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Magnetic resonance imaging of the knee and hip.

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Advances in the diagnosis and management of
rheumatic diseases have necessitated earlier and more
accurate noninvasive radiologic assessments. Magnetic resonance imaging (MRI), with its ability to
generate high contrast and high spatial resolution
images of the joints, muscles, ligaments, cartilage, and
synovium without the use of ionizing radiation (1,2),
has an important role in the radiologic evaluation of
joint disease. In this report, the potential rheumatologic and orthopedic applications of MRI of the knee
and hip joints are reviewed.
Technical principles
Clinical MRI is based on the behavior of hydrogen nuclei in a magnetic field (3,4). When placed in a
uniform magnetic field, hydrogen protons align their
fields and precess (spin) with the axis of the applied
magnetic field. An applied external radio frequency
pulse at the frequency of hydrogen precession or
resonance (the Larmor frequency) effects a change in
the net magnetic moment, tilting it away from alignment with the external field, typically to 90" with the
commonly used spin-echo technique. Realignment
with the magnetic field produces a radio frequency
From the Departments of Kadiology, Medicine, and Orthopaedic Surgery, University of California. San Francisco.
David W. Stoller, MD: Department of Radiology; Harry K.
Genant, MD: Departments of Radiology, Medicine, and Orthopaedic Surgery.
Address reprint requests to Harry K. Genant, MD, Department of Radiology, University of California, San Francisco, CA
Submitted for publication June 5, 1989: accepted in revised
form October 9, 1989.
Arthritis and Rheumatism, Vol. 33, No. 3 (March 1990)
signal, which when detected and recorded, is the basis
for MRI.
The T1 and T2 relaxation times characterize the
behavior of the excited hydrogen protons relative to
the external magnetic field and the interaction between
neighboring protons, respectively. The TI relaxation
time represents the return to equilibrium, after a 90"
pulse, where a greater number of hydrogen nuclei are
aligned in the direction of the main magnetic field. It is
dependent on the presence of large macromolecules
that vibrate near the Larmor frequency. Tissues with a
strong interaction between hydrogen nuclei and the
electromagnetic vibrations of macromolecules (fat tissue) exhibit a short T1 relaxation time and, thus, are
bright on a T1-weighted image. Fluids such as cerebrospinal fluid or synovial fluid, which have little
interaction between macromolecules and the protons
of water, are characterized by a long T1 relaxation
time, and display dark signal intensity on a T1weighted image.
T2 relaxation processes are responsible for
causing the hydrogen nuclei to precess out of phase
with each other. Free water molecules (not attached to
adjacent macromolecules) display long T2 relaxation
times compared with normal tissue and display bright
signal intensity on T2-weighted images. Many abnormal processes, such as neoplasia and inflammation,
produce increased free water within tissues. T2weighted pulse sequences emphasize the contrast difference between tissues based on T2 relaxation times,
and tissues with long T2 relaxation times are seen as
bright images.
By designating the TR (repetition time; the time
between 90" excitation pulses) and T E (echo time; the
time between a 90" excitation pulse and the production
Figure 1. Magnetic resonance imaging of the knee, showing the variation in signal intensity contrast of cartilage, synovial fluid, and bone with
the imaging technique used. A, TI-weighted image. B, T2-weighted image. C, T2*gradient echo image. D, Inversion recovery pulse sequence.
A conventional T1-weighted image poorly ditferentiates the fluid (black arrows) and the cartilagefluid interface (open arrows). The T2-weighted
image poorly defines the demarcation between cartilage and subarticular cortical bone (white arrows). The cartilage-fluid interface is distinct
on the T2* gradient echo image and on the inversion recovery pulse sequence (curved arrows), with the former revealing bright fluid and the
latter revealing dark fluid.
of a spin echo), TI and T2 tissue contrast can be
selectively emphasized. Short TR and TE sequences
produce T1-weighted images, while longer TR and TE
sequences generate TZweighted images. T 1-weighted
images usually demonstrate superior anatomic detail,
whereas TZweighted images enhance tissues with a
greater proportion of free or unattached water molecules, such as fluid, inflammation, edema, and tumors.
Cortical bone, fibrocartilage, tendons, and ligaments, all of which have few mobile hydrogen protons,
are imaged with low signal intensity (dark) on T1- and
T2-weighted images. Articular cartilage and hypertrophied synovium demonstrate an intermediate signal
intensity on conventional and TI- and T2-weighted
images, while fluid and edema show increased signal
intensity on TZweighted images.
