IMAGING MODALITIES IN ORTHOPAEDICS

Magnification radiography is occasionally used to
enhance the bony details not well delineated on the standard
radiographic projections. This technique involves a special screen–film
system and geometric enlargement that yields magnified images of the
bones and joints with increased sharpness and bony detail (28).
The technique is particularly effective for demonstrating early changes
in some arthritides and metabolic disorders. It may be useful for
demonstrating subtle fracture lines not seen on routine projections.

Stress views are important in the evaluation of
ligamentous tears and joint stability. In the hand, obtain
abduction-stress film of the thumb if gamekeeper’s thumb, resulting
from disruption of the ulnar collateral ligament of the first
metacarpophalangeal joint, is suspected. In the lower extremity, the
stress views of the knee and ankle joints are frequently obtained.
Evaluation of knee instability due to ligament injuries may require
this technique in cases of suspected tear of the medial collateral
ligament, and less frequently for evaluating insufficiency of the
anterior and posterior cruciate ligaments.

To obtain a stress film of the knee joint for evaluation
of the medial collateral ligament, position the patient supine with the
knee flexed about 15° to 20°. With the thigh fixed, apply pressure
against the medial aspect of the leg below the knee, forcing the knee
into valgus. The films thus obtained show the anteroposterior
projection. To obtain a stress film of the knee joint for evaluation of
the anterior cruciate ligament, position the patient on his side, with
the knee flexed 90°. Apply pressure against the posterior aspect of the
upper leg (anterior-drawer stress). The films show the lateral
projection.

Evaluation of ankle ligaments also requires stress radiography. Inversion (adduction) (Fig. 4.5) and anterior-drawer

stress films are most frequently obtained; only rarely is an eversion
(abduction) stress examination required. To obtain an inversion stress
film of the ankle joint, position the patient supine with the lower leg
fixed, and apply pressure on the lateral aspect of the foot, adducting
the heel and forcing the foot into varus position. For anterior-drawer
stress examination, position the patienton her side with the lower leg
fixed, and apply posterior stress to the heel. The films show the
lateral projection.

Scanogram is the most widely used method for measurement
of limb lengths. This technique requires a slit-beam diaphragm with a
1.5 mm opening attached to the radiographic tube, and a long film
cassette. The radiographic tube can move in the long axis of the
radiographic table. During the exposure, the tube traverses the whole
length of the film, scanning the entire extremity. This technique
allows the x-ray beam to intersect the bone ends perpendicularly, and
limb lengths can be compared (4,52).
A modified technique may be used with three separate exposures over the
hip joints, knees, and ankles if a motorized radiographic tube is not
available. In this technique, an opaque tape measure is placed
longitudinally down the center of the radiographic table (4).

Occasionally, a so-called orthoroentgenogram is
indicated. For this technique, position the patient supine with the
lower limbs on a 3-ft-long cassette and a long ruler at one side. Make
a single exposure, centered at the knees and including the entire
length of both limbs and the ruler.

Fluoroscopy is a fundamental diagnostic tool for many
radiologic procedures, including arthrography, tenography, bursography,
arteriography, and percutaneous bone or soft-tissue biopsy. Fluoroscopy
combined with videotaping is useful in the evaluation of the kinematics
of joints. Because it delivers a high dose of radiation, it is used
only occasionally (e.g., in evaluating joint movement and detecting
transient subluxation). It occasionally is used in follow-up
examination of the healing process after fractures to evaluate the
degree of bone union. Fluoroscopy is still used in conjunction with
myelography if it is important

Digital (computed) radiography is the process of digital
image acquisition using an x-ray detector. The detector comprises a
photostimulable phosphor imaging plate and image reader/writer that
processes the latent image information for laser printing on film. A
major advantage of computed radiography over conventional film and
screen radiography is that, once acquired, the digital image data are
readily manipulated to produce alternative renderings. In addition,
energy subtraction imaging may be acquired: Two images, acquired either
sequentially or simultaneously with different filtration, are used to
reconstruct a soft tissue–only image or a bone-only image.

