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Section III – Musculoskeletal Radiology

Figure 1A

Figure 1B

90. A 15-year-old woman presents with aching knee pain and no history of trauma. You are shown an AP radiograph
(Figure 1A) and non-contrast CT image (Figure 1B). Which one of the following is the MOST likely diagnosis?
A. Osteomyelitis
B. Osteosarcoma
C. Chondroblastoma
D. Osteoid Osteoma
E. Ewing’s Sarcoma

Diagnostic In-Training Exam 2003 1

Question #90
Findings: The standing AP radiograph of the femur shows a very dense but smooth and benign appearing periosteal
reaction at the medial aspect of the distal femoral Meta diaphysis. No destructive or aggressive features are present on
the image. The single axial, non-contrast CT image shows the dense reactive bone involving the medial and posterior
cortex as well as a small lucent nidus with central calcification within the cortex itself. There is no soft tissue mass or
any disruption of soft tissue planes on the CT image.

Rationales:
A) Incorrect. The pattern of osteomyelitis depends on the age of the patient and the mode of infection. In infants,
perforating vessels cross the open growth plate and hematogenously spread infection can extend to the epiphysis.
In childhood and early adolescence, those perforating vessels regress and there is not communication across the
open growth plate and infection more commonly involves the metaphysis or Meta diaphysis. As the growth plate
closes, there are again patent vessels, which allow communication between the metaphysis and epiphysis, but it
is far more common for adults to acquire osteomyelitis from a source of infection in the contiguous soft tissues
rather than from a hematogenous source. Radiographically, osteomyelitis usually presents as lucency in the bone.
Depending on the infectious agent and the chronicity of infection, there may be more or less reactive bone near
the lesion. With subacute or chronic infection, well-defined intraosseous abscesses with a sclerotic margin may
develop (Brodie’s abscess) and sinus tracts may be seen. Bony sequestra, which are intracortical pieces of necrotic
bone surrounded by granulation tissue, may also be seen. In the case of bone infection from an adjacent soft
tissue source, the key diagnostic feature distinguishing osteomyelitis from reactive change to cellulitis is cortical
destruction. In the early phases of infection, subtle changes in the bone may be imperceptible radiographically.
MRI is both more sensitive and more specific for the bone changes in addition to identifying the accompanying
soft tissue abnormalities. The specificity of MRI may be improved by the addition of nuclear medicine studies
using labeled white cells to localize infection. In our case, the calcified nidus might possibly be taken for a small
sequestrum and osteomyelitis is a differential diagnostic consideration here. However, the growth plate in this
adolescent girl is closed, making the metaphysis an unlikely location for infection in this patient and the dense,
benign reactive bone would be atypical for even the most chronic infection.
B) Incorrect. Osteosarcoma is second only to myeloma in frequency as a primary malignancy of bone. Commonly
presenting in the second and third decades, approximately 50-75% of the most common variant, conventional
osteosarcoma, occurs about the knee. The tumor typically has an aggressive appearance, with destruction of the
underlying bone and variable production of malignant appearing osteoid. Radiographically osteosarcoma is an
ill-defined, destructive intramedullary, metaphyseal lesion with an associated soft tissue mass. Due to rapid
growth, there is commonly “sunburst” periostitis or Codman’s triangles. X –rays are preferred for the initial
diagnosis, but MR is superior to CT in the evaluation of the intra and extraosseous extent of disease. The
metaphyseal location and age of the patient in question would be appropriate for osteosarcoma, but the smooth,
benign reactive bone and lack of any destructive changes or a soft tissue mass are consistent with a non-
aggressive process.
C) Incorrect. Chondroblastoma is a rare, benign primary tumor of bone that most commonly seen in the second
decade. It typically presents as a lytic lesion in the epiphysis or apophysis of a long bone with a well-defined
sclerotic margin. Matrix calcification is present in up to 50%. MRI may show edema in the surrounding bone
and soft tissues due to the prostaglandins secreted by the tumor and clinically they may mimic osteoid osteoma.
Benign periostitis or joint effusions may be seen, most commonly when the lesion is located in the capital
femoral epiphysis within the hip joint capsule. The lesion may appear expansile, most commonly when a
secondary aneurysmal bone cyst is coexistent. Rarely chondroblastoma may metastasize to the lungs. Treatment
is usually curettage with bone grafting with image-guided radiofrequency ablation being used at some centers.
Although the age of the patient in the index case would be appropriate for chondroblastoma, the location in the
Meta diaphysis of the femur and the large amount of reactive bone would not be.

