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1 Department of Radiology, Mayo Clinic College of Medicine, 5777 E Mayo Blvd.,
Phoenix, AZ 85054.
2 Department of Radiology, Thomas Jefferson University Hospital, Philadelphia,
PA.
Received June 19, 2007;
accepted after revision September 17, 2007.
Address correspondence to C. C. Roberts
(roberts.catherine{at}mayo.edu).
Abstract
Keywords: ankle • CT • foot • infection • mass • MRI
REQUIRED ACTIVITIES (available at www.arrs.org)
 TopAbstractINTRODUCTIONEDUCATIONAL OBJECTIVES REQUIRED ACTIVITIES (available... INSTRUCTIONSQUESTION 1QUESTION 2QUESTION 3QUESTION 4QUESTION 5QUESTION 6QUESTION 7QUESTION 8QUESTION 9References |
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Solution to Question 1
Ganglion cyst is the most common soft-tissue mass of the foot and ankle
[1]. Ganglion cysts are focal
collections of mucoid material found near a joint or tendon sheath
[2]. Option C is the best
response. Morton's neuroma is less common than ganglion cyst. Morton's
neuromas are a fibrosing degenerative process surrounding a plantar digital
nerve, not a true neuroma. Option A is not the best response. Pigmented
villonodular synovitis (PVNS) is less common than ganglion cysts. PVNS is a
proliferative synovial disorder resulting in a single or multiple
intraarticular masses. Giant cell tumor of tendon sheath is the focal form of
PVNS affecting a tendon sheath or bursa. These lesions can show intense
heterogeneous enhancement. The multiple synovial masses in PVNS contain
regions of low T1-and T2-weighted signal, which bloom on gradient-echo
sequences because of hemosiderin. Option B is not the best response. Plantar
fibromatosis consists of aggregates of fibroblasts in the plantar fascia. The
classic location involves the superficial medial aspect of the plantar fascia.
Option D is not the best response. Hemangiomas are not the most common mass in
the foot, although they are the most common tumor of vascular origin
[3]. Option E is not the best
response.
Solution to Question 2
Masses predominantly composed of fibrous tissue can have homogeneously low
T1-and T2-weighted signal with enhancement. This is most commonly seen in
fibrous masses containing mature collagen. Fibrosing masses containing
immature fibrous tissue or fibroblasts can have intermediate signal intensity.
Fibrosing masses include plantar fibromatosis and fibroma of tendon sheath.
Option D is the best response. Morton's neuromas typically have
intermediate signal that is isointense to muscle on T1-weighted and low signal
on T2-weighted sequences with variable enhancement
[4]. Option A is not the best
response. Lipomas follow fat signal intensity. They typically have high T1-and
T2-weighted signal and low signal on fat-suppressed sequences
[5]. Option B is not the best
response. The typical appearance of a ganglion cyst on MRI is a well-defined
mass with low T1-and high T2-weighted signal. Uncomplicated ganglion cysts do
not have central enhancement, although a thin rim of enhancement may surround
the ganglion. If a suspected ganglion cyst has central enhancement, then
malignancy must be excluded. Option C is not the best response. Hemangiomas
have mixed signal on T1-and T2-weighted sequences because of the presence of
vessels, fat, and fibrous tissue
[3]. The vascular portions of
hemangiomas homogeneously enhance. Option E is not the best response. Densely
calcified masses can have low T1-and T2-weighted signal, but would not be
expected to enhance.
Solution to Question 3
MRI is the study of choice for the evaluation of soft-tissue neoplastic
masses in the foot [6]. MRI
signal characteristics combined with the location of the mass can reveal a
characteristic appearance for several entities. Enhancement characteristics
can be important when assessing soft-tissue masses in the foot and are most
helpful in differentiating benign cysts from solid masses
[6]. Malignant masses in the
foot can be well defined and have T1-and T2-weighted signal similar to cysts,
thus making the presence or absence of enhancement critical for
differentiation [7]. Unenhanced
MRI is also widely used and, for some foot masses, performs as well as
enhanced MRI. Option A is the best response. PET, especially when
combined with CT, has great promise for staging musculoskeletal neoplasms but
has not been proven to characterize masses more specifically than MRI. Option
B is not the best response. CT is not the diagnostic imaging study of choice.
