Friday, December 19, 2008

Hunchbacks and Bony Variants

Photo by TCM Hitchhiker

The Hunchback of Notre Dame is a novel by Victor Hugo, set in 1482 in Paris, in and around the cathedral of Notre Dame de Paris. It has had numerous theatrical and movie interpretations. I'm no literary snob, and have to confess that my only real exposure to this story is via Walt Disney. One of the perils of having several children is unrelenting exposure to Uncle Walt and his minions.

There are hunchbacks in the medical world as well. The os styloideum was first described by the Frenchman Fiolle, in 1932. He termed this entity the "carpe bossu", which can be translated as "hunchback carpal bone". This anatomic variant is located on the dorsal side of the wrist, at the base of the third metacarpal. It is typically a separate ossicle, but can fuse with the adjacent bones, typically the second and third metacarpals, but sometimes with the trapezoid or capitate.

The carpe bossu is usually an incidental finding, but can cause symptoms in some patients, and can be confused with a ganglion cyst. When troublesome, patients will complain of focal dorsal pain, and a tender bone mass. The lesion usually becomes symptomatic in adulthood, but can rarely present in childhood.

In this case, a fourteen year old boy presented with wrist pain, and was discovered to have a scaphoid fracture on MRI. Review of the coronal images along the dorsal aspect of the hand reveals a carpal boss (red arrows) located between the bases of the second and third metacarpals:

The carpal boss is also well seen on this sagittal T1-weighted image:

The patient also had edema-like changes within the os styloideum and the base of the third metacarpal, seen on this coronal T2-weighted image:

Interestingly, the patient did not report any symptoms referable to this region. One wonders whether this area will eventually become symptomatic, given the degree of abnormal signal in this area.

Vic David MD

Friday, December 12, 2008

Lumbar Spine MRI— See More, Miss Less

What do you see in this diagram?

Some see a vase while others see two faces looking at each other. They are both there.

"You see what you look for and recognize what you know". This is a hoary but true dictum in radiology.

When looking at an MRI, radiologists will detect a greater percentage of abnormalities if they are specifically looking for them. Radiologists are in part a product of their training, and carry with them biases learned during that process.

One place where this tends to show up is in lumbar spine MRI scans, which are interpreted by radiologists with significant differences in their training. It's easy to miss subtle abnormalities that are clinically relevant on this exam.

Early in their post-training career, musculoskeletal-trained radiologists tend to miss small disk herniations, while neuroradiology-trained radiologists tend to miss bone findings such as stress reactions and stress fractures.

Here is an example of a subtle right foraminal disk herniation:

Here is an example of stress-related changes in the posterior elements of L4:

Both are subtle findings, but in my experience, musculoskeletal-trained radiologists will be more likely be miss the first example, while neuroradiology-trained radiologists will be more likely to miss the second example.

If you keep this in mind, whatever your training, you will be less likely to miss either type of finding.

Vic David MD

Friday, December 5, 2008

Osteomyelitis and Fly Larvae

Musculoskeletal infections can be a vexing, serious problem. For example, consider knee replacement surgery, a common procedure that is done over 400,000 times a year worldwide. If postoperative infection is avoided, the outcome is typically excellent. A post-operative infection, however, will often spell trouble, and a prolonged recovery.

Over time, we have gotten better at treating infections of bones and joints, particularly with the advent of antibiotics. In the pre-antibiotic era, some novel treatments were tried, including fly larvae:

In World War II, combat induced wounds to the extremities had a 20-25% rate of osteomyelitis. By the Vietnam War, this figure had dropped to about 8%, thanks to better treatment.

In the civilian world, infection is much rarer, but can still occur. Radiologists are often asked to assess for the possibility of bone, joint, or soft tissue infection, and the define its extent.

The question of whether or not osteomyelitis is present comes up most often in the foot. Collins el al. wrote an excellent article on the use of T1-weighted images when looking for pedal osteomyelitis (AJR 185:386-393, 2005). While signal abnormalities on T2-weighted and STIR images tend to be nonspecific, they noted that T1 signal abnormality that is medullary and confluent is highly suspicious for osteomyelitis.

Since then, I have used the principles outlined in their article when analyzing pedal osteomyelitis, and have had good success. I am not aware of any articles that address the MRI analysis of osteomyelitis in the hand and wrist. Given that the hand and foot are ontogenic homologs, I have transferred the principles of Collins et al. to analysis of osteomyelitis of the hand and wrist.

In this case, a 37 year-old man was sent for a finger MRI, due to pain and swelling. He suffered an injury at his work five weeks before the MRI.