2 American College of Radiology

D) Correct. Osteoid osteoma is a benign bone-forming neoplasm consisting of a central core of vascularized
osteoid surrounded by densely sclerotic bone. The clinical history is often suggestive, with pain, which is worse
at night and relieved by prostaglandin inhibiting agents such as aspirin. Age at presentation is usually in the
second or third decade. Lesions in long bones are commonly cortically based where they typically present as
lucency, the “nidus”, which may or may not contain calcification. The nidus is usually located at the center of
the reactive sclerotic bone. In the small bones of the hands and feet, the lesions tend to be intramedullary with
an associated periosteal reaction. Subperiosteal lesions can be seen and may have less prominent reactive changes.
The most common location is in the long tubular bones, typically in the diaphysis or metaphysis. Vertebral
lesions, often associated with a painful scoliosis, are usually located in the posterior elements. MR imaging of
osteoid osteoma shows edema in the bone and soft tissues, which may be deceptively aggressive in appearance.
Scintigraphy has been used in the past for its high sensitivity but it remains low in specificity. Plain films usually
show the benign reactive bone. High resolution CT is best for showing the nidus itself. CT may also be used
for pre-operative localization or definitive treatment with radiofrequency ablation. Our case shows the classic
appearance of osteoid osteoma with the dense but benign reactive bone with a subtle lucency on x-ray. The CT
demonstrates a well-defined nidus with central calcification. This case was subsequently successfully treated with
CT-guided radiofrequency ablation with complete resolution of symptoms.
E) Incorrect. Ewing’s Sarcoma is a primary malignancy of bone that chiefly affects young children, often under the
age of 10. It most commonly affects the femur and in general is more common in the lower part of the body.
In long bones, the Meta diaphysis or diaphysis are typical locations in the bone. Radiographically the lesion is
primarily lytic and may have a permeative appearance that may be mistaken for infection. A malignant periosteal
reaction is present which may appear laminated (“onion-skin” pattern) and a large soft tissue mass is usually
seen. The lesion appears central in the bone reflecting its origin from bone marrow. Ewing’s Sarcoma often
presents with constitutional symptoms such as fever, which may delay diagnosis. Scintigraphy is sensitive but
non-specific. Radiographs are the usual modality for primary diagnosis with MRI showing the extent of disease
within the bone marrow and any associated soft tissue mass. MRI is also often used to monitor response to
treatment with chemotherapy. CT is especially helpful when flat bones such as the pelvis or skull are involved.
In our case, the lesion is cortically based with a very benign and dense periosteal reaction. This would not be
consistent with a malignant process. The CT scan shows no soft tissue mass or disruption of soft tissue planes.

Citations:
Torriani M, Rosenthal DL. Percutaneous radiofrequency ablation of osteoid osteoma. Pediatric Radiology 32(8):
615-8, 2002.
Ho AC, Horton KM, McCarthy EF, Fishman EK. The role of imaging in the diagnosis and management of osteoid
osteoma: a pictorial review. Critical Reviews in Diagnostic Imaging 42(6): 357-77, 2001.



Figure 2

91. You are shown an axial T2-weighted, fat suppressed MR image in a 25-year-old woman who presents with
pain after kicking the ball while playing soccer (Figure 2). Which one of the following is the MOST likely
diagnosis?
A. Hamstring avulsion
B. Insufficiency fracture
C. Adductor strain
D. Gluteal myositis
E. Denervation injury


Question #91
Findings: The image presented is a fat suppressed, T2 weighted axial MR image of the lower pelvis at the level of
the ischial tuberosities. The image shows a focal area of increased T2 signal between the hamstring tendons on the
right and the tip of the ischial tuberosity. The marrow signal in both inferior pubic rami is normal, as is the signal
in the musculature.