Digital subtraction radiography uses a video processor
and a digital disk added to a fluoroscopy imaging system to provide
on-line viewing of subtraction images (23).
This technique is most widely used to evaluate the vascular system; it
is occasionally used in conjunction with arthrography. Use of
high-performance video cameras with low-noise characteristics allows a
single video frame of precontrast and postcontrast images to be used
for subtraction. Spatial resolution can be maximized using a
combination of geometric magnification, electric magnification, and a
small anode-to-target distance. The subtraction technique removes
surrounding anatomic structures and isolates the opacified vessel or
joint, making it more conspicuous.

Use nonvascular digital radiography to evaluate bone
abnormalities and, in conjunction with contrast injection (i.e.,
digital arthrography), to evaluate subtle abnormalities of the joints,
such as tears of the lunate-triquetral ligament or the abnormalities of
the triangular fibrocartilage in the wrist (Fig. 4.6).
Use it also to evaluate the stability of a prosthesis. Digital
radiography offers improved image quality, contrast sensitivity, and
exposure latitude. It is also an efficient mode in which to store,
retrieve, and transmit radiographic image data. Digital images can be
displayed on film or on a monitor. A significant advantage of image
digitization is its ability to produce data with low noise and a wide
dynamic range. Image digitization is suitable for window-level analysis
in a manner comparable to that employed in a CT scanner (56).

Choose digital subtraction angiography (DSA), the most
frequently used variant of digital radiography, to evaluate trauma and
tumors of the bone and soft tissue, and to view the vascular system. In
trauma to the extremity, DSA is effectively used to evaluate arterial
occlusion, pseudoaneurysms, arteriovenous fistulas, and transection of
the arteries (30). The advantage of DSA over
conventional film techniques is the rapidity of the study with the
ability to obtain multiple repeated projections. Bone subtraction is
useful in clearly delineating the vascular structures. In evaluating
bone and soft-tissue tumors, DSA is an effective tool for mapping tumor
vascularity.

Tomography is body-section radiography that enhances
visualization of lesions too small to be seen on conventional
radiographs. It is also effective for demonstrating anatomic detail
that is obscured by overlying structures. It employs continuous motion
of the radiographic tube and film cassette in opposite directions
throughout exposure, with the fulcrum of the motion located in the
plane of interest. The technique blurs structures above and below the
area being examined, thereby sharply outlining the object to be studied
on a single plane of focus. The newly developed tomographic units can
better localize the image and can detect lesions as small as 1 mm in
diameter.

The simplest tomographic movement is linear, with the
radiographic tube and film cassette moving on a straight line in
opposite directions. This linear movement has little application in the
study of bones because it creates streaks that often interfere with
radiologic interpretation. Resolution of the plane of focus, much
clearer if there is uniform blurring of undesired structures, requires
a multidirectional movement, as in zonography or circular tomography.
In this type of tomography, the radiographic tube makes one circular
motion at a preset angle of inclination. More complex multidirectional
hypocycloidal or trispiral movements increase the distance of excursion
of the tube and create a varying angle of projection of the x-ray beam
during exposure. These complex movements are advantageous because they
produce even greater blurring outside the field of interest and yield
the sharpest focal plane images.

Trispiral tomography is superior to conventional radiographs in the visualization of subtle fractures (35). It is helpful for delineating the extent of the fracture line (Fig. 4.7), evaluating the healing process, and evaluating nonunions (51).
It is also useful in evaluating tumor and tumorlike lesions (e.g., the
nidus of an osteoid osteoma, a calcific matrix in an enchondroma or
chondrosarcoma). Small cystic and sclerotic lesions and subtle erosions
are well demonstrated. Tomograms are better interpreted with plain
radiographs for comparison.

Computed tomography is an indispensable modality in the
evaluation of many traumatic conditions and various bone and
soft-tissue tumors because of its cross-sectional imaging capability.
In trauma, CT is extremely useful because of its abilities to define
the presence and extent of fracture or dislocation, to evaluate such
intra-articular abnormalities as damage to the articular cartilage or
presence of noncalcified and calcified osteocartilaginous bodies, and
to evaluate adjacent soft tissues. CT is important in the detection of
small bony fragments displaced into the joints after trauma, in the
detection of the small displaced fragments of the fractured vertebral
body (Fig. 4.8), and in the assessment of concomitant injury to the cord or thecal sac (14).