MRI of the knee
MRI is rapidly replacing arthrography and computed tomography (CT) for the evaluation of internal
knee derangements and nontraumatic inflammatory
disorders of the knee (4). MRI can visualize both
hyaline and fibrocartilage and has been instrumental in
characterizing synovial-based and cartilage-based disorders (5-7).
Imaging protocols for the knee. The routine
protocol for the evaluation of internal knee derangements uses TI-weighted images in the axial, sagittal,
Figure 2. Magnetic resonance imaging of the knee of a patient with Lyme disease. A, T1-weighted image. B, T2-weighted image. Large joint
effusion and synovial irregularity are apparent (arrows).
and coronal planes (8). A variety of different pulsing
sequences for exciting protons and generating signal
have been used. In addition to spin echo, other newer
MRI techniques have been developed. These include
gradient echo and inversion recovery sequences, each
of which offers different advantages relative to the
examination time required, to the tissue contrast, and
to the signal generated (Figure I). Gradient echo or
recall techniques have become popular because of
their ability to increase the rate of data acquisition and
decrease scan times. The radio frequency excitation
pulse typically possesses a flip angle of <90".
Inversion recovery sequences are a technique
for T1-weighted imaging. The initial excitation pulse is
a 180" excitation pulse that is followed by a standard
spin-echo pulse sequence at an inversion recovery
time (TI) after the initial 180" pulse. Short TI inversion
recovery sequences have been used to eliminate or
null fat signal intensity, and are sensitive for diseases
within medullary bone because of their ability to
suppress normal surrounding medullary fat signals.
Recently developed 3-dimensional Fourier transform
techniques use volumetric image acquisition to reduce
imaging time and slice thickness, and may have future
applications in cases of arthritis and trauma.
Arthritis. General principles. Joint effusions,
sy novial proliferations, juxtaarticular cysts, osteonecrosis, and articular and fibrocartilage erosions can
be identified by MRI, even in cases where conventional radiographs reveal no abnormalities.
sequences. However, acute inflammatory synovitis
associated with increased vascularity will show increased signal intensity on TZweighted images.
Rheumatoid arthritis and juvenile chronic arthritis. Patients with RA and juvenile chronic arthritis
(JCA) have been evaluated by MRI to assess synovial
inflammation, pannus, cartilage and subchondral destruction, and the presence of synovial fluid. Joint
effusions and popliteal cysts are commonly associated
findings in patients with RA and JCA. Suprapatellar
effusions or knee joint fluid will image with low signal
intensity on TI -weighted images and with bright signal
intensity on corresponding T2-weighted images (10).
Joint fluid will demonstrate a “saddlebag” distribution
in the axial plane (Figure 1C) as the fluid tracks along
Figure 3. Magnetic resonance imaging of the knee of a rheumatoid
arthritis patient with pancompartmental disease (TI-weighted sagittal image). Advanced erosive changes are seen in the patellofemoral
joint (arrow) and on the femoral condylar surfaces.
Cartilage. Normal hyaline cartilage, which has
a higher water content, demonstrates an intermediate
signal intensity compared with the low signal intensity
of cortical bone and the fibrocartilaginous menisci
(4,5,9). While hyaline articular cartilage normally generates intermediate signal intensity on conventional
T2-weighted images, new fast scan TZweighted gradient echo techniques have been used to highlight
hyaline cartilage by producing relatively high signal
Synovium. Although normal intact synovium is
not visible on MRI, early synovial proliferation is
imaged as a change in the contour of synovial surfaces
(4,5,9). The irregular infrapatellar synovial lining seen
with a variety of synovial inflammatory processes is
characterized by irregularity, with loss of the smooth
posterior concave free border of the infrapatellar fat
pad. Patients with hemophilia, rheumatoid arthritis
(RA), pigmented villonodular synovitis, Lyme disease, inflammatory osteoarthritis, and hemorrhagic
effusions have demonstrated synovial hypertrophy
(lC1’) (Figure 2)’ Fibrous
pannusy and a
thickened synovium will demonstrate low to intermediate signal intensity on either TI- or TZweighted
Figure 4. Magnetic resonance imaging of the knee of a patient with
pigmented villonodular synovitis (T2-weighted). Large effusion and
prominent synovial masses in the suprapatellar and posterior capsular regions are seen (arrows).
the medial and lateral extensions into the suprapatellar
In RA, bicompartmental and tricompartmental
disease is displayed by MRI on midcoronal images and
on axial sections through the patellofemoral joint.