A sagittal STIR image shows extensive marrow edema (red arrows) in the middle and distal phalanx, as well as a distal interphalangeal joint effusion (white arrow):

Marrow edema and excess joint fluid are worrisome findings, but tend to be nonspecific. Next, we examine the corresponding T1 weighted image:

The T1-weighted image depicts confluent medullary signal abnormality in the head of the middle phalanx (red arrow), and depicts extensive soft tissue edema. The presence of confluent medullary signal abnormality allows us to predict the presence of osteomyelitis with a high degree of confidence. Osteomyelitis and septic arthritis were confirmed at surgery.

Vic David MD

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

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

Saturday, September 27, 2008

Efficient PACS Reading

Radiologists are reading more cases every year, and anything that makes our day more efficient is a good thing. In a computer-centric environment, the workday revolves around digital imaging, the monitor, mouse, and keyboard.

A great scripting program called Autohotkey can save you thousands of repetitive mouse clicks and keystrokes a year. If you are a heavy computer user like me, it can make your computing experience significantly better, and save you from repetitive strain injury.

It's simple to learn, if you let your inner geek run wild and free. It can be run off a USB stick, so administrative privileges are not required.

Learn more about it:

Autohotkey in Radiology

Using AHK


Vic David MD

Sunday, September 14, 2008

Heart Surgeons, Abscesses, and Lytic Lesions

Michael DeBakey M.D. died at the age of in July 2008. He was best known as a pioneering heart surgeon, operating on celebrities and commoners alike, saving thousands of lives.

Debakey was a creative surgeon even early in his career. With his mentor Alton Ochsner, he devised a new way to drain subphrenic abscesses, through a transthoracic approach. An abscess is a collection of pus (infected fluid), and is typically found within the soft tissues. In some cases, however, abscesses can also occur within the bone.

In 1832 the surgeon B.C. Brodie described three cases of a chronic abscess within the tibia:

The patients were young adults, and each presented with chronic tibial pain and swelling. Brodie was a good writer, and his description is compelling:

"The lower extremity of the left tibia was considerably enlarged; the skin covering it was tense, and adhered closely to the parts below. The patient complained of a constant aching pain, which he referred to the enlarged bone. Once in two or three weeks there was an attack of pain more severe than usual, during which his sufferings were excruciating, lasting several hours, and sometimes one or two days, and rendering him altogether incapable of following his usual occupations. The pain was described as shooting or throbbing, worse during the night, and attended with such exquisite tenderness of the parts in the neighborhood of the ankle that the slightest touch was intolerable."

Recently, I saw a case of a 12 year-old girl with a lytic lesion in the tibia. Of course, the clinical history on the prescription was a little less eloquent than the description above: "Pain, MRI ankle".

Sagittal T1-weighted and STIR images depict an oval lesion (red arrows) in the distal tibial metaphysis. The lesion crosses the physis (yellow arrow), to involve the epiphysis as well. There is a great deal of marrow edema surrounding the lesion. A close-up of the lesion reveals that the lesion is heterogeneous, with a rind of T2 hyperintensity representing granulation tissue (red arrow) surrounding a relatively hypointense core (blue arrow):

The appearance is highly suggestive of a chronic bone abscess (Brodie's abscess), which was confirmed at surgery. In some cases, the lesion can be difficult to distinguish from a tumor, such as osteoid osteoma. Conventional radiographs and CT are often helpful, and depict a lytic lesion with surrounding sclerosis. This sclerotic response typically has a sharp interface with the lesion, but merges gradually with the surrounding bone (Musculoskeletal Imaging: A Teaching File. Felix S. Chew, Catherine C. Roberts; Lippincott Williams & Wilkins, 2005)

A Brodie's abscess develops when osteomyelitis is contained by the host immune response, but is not cured. The infection is walled off, but remains active. The lesion usually is within the metaphysis, but can occur anywhere. When it occurs in the epiphysis, it can be mistaken for a chondroblastoma.

Nearly 200 years have flashed by since Brodie's original description, but chronic bone abscess remains an important medical diagnosis. These lesions are eminently treatable, once the diagnosis is established.

Vic David MD

Friday, September 12, 2008

Diving and the Thumb CMC Joint

Platform diving is acrobatic and beautiful:

Photo by shutterhack

Inevitably, there are some serious g forces exerted on the human body, especially if the rapid deceleration is less like a knife-like entry into the water, and more like one of my dives, the classic belly-flop:

Photo by j. cliss

Recently, a hand surgeon called me and told me about a set of patients who are platform divers, with intractable pain at the base of the thumb, at the thumb carpometacarpal (CMC) joint. She remarked that it would be nice to get some imaging information about the ligamentous structures supporting this joint, particularly along the dorsal aspect of the joint, a region that is difficult to visualize at arthroscopy.