Advantages of CT over conventional radiography are its ability to provide excellent contrast resolution, accurate

measurement of the tissue attenuation coefficient, and direct
transaxial images. Another advantage is its ability to image the bone
in coronal, sagittal, and oblique planes, using reformation technique.
This multiplanar reconstruction is particularly helpful in evaluating
vertebral alignment (Fig. 4.9),
demonstrating horizontally oriented fractures of the vertebral body,
and evaluating complex fractures of the pelvis or calcaneus,
abnormalities of the sacrum and sacroiliac joints, sternum,
sternoclavicular joints, temporomandibular joints, and wrist.

Modern CT scanners employ collimated fan beams directed
only at the tissue layer under investigation. Recently developed spiral
(helical) scanning permits three-dimensional reconstruction for
analysis of regions with complex anatomy, such as the face, pelvis,
vertebral column, wrist, foot, and ankle (Fig. 4.10) (11,27,42).
The CT data may be used to create three-dimensional plastic models of
the area of interest, which can facilitate operative planning and allow
rehearsal of complex reconstructive procedures.

Although CT by itself is rarely helpful in making a
specific diagnosis of bone and soft-tissue tumors, it can provide a
precise evaluation of the extent of the bone lesion and may demonstrate
a break through the cortex and the involvement of surrounding soft
tissues (Fig. 4.11). Moreover, CT is helpful
for delineating a tumor in bones having complex anatomic structures,
such as the scapula, pelvis, and sacrum, which may be difficult to
image fully with conventional radiographic techniques or even
conventional tomography. CT examination is crucial in determining the
extent and spread of a tumor in the bone and surrounding soft tissues
if limb salvage is contemplated, allowing a safe margin of resection to
be planned. It is also useful for monitoring the results of treatment
and evaluating recurrence of resected tumor.

Computed tomography is an important modality for

successful aspiration or biopsy of bone or soft-tissue lesions because
it provides visual guidance for precise placement of biopsy needles
within the lesion.

The ability of CT to measure the attenuation
coefficients of each pixel provides a basis for accurate quantitative
bone mineral analysis in cancellous and cortical bone (8,26).
Evaluation of bone mass measurement underscores continuing progress in
our understanding of bone metabolism and provides valuable insight into
improvement of evaluation and treatment of osteoporosis and other
metabolic bone disorders (55).

Disadvantages of CT include the so-called average volume
effect, which results from insufficient resolution to describe
inhomogeneities of the tissue. Because of this, the Hounsfield’s units
represent average values of the different components of the tissue.
This effect becomes particularly important if normal and pathologic
processes interface within a section under investigation.

Another disadvantage of CT is poor tissue
characterization. Despite the ability of CT to discriminate some
density differences, a simple analysis of attenuation values does not
permit precise histologic characterization. Moreover, if the patient
moves at all, the result is artifacts that degrade the image quality.
Similarly, metal (e.g., prosthesis, rods, screws) produces significant
artifacts. Finally, the radiation dose may occasionally be high,
particularly if contiguous and overlapping sections are obtained during
an examination.

Arthrography is the introduction of a contrast agent into the joint space. Positive contrast describes iodide solution, and negative
contrast refers to air. Despite the evolution of newer diagnostic
imaging modalities, such as CT and MRI, arthrography has retained its
importance in daily radiologic practice (25). Arthrography is technically simple, and interpretation is much easier than interpretation of ultrasound, CT, or MRI (33).
Although virtually every joint can be injected with contrast,
arthrography is most frequently performed in the shoulder, wrist,
ankle, and elbow.

It is important to obtain preliminary films before any
arthrographic procedure, because contrast may obscure some joint
abnormalities (e.g., osteochondral body) that can be easily detected on
the plain film. Arthrography is particularly effective in demonstrating
rotator cuff tear (Fig. 4.12) and adhesive
capsulitis in the shoulder, osteochondritis dissecans, osteochondral
bodies, and subtle abnormalities of the articular cartilage in the
elbow joint. In the wrist, it is still a very effective procedure for
evaluating abnormalities of the triangular fibrocartilage complex (53).
The introduction of three-compartment injection techniques combined
with digital technique and postarthrographic CT examination has made
this modality almost always the procedure of choice in evaluating a
painful wrist (40).