Femoral and tibial loss of articular cartilage and low
signal intensity subchondral erosions are easily identified on sagittal MRI (4,ll) (Figure 3).
The initial presentation of patients with JCA
(11) and RA is often characterized by early synovitis,
with an irregular infrapatellar fat pad. Before the joint
space narrowing becomes identifiable by conventional
radiography, the focal articular cartilage erosions and
synovial hypertrophy can be identified by MRI. Synovial thickening of the suprapatellar bursa can be
visualized with low signal intensity on T1- and T2weighted images. MRI can detect subarticular cysts
Figure 6. Magnetic resonance imaging of the knee of a patient with
synovial osteochondromatosis (Tl-weighted sagittal image). Multiple low signal intensity, synovial-based osteochondral fragments are
evident (arrows).
Figure 5. Magnetic resonance imaging of the knee of a patient with
osteoarthritis. Marginal osteophytes and low signal intensity subchondral degeneration (sclerosis) (black arrows) in the medial femoral condyle and tibial plateau are seen in association with a
posterior horn medial meniscus tear (white arrow).
and subchondral sclerosis on both the femoral and
tibial surfaces. These findings frequently are not evident on conventional radiographs. Patients with JCA
have demonstrated hypoplastic menisci, a finding that
may be related to an alteration in the fluid composition
of synovial fluid and may impair normal fibrocartilage
Pigmented villonodular synovitis. Pigmented villonodular synovitis is a monarticular synovial proliferative disorder that usually causes nonpainful soft
tissue swelling. The knee joint is the most frequent site
of involvement. Hyperplastic synovial masses contain
deposits of hemosiderin-laden macrophages. The
hemosiderin-infiltrated synovial masses image with
low signal intensity on T1- and TZweighted images,
secondary to the paramagnetic effect of iron (12).
However, associated synovial fluid is shown as bright
signal intensity on TZweighted images (Figure 4).
Osteoarthritis. MRI findings in osteoarthritis
(OA) are characterized by osteophytic spurring, compartment collapse, subchondral cysts, and decreased
marrow signal intensity in areas of subchondral sclerosis (7,9,16) (
~5 ) . ~i~~~~
~ of hyaline
cartilage surfaces gives MRI an advantage over conventional radiography in preoperative planning for
joint arthroplasty procedures. In more advanced de-
sity that is attributed to imbibed synovial fluid (8)
(Figure 8). Several systems have been developed for
assessing meniscal pathology. These are based on the
distribution of focal increased signal intensity within
the otherwise low signal meniscus, relative to its
extension to an articular surface (exclusive of the
peripheral capsular margin of the meniscus) (8,18).
Meniscal tears may communicate or decompress into meniscal cysts, which are collections of
mucinous or synovial fluid traceable to the joint line.
These cysts are imaged with uniform low signal intensity on T1-weighted images and with high signal intensity on T2-weighted images.
MRI of the hip
Because of its excellent spatial and contrast
resolution characteristics, MRI facilitates the early
detection and evaluation of abnormalities of the femoral head in osteonecrosis and of the hyaline articular
Figure 7. Magnetic resonance imaging of the knee of a patient with
chondromalacia patellae (TI-weighted axial image), showing eroded
and attenuated articular cartilage (arrows).
generative disease, free intraarticular fragments may
be imaged with the high signal intensity of marrow fat.
Multiple synovial-based chondral fragments visualized with low to intermediate signal intensity characterize synovial osteochondromatosis (Figure 6).
These metaplastic fragments are generally of similar
size in primary chondromatosis and a variety of sizes
in secondary chondromatosis.
In chondromalacia patellae, axial MRI shows
early cartilage attentuation and erosions. Sagittal
MRI, which is less sensitive to cartilage erosions, may
show a straightening or loss of the normal convex
curve seen in patellar hyaline cartilage when viewed
in profile (4,17) (Figure 7). TZweighted images are
useful for demonstrating decreased signals from patellar cartilage in areas of focal degeneration. Sclerosis,
imaged as subchondral low signals, may be associated
with irregular surface erosions. Patellar subchondral
cysts are sometimes visualized in the early stages
of patellar cartilage fibrillation, preceding cartilage
and tears of the meniscus show increased signal inten-
effusion (large arrow).
cartilage and capsular structures in arthritis (4). As a
result, a variety of traumatic and degenerative processes can be thoroughly assessed.
