Magnetic resonance imaging (MRI) has become the musculoskeletal modality of choice for evaluation of bone and soft-tissue pathology. MRI of the hip is used frequently in symptomatic patients whose conditions cannot be resolved with plain films. This article reviews the imaging techniques and MRI manifestations of a variety of conditions in the adult hip.
is the Chairman of and
is a Resident in the Department of Radiology, Maimonides Medical
Center, Brooklyn, NY.
are medical students at New York College School of Medicine,
Recent technologic advances in gradient strength, pulse
sequence, and surface coil design allow for unprecedented
soft-tissue resolution with magnetic resonance imaging (MRI). The
musculoskeletal system is only one of the areas for which MRI has
become the modality of choice for evaluation of bone and
soft-tissue pathology. The hip is one of the areas most frequently
imaged with MR in symptomatic patients whose conditions cannot be
resolved with plain films. Common indications for hip MRI include:
trauma with suspected occult fracture, stress and insufficiency
fractures, labral tears, osteochondritis
dissecans, tendon tears, septic arthritis, osteomyelitis,
osteonecrosis, transient osteoporosis or bone marrow edema, and
other conditions common to all joints, such as bursitis, synovial
chondromatosis, synovial osteochondromatosis, inflammatory
conditions, and bone and soft-tissue tumors. In this article, the
imaging technique and MRI manifestations of these entities in the
adult hip are summarized.
The first decision to make with hip MRI is whether to image both
hips simultaneously or only the symptomatic hip. It is an important
decision since it will influence other decisions such as coil and
pulse sequence selection, as well as whether or not to inject
contrast material, intra-articularly or intravenously. This
decision is compounded by the common situation in which no clinical
information is provided to the radiologist. As a general guideline,
imaging of both hips simultaneously may be appropriate if one is
looking for osteonecrosis (given the frequency of bilateral
involvement) or metastasis. In most other clinical situations,
single hip imaging provides better spatial and contrast resolution,
since a dedicated surface coil can be used.
When bilateral hip imaging is chosen, the body coil, preferably
phase array, is used.
The following set of pulse sequences is recommended: T1-weighted
coronal and fast-spin echo (FSE) T2-weighted or short tau inversion
recovery (STIR) axial. If there are positive findings, one may
choose at this time to stop or to switch to single hip imaging.
This is done by using a dedicated surface coil, such as a flexible
coil, for better anatomical resolution of small structures such as
the acetabular labrum, or for better evaluation of the articular
surfaces or subchondral area of the femoral head. When single hip
imaging is indicated, the following pulse sequences are
recommended: Coronal T1-weighted high resolution with 512 matrix,
axial T2-weighted FSE or STIR and sagittal two-dimensional (2D) or
three-dimensional (3D) gradient-recalled echo (GRE) (fast low-angle
shot [FLASH], spoiled gradient-recalled acquisition in a steady
state [SPGR]) or T2-weighted or STIR sequences.
Intravenous contrast material may be useful to assess tissue
viability in osteonecrosis and in cases of suspected infection or
bone or soft-tissue tumors. T1-weighted high-resolution images with
fat saturation in the axial sagittal and coronal planes are
recommended when using intravenous contrast material. The most
recent innovation is the so-called "indirect" MR arthrogram (IMRA),
replacing the intra-articular or "direct" MR arthrogram (DMRA).
Indirect MR arthrogram is performed by injecting, intravenously, a
standard dose of a gadolinium chelate (1 mmol/kg). This is then
followed by 10 to 15 minutes of gentle exercise before T1-weighted
fat-saturated imaging with high resolution in all three orthogonal
planes, in addition to a T2-weighted sequence, preferably in the
axial plane. This technique provides enough diffusion of contrast
material through the synovium to outline the articular surfaces and
produce some distension of the capsule and its recesses. An added
advantage of IMRA is that it produces enhancement of hyperemic
tissues. Furthermore, if a pre-existing joint effusion is present,
the injected contrast material mixes with the joint fluid, giving
an excellent arthrographic effect with high resolution provided by
the use of the T1-weighted fat-saturated images with 512
The DMRA technique entails the placement of a needle into the
hip joint, generally using fluoroscopic guidance, and injecting 10
to 15 mL of dilution of a gadolinium chelate (0.1 mL) and saline
(20 mL). The patient is then taken immediately into the MRI suite,
and the same pulse sequences described for IMRA are used. The main
advantage of DMRA over IMRA is the full distension of the joint
capsule and its recesses. The disadvantages, however, are
significant: DMRA requires additional scheduling for fluoroscopy
and is an invasive procedure with added morbidity.
