Sunday, December 20, 2009

Hydrogen Precession Frequencies and a Trillion Dollars

The microcosmic, relativistic, and quantum worlds often work in counter-intuitive ways. The twin paradox is a famous example of a puzzling, non-intuitive phenomenon in the theory of special relativity.

Photo by phil h

Our real-world experiences often do not apply to the quantum universe, where strange particles such as quarks and leptons rule the roost. The scale of this world is difficult to comprehend, let alone the bizarre behavior that one encounters in this realm.

In this world, we run into the terms "micro" and "million", and often encounter the terms "nano, "pico", "billion", and "trillion". These are hard terms to understand at a gut level.

Clinical MRI depends on the physical properties of the hydrogen atom, which has a precession frequency of 64 million hertz at 1.5 Tesla. Perhaps we can grasp the size of this number by looking at something that has been in the news over the last few months— trillion dollar financial deficits in the United States. How bad is that, really?

Some time ago, I heard a story on NPR radio that helped me better understand how big these numbers are.

Let's ask a simple question: how long does it take to count to a million, if we assume that it takes one second to say each number out loud? How about a trillion? Here is a table that gives the results:

(click on image to enlarge)

It takes 12 days to count to 1 million, 32 years to count to 1 billion, and 32,000 years to count to 1 trillion.

If we step back twelve days, what we were doing is easily in our memory. If we step back thirty two years, that is in the memory of most readers of this blog (even if that memory is a bit dim). If we step back thirty two thousand years, we would find Neanderthals huddling in caves!
Hydrogen atoms spinning 64 million times a second are the basis of clinical MRI at 1.5 Tesla. Pretty fast, but not as fast as a government can spend money every year....

Vic David MD

Wednesday, November 25, 2009

Hemarthrosis and a Stable Knee

Photo by speedye

November is football season in America, and physicians will see a surfeit of gridiron-related knee mishaps in this month. The usual culprits are injuries to the anterior cruciate ligament, medial collateral ligament, and the menisci, but occasionally one will see rarer injuries.

In this case, a 16 year-old high school football player felt a "pop" over the lateral aspect of his knee, and presented with pain and swelling, but had a stable knee on exam.

Sagittal T2 fatsat image reveals a hemarthrosis with a fluid-fluid level (red arrow) and moderate edema within the popliteus muscle (yellow arrow).

The popliteus muscle originates from the posterior shaft of the tibia, and inserts on the lateral femoral condyle:

Axial images from (A) the patient and (B) a normal individual for comparison reveal an irregular popliteus tendon (red arrows) in the injured football player. Compare with a normal popliteus tendon (green arrow):

Coronal intermediate images from (A,B) the patient and (C,D) a normal comparison individual delineate the nature of the injury:

(click on image to enlarge)

There is an avulsion fracture involving the popliteus (red arrows = abnormal popliteus; green arrows = normal comparison popliteus). Note the hypointense bone avulsion fragment (blue arrows), which was confirmed on conventional radiographs (not shown)

More coronal images, posterior to anterior:

(click on image to enlarge)

Red arrows = popliteus tendon; black arrows = fibular collateral ligament; blue arrows = avulsed bone fragments; pink arrow = combination of torn tendon and bone.

The patient had no other injuries, with normal menisci, collateral ligaments and cruciate ligaments. Isolated avulsion of the popliteus tendon is a rare injury. It was originally described by Naver and Aalberg in 1985, and this injury was also reported in two adolescent patients by Garth et al. in 1992 (JBJS 74:130, 1992). They stated that isolated avulsion of popliteus should be considered in the setting of a traumatic hemarthrosis. One may see a small flake of bone next to the lateral femoral condyle on radiographs, a clue to the diagnosis. In some patients, the tendon may retract out of the joint.

in the setting of lateral pain, traumatic hemarthrosis and a stable knee, one should examine the popliteus tendon carefully for signs of an avulsion injury.

Vic David MD

Friday, October 30, 2009

Weightlifting and the Distal Clavicle

Photo by greg westfall

Weightlifting is an ancient sport, dating back to ancient China, Egypt, and Greece. I'm sure that even before these recorded competitions, there were plenty of contests of brute strength, including lifting heavy rocks:

The original Olympics did not include weightlifting as an event, but it was still a popular pastime in ancient Greece. Weightlifting continues to this day, and one popular exercise is the bench press:

Photo by Justin Berndt

This exercise places a great deal of strain on the glenohumeral (shoulder) joint and the adjacent acromioclavicular (AC) joint. One consequence of repeated increase strain on the AC joint can be osteolysis of the distal clavicle.