Imaging techniques for the hip. MRI of the hip
uses the body coil and large fields of view to allow
comparison of the hips. T1-weighted images are acquired in axial, sagittal, or coronal imaging planes,
generally using 5 mm-thick sections. Three millimeterthick sections are preferred in pediatric patients or
when precise assessments are required to image the
thin articular cartilage surfaces and the labrum. T2weighted images are especially useful when evaluating
arthritis, infection, and neoplasia (1,2,4,10).
Rheumatoid arthritis and juvenile chronic arthritis. Although MRI of the hip has thus far been used
only to a limited extent in patients with RA, the
technique affords the evaluation of soft tissues, pannus, erosions, and effusions, as well as articular cartilage congruity (Figure 9).
In JCA, MR1 has demonstrated irregularities of
the femoral capital epiphyses and growth plate, as well
as osseous erosions that were underestimated on conventional radiographs ( I 1). Sagittal images separately
define the femoral and acetabular cartilage. Synovial
fluid-filled cystic changes demonstrate increased signal
intensity on T2-weighted images. Focal thinning of the
hyaline articular cartilage may be identified on coronal
and sagittal images, prior to radiographic evidence
of joint space narrowing. Synovial hyperthrophy is
visualized as masses of low to intermediate signal
Osteoarthritis. Changes characteristic of OA
can be demonstrated in all orthogonal planes (4). Prior
to evidence of subchondral sclerosis by conventional
radiography, thickened trabeculae are imaged as regions of low signal intensity on T1- and T2-weighted
images. Subchondral cystic changes, with or without
joint space narrowing and osteophytosis, are frequently identified by MRI.
Osteonecrosis. The accuracy of MRI in the
detection of osteonecrosis (ON) of the hip has been
reported to exceed that of CT and radionuclide bone
scintigraphy (19-21). MRI techniques are also effective in assessing associated joint effusions, marrow
conversion, edema, and articular cartilage congruity.
This is not possible with conventional radiography,
bone scintigraphy, or CT.
MRI classification systems have been based on
region Of MR1
intensity in the
osteonecrotic focus (19,221. In the early stages of ON,
the osteonecrotic focus has shown imaging character-
Figure 9. Magnetic resonance imaging of the left hip of a patient with
advanced rheumatoid arthritis. A, TI-weighted coronal image, showing
cystic erosions (arrnw) and uniform joint space narrowing. B, TIweighted and C, T2-weighted axial images, demonstrating increased
signal intensity of fluid (straight arrow), while fibrous pannous remains
at a low to intermediate signal intensity (curved arrow).
Figure 10. Magnetic resonance imaging of the hips, showing bilateral osteonecrosis of the femoral heads. A, T1-weighted axial image,
demonstrating more advanced disease and associated joint effusion of the right femoral head (arrows). B, Tl-weighted sagittal image of the right
hip, showing the anterior to posterior extent of involvement.
istics of normal fat and blood constituents (intermediate to high signal intensity) on Tl- and TZweighted
images (19). In the later stages of ON, fibrous tissue is
identified with low signal intensity on T1- and/or
TZweighted images (Figure 10).
The majority of osteonecrotic lesions seen on
TZweighted images demonstrate an inner border of
high signal intensity, paralleling the low signal intensity of the periphery. The central focus of normal
signal intensity in ON corresponds to necrosis of bone
and marrow, prior to development of capillary and
mesenchymal ingrowth (23). The peripheral band of
low signal intensity corresponds to a sclerotic margin
of reactive tissue at the interface between necrotic and
viable bone. Decreased or low signal intensity on
T1-weighted images and intermediate to high signal
intensity on T2-weighted images can be attributed to
the high water content of mesenchymal tissue and the
thickened trabecular bone. The hyperemic or inflammatory response with granulation tissue inside the
reactive bone interface is believed to represent the
source of the high signal.
In children with Legg-CalvC-Perthes disease,
MRI demonstrates the low signal intensity focus of
osteochrondrosis in the osseous femoral epiphysis and
defines the integrity of the cartilaginous surface.
MRI has evolved rapidly and remarkably in
recent years, with major improvements gained in
spatial and contrast resolution, and in imaging time.
The potential of this technology in selected rheumatologic and orthopedic applications has been reviewed in
the context of imaging of the knee and hip. While
certain applications of MRI, such as the detection of
ligamentous and meniscal tears of the knee and the
assessment of osteonecrosis of the hip, are now well
established, its ultimate clinical role in the evaluation
of inflammatory and degenerative joint disease remains to be determined. The many unique imaging
characteristics offered by MRI provide a broad array
of important and exciting musculoskeletal and articular applications to be explored and further delineated.
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