The most common indication for MR imaging of the foot and ankle is for the evaluation of tendon and bone abnormalities, such as osteomyelitis, occult fractures, and partial and complete
Trang 1Magnetic resonance (MR) imaging
of the foot and ankle has lagged
behind MR imaging of other joints
in clinical acceptance and utility,
because of the complex anatomy of
the foot and ankle and the need for
small-field-of-view, high-resolution
images Recent advances in both
hardware and software, however,
have made possible the acquisition
of high-resolution images This
feature, combined with the degree
of soft-tissue contrast that can be
achieved with MR imaging and the
ability to obtain images in multiple
planes, has led to the increasing
importance of this modality in
imaging of the foot and ankle for
both diagnosis and surgical
plan-ning It is important that
physi-cians understand the common
clin-ical MR imaging techniques and
their role in evaluating disorders
of the foot and ankle to most
effec-tively utilize this diagnostic
mo-dality
Technique
Because of the complex anatomy of the foot and ankle and the small size of many of its structures, the acquisition of high-resolution im-ages is necessary To obtain such images, the foot and ankle should
be imaged separately by using a surface coil When a comparison view of the unaffected foot is needed
to assess tendon or ligament sym-metry, this should be accomplished
by scanning each foot separately in
a surface coil Attempting to scan both feet together in a body coil or a head coil saves time but at the cost
of having to use a large field of view, which results in low-resolution im-ages that are often nondiagnostic
The optimal field of view of the im-ages should be no larger than 16
cm2, and the matrix should range from 192 x 256 to 256 x 512 Section thickness depends on the pulse sequence used but should be 3 to 4
mm when using spin-echo (SE) se-quences and 1 to 2 mm when using gradient-echo (GRE) sequences For the purpose of MR imaging, the foot and ankle can be divided into three zones: the ankle and hindfoot, the midfoot, and the fore-foot.1 The midfoot is adequately examined by imaging both the hindfoot and the forefoot; there-fore, examination protocols can be further simplified into two zones: ankle-hindfoot and forefoot
To image the hindfoot and ankle, the patient is placed in a supine position with the medial malleolus centered in the coil The foot is allowed to rest in a relaxed posi-tion, generally in 10 to 20 degrees of plantar-flexion and 10 to 30 degrees
of external rotation The foot posi-tion may need to be altered when imaging specific ligaments, such as the calcaneofibular ligament For forefoot examinations, the patient can be either supine or prone with
Dr Recht is Section Head, Outside Imaging, Department of Diagnostic Radiology, Cleve-land Clinic Foundation, CleveCleve-land, Ohio Dr Donley is Staff Surgeon, Department of Ortho-paedic Surgery, Cleveland Clinic Foundation Reprint requests: Dr Recht, Department of Diagnostic Radiology, Cleveland Clinic Foundation, A21, 9500 Euclid Avenue, Cleveland, OH 44195.
Copyright 2001 by the American Academy of Orthopaedic Surgeons.
Abstract
Magnetic resonance (MR) imaging of the foot and ankle is playing an
increas-ingly important role in the diagnosis of a wide range of foot and ankle
abnormali-ties, as well as in planning for their surgical treatment For an optimal MR
study of the foot and ankle, it is necessary to obtain high-resolution,
small-field-of-view images using a variety of pulse sequences The most common indication
for MR imaging of the foot and ankle is for the evaluation of tendon and bone
abnormalities, such as osteomyelitis, occult fractures, and partial and complete
tears of the Achilles, tibialis posterior, and peroneal tendons Magnetic
reso-nance imaging has also been shown to be helpful in the diagnosis of several
soft-tissue abnormalities that are unique to the foot and ankle, such as plantar
fasci-itis, plantar fibromatosis, interdigital neuromas, and tarsal tunnel syndrome.
