Friday, November 28, 2008

SOS and a Rheumatoid Knee

The phenomenon of "satisfaction of search" (SOS) is the bane of all radiologists. ("Bane" comes form the old English word for "slayer, murderer"; it means "that which causes ruin or woe", so it is a felicitous choice for this context). In SOS, the radiologist detects an obvious lesion, becomes satisfied with his perceptions, and misses less obvious abnormalities on the same study.

Satisfaction of search has been described by several authors, including Samuel et al. (Radiology 194:895-902, 1995), who studied this phenomenon in the detection of nodules on chest radiographs. They concluded that "obvious abnormalities capture visual attention and decrease vigilance for more subtle abnormalities". SOS also occurs in musculoskeletal radiology, as documented by Ashman et al (AJR 175:541-544, 2000).

Not surprisingly, radiologists continue to wrestle with this phenomenon. The more striking the findings, the more you have to increase your vigilance for subtle findings.

In this case, a 22 year-old female with rheumatoid arthritis presents with knee pain:

Sagittal and coronal T2 fat sat images show a large joint effusion with rice bodies (yellow arrow) and inflammatory pannus (green arrows). There is also a bone infarct in the tibia (red arrows). There seem to be more than enough findings to explain the patient's pain!

Mindful of the SOS phenomenon, a careful inspection of the entire knee was done. This revealed a tear of the posterior horn of the lateral meniscus:

This case reinforces the idea that the bigger and more spectacular the findings, the more you should look carefully at the entire study. Otherwise, human nature and SOS will tend to lead you astray.

Vic David MD

Friday, November 21, 2008

Hamstrings and Filet Mignon

The reason I gave up eating red meat can be traced back to my experiences as a medical student dissecting cadavers in gross anatomy. I learned that filet mignon was a cut from the psoas major muscle of the steer. Knowing that, and looking simultaneously at the psoas major of a human (albeit a dead one), did the trick— instant vegetarian. Eventually I gained back my appetite for meat, but it's now pretty much restricted to fish and chicken.

The "hamstrings" are the muscles that compose the posterior thigh. Roughly speaking, they run from the pelvis to the knee, and are composed of the biceps femoris, semitendinosus and semimembranosus muscles. The ham of the dinner table is typically the hamstring of the pig:

Photo by cactusbones
The verb "hamstring", meaning "to disable or render useless", derives from the observation that cutting the hamstring tendons renders a person or animal lame.

Hamstring injuries are common in sports. Many people consider these to be fairly innocuous injuries, but Askling et al. recently pointed out that these can be extremely debilitating injuries (AJSM 36:1799-1804,2008).

Consider this case, a 55 year-old female athlete, with sustained a sports injury three weeks ago:

A coronal T1-weighted image depicts a near-complete avulsion of the right hamstring tendon (red arrow) from the ischial tuberosity. Note the normal left hamstring (green arrows).

An axial T2-weighted image with fat saturation shows the nearly naked right ischial tuberosity (red arrow), where the hamstring tendon has been avulsed. Note the normal left hamstring tendon (green arrow):

Askling et al. examined 30 subjects from 21 different sports. All the hamstring injuries they studied were located proximally in the posterior thigh, close to the ischial tuberosity. They found that a large percentage of patients (47%) actually chose to give up their sport after an extended time of rehabilitation (median, 63 weeks). Of those patients that returned to sports, 88% of the subjects still reported symptoms from the injury.

Thus, high-grade partial or complete proximal hamstring tears can be an extremely debilitating, serious injury that can have a prolonged recovery time. It is important to give an accurate description of the location and degree of tear when interpreting the MRI examination.

Vic David MD

Friday, November 14, 2008

Iliotibial Band Friction Syndrome (or Not)

Running is a great way to get exercise, if your ankles and knees will tolerate it:

Photo by ziga-zaga
Runners can develop a condition called, iliotibial band friction syndrome, as can cyclists. In this condition, there is inflammation of the tissues around the iliotibial band (ITB). Patients present with lateral knee pain. On MRI, one can see edema around the iliotibial band, typically deep to the iliotibial band (arrows), as in this example:

Recently, I ran across a case of a young female patient with knee pain, with edema around the iliotibial band:

Although the edema is closely associated with the ITB, the preponderance of edema is superficial to the ITB, rather than deep to it, as would be expected for ITB friction syndrome.

This seemed a bit odd, so I picked up the phone and called the patient. She informed me that she had received a cortisone shot in this location about 3 hours prior to the MRI, and had a bruise in that location. A little clinical history can go a long way...

Thus, if you see edema concentrated in a focal area in the superficial soft tissues, without a history of trauma or infection, consider the possibilty of iatrogenic mischief. This is also true for other joints, such as the shoulder.

Vic David MD

Friday, November 7, 2008

Icebergs and Fractures

The word "fracture" comes from the Latin word fractura, meaning "a breach, break, cleft". The word is used in various ways— for example, when icebergs fracture from a glacier, that process is known as "calving":

In medicine, "fracture" is usually used in the context of a bone break. MRI is excellent for depicting acute fractures, because most acute fractures are accompanied by extensive bone marrow edema, as in this radial head fracture in my 14 year-old son:

In the subacute or chronic setting, however, MRI becomes less sensitive for fractures, in part because bone marrow edema often subsides or resolves entirely. This can be particularly problematic in small joints. The bones are smaller, and are often curving in character, creating volume-averaging artifact.

One way to increase the sensitivity of the exam for small chip or avulsion fractures is to use a thin-section 3D (volume) gradient echo sequence. This is often the best sequence for picking up these subtle injuries.

Here are MRI and CT images from a 35 year-old male with a history of trauma four months ago:

(click on image to enlarge)

There is a subtle chip fracture of the lunate (red arrows), which is seen best on the GRE (gradient echo) and CT images. Note how difficult the fracture is to see on the intermediate fatsat (Int FS) image. When reading wrist MRI, I try to scrutinize the 3D GRE images very carefully, as this can often be the only sequence where a fracture is well-seen.

Thus, it's generally a good idea to run a thin-section GRE sequence for wrist MRI. Fractured icebergs off glaciers are easy to see, but small bone chip and avulsion fractures can be hard to detect without this sequence.

Vic David MD