Friday, August 29, 2008

Lost in Translation and a Hip Tumor

Some things are easily lost in translation. Witness the ineptly named Chinese Barbie-doll knock-off, "Benign Girl":

Something tells me that the manufacturer was trying to conjure up resonances of something besides a benign tumor when they named this product.

Clearly, language is important. For example, what is the difference between a benign and malignant tumor? The essential difference is their biologic behavior— benign tumors classically do not metastasize (spread to other sites), and typically grow more slowly than their malignant counterparts. Despite their less aggressive behavior, however, benign tumors can still be quite symptomatic.

Teenage boy who presents with left hip pain:
Coronal STIR image reveals a rounded, mixed-signal mass (red arrow) in the epiphysis of the left femoral head. There is extensive surrounding bone marrow edema, as well as a joint effusion.

(A) Axial T1-weighted and (B) T2-weighted images confirm the presence of the lesion, which is nearly isointense to the edematous cancellous bone:

Intravenous gadolinium was administered, and a subtraction image was obtained, confirming the presence of an enhancing tumor in the femoral epiphysis:

A CT scan was obtained for lesion characterization:

The lesion has internal matrix, with a pattern of rings and arc calcifications, characteristic of chondroid matrix. The lesion is well defined, and has a sclerotic rim. A pathologic fracture (blue arrow) is also identified.

The overall findings are highly suggestive of a chondroblastoma, which was confirmed at pathologic analysis.

Chondroblastomas typically arise in patients between the ages of 10 and 30. They almost always arise in the epiphysis. Although are regarded as a benign lesion, a small percentage can metastasize to the lungs.

On MRI, chondroblastomas are typically heterogeneous, but will often have some dark areas on T2-weighted images. They tend to generate a great deal of bone marrow edema, and will enhance after the administration of gadolinum. A subchondral epiphyseal tumor in the immature skeleton, associated with significant edema, is usually a chondroblastoma (Kan,J.H., & Kleinman,P.). The differential diagnosis includes a Brodie's abscess and Langerhans histiocytosis. Also to be considered, but much less likely, is an epiphyseal osteosarcoma or ganglion cyst filled with granulation tissue.

Vic David MD

Friday, August 22, 2008

Snapping Hip- Internal or External?

High-performance athletes learn to live with many injuries. I am always amazed to see what the joints of some professional athletes look like- many of them are walking around with pain that would make me admit myself to the hospital for a pain pump.

Some ailments are more confusing to diagnose than others. In the "snapping hip" syndrome, there is an audible snap or click that occurs in the region of the hip joint. Athletes that are particularly prone to this sydrome include ballet dancers, track and field competitors, soccer players, and gymnasts, where repetitive hip flexion is common.

Snapping hips can be caused by intraarticular and extraarticular conditions. The most common intraarticular cause is a tear of the acetabular labrum. There are many extraarticular causes, including movement of the iliotibial band over the greater trochanter, snapping of the iliopsoas tendon, and movement of the gluteus maximus tendon over the greater trochanter.

15 year-old female with snapping left hip, referred for MRI to see if snapping is intraarticular (usually due to a labral tear) or extraarticular in origin. Oblique axial T2-weighted image reveals local soft tissue edema (red arrow) associated with the iliopsoas tendon (yellow arrow):

The iliopsoas tendon can snap over the lesser trochanter, the joint capsule, or the iliopectineal eminence. With reptitive snapping, the local soft tissues can become edematous, as in this case. The iliopsoas bursa can also become inflamed.

Oblique coronal T2-weighted image confirms the edema (red arrow) tracking along the iliopsoas tendon (yellow arrow):

Axial T2-weighted image depicts the unilateral nature of the edema (red arrow) associated with the left iliopsoas tendon (yellow arrow). Note the normal right iliopsoas tendon (green arrow):

Snapping can be asymptomatic, but can be painful in some patients. Althoug nonoperative treatment is usually successful, in recalcitrant cases, one can perform a total or partial release of the posteromedial tendinous portion of the iliopsoas muscle at the pelvic brim.

Vic David MD

Friday, August 15, 2008

Volume Averaging Artifact

"Partial volume artifact" and "volume averaging" are terms that are thrown about willy-nilly by radiologists, because we run into this phenomenon every day. Here, we aim to give a brief explanation of this phenomenon.

This artifact occurs when an object is only partially within the slice (i.e. volume) that is imaged. When this occurs, the pixel value at that location is the average of the object and its surroundings.

To reify this concept, let us conduct an experiment. We will take a piece of plastic shaped like this:

Place this plastic piece in a water bath, and put the water bath into an MRI scanner. Next, perform two MRI pulse sequences, varying only slice thickness, keeping all other parameters the same:

(click on image to enlarge)

Each row contains consecutive slices through our plastic piece. At 0.7 mm slice thickness, the margins of the plastic object are relatively sharp, particularly at the center of the object.

At 1.2 mm slice thickness, however, we see something different. As we get to the edge of the object, there is marked blurring (red arrows). Why does this occur?

Remember that in this area, the slice encompasses both the object and the surrounding water. Thus, the signal value of the pixels in this area is the average of the pixel value of the object and the pixel value of the surrounding water. Voila, volume averaging artifact.