J Am Acad Orthop Surg 2001;9:187-199 the Foot and Ankle
Michael P Recht, MD, and Brian G Donley, MD
Trang 2the toes centered in the coil It is
im-portant to image in all three planes
(transaxial, sagittal, and coronal) for
all indications However, for
ten-don and ligament disorders as well
as for soft-tissue masses, the
trans-axial plane of imaging is the most
useful; most sequences should be
acquired in this plane For bone ab-normalities, particularly those of the talar dome, the sagittal and coronal planes provide the greatest amount
of information
A variety of pulse sequences can
be utilized in the examination of the foot and ankle T1-weighted (short
repetition time [TR]/short echo time [TE]) SE images provide excellent anatomic detail and information about the integrity of the bone mar-row T2-weighted (long TR/long TE) SE images allow detection of the increased water content seen with most pathologic processes as abnor-mal high signal intensity Fast SE T2-weighted sequences have largely replaced conventional SE T2-weighted sequences because of the ability to ob-tain images in a shorter time period with higher resolution Gradient-echo sequences allow the acquisition
of thin contiguous sections that can
be reformatted in multiple planes These sequences have been shown
to be useful in the detection of carti-lage abnormalities.2,3
Short-tau inversion recovery (STIR) imaging is a method of fat suppression that has proved very sensitive in detecting marrow ab-normalities, as well as increased water content in soft tissues.4 Cur-rently, most STIR sequences also use
a variant of the fast SE technique, which allows the images to be ac-quired in a shorter period of time Another method of fat suppression is the use of chemical-selective fat sup-pression, which takes advantage of the differences in resonance
frequen-cy between fat and water protons When evaluating a soft-tissue or osseous mass, T1-weighted chemical-selective fat-suppressed imaging after injection of intravenous con-trast material (e.g., gadolinium– diethylenetriaminepenta-acetic acid [DTPA]) improves the conspicuity
of the mass and facilitates the differ-entiation of a solid soft-tissue mass from a fluid-filled cyst.4 Magnetic resonance arthrography may play a role in the detection of ligament abnormalities and intra-articular bodies and in the staging of osteo-chondral defects.5,6
The foot and ankle can be im-aged with high-field-strength (>0.5-T) or low-field-strength (≤0.5-(>0.5-T) magnets A high-field-strength
Definitions of Radiologic Terms
Chemical-selective The use of chemically selective radio-frequency
fat suppression pulses to eliminate fat signal by taking advantage
of the difference in resonance frequency between fat and water protons
Echo time (TE) The time between the middle of the excitation pulse
and the middle of the spin echo Gradient-echo (GRE) Describing a sequence in which an echo is produced
by a single radio-frequency pulse followed by a gradient reversal
Proton-density- An image acquired with a long TR (e.g., 2,000-3,000
weighted image msec) and a short TE (e.g., 20 msec) to emphasize
differences in proton density and minimize T1 and T2 differences between tissues
Repetition time (TR) The time between successive excitations of a section
Short-tau inversion Describing a sequence that suppresses fat signal by
recovery (STIR) the use of a 180-degree inversion pulse and a short
inversion time Spin-echo (SE) Describing a sequence in which an echo is produced
by a 90-degree radio-frequency pulse followed by one or more 180-degree radio-frequency pulses T1 Spin-lattice or longitudinal relaxation time The
time constant for magnetization to return to the longitudinal axis after application of a radio-frequency pulse
T1-weighted image An image acquired with a short TR (e.g., 400-600
msec) and a short TE (e.g., 10-20 msec) to emphasize differences in T1 relaxation rates between tissues
Fat is of high signal intensity, and fluid is of low signal intensity on T1-weighted sequences
T2 Spin-spin relaxation time The time constant for loss
of phase coherence of a group of spins and the resulting loss in the transverse magnetization signal
T2-weighted image An image acquired with a long TR (e.g., 2,000-3,000