There is not a great deal written about MR imaging of the thumb CMC joint. One good article was written by Connell et al.; there is also a Wandering Radiologist blog post.

There is some variability in how these ligaments are named, but if we follow the convention of Connell et al., there are four main ligaments:

1. Anterior oblique (beak) ligament- extends from the trapezium to the first metacarpal base, on the volar side of the joint.

2. Posterior oblique ligament- extends from dorsoulnar trapezium to the first metacarpal base.

3. Dorsoradial ligament- extends from the dorsoradial aspect of trapezium to the first metacarpal base. This ligament is reinforced by the abductor pollicis longus tendon.

4. Intermetacarpal ligament- runs from the radial base of the index metacarpal to the ulnar aspect of the thumb metacarpal base.

The combination of direct coronal and
oblique sagittal images of the CMC joint visualize the first three of ligamentous structures quite well, at 1.5 Tesla. The intermetacarpal ligament can also be seen, but is more wispy in character.

Consecutive oblique sagittal intermediate-weighted images of a normal volunteer:

The anterior oblique ligament (red arrows) is well seen, as is the posterior oblique ligament (green arrows). The dorsoradial ligament is depicted on this next set of images:

(A) Oblique sagittal and (B) direct coronal images delineate the normal dorsoradial ligament (pink arrows).

Direct coronal images are usually the best way to visualize the anterior oblique (beak) ligament:

The MRI technologists I work were instrumental in determining the optimal imaging planes for us to visualize these structures. This post illustrates once again the vital role the MR technologist plays in producing high-quality MR images.

Vic David MD

Friday, September 5, 2008

Movies and ACL Tears

Movement is essential to life. Our brain is hard-wired to detect motion, and we respond to video much more than static pictures. Consider these two different depictions of a horse galloping:

Now, the video version:

Clearly, the video version is more appealing. Moreover, the video version contains important information that the still picture does not. This video, made by Eadweard Muybridge in 1878, was the first motion picture ever made. Muybridge was commissioned by Leland Stanford (California governor/ Stanford University) to answer a popularly debated question of this era— are all four of a horse's hooves ever off the ground at the same time while the horse is galloping? Muybridge's time-motion photography proved they were off the ground simultaneously. Video analysis had answered a scientific question for the first time in history.

Although cross-sectional imaging has revolutionized radiology, video representations of anatomic detail are the exception in MRI and CT. Volume-rendering techniques enable us to depict 3D data sets, but these techniques are diffusing into clinical practice in a sluggish fashion.

Almost everyone reads off PACS now, which is clearly superior to reading from film for many reasons. One advantage of PACS is the ability to rapidly scroll through a stack of images. This video-like representation of data sets will sometimes enable you to quickly recognize abnormalities that are subtle and easily missed on static images.

One example is the analysis of the anterior cruciate ligament (ACL), a key stabilizer of the knee. It is important to recognize tears of this structure, as the presence of an ACL tear will often change the treatment algorithm. There have been many excellent articles written about the MRI analysis of ACL tears, and these tears are usually easy to recognize on MRI.

The key word here is usually. These tears can be subtle in some patients. The ACL should be examined in the sagittal, axial, and coronal planes to maximize your accuracy. One common error is to rely solely on the sagittal plane to determine the status of the ACL. The axial and coronal planes will often yield additional information about this critical structure.

In the coronal plane, the normal ACL has an oblique course, arising from the lateral femoral condyle, and coursing anteriorly and medially before inserting on the anterior tibia. In the following figure, the green arrow depicts the normal oblique course of the ACL:

Any significant deviation from this normal oblique course of the ACL on coronal images is abnormal.

Here is a movie of a stack of coronal images, depicting a normal ACL. Note how the ACL tracks from the top left of the figure to the bottom right, following its normal oblique course:

Next, let's look at a sagittal image from a 37 year-old patient with knee pain:

The ACL is visualized (red arrow) throughout most of its course. The femoral origin (green arrow) is hazy , but on the adjacent image, this part of the ACL can be seen:

The ACL looks "funny", but it would not shock me if a busy reader passed this off as volume averaging, or a partial tear of the ACL.

A complete analysis of the ACL, however, requires scrutiny of the axial and coronal planes as well, particularly in complex cases. Here is a movie of a stack of coronal images from the same patient. T
he ACL (red arrow) no longer follows its normal oblique course through the intercondylar notch. Its path is now curvilinear, and the inferior aspect of the ACL is deviated laterally:

With these additional images, we can confidently diagnose a complete tear of the ACL, which was subsequently confirmed at arthroscopy.

One of the less-advertised benefits of PACS is its ability to give us video-like representations of the imaging data.

Vic David MD