Although arthrography of the knee has been almost

completely replaced by MRI, it still may be used to demonstrate
injuries to the soft-tissue structures, such as the joint capsule,
menisci, and various ligaments (Fig. 4.13).
It also provides important information on the status of the articular
cartilage, particularly if the surgeon suspects subtle chondral or
osteochondral fracture or requires confirmation of osteochondral bodies
(e.g., osteochondritis dissecans).

In the examination of any joint, arthrography can be
combined with tomography (i.e., arthrotomography), with CT (i.e.,
arthro-CT), or with digitization of an image (i.e., digital subtraction
arthrography) to provide additional information (Fig. 4.6) (43).

There are relatively few absolute contraindications to
arthrography. Even hypersensitivity to iodine is only a relative
contraindication, because a single contrast study using only air can be
performed.

Contrast material is occasionally injected into the
tendon sheath to evaluate the integrity of a tendon, in a procedure
known as a tenogram. CT and MRI have superseded this procedure, but it
is still used, mainly to evaluate traumatic or inflammatory condition
of tendons of the lower extremity, such as peroneus longus and brevis (Fig. 4.14),
tibialis anterior and posterior, and flexor digitorum longus, and to
outline the synovial sheaths within the carpal tunnel in the upper
extremity.

Bursography involves the injection of contrast agent
into bursae. This procedure has mostly been abandoned, and only
occasionally the subacromial-subdeltoid bursae complex is directly
injected with contrast agent to demonstrate partial tears of the
rotator cuff.

In arteriography, contrast agent is injected into the
arteries and films are obtained, usually in a rapid sequence. In
venography, contrast is injected into the veins. The technique is used
most frequently to detect vascular trauma.

In evaluating tumors, arteriography is used mainly to
map bone lesions, to demonstrate the vascularity of the lesion, and to
assess the extent of disease (Fig. 4.15). It is
also used to demonstrate the vascular supply of a tumor and to locate
vessels suitable for preoperative intraarterial chemotherapy. It is
useful in demonstrating the area suitable for open biopsy, because most
vascular parts of a tumor contain the most aggressive component of the
lesion. Occasionally, arteriography can be used to demonstrate abnormal
tumor vessels, corroborating findings with plain film radiography and
tomography. Arteriography is often extremely helpful in planning
limb-salvage procedures by demonstrating the regional vascular anatomy
and permitting a plan to be drawn for the resection procedure. It is
sometimes used to outline the major vessels before resection of a
benign tumor. It can be combined with an interventional procedure, such
as embolization of hypervascular tumors, before surgery. It also plays
a major role in assessing the effectiveness of chemotherapy in the
treatment of malignant bone and soft-tissue tumors.

During myelography, water-soluble contrast agents are
injected into the subarachnoid space, mixing freely with the cerebral
spinal fluid to produce a column of opacified fluid with a higher
specific gravity than the clear fluid. Tilting the patient allows the
opacified fluid to run up or down the thecal sac under the influence of
gravity. The puncture usually is done in the lumbar area, at L2–L3 or
L3–L4 levels. For examination of the cervical area, a C1–C2 puncture is
performed. Myelographic examination has been almost completely replaced
by high-resolution CT and high-quality MRI.

Discography is an injection of contrast material into
the nucleus pulposus. Although many surgeons have abandoned the use of
this controversial procedure, under tightly restricted indications and
immaculate technique a discogram can yield valuable information (29,59).
Discography is a valuable aid for determining the source of a patient’s
low-back pain. It is not a pure imaging technique, because the symptoms
produced during the test (e.g., pain during the injection) are
considered to have even greater diagnostic value than the radiographs
obtained (66). Always combine it with CT examination (Fig. 4.16).
According to the official position statement on discography by the
Executive Committee of the North American Spine Society in 1988, this
procedure “is indicated in the evaluation of patients with unremitting
spinal pain, with or without extremity pain, of greater than 4 months
duration, when the pain has been unresponsive to all appropriate
methods of conservative therapy” (21).
According to the same statement, before discography is performed, the
patient should have undergone investigation with other modalities
(e.g., CT, MRI, myelography), and surgical correction of the patient’s
problem should be anticipated.