msec) and long TE (e.g., 80-100 msec) to emphasize differences in T2 between tissues Fluid is bright on T2-weighted sequences
Trang 3magnet, with its higher
signal-to-noise ratio, allows the acquisition of
high-resolution images in a shorter
time period, thus decreasing the
potential for patient motion In
addition, chemical-selective fat
sup-pression is not available with
low-field-strength magnets
Tendons
Ten tendons cross the ankle joint on
their path from the lower leg into
the foot (Fig 1): the Achilles
ten-don posteriorly; the peroneus
bre-vis and longus laterally; the tibialis
posterior, flexor digitorum longus,
and flexor hallucis longus medially;
and the tibialis anterior, extensor
digitorum longus, extensor hallucis
longus, and peroneus tertius
anteri-orly Tendons are composed
primar-ily of collagen, elastin, and reticulin
fibers
Normal tendons appear as ho-mogeneous low signal intensity on
MR imaging because of their lack
of mobile protons.7 However, on T1-weighted and proton-density-weighted (long TR/short TE) im-ages, normal tendons can have inter-mediate signal intensity because of the “magic angle effect.”8 This effect occurs with short-TE sequences because the signal intensity of struc-tures with poorly hydrated protons, such as tendons, depends in part
on the orientation of the structure
in relation to the main magnetic field When a structure is oriented obliquely in relation to the main magnetic field, its signal intensity is increased on short-TE sequences;
this increase is greatest when the structure is oriented at 55 degrees in relation to the main magnetic field
The increase in signal intensity is not seen on long-TE (T2-weighted) sequences The magic-angle effect is noted in most of the tendons of the foot and ankle as they curve across the ankle to pass into the foot How-ever, this normal increase in signal intensity can usually be differenti-ated from pathologic changes within the tendon if one is aware of the characteristic location of the magic-angle effect, the lack of high signal intensity on T2-weighted images, and the normal morphology of intact tendons
The transaxial plane is the most useful for evaluating the integrity of tendons To obtain a true transaxial image of the tendons that traverse the ankle to pass into the foot, it is necessary to select a plane perpen-dicular to the long axis of the ten-dons as they curve about the ankle (Fig 2) A useful protocol for tendon pathology includes transaxial T1-weighted SE and STIR images or fat-suppressed fast SE T2-weighted images (obtained with the same sec-tion thickness and posisec-tions to allow comparison), coronal T1-weighted, and sagittal fat-suppressed fast SE T2-weighted sequences A sagittal
T1-weighted sequence is added when examining the Achilles tendon Pathologic changes that can be seen in and about tendons on MR imaging include tenosynovitis, tendinopathy, and tendon tears Tenosynovitis is best visualized on T2-weighted images as high-signal-intensity fluid surrounding a normal-appearing tendon (Fig 3) “Ten-dinopathy” or “tendinosis” is the term currently favored to describe tendons that are abnormal but not torn Although some authors still use the term “tendinitis,” studies have not shown a true inflammatory process in tendons.9,10 Rather, histo-logic studies have demonstrated hyperplasia, fibrosis, and vacuolar, mucoid, eosinophilic, and fibrillary degeneration On MR imaging, tendinopathy is characterized by altered tendon morphology,
usual-ly thickening There may also be areas of mildly increased signal intensity on short TE (T1-weighted
or proton-density-weighted) se-quences, but this signal intensity usually decreases on T2-weighted images unless severe tendinopathy
is present
Three MR patterns of tendon rupture have been described.11
Figure 1 Transaxial T1-weighted image
at the level of the distal tibia (T) and fibula
(F) Note the homogeneous low signal
intensity of the tendons A = Achilles
ten-don; ED = extensor digitorum longus; EH
= extensor hallucis longus; FD = flexor
dig-itorum longus; FH = flexor hallucis longus;
PB = peroneus brevis; PL = peroneus
longus; TA = tibialis anterior; TP = tibialis
posterior.
TA
T
F
TP
FD
A
FH
PL PB
EH ED
Figure 2 True transaxial images of the peroneal tendons The sections are graphi-cally prescribed off a sagittal image as the tendons curve around the ankle.