This has real world implications in radiology- to minimize volume averaging artifact, we want the smallest possible pixels. Unfortunately, as we make pixels smaller, the signal from those pixels also decreases. Thus, one must balance the desire for high resolution with the need for adequate signal. If done correctly, optimal image quality and diagnostic information is achieved.

Vic David MD

Friday, August 8, 2008

Swimmers and Unstable Shoulders

The 2008 Summer Olympics are coming up, and one of the main events will be swimming. Many eyes will be on Michael Phelps, who won eight medals at the 2004 Summer Olympics:

Swimmers like Michael Phelps are athletes who repeatedly stretch the shoulder girdle, and the capsular attachments of the glenohumeral joint. This can lead to ligamentous laxity, particularly in patients predisposed to this condition on a genetic basis. Ligamentous laxity can lead to shoulder instability, with repeated subluxations and dislocations.

Shoulder instability can be in one direction (anterior or posterior), or be multidirectional. Most cases of multidirectional instability are atraumatic in nature, and are related to increased joint volume and capsular laxity.

22 year-old male with history of multidirectional instability. An MR arthrogram was requested to evaluate the capsule and labrum. Axial image reveals a markedly capacious posterior joint:

For comparison, here is a patient without a history of posterior instability; note that the posterior capular volume is markedly smaller:

Sagittal image from the patient with multidirectional instability confirms the patulous nature of the posterior joint, particularly posteroinferiorly (yellow arrows). Green arrow= coracoid process, an anterior structure:

Dewing et al. published an analysis of this issue recently (AJSM 36:515-522, 2008). They analyzed capsular area in patients with anterior, posterior, and multidirectional instability. They found that capsular area is increased in the setting of posterior and multidirectional instability, but not in patients with anterior instability. They pointed out that it may be difficult to reproducibly measure joint volume, due to variability in the amount of contrast injected between patients. In addition, they also noted that gravity might play a role in the relative size of the anterior aspect of the joint. (Patients are scanned in the supine position, and it is unclear what would happen to the relative volume of the anterior and posterior aspect of the joint in the prone position).

My own experience suggests that one should examine the capacity of the glenohumeral joint when analyzing an MR arthrogram, particularly in patients with a history of multidirectional instability. Some of these patients will have no labral abnormality whatsoever, but will have clear evidence of increased posterior or posteroinferior joint capacity. I will comment on this in my report, noting the absence of a labral tear, but the presence of an abnormally increased joint volume. In addition, my anecdotal observations of increased anterior joint volume in the setting of repeated anterior dislocations are consonant with the observations of Urayama et. al, (
AJSM 31:64-67, 2003) who reported capsular elongation in patients with recurrent anterior dislocation.

Vic David MD

Friday, August 1, 2008

Korn and the Extensor Pollicis Longus

My teenage son recently went through his musical Korn phase. Delicate music, this is not. Lots of ear-splitting, pounding guitar, twisted transistors, and a drummer that loves to pound his kit:

Photo by Mark McLauglin

Drumming has been around for a few hundred years, and it should come as no surprise that drummers can get injured from their repetitive acrobatics. In 1876 Duplay described "drummer boy's palsy" in Prussian drummers, due to rupture of the extensor pollicis longus (EPL) tendon. Patients with rupture of the EPL tendon present with pain and loss of thumb extension.

The EPL tendon is classically torn as a result of a distal radius fracture. In this setting, the tendon usually ruptures at the level of Lister's tubercle, typically one to three months following the fracture. The EPL can also experience attritional tearing in the setting of systemic diseases such as rheumatoid arthritis, lupus, and gout. Rarely, the EPL tendon can be torn at the level of the distal phalanx of the thumb.

The EPL tendon can also rupture due to trauma, although this is less common. Twenty-eight year old gentleman, who sustained an injury playing lacrosse, and complained of dorsal wrist pain, radiating to the thumb. Axial T2-weighted image reveals a small amount of fluid surrounding the EPL tendon, which is markedly abnormal in signal:

On sequential coronal images, the EPL tendon (yellow arrows) is still continuous, but has a striated appearance, and it is surrounded by a thin rim of increase fluid:

Note the normal cross-over of the EPL tendon over the extensor carpi radialis brevis [ECRB] tendon (green arrows). These images define the presence of a partial tear of the tendon, with reactive inflammatory fluid in the tendon sheath.

The patient was treated conservatively, but several weeks later suffered an additional traumatic event, and presented with acute loss of thumb extension, with a recurrence of dorsal wrist pain radiating to the thumb. An EPL rupture was diagnosed clinically, but an MRI was obtained to exclude a concurrent intercarpal ligament injury. An axial T2-weighted image reveals that the third dorsal compartment, which normally houses the EPL tendon, is nearly empty, with only a small amount of debris in the tendon sheath:

A coronal image better depicts the fluid in the essentially empty tendon sheath:

A slightly more anterior coronal image identifies the torn, retracted EPL tendon (red arrow).

Ruptures of the EPL tendon are not repaired directly. This situation has not changed for many decades, as demonstrated from this letter in the British Medical Journal in September, 1937:

EPL ruptures are treated with a tendon transfer, typically using the extensor indicis.

The EPL tendon can be challenging to image on MRI, due to the thin nature of the tendon, and the complications of magic angle artifact. Nonetheless, with high-resolution, thin section MR imaging, much information can be gained.

Vic David MD