1. health and fitness
2. disease

Imaging of Lumbar Degenerative Disc Disease
John A. Carrino and William B. Morrison
A
lmost every medical imaging modality has
been applied to the evaluation of degenerative lumbar disc disease, including radiography,
fluoroscopy, and techniques modified with contrast (myelography discography), computerized
tomography (CT), magnetic resonance imaging
(MRI), and scintigraphy (isotope bone scan). The
medical image serves as a surrogate record of
morphology and sometimes physiology. A probe,
or energy source, is applied to a patient whereby
there is a physical interaction that alters the
probe (changes the energy output), and a detector records this. Some modalities use ionizing
radiation, either from electrons (radiography,
CT) or from the nucleus (scintigraphy), while
others use nonionizing sources, such as radiofrequency (MRI) or sonication (ultrasound). Acquisition may be projectional (a “shadowgram”),
such as with the radiographic techniques, or
cross-sectional (“bread loafing”), such as with CT
and MRI. The 3 pertinent parameters of spatial
resolution, contrast resolution, and temporal resolution are generic descriptors of performance
capability of an imaging system.
Spatial resolution refers to the ability to see
spatial detail (ie, resolve 2 points as different).
Radiography has the highest spatial resolution (5
to 10 line pairs per mm), with intermediate resolution for CT and MRI (0.5 line pairs per mm, or
approximately 1 mm resolution). Although with
the advent of multi-detector CT, isotropic submillimeter voxels can be acquired. Scintigraphy
typically has the least resolution, approximately 1
to 2 cm, unless single photon emission computerized tomography (SPECT) techniques are used.
Contrast resolution refers to the ability to distinguish between signal values at different locations and requires some change in luminance or
signal intensity over the background. The usual
range is from 0.5% to 10% of some reference
signal. MRI is advantageous because of its ability
to perform different pulse sequences to exploit
different types of soft tissue contrast that are not
available by other modalities (eg, T1-weighted, T2weighted, and intermediate weighted images). MRI
has superb soft tissue contrast resolution, being
able to detect 5% to 10% differences in nuclear
magnetic relaxation times.
CT can detect 0.5% differences in radiograph
attenuation regarding water, which is the reference standard (the Hounsefield Unit of water is
calibrated to zero). The physical interaction is
based on the linear attenuation coefficient, and
this is roughly proportional to density (that is
why ligamentous structures such as the anulus
fibrosus are hyperattenuating and subcutaneous
fat is hypoattenuating). Therefore, for CT, contrast is best between very dense structures (ie,
bone), highly compact soft tissues (eg, tendons,
ligaments, anulus fibrosus), water-containing tissue (eg, muscle, thecal sac), low-density tissues
(eg, fat) and gas. This is an improvement over
projectional radiography, which requires approximately a 10% change of full scale to detect contrast differences. One mechanism to improve
contrast resolution is to administer a “contrast”
agent, which can be performed through several
different routes. The most commonly used routes
for spine imaging are intravenous, intrathecal,
and intradiscal. Nuclear medicine techniques can
also detect approximately 10% difference in radioactivity as an emission phenomenon from the
administered agent. The agent most commonly
used for isotope bone scanning is technetiumlabeled methylene diphosphonate (MDP). This
agent works by chemisorption onto the bone matrix.
MRI deserves a more in-depth description because it is widely used in the evaluation of spine
degenerative disc disease, and it has more complex features than other modalities. There are
several different types of MRI platforms, which
may be referred to as “closed” and “open.”
From the Departments of Radiology, Harvard Medical School,
Brigham and Women’s Hospital, Boston, MA; and Jefferson Medical
College, Thomas Jefferson University Hospital, Philadelphia, PA.
Address reprint requests to John A. Carrino, MD, MPH, Assistant
Professor of Radiology, Harvard Medical School, Brigham and Women’s Hospital, Department of Radiology, 75 Francis Street, Boston,
MA 02115. E-mail: [email protected].
© 2003 Elsevier Inc. All rights reserved.