Trang 4Type 1 is characterized by
hypertro-phy of the tendon with partial tears
oriented primarily longitudinally
The tendon is enlarged (Fig 4, A),
with foci of increased signal inten-sity on T2-weighted and STIR images (Fig 4, B) Type 2 is characterized
by a partially torn atrophic tendon;
the appearance is of a small tendon with foci of increased signal
intensi-ty on T2-weighted or STIR images (Fig 4, C) Type 3 tears are com-plete tendon ruptures (Fig 4, D)
Although all of the tendons of the foot and ankle can be studied with
MR imaging, the tendons most often associated with injury or dis-ease are the Achilles, tibialis poste-rior, and peroneal tendons
Achilles Tendon
The Achilles tendon is the largest and strongest tendon in the body, ranging in length from 10 to 15 cm
The Achilles tendon does not pos-sess a synovial sheath but rather is invested by loose connective tissue (peritenon) The normal Achilles tendon appears as homogeneously low signal intensity on all pulse sequences1(Fig 5, A and B) It has
a flat or concave anterior margin on transaxial images, giving it a cres-centic shape At its insertion onto
the calcaneus, the tendon becomes more ovoid, with a flattened anterior margin
Acute peritendinitis is manifested
by loss of the sharp interface be-tween the tendon and the pre-Achilles fat, with high signal inten-sity on T2-weighted and STIR images about the tendon, but with preservation of the low signal in-tensity of the tendon itself Chronic Achilles tendinopathy has the appearance of a thickened enlarged tendon (Fig 5, C and D) There may be increased signal intensity within the tendon on T1-weighted and proton-density-weighted im-ages, but the signal usually de-creases in intensity on T2-weighted and STIR images Although mea-surements of the thickness of the Achilles tendon have been pub-lished (normal, <8 mm),1 careful assessment of the anterior margin
of the tendon on transaxial views may be more useful Loss of the normal concave margin of the ante-rior aspect of the tendon is a sign that the tendon is thickened and abnormal
Figure 3 Tenosynovitis of the flexor
hallu-cis tendon Note the high-signal-intensity
fluid (arrows) surrounding the normal
low-signal-intensity tendon and the
metal-lic artifact about the tibia secondary to
pre-vious hardware placement.
Figure 4 Patterns of tendon rupture A, Type 1 tear of the tibialis posterior tendon Transaxial T1-weighted image at the level of the sustentaculum tali demonstrates an enlarged, irregularly shaped tibialis posterior tendon (arrow) B, Transaxial STIR image of the same ankle demonstrates high signal intensity within the enlarged tibialis posterior tendon (arrows) C, Type 2 tear of the tibialis posterior
ten-don Transaxial T1-weighted image demonstrates a small atrophic tibialis posterior tendon (white arrows) approximately half the
diame-ter of the flexor digitorum tendon (black arrow) D, Type 3 tear of the Achilles tendon Sagittal T2-weighted fast SE image demonstrates
discontinuity of the Achilles tendon There is retraction of the torn edge of the tendon (arrows) and high signal intensity in the gap between the tendon edge and the calcaneus.
Trang 5Ruptures of the Achilles tendon
most frequently occur 3 to 4 cm
above its insertion onto the
calca-neus.7 The MR findings of a partial
rupture include focal areas of high
signal intensity on T2-weighted
and STIR images within the tendon
substance but with preservation of
some tendon continuity4(Fig 5, E
and F) Acute complete ruptures
demonstrate loss of tendon
conti-nuity, with the gap in the tendon
appearing as an area of high signal
intensity on T2-weighted and STIR
images, representing blood or
ede-ma4 (Fig 4, D) In chronic
com-plete ruptures, the gap may be
filled with low-signal-intensity
fibrotic tissue
Tibialis Posterior Tendon
Tibialis posterior tears are most
commonly seen in middle-aged
women, who present with an
ac-quired, painful flatfoot; these are
generally chronic tears.11,12 The
tib-ialis posterior tendon is the most
medial tendon in the posterior
com-partment at the level of the ankle
The tendon continues into the foot,
where it inserts onto the navicular,
A
C
E
B
D
F
Figure 5 A and B, Normal Achilles
ten-don A, On T1-weighted sagittal image,
the normal Achilles tendon is of
homoge-neous low signal intensity and has sharp
interfaces with the surrounding soft tissue.
B, Transaxial T1-weighted image at the
level of the distal Achilles tendon The
concave anterior surface of the tendon
gives a crescentic shape to the distal
por-tion (arrowheads) C and D, Chronic
tendinopathy of the Achilles tendon
T2-weighted sagittal (C) and T1-T2-weighted
transaxial (D) images demonstrate a
thick-ened Achilles tendon (arrows), which is of
low signal intensity Note the loss of the
normal concave anterior surface of the
ten-don on the transaxial image E and F,
Partial tear of the distal Achilles tendon.