Ultrasound is a safe, noninvasive modality that relies
on the interaction of propagated sound waves with tissue interfaces in
the body. Whenever the directed pulsing of sound waves encounters an
interface between tissues of different acoustic impedance, reflection
or refraction occurs, which is then recorded with a sensitive
transducer

and
converted into images. An advantage of this technique is that, because
it does not use ionized radiation, it is safe to use on pregnant women
and infants. Modern probe technology, development of real-time imaging,
and color Doppler imaging have extended to some degree the use of
ultrasound in orthopaedic radiology. Higher-frequency transducers of
7.5 and 10 MHz have excellent spatial resolution and are ideal for
imaging of the appendicular skeleton. Some applications of ultrasound
in orthopaedics include evaluation of the rotator cuff and injured
tendons (e.g., Achilles tendon) (7,24,60,64). Ultrasound is occasionally used during surgery to assess reduction of spinal fractures (65). It is also used to image soft-tissue tumors (e.g., hemangioma) (15).

The most effective application, however, is in
evaluation of the infant hip. In this case, ultrasound has become the
imaging modality of choice, because of the hip’s cartilaginous
composition, ultrasound’s real-time capability for studying motion and
stress, the absence of ionizing radiation, and the relative cost
effectiveness. The newest development in this area is the introduction
of three-dimensional ultrasound for evaluation of developmental
dysplasia of the hip. Three-dimensional sonography provides functional
utility in the evaluation of the joint in the added sagittal plane
(section image) and craniocaudal projection (revolving spatial image).
This technique permits excellent demonstration of the femoral
head–ace-tabulum relationship and femoral head containment.

Sonography has been applied to certain areas in
rheumatic disorders, in particular to differentiation of popliteal
fossa masses (e.g., aneurysm, Baker’s cyst, hypertrophied synovium).
More recent ultrasound techniques such as Doppler ultrasound or
colorflow imaging, which expresses motion from moving red blood cells
in color, have found limited applications in orthopaedic radiology.
This modality is used mainly to detect arterial narrowing and venous
thrombosis. Also, there have been a limited number of reports regarding
the use of this technology in measuring tumor vascularity within soft
tissue masses.

One of the major advantages of skeletal scintigraphy
(i.e., radionuclide bone scan) over all other imaging techniques is its
ability to image the entire skeleton at once. A bone scan can confirm
the presence of the disease, demonstrate the distribution of a lesion (Fig. 4.17),
and help to evaluate the activity of a pathologic process. Indications
for skeletal scintigraphy include traumatic conditions, tumors (primary
and metastatic), arthritides, infections, and metabolic bone disease.
The detected abnormality may consist of decreased uptake of
bone-seeking radiopharmaceutical (e.g., early stage of osteonecrosis)
or increased uptake (e.g., fracture, neoplasm, focus of osteomyelitis).
Some

structures under normal conditions may show increased activity (e.g., sacroiliac joints or normal growth plates).

Scintigraphy is a sensitive imaging modality, but it
lacks specificity. It is impossible to differentiate between various
processes that can cause increased uptake. Occasionally, however, the
bone scan may yield very specific information and even suggest the
diagnosis, as in multiple myeloma or osteoid osteoma. In the search for
myeloma, scintigraphy is helpful for differentiating bony metastases.
In most cases of myeloma, there is no significant increase in the
uptake of the radiopharmaceutical; in skeletal metastases, however,
invariably the uptake of the tracer is significantly elevated. In
osteoid osteoma, there is a typical appearance of the bone scan that
demonstrates a “double-density sign.” There is more increased uptake in
the center, due to the nidus, and less increased uptake at the
periphery, related to the reactive sclerosis surrounding the nidus.

Radionuclide bone scan is an indicator of mineral
turnover; because there is usually an enhanced deposition of
bone-seeking radiopharmaceuticals in areas of bone undergoing change
and repair, a bone scan is useful in localizing tumors and tumor-like
lesions in the skeleton. This is particularly helpful in fibrous
dysplasia, eosinophilic granuloma, or metastatic cancer, in which more
than one lesion develop. It also plays an important role in localizing
small lesions, such as osteoid osteoma, which may not always be seen on
the plain film studies. In most instances, radionuclide bone scan
cannot differentiate benign lesions from malignant tumors, because both
conditions have an increased blood flow with resultant increased
isotope deposition and increased osteoblastic activity. It may,
however, permit differentiation of benign lesions, such as bone
islands, that usually do not absorb the radioactive isotope.