CT can be useful for assessing the underlying bone, but does not best
characterize soft-tissue masses. However, CT can confirm the presence of fat
in a mass. Option C is not the best response. Sonography of soft-tissue masses
is relatively nonspecific when compared with enhanced MRI. Option D is not the
best response. Radiography poorly characterizes soft-tissue masses, but can
assess the underlying bone. Option E is not the best response.
Solution to Question 4
Subchondral cysts are a hallmark of degenerative arthritis; osteoarthritis
is common at Lisfranc's (tarsometatarsal) joint. In the setting of diabetes,
chronic neuropathic osteoarthropathy should also be considered. Acute
inflammatory conditions such as septic arthritis do not result in cyst
formation. Therefore, the presence of subchondral cysts argues against septic
arthritis in patients with marrow abnormality. Osteoarthritis with
superimposed infection remains a possibility, but infection rapidly destroys
the cartilage and subchondral plate, resulting in the initial disappearance or
obscuration of cysts [8]. After
infection clears, secondary osteoarthritis results in the reappearance of
cysts. Option C is the best response. Although enhancement is present
in the medial plantar tissues around the ulcer, no enhancement would be seen
with devitalization of the soft tissues
[9,
10]. This finding represents
cellulitis. Option A is not the best response. In general, in the setting of
diabetic foot ulceration when there is communication of the skin surface and
bone (via deep ulceration or sinus tract), osteomyelitis is often present
[9,
11,
12]. Enhancement of the first
cuneiform adjacent to the sinus tract disproportionate to the rest of the
Lisfranc joint suggests early osteomyelitis. Option B is not the best
response. The fifth metatarsal bone shows normal signal, which is low on this
fat-suppressed T1-weighted image
[12]. Option D is not the best
response. As with option C, cysts are compatible with a neuropathic joint.
However, a sinus tract extending from an ulcer to the medial cuneiform with
adjacent marrow enhancement should suggest the presence of superimposed
osteomyelitis [9,
11,
12]. Option E is not the best
response.
Solution to Question 5
Heterogeneous fat suppression can result from a variety of factors,
including nearby metal or a large field of view. If presaturation of fat
resonance frequency results in a heterogeneous signal, an inversion recovery
sequence, which provides more homogeneous fat suppression, should be performed
[9,
12]. Option D is the best
response. The smallest coil available to image the desired field of view
should always be used [9].
Imaging both feet with a head coil appears more efficient but results in
suboptimal imaging of both sides. Each foot should be imaged separately.
Option A is not the best response. The calf should be included if there is
clinical concern for proximal spread of infection. However, this is relatively
rare [11], and the large field
of view renders interpretation of the small bones of the foot limited because
they are subject to volume averaging effects. Option B is not the best
response. Infection results in edema: marrow edema in osteomyelitis and
soft-tissue edema in cellulitis
[12]. On T2-weighted spin-echo
and especially fast spin-echo imaging, fat in marrow and subcutaneous tissues
is hyperintense, which can obscure subtle areas of edema
[9]. Fat suppression should be
used when available unless there is significant artifact
[9,
10,
12]. Option C is not the best
response. IV gadolinium contrast material facilitates identification of
abscesses, sinus tracts, and devitalized regions; differentiation of
cellulitis from diabetic soft-tissue edema; and differentiation of bland fluid
from septic arthritis and septic tenosynovitis
[9–12].
Option E is not the best response.