The indications for IMRA or DMRA include assessment of the
articular surfaces when studying osteochondritis dissecans and
assessment of the acetabular labrum.
The hip is a ball-and-socket joint composed of the acetabulum
and the femoral head. The acetabulum is partially covered by
articular cartilage, a horseshoe shape opened inferiorly. The
central portion or acetabular fossa is filled by fibrofatty tissue.
The teres ligament originates in this fossa and inserts into the
fovea capitis of the femoral head. A rim of fibrous tissue called
the "acetabular labrum" surrounds the acetabulum. The joint capsule
extends from the acetabular margin to the femoral neck, and it is
reinforced anteriorly and posteriorly by a series of capsular
ligaments. Figures 1, 2, and 3 depict the normal MRI anatomy of the
hip in the three orthogonal planes.
The normal patterns of the trabecular structure and bone marrow
of the proximal femur vary with the age of the patient. As the
patient becomes older, the trabecular pattern, formed by the
primary and secondary trabeculae, becomes thinner and tends to be
less conspicuous on MRI, due to osteoporosis. Similarly, the red
marrow, which in young patients is very prominent in the femoral
neck and intertrochanteric region, diminishes in the elderly
Occult fractures in the elderly are one of the most frequent
indications for MRI of the hip (Figure 5). In these generally
osteoporotic patients, plain films may not demonstrate the fracture
line, and bone scintigraphy may demonstrate increased uptake only
several days after the trauma. MRI is highly sensitive for
detecting the fracture and the surrounding edema immediately after
the traumatic event.
Subchondral fractures of the femoral head are now a well-recognized
entity that may lead to extensive bone-marrow edema and the
eventual collapse of the femoral head with subsequent bone
resorption and rapid development of osteoarthritis. Subchondral
fractures are mostly occult on plain films and may be related to
stress, rather than direct trauma. This entity may be misdiagnosed
as osteonecrosis or transient osteoporosis of the hip (TOH) because
of the extensive bone-marrow edema that is seen during the first
few months. In fact, TOH and subchondral fracture may be the same
condition. Another disease that may be linked to subchondral
fracture of the femoral head is rapidly destructive osteoarthritis
or Pastel disease. MRI demonstrates bone-marrow edema, joint
effusion, and a faintly seen subchondral hypointense line that
represents the fracture. This line is better seen once the
bone-marrow edema subsides (Figure 6).
Osteochondritis dissecans or osteochondral fractures are
relatively rare in the femoral head, but when they occur,
radiographic findings may be subtle. MRI shows the lesion quite
well (Figure 7), as well as other important data such as size,
location, stability, joint surface congruity, and viability of the
fragment can be assessed.
Post-traumatic lesion of the soft tissues can also be studied
with MRI. Among the tendon lesions around the hip, hamstring
avulsion injuries and tears are frequent in certain sports that
involve extreme abduction of the hip. MRI can demonstrate the
lesion as well as the degree of tendon retraction (Figure 8).
Other soft-tissue lesions that may be related to trauma or
degeneration are the labral tears. These tears occur more
frequently at the level of the superior aspect of the acetabular
labrum and may be difficult to diagnose unless a high-quality MRI
is performed or intra-articular or intravenous injection of
contrast material is used (Figure 9).
Osteonecrosis or avascular necrosis (AVN) of the femoral head is
one of the most frequent indications for MRI of the hip. MRI has
been proven highly sensitive and accurate for the detection of this
condition. The findings include an irregular low-signal intensity
line that demarcates the peripheral aspect of the necrotic segment,
which histologically correlates with the reactive interphase. A
high-intensity line paralleling the hypointense line and termed the
"double-line sign" is often seen and has been quite characteristic
of AVN (Figure 10).
In the recent years, emphasis has been placed in the assessment
of the size of the lesion as compared with the size of the
weight-bearing portion of the femoral head. This measurement helps
to determine the potential outcome if minimally invasive treatment,
such as core decompression, is considered. Large areas of AVN will
not respond to core decompression and the femoral head will
eventually collapse, leading to osteoarthritis and subsequent need
for joint replacement. Small lesions (<25% of the weight-bearing
portion of the femoral head) have a better chance to remain stable,
without collapse, following core decompression.