In traumatic conditions, scintigraphy is extremely
helpful in the early diagnosis of stress fractures, which may not be
seen on conventional radiographs or tomographic studies. It also has
value in diagnosing fractures of the scaphoid or the femoral neck in
elderly patients if the routine radiographic examinations appear normal
(13,39).

In metabolic bone disorders, bone scintigraphy is
helpful in establishing the extent of skeletal involvement in Paget’s
disease and assessing the response to treatment. Although it is of no
use in the evaluation of patients with generalized osteoporosis, it may
occasionally be helpful in differentiating osteoporosis from
osteomalacia and in differentiating multiple vertebral fractures
resulting from osteoporosis from those occurring in metastatic
carcinoma.

Skeletal scintigraphy is frequently used to evaluate infections. In particular, ^99mtechnetium–methylene diphosphonate (^99mTc-MDP) and ¹¹¹indium (¹¹¹In) are highly sensitive in detecting early and occult osteomyelitis (2). For detecting recurrent active infection in patients with chronic osteomyelitis, ¹¹¹In-labeled leukocytes appear to be the tracer of choice. It must be stressed, however, that ¹¹¹In-labeled
leukocytes accumulate in active bone marrow, and this tendency limits
their sensitivity for the detection of chronic osteomyelitis. To
improve the diagnostic ability of this technique, some advocate a study
that combines ^99mTc-sulfur colloid bone marrow with ¹¹¹In-labeled leukocytes. In chronic osteomyelitis, imaging with ⁶⁷gallium (⁶⁷Ga) citrate is more accurate in detecting the response or lack of response to treatment than ^99mTc-phosphate
bone imaging. A three-phase technique can be effectively used to
differentiate soft-tissue infection (cellulitis) and osseous infection
(osteomyelitis).

In neoplastic conditions, the detection of skeletal
metastasis is probably the most common indication for skeletal
scintigraphy. It also is frequently used to determine the extent of a
lesion or the presence of skip lesions or intraosseous metastases. It
is not the method of choice to determine the extent of the lesion in
the bone. Scintigraphy alone cannot diagnose the type of the tumor, but
it may

be useful in detecting and localizing some primary tumors and multifocal lesions (e.g., multicentric osteosarcoma).

^99mTc-MDP scans are used primarily to
determine whether a lesion is monostotic or polyostotic, information
essential in the staging of a bone tumor. Although the degree of
abnormal uptake may be related to the aggressiveness of the lesion,
this finding does not correlate well with histologic grade. Gallium-67
may be taken up in a soft-tissue sarcoma and may help to differentiate
a sarcoma from a benign soft-tissue lesion.

Although a bone scan may be useful in demonstrating the
extent of the primary malignant tumor in bone, it is not as accurate as
CT or MRI. It may be useful in the detection of local recurrence of
tumor and occasionally may indicate the response or lack of response to
radiotherapy or chemotherapy.

In the evaluation of arthritides, a bone scan can
demonstrate the distribution of the lesion in the skeleton; this method
has completely replaced the previously used radiographic joint survey.
Scintigraphy can determine the distribution of arthritic changes in
large and small joints and in areas usually not detected by standard
radiography, such as the sternomanubrial and temporomandibular joints (38).

Several bone-seeking tracers are available for scintigraphic imaging (44).

Pathologic processes produce significantly different,
characteristic, nonuniform distribution of administered
radiopharmaceutical tracers. Imaging the distributions yields
information about the functional status of bony tissues. A perfect
image of such an object would be an exact, three-dimensional replica of
the distribution of activity within

the area of interest (6).
This goal has been partially achieved through the tomoscanner and dual
Anger-type scintillation cameras operated in the conjugate counting
mode, or single photon emission computed tomography (SPECT).

