse variations in the fascicular structure of the ATFL for a tear of this ligament.

Vic David MD

Sunday, October 19, 2008

Skier's Thumb and Partial Stener Lesions

Skiing in the crisp mountain air, beneath a blue sky, is one of life's true pleasures.

Photo by Spatial Mongrel
Eventually, every skier falls backward in a fall, and hyperabducts the thumb MCP joint. That places them at risk for "skier's thumb", the eponym for a tear of the ulnar collateral ligament. Historically, the UCL was often chronically injured in gamekeepers, who killed rabbits and other small game by breaking their necks between the ground and their thumbs and index fingers.

Bertil Stener was a Swedish hand surgeon who elucidated the cause of chronic instability in patients with ulnar collateral ligament tears:

Stener noted that in some patients with UCL tears, the torn ligament would retract superficial to the aponeurosis of the overlying adductor pollicis muscle. The interposed adductor aponeurosis prevented healing of the torn UCL.

In most cases, a Stener lesion represents a complete rupture of the UCL. This is not always true however, as pointed out by Romano et al. (Can Assoc Radiol J 2003;54(4):243-8) in an excellent article on the spectrum of ulnar collateral ligament injuries. They observed that in some patients, "a large component of the redundant ligament was displaced proximally and dorsally, as would be found in a Stener’s lesion, but the distal end remained beneath the aponeurosis, which distinguished it from a classic Stener’s lesion".

This article goes on to note that "this contradicts previous claims that adductor aponeurosis interposition cannot occur in partial ruptures".

Twenty-seven year old patient with trauma to the thumb, and clinical evidence of a thumb UCL tear, who was referred for an MRI, to rule out a Stener lesion:

Coronal T2-weighted images with fat saturation reveal a torn, retracted UCL (red arrows), superficial to the adductor aponeurosis (yellow arrows). Note that although the majority of the UCL is retracted proximally, there remains a portion of the ligament (white arrow) that remains underneath the aponeurosis.

Thus, although a Stener lesion typically reflects a complete tear of the UCL, in a minority of cases one can have partial tears of the UCL leading to a Stener configuration.

Vic David MD

Saturday, October 11, 2008

Profundus Laceration

The word "laceration" comes from the latin "laceratio", Latin for "tearing, lacerating". Fingers are often cut:

Photo by amanky
These injuries are often treated in the emergency room, as described by in this blog entry by an an ER doc in Texas. Worth reading....

Finger lacerations are often superficial, but a deep injury can transect a tendon. In this case, a thirty-two year old female lacerated her ring finger on a food preparation machine, and presented to a hand surgeon. The clinical examination was difficult, and the patient was referred for an MRI to better delineate the nature of the flexor tendon injury.

Sagittal gradient-recalled echo image:

The flexor digitorum profundus tendon is completely cut, with a 7 mm gap (red arrow) in the distal tendon. Interestingly, there is a second gap in the tendon (yellow arrow), just proximal to its insertion. Thus, there are two separate lacerations of the tendon, with a free-floating segment. These findings were confirmed at surgery. At surgery, it was discovered that the volar plate of the PIP joint was also cut.

Compare this to a normal flexor digitorum profundus tendon:

Lacerations usually are not referred for MR imaging, but when the clinical exam is difficult or confusing, a high-resolution MRI can often yield valuable pre-operative information.

Vic David MD

Saturday, October 4, 2008

Beckham and the Posterior Tibial Tendon

Tears of the posterior tibial tendon (PTT) are rare in athletes. Overuse syndromes leading to tendinopathy and tenosynovitis are more common in sports such as tennis and soccer, which require a great deal of side to side movement. As long as the feet are happy, stars like David Beckham can make great plays:

An uncommon ankle injury worth remembering is dislocation of the posterior tibial tendon. Thirty year-old male with medial ankle pain:

Axial proton-density (PD) and T2-weighted images reveal a medially dislocated posterior tibial tendon (red arrows).
Here are comparison images from a normal individual, depicting the normal posterior tibial tendon (green arrow), behind the medial malleolus (yellow arrows). (The bright oval laterally is a skin marker, and can be ignored):

A coronal intermediate image from the abnormal patient confirms the abnormal position of the PTT, superficial and medial to the medial malleolus:

Dislocation of the PTT is a rare condition, and diagnosis is often delayed. The MRI diagnosis of this condition has been described by Bencardino et al. (AJR 169:1109-1112, 1997). Most lesions involve tearing of the flexor retinaculum, but some cases are due to an incompetent flexor retinaculum (AJSM 29:656-689, 2001). Conservative therapy is not effective, and surgical repair is usually necessary.

Vic David MD