T1-weighted (E) and T2-weighted (F)
sagit-tal images demonstrate an enlarged,
thick-ened distal Achilles tendon On the
T2-weighted image, there is high signal
insity (arrow) within the distal Achilles
ten-don, representing a partial tear.
Trang 6medial cuneiform, metatarsal bases,
and sustentaculum tali The normal
tibialis posterior tendon has
homo-geneously low signal intensity
ex-cept for its most distal segment,
which may demonstrate
intermedi-ate signal intensity on T1-weighted
sequences It should be twice the
diameter of the flexor digitorum
longus and flexor hallucis longus
tendons distal to the level of the
medial malleolus.12
Type 1 tears of the posterior
tib-ialis tendon, which are the most
common type of tear, are manifested
by an enlarged tendon, which may
be four to five times the size of the
flexor digitorum tendon (Fig 4, A
and B) There is increased signal
intensity within the tendon on
short-TE images, which often remains
high on T2-weighted and STIR
images Type 2 tears present as a
smaller than normal tendon, often
the same size as or smaller than the
flexor digitorum longus (Fig 4, C)
Type 3 tears are complete tendon
ruptures
Peroneal Tendons
The peroneus longus and brevis tendons occupy a common synovial sheath up to the level of the calca-neocuboid joint, beyond which the sheath bifurcates At the level of the lateral malleolus, the peroneus bre-vis tendon is anteromedial or ante-rior to the peroneus longus tendon
The posterior edge of the fibula is normally concave in this region, forming a groove within which the tendons lie The tendons are kept within this groove by the superior peroneal retinaculum
On MR imaging, normal peroneal tendons are of similar size and ho-mogeneous low signal intensity
The peroneus longus tendon is ovoid, but the peroneus brevis ten-don may have a flattened appear-ance in the retromalleolar groove
The superior peroneal retinaculum
is usually identifiable as a discrete structure
Although complete ruptures of the peroneal tendons are uncom-mon, longitudinal splits of the
per-oneus brevis tendon have been increasingly recognized.13-15 Longi-tudinal splits are thought to be caused by either forced dorsiflexion
or repetitive peroneal subluxation, which leads to compression of the peroneal brevis against the posterior aspect of the fibula Interposition of the peroneus longus between the portions of the split peroneus brevis tendon can occur On MR imaging,
a split peroneus brevis appears either as a C-shaped structure at or below the level of the lateral malleo-lus, which partially wraps around the peroneus longus tendon, or as a completely bisected tendon (Fig 6, A) There may or may not be in-creased signal intensity within the tendon An osseous ridge at the lat-eral margin of the fibula has been associated with a split peroneus bre-vis tendon, and is considered to rep-resent changes secondary to repeti-tive subluxation of the peroneal ten-dons.13 Other MR findings that are associated with, and may predispose
to, splitting of the peroneus brevis
Figure 6 Lesions of the peroneal tendons A, Transaxial T1-weighted image obtained just distal to the lateral malleolus demonstrates a completely bisected peroneus brevis tendon (arrowheads) T1-weighted transaxial (B) and coronal (C) images show subluxation of the
peroneal tendons (arrows) so that they lie lateral to the malleolus, rather than posterior to it.