The recent development of single photon emission
tomography (SPET) and SPECT has tremendously increased diagnostic
precision in evaluating bone and joint abnormalities. The efficiency of
instrumentation for SPECT is improving with multiple crystal detectors,
fan beam and cone beam collimators, detection of a greater fraction of
photons, and improved algorithms. Compared with planar images, SPECT
provides increased contrast resolution, using a tomographic mode that
eliminates the noise from tissue outside the plane of imaging; in this
regard, it is similar to conventional tomography. It provides
quantitative and qualitative information on the uptake of bone-seeking
radiopharmaceuticals.

Magnetic resonance imaging is based on the reemission of
an absorbed radiofrequency signal while the patient is in a strong
magnetic field (31). An external magnetic field
is usually generated by a magnet with field strengths of 0.2 to 1.5
tesla. The system includes a magnet, radiofrequency coils (transmitter
and receiver), gradient coils, and a computer display unit with digital
storage facilities.

The ability of MR to image body parts depends on the
intrinsic spin of atomic nuclei with an odd number of protons or
neutrons (e.g., hydrogen), which generates a magnetic moment. Nuclei of
tissues placed within the main magnetic field tend to align along the
direction of that field. Application of radiofrequency (rf) pulses
induces resonance of particular sets of nuclei. The required frequency
of the pulse is determined by the strength of the magnetic field and
the nucleus under investigation. The energy absorbed during the
transition from the induced high-energy state to the normal low-energy
state is released; this can be recorded as an electrical signal, which
provides the data from which digital images are derived (58).

Two relaxation times, T1 and T2, are employed. The T1
relaxation time is a term used to describe the return of protons to
equilibrium after application and removal of the rf pulse. T2
relaxation time is a term used to describe the associated loss of
coherence or phase between individual protons immediately after the
application of rf pulse. A variety of rf pulse sequences can be used to
enhance the differences in tissue relaxation times T1 and T2, providing
the necessary image contrast.

The most commonly used sequences are partial saturation
recovery, inversion recovery, and fast scan technique. Inversion
recovery sequences can be combined with multiplanar imaging to shorten
scan time. Spin echo short pulse repetition (TR ≤ 0.5 sec) and short
echo delay (TE ≤ 40 msec) sequences (or T1) provide good anatomic
detail. Long TR (≥1.5 sec) and long TE (≥≥90 msec) pulse sequences (or
T2) provide good contrast, sufficient for evaluation of pathologic
processes. With a short inversion time in the range of 100 to 150 msec,
the effects of prolonged T1 and T2 relaxation times are cumulative, and
the signal from fat is suppressed. This technique is short time
inversion recovery (STIR), and it has been useful for evaluating bone
tumors (3).

Fast imaging techniques have become increasingly popular
because they provide advantages compared with much slower spin echo
imaging. In particular, gradient recalled echo (GRE) pulse sequences
using variable flip angles (5° to 90°) have gained rapid acceptance in
orthopaedic radiology because they represent the most effective means
of performing fast MR imaging.

Several different types of GRE methods in clinical use,
each of which relies on using a reduced flip angle to enhance the
signal with short TR. These techniques are known by a variety of
acronyms such as FLASH (fast low-angle shot), FISP (fast imaging with
steady procession), GRASS (gradient-recalled acquisition), and MPGR
(multiplanar gradient recalled).

In most examinations, obtain at least two planes (axial
and coronal or sagittal); for many injuries, all three planes are
necessary. For adequate MR imaging, surface coils are necessary because
they provide improved spatial resolution. Most surface coils are
designed specifically for different areas of the body, such as knee,
shoulder, wrist, and temporomandibular joints.

The musculoskeletal system is ideally suited for
evaluation by MRI because different tissues display different signal
intensity on T1- and T2-weighted images. The images displayed may have
a high, intermediate, or low signal intensity. High signal intensity of
fat planes and differences in signal intensity of various structures
allow separation of the different tissue components. Fat, bone marrow,
and hematoma display relatively high signal intensity on T1- and
T2-weighted images; cortical bone, air, ligaments, tendons, and
fibrocartilage display low signal intensity on T1- and T2-weighted
images; muscle, nerves, and hyaline cartilage display intermediate
signal intensity on T1- and T2-weighted images. Most tumors display low
to intermediate signal intensity on T1-weighted images, and high signal
intensity on T2-weighted images.

The use of MRI in orthopaedic radiology is confined primarily to trauma and tumors.