In this case a 36 year-old female felt the gradual increase of shoulder pain while increasing the number of pushups she was doing daily. She then experienced an abrupt increase in her pain after moving luggage from overhead compartment in a plane. Her orthopedic surgeon sent her for an MRI:

Sagittal T2 fatsat image reveals striking bone marrow edema (yellow arrow) within the distal clavicle.

On an axial T2 fatsat image, there is a linear, hypointense fracture line (yellow arrows) within the subchondral bone:

A coronal intermediate-weighted image confirms the presence of the subchondral fracture:

Repetitive stress can lead to bone marrow edema and a subchondral fracture line within the distal clavicle. Demineralization and bone resorption may follow. (Textbook of Arthroscopy; Miller, M. and Cole, B.; 178-179, 2004 and Kassarjian et al.; Skel. Rad. 36:17-22, 2007)

On physical examination, patients have point tenderness over the affected AC joint. The range of motion of the glenohumeral joint is typically not affected.

Thus, a subchondral stress fracture may be the earliest MRI manifestation of distal clavicular osteolysis.

Vic David MD

Saturday, October 3, 2009

Horse Racing and Foreign Bodies

Photo by raymond

Horse racing is an ancient sport. Its origins date back to about 4500 B.C, among the nomadic tribesmen of Central Asia (who first domesticated the horse). The sport has a history of nobility, but interest in the sport can be found at all rungs of the societal ladder. In the modern world, organized horse racing is done at a racetrack. Many horse racing tracks are quite old, and are constructed mostly of wood.

In this case, a 71 year old woman went to a race track four weeks ago, and felt a sharp pain as she ran her hand along a wooden railing. Her clinician suspected a wooden splinter, but was unable to detect one on clinical examination. She was sent for an MRI:

Sagittal T2 fat sat image reveals a linear structure (red arrow) surrounded by fluid.

Axial and coronal T2 fatsat images confirm the presence of a wooden splinter:

What is the best test for a foreign body? While MRI can reveal foreign bodies, it is not very sensitive for the presence of non-metallic foreign bodies. Thus, despite what we see in this case, if the clinical question is "rule out foreign body", one should generally start with an x-ray. If this is negative, one can go on to CT or ultrasound, with the choice depending many times on local clinical practice.

Remember— while MRI can detect foreign bodies, it is not the ideal test, particularly if the foreign body is non-metallic in nature.

Vic David MD

Sunday, September 6, 2009

Extensor Digitorum Brevis Muscle Edema

A 32 year-old male suffered an MVA 3 months ago, and was sent for an ankle MRI due to persistent ankle pain.

(A) Oblique coronal T1-weighted and (B) Oblique coronal T2 fatsat images at slightly different slice positions reveal mild atrophy and striking edema within the extensor digitorum brevis (EDB) muscle.

The traumatic event was three months ago, and it would be unusual for muscle edema to persist this long after an injury. After digging into the clinical history a little further, it came to light that the patient had suffered an injury to his common peroneal nerve at the level of the fibular head, due to a motor vehicle accident.

Thus, the atrophy and edema of the EDB muscle in this case is due to denervation. The differential diagnosis for intramuscular edema is quite broad, and includes trauma, infection, myositis of various etiologies, and systemic diseases such as dermatomyositis.

The most common cause of
extensor digitorum brevis muscle edema is trauma. In the proper clinical context, one should also consider the possibility of anterior tarsal tunnel syndrome. In this disorder, the deep peroneal nerve is trapped/compressed as it travels under the extensor retinaculum. Patients complain of dorsal foot pain. Examination will reveal a sensory deficit in the area between the first and second toes and paresis and atrophy of the extensor digitorum brevis. This syndrome is further discussed in a beautiful review of foot and ankle entrapment by Delfaut et al. (Radiographics 23:613, 2003).

Vic David MD

Saturday, July 18, 2009

Sesamoids and Lateral Ankle Pain

The term 'sesamoid', coined by Galen, is derived from the flat, oval seeds of the Sesanum indicum plant.

Sesame Plant

The largest sesamoid bone in the body is the patella, a sesamoid within the extensor mechanism of the knee.

The peroneus longus muscle plantar flexes and everts the foot. A sesamoid bone called the os peroneum may be present within the peroneus longus tendon. Many support the idea that the os peroneum is always present, but the os peroneum is completely ossified in only about 20% of the population. It may also be cartilaginous or fibrocartilaginous in nature. The os peroneum is multipartite in some individuals.