Rationales:
A) Correct. Hamstring injuries are commonly seen in athletes such as hurdlers or those who participate in sports
with powerful kicking. In children, these injuries are often associated with avulsions of the ischial apophysis; in
young adults, tendon avulsions without underlying fractures may be seen. Radiographs are often normal in the
absence of an associated fracture. MRI will show the relationship of the tendons to their attachment, associated
muscle injuries, and the presence or absence of osseous pathology. In our case, the T2 weighted fat suppressed
axial image shows high signal fluid between the hamstring tendons and the bone, with no such separation on
the contralateral normal side. The inferior pubic rami are normal on both sides with no evidence for fracture.
The muscles themselves are also normal. There is some fluid surrounding the right sciatic nerve, which is
otherwise normal, explaining why this patient may present with complaints of sciatica due to irritation of the
nerve related to its proximity to the tendon avulsion. These injuries are most often treated conservatively with
the exception of apophyseal avulsions in childhood, which may require fixation.
B) Incorrect. Insufficiency fractures are the result of normal stresses on bone that has lost its normal elastic
resistance. The pelvis is a common location for these fractures, which are usually seen, in elderly, osteoporotic
women. In particular, the inferior and superior pubic rami are often affected, with a subgroup of these patients
having avulsion insufficiency fractures of the ischial tuberosity. In this case, the most common presentation is
sciatica due to irritation of the nearby sciatic nerve. For this reason, diagnosis may be delayed as the potential
for lumbar spine pathology is evaluated. These fractures are usually visible on radiography. In the very acute
phase, non-displaced fractures may be difficult to identify. Scintigraphy is very sensitive but lacks specificity
and anatomic resolution. MRI will show the edema and any associated tendon or muscle injuries. CT will best
display the fracture, and is particularly helpful in excluding pathologic fractures in the sacrum. In our case, the
inferior pubic rami are well seen and normal bilaterally, excluding the possibility of acute or subacute fracture.
C) Incorrect. The adductor muscle group includes the adductor magnus, brevis and longus as well as the gracilis,
pectineus and Sartorious muscles. These muscles principally take their origin from the pubic ramus and are
located in the medial thigh, primarily acting to abduct the thigh although individual muscles in this group
contribute to actions such as hip flexion and extension. Muscle injuries in general may be divided into
contusions or strains, partial tears and complete tears or lacerations. Radiography is usually normal, but will
show associated osseous injuries. MRI is the preferred imaging modality for evaluating muscle injuries. A
muscle contusion or strain will appear as an intact muscle with increased T2 signal suggesting edema. Partial or
complete tears can also be identified. In the acute phase, MRI may show hemorrhage at the site of injury. In
the image shown for this question, some of the upper adductor musculature is visible, notably the adductor
magnus and brevis. These muscles are entirely normal in their signal characteristics and morphology with no
edema or loss of muscle bulk, effectively excluding a significant muscle injury.
D) Incorrect. Myositis is a non-specific term indicating inflammation within a muscle. Etiologies include bacterial,
viral, and parasitic infections, collagen vascular diseases such as SLE and even drug toxicities. Pyomyositis is a
distinct entity, which is often related to staphylococcal infection. The imaging appearance of myositis is also
non-specific. On MRI, muscles may have increased T2 signal and there may be loss of distinction between
tissue planes on T1 weighted sequences. Contrast-enhanced studies, using CT or MRI, will show any
associated abscesses such as those seen with pyogenic infections. Accurate diagnosis requires aspiration and
culture of the recovered material. Depending on the location, ultrasound or CT can be used for imaged guided
aspiration or drain placements. In our case, all of the muscles, including the gluteus group, are normal with no
bright T2 signal except at the sight of the tendon avulsion, making myositis an extremely unlikely diagnosis in
this example.