Trang 7tendon include a flat or convex
fibu-lar groove, a ligamentous tear, or
the presence of a peroneus quartus
muscle or the low-lying belly of the
peroneus brevis muscle.14
Traumatic peroneal subluxation
or dislocation is associated with
dis-ruption of the superior retinaculum
or stripping of the periosteum at its
attachment onto the fibula This is
not an uncommon injury in athletes
and may be misdiagnosed as an
ankle sprain.7 Traumatic dislocation
can also be seen with calcaneal
frac-tures The abnormally positioned
peroneal tendons are easily seen on
MR images lying lateral (Fig 6, B
and C), or in extreme cases anterior,
to the lateral malleolus
Ankle Ligaments
Magnetic resonance imaging can
demonstrate both intact (Fig 7) and
abnormal (Fig 8) ankle ligaments
However, its role in the evaluation
of ankle ligament injuries remains
uncertain, especially in cases of
acute ligament injury, which are
most often diagnosed on clinical
examination It may play a limited
role in defining which ligaments
are injured, the extent of such
in-jury in patients with chronic ankle
instability, and the presence of
os-teochondral injuries of the talar
dome in patients with chronic ankle
pain after ligament injuries
The ankle ligaments can be
grouped into three complexes: the
lateral complex, consisting of the
anterior talofibular, posterior
talo-fibular, and calcaneofibular
liga-ments; the deltoid ligament, which
has several components; and the
syndesmotic complex, composed of
the interosseous membrane, the
anterior and posterior tibiofibular
ligaments, and the transverse
tibio-fibular ligament To evaluate these
ligaments with MR imaging, it is
necessary to image them in a plane
parallel to their long axes This
plane varies for the different liga-ments, but a cadaveric study of the ankle ligaments demonstrated that particular planes were optimal for studying the various ligaments.16,17
The transaxial plane with the foot positioned in 10 to 20 degrees of dorsiflexion provides the best visual-ization of the anterior and posterior talofibular ligaments; the anterior,
A
C
B
D Figure 7 Appearance of normal ankle ligaments A, The intact anterior talofibular
liga-ment (arrowheads) is of low signal intensity on this T1-weighted transaxial image Note
the elliptical shape of the talus and the presence of the lateral malleolar fossa B, Intact
anterior (arrowheads) and posterior (arrows) tibiofibular ligaments are of uniform low sig-nal intensity The medial border of the lateral malleolus is flattened, indicating that this is
the level of the tibiofibular ligaments C, Intact tibiotalar component of the deltoid (arrow-heads) Note the osteochondral defect of the lateral talar dome D, Posterior talofibular
ligaments (arrowheads) on T1-weighted coronal image The deltoid and posterior talofibular ligaments have a striated appearance, rather than a homogeneous low-signal-intensity appearance like the anterior talofibular ligament.
Trang 8transverse, and posterior
tibiofibu-lar ligaments; and the various
com-ponents of the deltoid ligament
Coronal images allow visualization
of the full length of most of the
com-ponents of the deltoid ligament The
calcaneofibular ligament is best seen
on transaxial images with the foot in
40 to 50 degrees of plantar-flexion
It is difficult to image the foot in
all of these positions in a reasonable
time period Most MR examinations
of the foot and ankle are done in 10
to 20 degrees of plantar-flexion,
which is not ideal for imaging any of
the ankle ligaments Therefore, it is
important to clearly communicate in
advance which specific ligamentous
complexes need to be imaged The
MR sequences used to evaluate the
ankle ligaments include T1-weighted
SE, T2-weighted fast SE, and STIR
sequences In cases of chronic ankle
instability, MR arthrography may
also be useful
Normal ligaments are thin and of
low signal intensity on all MR pulse
sequences Occasionally, they have
a striated appearance, especially the deltoid and posterior talofibular and tibiofibular ligaments.18 Be-cause of the oblique course of the tibiofibular ligaments, the talus may
be seen on images demonstrating their fibular attachments This can lead to misidentification of the tibiofibular ligaments as the talo-fibular ligaments The best way to avoid this mistake is to identify the insertion of the ligaments The shape of the talus and the fibula in the transaxial plane can also be used
to correctly identify the two sets of ligaments.8 At the level of the tibio-fibular ligaments, the talus is rectan-gular, and the medial border of the fibula is flattened At the level of the talofibular ligaments, the talus is more elongated, the sinus tarsi is usually visible, and there is a deep indentation along the medial border
of the lateral malleolus (the malleo-lar fossa)
The MR findings in ligament injuries include complete tear of the ligament, ligament waviness or lax-ity, thickening or irregularity of the ligament, increased signal intensity within the ligaments, edema and hemorrhage about the ligament, and abnormal increased fluid
with-in the jowith-int and surroundwith-ing ten-dons.7,17 In cases of chronic insta-bility, some of the ancillary findings
of ligament injury, such as edema and hemorrhage and joint effusions, may not be present
Magnetic resonance arthrography has been shown to be more sensitive and more accurate than conventional
MR imaging in this situation.5 This
is because the torn, scarred ligament