Solution to Question 6
Neuropathic disease can lead to skin breaks due to minor trauma such as
toenail cutting. Also, decreased perception of injury and inflammation can
cause propagation of ulcers as well as superinfection
[9,
11,
12]. Neuropathic disease with
joint deformity and muscle imbalance can also lead to abnormal prominences
that result in callus formation. Ischemic calluses subsequently break down,
forming ulcers [8]. Option D
is the best response. After amputation, marrow signal is generally normal,
even shortly after surgery. Therefore, a diabetic patient presenting with
wound breakdown after amputation who has marrow edema and enhancement at the
amputation site on MRI should be considered highly suspicious for underlying
osteomyelitis [9]. Option A is
not the best response. Diffuse soft-tissue edema is quite common in the feet
of diabetic patients on MR images
[9,
10,
12]. This may be related to
vascular insufficiency or neuropathy, but it does not necessarily imply the
presence of inflammation. IV contrast material can distinguish "diabetic
edema" from inflammation if there is adequate blood flow
[10,
12]. Option B is not the best
response. In most areas of the body, the hematogenous route is the most common
mode for the spread of infection. However, in the diabetic foot, the
overwhelming mode (
90%) is contiguous spread from adjacent soft-tissue
ulceration [9,
11,
12]. Option C is not the best
response. The most common site for neuropathic osteoarthropathy in feet of
diabetic patients is Lisfranc's joint. The metatarsal bases subluxate
superiorly relative to the midfoot, leading to a rocker-bottom foot deformity.
The intertarsal joints, Chopart's joint, ankle, and subtalar joint also are
common sites of involvement. Neuropathic osteoarthropathy occurs at the
metatarsophalangeal joints but is relatively less common in this location
[8]. Option E is not the best
response.
Solution to Question 7
Bone fusion across more than 50% of the joint space indicates that the
arthrodesis is likely stable. Option E is the best response. To limit
metal artifact from the screw to the fewest number of slices, the scanning
plane should be aligned parallel to metal screws, not perpendicular. This will
concentrate all of the artifact on a few slices, leaving the rest of the
slices essentially undegraded. This can be useful when assessing an
arthrodesis of a small joint, such as the subtalar joint. Orienting the gantry
perpendicular to metal screws is another good positioning option. This will
spread the artifact over all of the slices, instead of concentrating it in a
few. Option A is not the best response. Raw CT data are the digital form of
scan data, not viewable in an image format until they are reconstructed with
operator-specified field of view, kernel, slice thickness, and spacing.
Multiplanar reformations (MPRs) are made from minimal-thickness axial source
images, not raw data [13].
Option B is not the best response. The ankle should be scanned in only one
plane. Two reconstructions should be made: one set of axial 2-to 3-mm thick
axial slices for routine review and a set of thin overlapping source images
that will be used to make MPRs in the other desired planes. Option C is not
the best response. Because the minimum slice reconstruction width is limited
by the detector collimation, increasing the collimation width will result in
thicker source images to be used for MPRs. The resulting MPRs will have
blurring of small structures such as bone trabeculae. Option D is not the best
response.
Solution to Question 8
Extension of a fracture into the talar dome is likely to lead to
posttraumatic degenerative joint disease, usually affecting both the ankle and
the subtalar joints [14].
Option B is the best response. Pilon fractures usually are due to axial
loading injuries, such as a fall from a height. Option A is not the best
response. The risk of avascular necrosis of the talus is increased with
subluxation or dislocation of the subtalar or ankle joint but is unrelated to
involvement of the anterior process. Option C is not the best response.
Fractures of the sustentaculum tali usually are due to vertical shear forces
from axial loading injuries. Option D is not the best response. The blood
supply to the talar dome consists of branches of the anterior tibial artery
that enter the bone at the talar neck. Option E is not the best response.
Solution to Question 9
A hypertrophic nonunion is thought to have hypervascularity and a capacity
for biologic activity; however, healing is hindered by a lack of mechanical
stability. Option C is the best response. Fracture nonunion is defined
as a lack of healing 9 months after the fracture occurred and 3 months without
progression of healing [15].
Option A is not the best response. Delayed union of a fracture is defined as
failure to unite completely as expected, but with continued biologic activity.
Option B is not the best response. An atrophic nonunion is thought to be
avascular and lacks the biologic capacity to heal, even with proper
stabilization. Debridement or a vascularized bone graft would be needed to
stimulate healing. Option D is not the best response.
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