E) Incorrect. Denervation injury to muscle can be the result of acute or chronic trauma to a nerve or other processes
such as inflammatory neuropathies. Compressive neuropathies such as the anterior interosseous nerve syndrome
or carpal tunnel syndrome also fall into this disease category. Initial imaging findings may be negative despite
positive clinical examinations or studies such as EMG. As the disease progresses, the muscles that are innervated
by the affected nerve may show some mild increased T2 signal. As the disease becomes chronic, there is often
loss of muscle bulk and fatty replacement. In the case of large peripheral nerves such as the sciatic or median
nerves, the abnormality in the nerve itself may be seen as increased size and T2 signal. In the case of smaller
nerves, which cannot be easily resolved on imaging studies, the key to diagnosis is recognizing the pattern of
muscle involvement relating to a specific nerve. In our case, the muscles are normal and even though there is
some fluid surrounding the right sciatic nerve, the nerve itself is normal and symmetric with the opposite side.

Citations:
Brandser EA, el-Khoury GY, Kathol MH, Callaghan JJ, Tearse DS. Hamstring injuries: radiographic, conventional
tomographic, CT, and MR imaging characteristics. Radiology 197(1): 257-62, 1995.



Figure 3A Figure 3B

92. You are shown PA (Figure 3A) and oblique (Figure 3B) radiographs of the hand in a 40-year-old woman with
hand pain. Which one of the following is the MOST likely diagnosis?
A. Gout
B. Systemic Lupus Erythematosus
C. Erosive osteoarthritis
D. Scleroderma
E. Rheumatoid arthritis


Question #92
Findings: PA and oblique radiographs of the hand show diffusely decreased bone density in this 40-year-old woman.
On the oblique view, there are multiple subluxed MCP and IP joints with dislocation of the 5th PIP joint on the
oblique image with near complete reduction on the PA view. No erosions or productive changes are seen.

Rationales:
A) Incorrect. Gout is a crystalline deposition disease. Both primary and secondary gout are related to hyperuricemia,
with the primary form representing an inborn error of metabolism and secondary gout arising from altered uric
acid metabolism associated with other clinical disorders. Primary or idiopathic gout is much more common in
men and typically presents in the fifth decade. It often begins as a monoarticular or oligoarticular disorder,
progressing to involve more joints over time. The most commonly involved joint is the first metatarsophalangeal
joint, which is, altered 75-100% of patients with gout. Most patients with biochemical and clinical evidence of
gout will not have bone changes due to early treatment of the metabolic disorder. In patients who do manifest
radiographic changes, the findings are usually classic and diagnostic of gout. The joint space is well preserved,
with erosions in intra-articular and para-articular locations. These erosions are well defined with sclerotic margins
and overhanging edges in up to 40%. The overhanging edges may be associated with gouty tophi in the adjacent
soft tissues, which often show increased density or even calcification on x-rays. Bone density is usually preserved.
The distribution in the skeleton is typical, with the feet most commonly involved. Hand and wrist, the knee and
elbow are also usual sites. Cross sectional imaging is rarely contributory in the evaluation of gout. MRI may be
helpful in early gout to identify synovitis and early erosions as well as the extent of soft tissue involvement. Gout
would not be an appropriate consideration in the case in question. There are no erosions or other destructive
changes present, effectively excluding a radiographic diagnosis of gout.
B) Correct. Systemic lupus erythematosus is an autoimmune connective tissue disorder affecting multiple organ
systems. In the musculoskeletal system, common manifestations are a deforming, symmetric polyarthritis,
myositis, tendon weakening, rupture, and osteonecrosis. As with most collagen vascular diseases, adult women
are most commonly affected. The hallmark of the arthritis associated with SLE is deformity without destruction.
The small joints of the hand are characteristically involved with multiple subluxed or even dislocated joints,
which are easily and usually completely reducible. In fact, the positioning of the patient for the PA radiograph
of the hand may itself reduce the subluxations making the disease less prominent. A relaxed oblique or “ball-
catcher’s” view often shows the subluxations to better advantage as in our case. Periarticular osteopenia is
common and reminiscent of rheumatoid arthritis, but the complete lack of any erosive changes should help to
distinguish these entities. Osteonecrosis may be present in the form of avascular necrosis, bone infarcts, or both
but this usually involves long bones rather than the small bones of the hands. Jacoud’s arthropathy, a sequela of
rheumatic fever, has an appearance that is identical radiographically to SLE. Fortunately, this has become very
rare in the era of antibiotic treatment of streptococcal infections, limiting differential diagnostic considerations.
C) Incorrect. Erosive osteoarthritis is an inflammatory variant of osteoarthritis, which is characterized by a
combination of erosive and productive changes typically in the DIP joints of the hands. This disease usually
affects middle-aged women. While other changes of degenerative osteoarthritis may be present within the
other joints of the hand and wrist, the DIP involvement is usually strikingly worse. The primary differential
considerations are other types of inflammatory arthritis, including psoriatic arthritis, Reiter’s syndrome, and
even metabolic disorders such as hyperparathyroidism. What distinguish erosive osteoarthritis from these entities
are the distribution of the radiographic changes and the lack of associated systemic disease. Psoriatic arthritis
may affect women in the same age group. This is usually not confined to the DIP joints and the pattern of
erosive changes is different. Reiter’s syndrome usually affects younger men and more typically involves the lower
extremities. Rheumatoid arthritis spares the DIP joints in nearly all cases. Other forms of inflammatory and
erosive arthritis also have typical radiographic patterns of disease. In our case, the complete lack of any erosive
or productive changes in the interphalangeal joints excludes erosive osteoarthritis as a possibility.