is closely applied to the bone and is better visualized when separated from the bone by the intra-articular injection of contrast material (Gd-DTPA) In addition, contrast extra-vasation through the torn ligament into the surrounding soft tissues serves as convincing evidence of dis-ruption of the ligament
Bones
Infection
Infection of the bones of the foot and ankle occurs most commonly in diabetic patients, usually due to direct extension of soft-tissue infec-tion Magnetic resonance images, especially STIR and fat-suppressed T1-weighted images acquired after intravenous contrast administration effectively depict the bone marrow changes that occur with osteomye-litis.19 However, these changes are not specific for osteomyelitis The differentiation of bone marrow changes due to infection from those due to edema or neuropathy has proved challenging
Although neuropathic tissue may
be visualized as low signal intensity
on all pulse sequences,20it may have a high-signal-intensity appear-ance on T2-weighted and STIR sequences.7 Enhancement after intravenous contrast administration
is also not specific, as it can be seen
in the presence of any process result-ing in increased vascular
permeabili-ty Findings useful in the diagnosis
of osteomyelitis include soft-tissue changes that extend to the skin surface adjacent to bone-marrow changes, cortical disruption, and periosteal abnormalities (Fig 9).4,7 Nonetheless, it is still often difficult
to reliably differentiate neuropathy from infection on MR imaging A useful protocol for the evaluation of osteomyelitis includes T1-weighted
SE, STIR, and fat-suppressed T1-weighted SE images obtained after contrast administration The images are acquired in at least two planes, which are determined on the basis
of the site of the suspected infection
Fractures
Magnetic resonance imaging has little role to play in the evaluation of acute traumatic bone injuries, as they are usually easily diagnosed on conventional radiography How-ever, MR imaging may be useful in
Figure 8 Chronic tear of the anterior
talofibular ligament This transaxial
T2-weighted image demonstrates the absence
of the anterior talofibular ligament, with
high-signal-intensity fluid (arrows) filling
the expected location of the ligament.
Trang 9identifying bone contusions, occult
nondisplaced fractures, and stress
fractures and stress reactions in the
foot and ankle Metatarsal stress
fractures can generally be
diag-nosed without MR imaging In
con-trast, stress fractures of other tarsal
bones, such as the navicular,
cunei-forms, and calcaneus, often present
as foot pain of unknown etiology If
conventional radiographs appear
normal, MR imaging is a valuable
modality, as it can depict pathologic
changes in both bone and soft tissue
Stress fractures are diagnosed most
readily on STIR and T1-weighted
SE images but can also be seen on
T2-weighted images (Fig 10) On
T1-weighted images, stress fractures
appear as a linear area of low signal
intensity compared with normal
bone marrow surrounded by a more
diffuse area of slightly higher, though
still low, signal intensity.20-22 On STIR
images, the linear component
re-mains of low signal intensity, but the
surrounding area is of high signal
intensity, consistent with bone
mar-row edema
In addition to stress fractures, MR imaging is also able to depict stress responses, which represent early changes in bone before the develop-ment of a fracture.22 Stress responses are characterized by globular areas of low signal intensity on T1-weighted images, which increase in signal intensity on STIR sequences They can be differentiated from fractures
by the lack of a linear component
Stress responses appear similar to bone bruises but can be differen-tiated from them by the lack of an antecedent acute traumatic event
Osteochondral Injuries
of the Talar Dome
Osteochondral injuries of the talar dome occur most commonly in the second to fourth decades of life and affect both the medial and lat-eral aspects of the dome.23 Most lesions are apparent on conventional radiographs; however, MR imaging can depict lesions too small to be seen on plain films and may be use-ful in evaluating the extent of the lesion and the stability of the
frag-ment.23 Increased signal intensity separating the lesion from the un-derlying bone on T2-weighted or STIR images is the most frequent
MR sign of instability, but this ap-pearance has also been reported in stable lesions.24 Other less fre-quently seen signs of instability are cartilage fractures, focal cartilage defects, and underlying cysts.24 Magnetic resonance arthrogra-phy has been shown to be more accurate in evaluating the stability
of the fragment than conventional
MR imaging in osteochondral le-sions of the knee,6because of the ability to see contrast material tra-versing the overlying cartilage de-fect and encircling the loose frag-ment More recently, cartilage-specific sequences, such as fat-suppressed T1-weighted GRE sequences, have been used to evaluate the overlying cartilage (Fig 11) Although MR imaging is highly accurate for eval-uating the cartilage in the knee,2,3
Figure 9 Osteomyelitis of the calcaneus A, Sagittal T1-weighted image demonstrates a
soft-tissue ulcer (white arrow) on the plantar surface of the foot adjacent to the area of
abnormal low signal intensity within the calcaneus (black arrows) B, Sagittal
fat-suppressed T1-weighted image shows enhancement (arrows) of the calcaneus and adjacent
soft tissues, as well as subtle cortical disruption (lower arrow).