Transient osteoporosis of the hip is another relatively frequent
bone marrow condition seen on MRI. The entity was described in
pregnant women during the third trimester of pregnancy, but is now
recognized to be more frequent in overweight middle-aged men. The
MRI findings include extensive bone-marrow edema of the femoral
head and neck, frequently associated with a joint effusion (Figure
11). The condition is generally self-limited, with clearing of the
clinical symptoms and MRI changes in 6 to 8 months. Some cases may
demonstrate a focal lesion in the subchondral area of the femoral
head once the bone marrow edema is clear. This finding suggests
that TOH and stress-related subchondral fracture are the same
entity, since excessive weight is a common denominator in these
Septic arthritis and osteomyelitis occasionally occur in the
hip. The MRI findings of septic arthritis are nonspecific. In the
early stages, a joint effusion is present. Periarticular
soft-tissue edema may also be noted (Figure 12). The characteristic
cartilage loss and surface erosions, also well-demonstrated on
plain films, follow these findings. The MRI manifestations of
osteomyelitis include bone-marrow edema and periostitis with
periarticular soft-tissue edema during the early stages, followed
by intraosseous abscess formation (Brodie's abscess). The value of
MRI is the detection of early infection, before the formation of
Among the inflammatory conditions, iliopsoas bursitis is a
relatively frequent and symptomatic entity. The iliopsoas bursa is
not normally seen on MRI since the amount of fluid it contains is
not sufficient enough to be detected. Under inflammatory
conditions, fluid accumulates in the bursa, which becomes
distended, attaining an hourglass shape, in part intrapelvic and in
part extrapelvic. This characteristic appearance is easily seen on
MRI (Figure 13).
Synovial osteochondromatosis occurs frequently in the hip joint.
MRI demonstrates the presence and location of the multiple
cartilaginous bodies within the recesses of the capsule, often
associated with an effusion and erosions of the surface of the
bone, particularly at the level of the femoral neck (Figure 14).
The differential diagnosis based on the MRI findings includes
pigmented villonodular synovitis (PVNS) since both entities can
display areas of low signal intensity within the joint capsule. In
synovial osteochondromatosis, the hypointense areas are related to
ossification of the cartilaginous bodies, whereas hemosiderin
deposition is responsible for the areas of decreased signal
intensity in various pulse sequences in PVNS (Figure 15).
Both diseases can also display surface erosions (apple core) of the
femoral neck and joint effusion.
In general, the primary diagnostic approach to bone tumors is
made by their radiographic characteristics and MRI is used for
evaluation of their intramedullary and soft-tissue extension,
articular extension, and neurovascular bundle involvement. Computed
tomography (CT) is used for assessment of cortical and periosteal
extension and detection of subtle matrix calcifications not seen on
plain films. Some tumors, tumor-like conditions, and miscellaneous
conditions have a special predilection for the proximal femur.
Frequent proximal femoral lesions include osteoid osteomas,
osteoblastoma, chondroblastoma, fibrous dysplasia, unicameral bone
cyst, chondrosarcoma, and Paget's disease. Special mention should
be made of osteoid osteomas, which are relatively frequent around
the hip. Osteoid osteomas are benign bone-forming tumors that
characteristically demonstrate cortical thickening surrounding a
nidus. These lesions are better evaluated with plain films and CT
than with MRI, since the purpose of the imaging is to locate the
small nidus prior to surgical removal or percutaneous ablation.
However, MRI may be used occasionally if the plain radiographs do
not demonstrate the lesion. Unfortunately, MRI findings may be
misleading, since these tumors produce extensive bone-marrow and
soft-tissue edema to the point that the edema may obscure the
Due to the exquisite soft-tissue contrast resolution of MR
combined with recent technologic developments, a variety of
conditions involving the hip joint and adjacent bony structures can
be well evaluated using MRI, with or without contrast material.
Among these conditions, hip trauma and AVN are the most common
indications. Other entities in which MRI has proven its usefulness
include inflammatory and infectious processes, intra-articular
lesions (such as synovial chondromatosis and pigmented villonodular
synovitis), and a variety of bone-marrow disorders.