D) Incorrect. Scleroderma, or progressive systemic sclerosis, is a rare disorder of connective tissue affecting multiple
organ systems most commonly affecting women in the third to fifth decades. In the musculoskeletal system,
the hands are the most common sites of involvement, with changes most pronounced in the digits. Progressive
atrophy of the soft tissues at the tips of the fingers creates a characteristically conical appearance to the finger.
Progressive erosion and resorption of the ungual tuft of the fingers is also commonly seen with amorphous
calcifications seen in the soft tissues. With time, the more proximal bones of the fingers may be involved as well
with further resorption giving a “pencil” appearance to the digit. The feet are usually not affected to the same
degree as the hands. Other sites where bony changes can be seen include the ribs, spine, and mandible. Soft
tissue calcifications often are more diffuse and periarticular tumoral calcinosis may be seen. In the index case,
the bones of fingers are normal except for their alignment and no soft tissue resorption or calcifications are seen.
E) Incorrect. Rheumatoid arthritis is a relatively common inflammatory arthritis affecting synovial joints, bursae,
and tendon sheaths. The primary process is one of synovial inflammation with secondary affects on the
underlying bone and cartilage. The small joints of the hands are commonly involved as are the cervical spine,
feet and other sites. In the early stages of disease the primary finding may be soft tissue swelling over the MCP
joints and ulnar styloid representing synovitis. Periarticular osteopenia may also be present early on reflecting
hyperemia at the inflamed joints. With progression, erosions are seen in characteristic locations such as the
ulnar styloid and MCP joints but no productive changes are present. The DIP joints are uniformly spared. As
the inflamed synovium destroys ligaments and tendons, subluxations such as the swan’s neck deformity become
common. Although the subluxations in our case could be seen in rheumatoid arthritis, the complete lack of any
destructive changes or soft tissue swelling makes this much less likely.

Citations:
Brower A, Flemming DJ, Bralow L. Arthritis in Black and White. 2cd edition, W.B. Saunders, Philadelphia, 1997.



Figure 4A Figure 4B

93. A 45-year-old man presents with ankle pain after playing basketball. You are shown sagittal T1 (Figure 4A)
and T2 and sagittal-T2 (Figure 4B) weighted MR images of the ankle. Which one of the following is the
most likely diagnosis?
A. Pilon fracture
B. Achilles tenosynovitis
C. Achilles tendon rupture
D. Os trigonum syndrome
E. Calcaneal stress fracture


Question #93
Findings: Sagittal T1 and fat suppressed T2 weighted MR images of the ankle show disruption of the Achilles
tendon with retraction of several centimeters. The visualized distal tendon is thickened with increased intra substance
signal. There is increased T2 signal in the gap and the adjacent soft tissues. A small ankle effusion is present. Osseous
structures are normal.