In this case, a 23 year old female presented with lateral ankle pain.

(A) Oblique axial intermediate and (B) Oblique axial T2 fatsat images depict globular areas of abnormal signal red arrows) within the peroneus longus tendon. There is adjacent soft tissue edema (yellow arrows)

Examination of a lateral radiograph of this area reveals a markedly enlarged, multipartite os peroneum (red arrows):

(A) Oblique axial intermediate and (B) Oblique axial T2 fatsat images immediately proximal to the enlarged, fragmented os peroneum identify focal longitudinal splits (red arrows) within the peroneus longus tendon.

This is an example of painful os peroneum syndrome, with superimposed partial tearing of the peroneus longus. Painful os peroneum syndrome is associated with a spectrum of conditions that includes one or more of the following: (1) acute os peroneum fracture or a diastasis of a multipartite os peroneum, either of which may result in a discontinuity of the peroneus longus tendon; (2) chronic os peroneum fracture or diastasis of a multipartite os peroneum with callus formation; (3) attrition or partial rupture of the peroneus longus tendon, proximal or distal to the os peroneum; (4) frank rupture of the peroneus longus tendon with discontinuity proximal or distal to the os peroneum; (5) enlarged peroneal tubercle on the lateral aspect of the calcaneus which entraps the peroneus longus tendon and/or the os peroneum during tendon excursion (Foot Ankle Int. 15:112-24, 1994).

One must be careful not equate abnormal signal within the peroneus longus tendon at the level of the calcaneocuboid joint with a tendon tear. I have had more than one phone call from a foot and ankle surgeon asking why a radiologist commented on a peroneus longus tear, and did not mention the os peroneum, which was clearly present on x-rays.

While the os peroneum is obvious on x-ray, it is often much less apparent on MRI, which is insensitive to the presence of calcification and mineralized bone (unless it contains fatty marrow).

When abnormal signal is present in the peroneus longus tendon at the level of the calcaneocuboid joint on MRI, one should consider the possibility of os peroneum syndrome, and correlate with conventional radiographs.

Vic David MD

Saturday, June 20, 2009

Cellular Hyperplasia and a Thumb Bump

Hypertrophy and hyperplasia are common medical terms. Doctors, like lawyers, tend to come up with single words or short phrases to summarize important concepts. Both professions develop a lexicon that is sometimes impenetrable to the uninitiated.

Fortunately, hypertrophy and hyperplasia are simple concepts. In hypertrophy, cells increase in size. In hyperplasia, cells increase in number:

In this case, a 15 year old patient presented with a thumb mass that had been present for about 6 months:

Axial images of the thumb reveal an oval mass (red arrow) corresponding to the clinically palpable abnormality. The mass has a tubular-appearing T2 hyperintense rim which enhances following the administration of gadolinium.

(A) Sagittal and (B) Coronal intermediate weighted images with fat saturation through the lesion reveal hypointense material within the center of the lesion.
Here are images from a different patient, also with a thumb mass:

(click on image to enlarge)

Multiple images through the lesion reveal that this lesion has an appearance almost identical to the first, with a tubular T2 hyperintense rim that enhances following the administration of gadolinium (red arrows). This lesion also has hypointense material more centrally (green arrows).

Both of these lesions were excised, and were confirmed to be cases of intravascular papillary endothelial hyperplasia (IPEH), also known as Masson's tumour. An excellent description of this condition is provided by Dr. Sampurna Roy.

IPEH is a reactive condition representing an unusual form of organizing thrombus. Masson's tumour may either occur in pure form (primary), as a focal change in a preexisting vascular lesion (hemangioma, pyogenic granuloma, or vascular malformation) and rarely in an extravascular location as a result of organization of a hematoma.

The lesion may occur in any blood vessel in the body, but is commonly located on the fingers, head and neck and trunk. It is typically seen as a small (less than 2 cm in diameter), firm, blue or purple nodule. This lesion has been reported to occur deep in the body, including the liver (J Korean Med Sci 19:305, 2004) and renal sinus (Jap J Clin Oncol 27:433, 1997)

Multiple small, delicate papillary structures project into the lumen and these are associated with thrombus. These projections and associated thrombus presumably give rise to the T2 hypointense material seen in the central aspect of the lesions presented here.

This lesion has an excellent prognosis and is usually cured by simple excision.

If you see a hand lesion with a tubular T2 hyperintense rim that enhances, with a T2 hypointense center, one should think of the diagnosis of IPEH.