Figure 10 Stress fracture of the navicular Coronal T2-weighted image demonstrates high signal intensity within the navicular surrounding a thin linear area of low signal intensity (arrowheads), which represents the fracture line.
Trang 10evaluation of talar cartilage has
been more difficult, even with both
sagittal and coronal images, because
the talar cartilage is considerably
thinner
Bone Tumors
Although MR imaging is very
sensitive in the detection of bone
tumors, it frequently lacks
specific-ity However, MR imaging can be
useful in evaluating the extent of
the tumor and the presence of an
associated soft-tissue mass The
imaging is done in all three planes
with a combination of T1-weighted
SE, STIR, and T2-weighted fast
SE sequences Occasionally, a
fat-suppressed T1-weighted sequence
is used after administration of
in-travenous contrast material, as it
increases the conspicuity of the
bone and soft-tissue abnormalities
and improves the differentiation of
necrotic tissue from viable tumor.25
In addition, some researchers have
suggested that dynamic
contrast-enhanced MR imaging may be
use-ful in assessing the response of
osteosarcoma and Ewing’s sarcoma
to chemotherapy.25,26
Associated Soft-Tissue Conditions
Plantar Fasciitis
The deep plantar fascia, or plan-tar aponeurosis, is a multilayered fibrous structure, subcutaneous in location, which extends as a thick, strong, dense tissue from the calca-neus posteriorly to the region of the metatarsal heads and beyond
Plantar fasciitis is one of the causes
of painful heel syndrome and may
be secondary to mechanical, degen-erative, or systemic conditions The plantar fascia is best visual-ized on sagittal and coronal images and normally should be 3 to 4 mm thick and of homogeneously low signal intensity on all pulse se-quences.27 In patients with plantar fasciitis, the plantar fascia is thick-ened (7 to 8 mm thick) and demon-strates areas of increased signal intensity on T2-weighted and STIR sequences4,27 (Fig 12) There fre-quently is also abnormally increased signal intensity in the adjacent sub-cutaneous tissue Increased signal intensity may also be seen in the cal-caneus at the insertion site of the plantar fascia, presumably second-ary to reactive edema
Plantar fasciitis is a clinical diag-nosis and rarely, if ever, warrants
MR imaging However, in patients with a painful heel syndrome, it can occasionally be helpful in excluding other etiologic possibilities, such as calcaneal stress fractures and tarsal tunnel masses
Plantar Fibromatosis
Plantar fibromatosis is character-ized by fibrous proliferation in the plantar fascia.4 An association be-tween plantar fibromatosis and other conditions associated with proliferation of fibrous tissue, such
Figure 11 Osteochondral injury of the talar dome A, T1-weighted coronal image
demon-strates deformity of the talar dome, with a focal area of low signal intensity within the
bone marrow of the talar dome (arrows) B, Fat-suppressed three-dimensional GRE
coro-nal image depicts articular cartilage as high sigcoro-nal intensity There is disruption of the
articular cartilage (arrows) overlying the signal abnormality within the talar dome.
Figure 12 Plantar fasciitis A, T1-weighted sagittal image shows thickened deep plantar
fascia at its insertion onto the calcaneus (arrow) There is also abnormal intermediate
sig-nal intensity within the deep plantar fascia B, Sagittal STIR image demonstrates abnormal
high signal intensity about the deep plantar fascia (arrow).