Rationales:
A) Incorrect. Pilon fractures of the ankle result from pronation-dorsiflexion injuries, which drive the talar dome
into the tibial plafond. Included in this complex are an oblique fracture of the medial malleolus and an
intraarticular fracture of the distal tibia, which may have more than one part. A fibular fracture may also be
present. These are uncommon fractures, representing less than 0.5% of all ankle fractures. Treatment decisions
are based on the degree of comminution of the fracture and the extent of intraarticular involvement and
displacement. Radiography and knowledge of the mechanism of injury are usually diagnostic, but CT scanning
with multiplanar reformatting may be helpful to assess f small intra-articular fragments and to better quantify
the degree of displacement and articular incongruence. MRI is generally not contributory to management in
the acute phase. In our case, the MRI of the ankle shows a normal tibia and talar dome, excluding fracture from
the diagnosis.
B) Incorrect. The Achilles tendon is the largest tendon in the body. It represents the confluence of the
gastrocnemius and soleus tendons, inserting on the posterior calcaneus. In adults, the Achilles tendon is
approximately 10-15cm long. Unique among the tendons of the ankle, the Achilles tendon does not have a
synovial-lined tendon sheath, so that tenosynovitis involving the Achilles tendon is not a possibility. Rather, it is
covered by a peritenon. Peri- or para tendonitis may be seen within the surrounding soft tissues, often associated
with tendinoplasty or partial tears of the tendon itself. Retrocalcaneal bursitis may also be seen just anterior to
the insertion of the Achilles tendon on the calcaneus. Our case shows the clear discontinuity of the Achilles
tendon with surrounding fluid related to the injury, but is not suggestion of a separate inflammatory process.
C) Correct. Achilles tendon rupture is most common in men between the ages of 30 and 50. The typical scenario
is a “weekend warrior” who participates in a sport such as basketball, which uses sudden, forceful dorsiflexion or
push off of the foot. Clinically there is sudden onset of pain and soft tissue swelling with an inability to stand on
tiptoe on the affected side. Radiographs are usually obtained to exclude fracture and may show loss of the soft
tissue planes surrounding the Achilles tendon or may even suggest disruption and retraction of the tendon itself.
MRI definitively shows the disruption as well as its location and the degree of retraction. The most common site
of a complete tear is approximately 2-6 cm proximal to the insertion of the tendon on the calcaneus. This site is
vulnerable both to partial and complete tears as a relatively avascular portion of the tendon. In addition to
trauma in unconditioned individuals, Achilles tendon rupture may be associated with chronic tendinoplasty and
partial tears of the tendon, rheumatoid arthritis, SLE, and the use of local or systemic corticosteroids. MRI may
show hemorrhage or fluid in the acute phase as well as the discontinuity of the tendon. When the problem is
subacute or chronic, as in our case, the MRI shows discontinuity and some mild retraction as well as a thickened
Achilles tendon with increased signal in the distal portion due to underlying tendinoplasty. Increased T2 signal
is seen surrounding the ruptured tendon consistent with edema and fluid.
D) Incorrect. The Os Trigonum or talar compression syndrome is a pain syndrome involving the posterior ankle.
The os trigonum is an accessory ossicle just posterior to the talus at the ankle. When fused to the posterior talus
it is referred to as Stieda’s process. The flexor hallucis longus (FHL) tendon lies immediately adjacent to the os
trigonum. In some cases where the os trigonum is enlarged or is relatively more mobile, irritation of the FHL
tendon or the posterior talus itself may be seen, with tenosynovitis of the FHL or even partial tears of the tendon
resulting. Radiography will show the enlarged ossicle, which may appear irregular, but MRI is diagnostic,
showing the edema surrounding the os trigonum and posterior talus and the associated abnormalities in the
FHL tendon. In our case, the posterior talus is normal with no edema and the FHL tendon is not included on
the images shown.