Vic David MD

Friday, June 5, 2009

Blogs and Google Analytics

One of the benefits of blogging on the Google platform is that you have access to Google Analytics. This service will give you insight into who is reading a given blog.

Here, for example, is a snapshot of where traffic to this blog comes from:

(click on image to enlarge)

Blogging is a great way to distribute information. It is not a replacement for other forms of communication, but is a valuable adjunct. You don't have to supersede a format to be relevant.

This blog has been included in the "Top 50 Radiology and Sonography Technician Blogs", and I thank them for the inclusion. I hope that this blog stimulates its readers, and helps medical caregivers take better care of their patients.

Saturday, May 23, 2009

Oily Fish and Enhancing Masses

This is a photograph of a herring, a small, oily fish of the genus Clupea found in the shallow, temperate waters of the North Pacific and the North Atlantic, including the Baltic Sea.

Most people are familiar with the term "red herring". In the investment world, a red herring is associated with initial public offerings (IPOs). A red herring is a preliminary registration statement that must be filed with the SEC describing a new issue of stock and the prospects of the issuing company.

Outside the financial world, a red herring is an item or event that distracts the observer from what is truly important. The origin of the term is somewhat controversial but most associate it with hunting. When herrings are cured, they turn red and acquire a distinctive smell. Hunting dogs were taught to follow a trial by following the scent of a herring that was dragged along the ground. Red herrings may also have been used to confuse the hounds in order to prolong a fox hunt or to test their ability to stay with a scent.

Occasionally, we run across red herrings in medicine as well. In this case, a 35 year-old female presented to her physician with a mass behind the elbow. She was sent for an MRI:

(A) Coronal T1-weighted and (B) T2 fatsat images reveal a T2 hyperintense mass (red arrows) corresponding to a clinically palpable abnormality.

An axial T2 fatsat image confirms the mass (red arrows) under a skin marker placed by the technologist (yellow arrow):

(A) Axial precontrast T1 fatsat and (B) postcontrast T1 fatsat images obtained immediately after IV contrast administration reveal strong enhancement associated with the lesion, which has irregular margins:

The history obtained by the MRI technologist reported no history of recent trauma. Given this, a differential of various neoplastic entities was offered, and the patient was referred to an orthopedic oncologist.

Up to this point, the patient had proceeded at a breakneck pace through the medical system. At the orthopedic oncologist, things finally slowed down. The astute orthopedic oncologist obtained a careful history, which included a possible history of a bleeding disorder. After careful consideration, he opined that this was likely a hematoma, despite the strong enhancement seen on MRI.

Over the next several weeks, the mass decreased in size, and a follow-up scan eight weeks later confirmed resolution of the hematoma:

This case illustrates many things, not the least of which is the importance of an accurate clinical history. It is also a good example of how hematomas can rarely enhance, particularly along their periphery. Cases have been described of chronically expanding hematomas which enhance, simulating a neoplasm (Skel Rad 35:1432, 2006)

Although gadolinium enhancement is often associated with neoplasms, in some cases it can be a red herring, as this case illustrates.

Vic David MD

Friday, May 8, 2009

Shoulder MR Arthrography and Butterflies

Insects often uses camouflage to hide from predators. Can you find the butterfly in this picture?

Photo by plj.johnny
Click on image to enlarge

Although arthrography (the placement of contrast within a joint) generally increases the accuracy of MRI, on occasion the contrast can camouflage important findings.

In this case, a patient with chronic shoulder pain was sent for an MR arthrogram. Here is a T1-weighted image with fat saturation:

Here is the corresponding T2-weighted image with fat saturation:

Do you see anything important?

Here are the two images next to each other:

(A) T1-weighted image with fat saturation and (B) T2-weighted image with fat saturation reveal a lobulated paralabral cyst (red arrows) adjacent to the anterosuperior labrum. Note how the hyperintense fluid within the joint on makes this lesion more difficult to perceive on the T2-weighted image. As expected, the paralabral cyst is hypointense on the T1-weighted image.

I have seen several cases where paralabral cysts are missed, because they blend in with the hyperintense joint fluid on T2-weighted images. This is unfortunate, as the presence of a paralabral cyst in the shoulder is highly predictive of a labral tear.

Axial image from the same patient identifies the paralabral cyst (red arrow) and the adjacent tear of the anterosuperior labrum (yellow arrow):

Thus, on MR arthrograms, be careful not to miss paralabral cysts on T2-weighted images, due to the presence of adjacent bright joint fluid.

Did you manage to find the butterfly? Here it is (yellow arrows):

Vic David MD

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