Saturday, April 25, 2009

Shoulder MR Arthrography

Photo by Cesar R.

Many different people from all walks of life look at this blog. The kaleidoscopic panoply that the web offers to both the writer and viewer is astounding.

The intended main audience of this blog is the radiology community, but orthopedic surgeons, other health care professionals and patients also view this blog. In this entry, we will address an area that will interest primarily orthopedic surgeons— the interpretation of shoulder MR arthrograms.

A potential point of confusion in MR arthrography is the similar image contrast of two pulse sequences that are often used: T1-weighted image with fat saturation, and T2-weighted images with fat saturation.

Note how similar these images appear:

(A) T1-weighted image with fat saturation and (B) T2-weighted image with fat saturation depict hyperintense contrast within the joint
(yellow arrows) and fluid in the subacromial-subdeltoid bursa (green arrows).

It is quite important not to confuse T1-weighted images with T2-weighted images. Pure fluid will be bright only on the T2-weighted image, while contrast will be bright on both pulse sequences.
In this example, the bursal fluid is hypointense on the T1-weighted image, and hyperintense on the T2-weighed image, while contrast in the joint is hyperintense on both pulse sequences, as expected.

Since the overall appearance of these pulse sequences is so similar, how is one to quickly distinguish between these two images? For coronal images, most centers use spin echo images, and do not use gradient echo in the coronal plane. With this assumption, one can easily identify a T1-weighted image simply by looking at the TR that is used. If the TR is less than 900, it can be considered a T1-weighted image:

The TR in this case is 566 (yellow arrow). Thus, we know that this is a T1-weighted image. Again, this rule holds true for spin echo images, not for gradient echo images.

In MR arthrography, one reliable sign of a rotator cuff tear is contrast entering the tendon. If the tear is full-thickness, the contrast will leak into the subacromial-subdeltoid bursa.

There are two exceptions to this rule. First, in the case of a partial-thickness bursal surface tear (a tear of surface of the tendon facing away from the joint), contrast will not enter the tendon. The tear is on the bursal surface and contrast is contacting the intact articular surface of the tendon. The other exception to this rule is when there is an intrasubstance tear of the tendon; contrast cannot enter the tear because there is a layer of intact tendon separating the tear from the contrast within the joint.

Consider this full-thickness tear:

(A) T1-weighted image with fat saturation and (B) T2-weighted image with fat saturation depict contrast within the joint (yellow arrows). Note the presence of contrast within the subacromial-subdeltoid bursa (red arrows) due to the full-thickness tear within the distal supraspinatus tendon (green arrow)

This is contradistinction to this case of a partial-thickness, articular surface tear of the supraspinatus tendon:

(A) T1-weighted image with fat saturation and (B) T2-weighted image with fat saturation depict a partial-thickness articular surface tear (red arrows) of the supraspinatus tendon. Contrast enters the tear, and the tear is seen as a hyperintense area on both pulse sequences. Although there is a small amount of hyperintense fluid in the subacromial-subdeltoid bursa on the T2-weighted image, no contrast enters the bursa. Thus, this is a partial thickness tear.

Finally, consider this case:

(A) T1-weighted image with fat saturation and (B) T2-weighted image with fat saturation depict contrast within the joint (yellow arrows). Note the intrasubstance tear (red arrow) of the distal supraspinatus tendon, seen as a hyperintense zone on the T2-weighted image. Note that contrast does not enter this area since the tear does not communicate with the articular surface of the tendon. There is hyperintense fluid in the subacromial-subdeltoid bursa on the T2-weighted image, but this area is hypointense on the T1-weighted image. This fluid should not be confused with contrast in the bursa.

Accurate interpretation of MR arthrograms hinges on your ability to distinguish between various pulse sequences. It's worth remembering this little trick of simply looking at the TR to distinguish between T1-weighted and T2-weighted images.

Vic David MD

Saturday, April 11, 2009

Fruit Flies and Tarsal Coaltion

Those of you who took genetics in college might recognize this little creature:

This is the fruit fly, Drosophila melanogaster, which has delighted geneticists and tortured premed students for decades. I still can smell the ether we used to anesthetize these buggers as we struggled to understand Mendelian inheritance in my college genetics class.

Far apart in the tree of life, humans and Drosophila nonetheless share major portions of DNA. One such DNA sequence is the homeobox, which encodes transcription factors that play a major role in limb development. These DNA sequences are conserved across vast distances in the phylogenetic tree— for example, a fly can function perfectly well with a chicken homeotic gene in place of its own.

Hox genes are a subgroup of homeobox genes. In vertebrates these genes are found in gene clusters on the chromosomes. In mammals four such clusters exist, on four different chromosomes.

Mutations in hox and other genes can cause multiple genetic anomalies, including segmentation errors. Segmentation errors can lead to fusion of bones in the foot, a phenomenon known as tarsal coalition.

Tarsal coalition has been know about for hundreds of years, although the genetic basis is only being investigated in the modern era. The first written description of tarsal coalition was by Buffon in 1769. The first radiologic depiction of tarsal coalition took place in 1898, only three years after Roentgen described x-rays.

The most common types of tarsal coaliton are calcaneonavicular and talocalcaneal coalitions, These variants are commonly seen by every busy radiologist that reads MRI scans of the foot and ankle.

39 year old female training for 10 mile run, who recently increased running up to seven miles a day, and complained of distal leg pain:

(A) Sagittal T1 and (B) Sagittal T2 fatsat images depict a stress fracture (red arrow) of the distal tibial metaphysis. Note the striking marrow edema, seen best on the T2 fatsat image.

One must be cautious about satisfaction of search, however, and examination of the remainder of the examination reveals a second finding:

(A) Sagittal T1 and (B) Sagittal T2 fatsat images display a predominantly fibrous coalition (red arrow) between the navicular (green arrow) and the cuboid (blue arrow) bones.

The coalition (red arrow) is nicely seen between the navicular (green arrow) and the cuboid (blue arrow) on this coronal intermediate image:

An oblique axial T2 fatsat image reveals marrow edema on both sides of the abnormal joint, reflecting abnormal stress:

Coalitions between the cuboid and navicular are rare, accounting for less than one percent of tarsal coalitions. Coalitions are often treated nonsurgically, but when necessary, they can be surgically resected.

Humans run and flies use their wings to get from place to place, but they both share common DNA. Errors in the DNA code in critical areas of either species can lead to segmentation anomalies in both.

Vic David MD