E) Incorrect. Stress fractures are the result of repetitive loading on bone, which may be normal or abnormal.
Fatigue fractures involving normal bone may be due to a novel, strenuous activity, which places repetitive stress
on a specific bone; an example would be the march fractures of the metatarsal seen in new military recruits. In
the case of abnormal underlying bone, such as osteoporosis, the term insufficiency fracture may be applied.
Common sites for insufficiency fractures include the pelvis and calcaneus. In the calcaneus, radiographs show
the underlying osteopenia with crescentic area of sclerosis usually in the posterior calcaneus. The sclerosis
probably represents a combination of impaction and healing. In the very acute phase, these fractures may be
radiographically occult. In this case, MRI would show the reactive edema on T2 weighted imaging as well as
the fracture itself, which appears as a linear area of low signal intensity on T1 weighted images.
Scintigraphy and MRI have similar sensitivity, but MR is significantly more specific. CT with multiplanar
reformatting will also show the fracture before it is radiographically evident. In our case, the calcaneus is
normal on both T1 and T2 weighted images with normal marrow signal throughout and no edema to suggest
fracture.

Citations:
Schepsis AA, Jones H, Haas AL. Achilles tendon disorders in athletes. American Journal of Sports Medicine 30(2):
287-305, 2002.
Movin T, Kristoffersen-Wiberg M, Rolf C, Aspelin P. MR Imaging in chronic Achilles tendon disorder. Acta
Radiologica 39(2): 126-32, 1998.



Figure 5A Figure 5B

Figure 5C

94. This 50-year-old man presented with a mass along his right chest wall. You are shown coronal T1 (Figure 5A),
and fat-saturated post gadolinium T1 (Figure 5B) weighted images and a non-contrast axial CT scan (Figure
5C) obtained 4 weeks after the MRI. Which one of the following is the MOST likely diagnosis?
A. Myositis ossificans
B. Malignant fibrous histiocytoma
C. Desmoid tumor
D. Chondrosarcoma
E. Intramuscular myxoma


Question #93
Findings: Coronal T1 and fat suppressed T1 post-gadolinium MR images of the right chest wall show a mass
within the lateral chest wall, which is isointense to muscle on the non-contrast examination and shows avid
enhancement with intravenous gadolinium. A small central area in the mass does not enhance and may be necrotic
or cystic. A single axial non-contrast CT image obtained four weeks later shows benign, peripheral calcification
within the teres major muscle. There has been no interval change in the size of the mass.

Rationales:
A) Correct. Myositis Ossificans (MO) is a localized soft tissue calcification most often related to antecedent injury.
Up to 75% of patients who develop MO can relate a clear history of trauma to the affected area. Other causes
may include burns, neurologic conditions, and systemic disorders associated with soft tissue calcification.
Rarely the lesion may develop spontaneously. Common sites of MO include areas exposed to injury such as
the buttocks, elbow, thigh, and calf. A soft tissue mass appears at the site of injury within about 10 days, with
calcification usually appearing by 6 weeks. The calcification typically forms at the periphery of the soft tissue
lesion, forming a smooth, complete border when mature. The peripheral calcification is an important feature
distinguishing developing MO from malignancies such as osteosarcoma or chondrosarcoma. If the lesion is
imaged with MR early in the process, the appearance can be very worrisome with avid contrast enhancement
and features consistent with a soft tissue sarcoma such as a malignant fibrous histiocytoma. Even biopsy may
be misleading if obtained from the cellular central portions of the mass. As with imaging, the more mature
peripheral zone shows the benign nature of the lesion. Radiography will often show the mature peripheral
calcification well enough for diagnostic confidence. If there is any question regarding the nature of the
calcification (with an appropriate clinical history, of course) CT can be obtained which will show the benign
calcification to best advantage. In our case, the patient was involved in a car accident with an injury to his chest
wall approximately two weeks prior to the MRI. The MRI was felt to be worrisome for malignancy, but because
of the history of trauma, biopsy was not immediately performed. CT obtained 4 weeks later shows the mature
calcification of MO and no further evaluation was required.
B) Incorrect. Malignant fibrous histiocytoma (MFH) is the most common soft tissue sarcoma in adults. Rarely
(5%) it may arise as a primary bone tumor. MFH presents as a painless soft tissue mass. Radiographs may show
the tumor in the soft tissue if large, but the preferred modality for evaluation of soft tissue tumors is MRI.
On MRI, the mass will be low in signal intensity on T1 weighted imaging and bright on T2. If contrast is
administered, there is usually avid and homogeneous enhancement. In very large tumors there may be non-
enhancing areas centrally representing necrosis. MRI best demonstrates the anatomic compartments involved
and the relationship of the tumor to neurovascular structures, bones and joints. CT with contrast may be used
if the patient is unable to undergo MRI. Ultrasound may be helpful for image-guided biopsy. Imaging is non-
specific; high-grade liposarcomas and other types of primary sarcomas and metastases may have similar imaging
features. MFH may rarely calcify, with the calcification located within the mass and appearing irregular. If the
mass is adjacent to a bone, that bone may undergo pressure erosion. MFH was a consideration when the MRI
was obtained in our patient, but because of the history of trauma, biopsy was deferred in favor of a follow-up
CT.
C) Incorrect. Desmoid tumor is the name given to a benign fibrous proliferation arising in the abdominal and
extra-abdominal musculature, often growing slowly and engulfing the surrounding tissue in an insidious fashion.
While histologically benign, these tumors commonly recur after excision and are difficult to treat. As with most
of the fibromatoses, they are locally aggressive and may cause erosion of adjacent bones with a periosteal reaction
sometimes seen. They are usually solitary with a predilection for the shoulder girdle. Radiography may show the
soft tissue mass if large enough as well as any effects on adjacent bone, but MRI is the preferred modality for
diagnosis, with the dense fibrous tissue having characteristic signal intensity which is low on both T1 and T2
weighted imaging. If intravenous gadolinium is given, there is usually marked enhancement throughout the
lesion. Calcifications may be seen but are rare. In our case, the signal intensity of the lesion on the unenhanced
T1 image is not as low as would be expected with a desmoid tumor. The post-gadolinium image shows the mass


to be well defined; desmoid tumors often have a less well-defined, more infiltrative appearance. The CT showing
peripheral calcification in the mass is diagnostic of myositis ossificans and would be extremely unusual for a
desmoid. The combination of history and the CT appearance effectively excludes the diagnosis of a desmoid
tumor.
D) Incorrect. Chondrosarcomas are tumors of cartilaginous origin, which may occur primarily, or secondary to
pre-existing lesions such as an enchondroma or osteochondroma. They may also be categorized according to
their location in bone or by their histologic characteristics. Extraskeletal chondrosarcomas are rare. The most
common location for chondrosarcoma is in the long tubular bones, with the femur being the most frequently
affected bone. They are usually located in the metaphysis but extension to the epiphysis can be seen. Most
chondrosarcomas are of low histologic grade. Radiographically, these tumors may show a primarily lytic area
with endosteal scalloping and chondroid calcification within the lesion. Cortical thickening may also be seen.
Higher grade or dedifferentiated chondrosarcomas will have a more aggressive appearance, with cortical
breakthrough and large soft tissue masses. Radiography is the primary diagnostic modality. When the flat
bones such as the pelvis and scapula are involved, CT may be preferred both to define the extent of the lesion
and to characterize the calcifications. MRI may also be used to evaluate the extent of soft tissue and marrow
involvement and to establish which anatomic compartments are involved for surgical planning. In the unusual
case of an extraskeletal chondrosarcoma, a soft tissue mass with c chondroid calcification is seen. Our case shows
smooth peripheral ossification, making a cartilaginous process extremely unlikely.
E) Incorrect. Intramuscular myxoma is an uncommon benign soft tissue mass. As its name would suggest, the mass
is located within a muscle, most often in an extremity. Radiography rarely shows these lesions. MRI will show
a well-circumscribed mass within a muscle, which is very low in signal intensity on T1, and very bright on T2
weighted images. Myxomas rarely enhance except for a smooth, peripheral rim. These lesions almost never
calcify. In our case, although the mass is located within the chest wall musculature, the mass enhances
homogeneously which would be inconsistent with a myxoma, as would be the peripheral calcification.

Citations:
Jepsen MC, Graham SM. Traumatic myositis ossificans of the levator scapulae muscle, American Journal of
Otolaryngology 19(5): 345-348, 1998.
Resnick, D. “Soft tissues” in Resnick, D. Bone and Joint Imaging, 2cd edition, 1261-1263, W.B. Saunders,
Philadelphia, 1996.


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