1040-7383/03/1504-0003$30.00/0
doi:10.1053/S1040-7383(03)00070-4
Seminars in Spine Surgery, Vol 15, No 4 (December), 2003: pp 361-383
361
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Carrino and Morrison
Closed MRI systems are high field strength (1.5
T or higher) with a signal-to-noise ratio that
produces images of overall better quality than
open low field strength magnets (usually from 0.3
to 1.0 T). In addition, frequency selective fat
suppression can be used. However, to obtain images using a high field strength scanner, the bore
needs to be smaller to accommodate the gradients and some other hardware. Open MRI systems may produce diagnostic quality studies of
the lumbar spine but are best reserved for patients with severe claustrophobia or a body habitus that precludes the use of a closed system
because the image quality is generally significantly inferior to a closed system. Overall, the
installation and maintenance of open MR systems is simpler, making open systems more economical. A coil of wiring acts as an antenna to
receive the signal from the patient’s body. A dedicated spine coil is useful to improve the signalto-noise 3 to 5 times over the scanner’s built-in
“body coil.”
The type of image “weighting” used during
image acquisition determines MR contrast. The
most commonly used images for lumbar spine
applications are T1-weighted, T2-weighted, intermediate-weighted (proton density), and short
tau inversion recovery (STIR). These types of
images may be generated using various sequence
designs. Commonly used mechanisms are (1)
conventional spin echo (CSE); (2) fast spin echo
(FSE), also known as turbo spin echo, depending
on the magnet vendor; and (3) gradient echo
(GE). CSE is the traditional way of acquiring
images and is the standard way to obtain T1weighted images. Traditional T2-weighted images have been largely supplanted by FSE. The
pulse sequences reflect different ways of interrogating tissue, and are related to different patterns of applied radiofrequency pulses and signal
collection. They extract different magnetic properties that infer specific tissue composition. It
should be noted that there is some variability in
protocols, and this depends on several factors,
including the operator’s familiarity with certain
pulse sequences. However, a general-purpose
protocol for lumbar spine imaging is commonly
used (Table 1). Axial sections are often used
parallel to the disc, to help facilitate identification of contour abnormalities. In addition, socalled “stacked” axial sections through the lower
lumbar spine are used to facilitate detection of
other pathology and/or displaced or sequestered
disc fragments.
T1-weighted images are useful for detection of
fat, and fat acts as a natural contrast agent for
detection of epidural or paraspinal lesions, marrow infiltration or replacement, focal bone lesions, and also the diagnosis of lipid-containing
lesions, especially hemangiomas. T1-weighted
images are also used after the administration of
contrast material. In this case, fat suppression is
often used to increase conspicuity of the contrast
agent deposition.
On T1-weighted images, fat is bright (unless it
has been suppressed, such as in postcontrast
studies), and fluid is dark. Therefore, one strategy for identifying T1-weighted images is to identify a known fluid containing structure, such as
the cerebrospinal fluid (CSF) in the thecal sac or
fluid in the urinary tract or gall bladder, and note
the signal intensity for the known fluid. T1weighted images make fat conspicuous and help
identify anatomic cleavage planes, an essential
part to study bone marrow. In addition, T1
weighted images provide a high intrinsic signalto-noise ratio because of the short echo time,
which allows anatomic detail. Intermediateweighted or proton density weighted images are
called such because they minimize T2-weighting
(by having a short echo time) and minimize T1weighting (by having a long repetition time), and,
thus, have a contrast that is intermediate to T1
and T2. Intermediate-weighted images have the
highest signal-to-noise ratio, but, because the
contrast properties often do not add much to
other sequences, it is not commonly used for
routine lumbar spine imaging.
Fluid appears bright on T2-weighted images,
so, again, one strategy is to look for known fluidcontaining structures, such as CSF or urinary
bladder, and identify the “bright” hyperintense
fluid signal. Fat is variable, and can depend on
whether the image is acquired as CSE versus FSE
and whether spectral fat suppression is used.
Spectral fat suppression can only effectively be
used at moderate to high field strengths (1.0 T or
higher). T2-weighted images are useful for the
detection of areas of bone marrow edema and are
critical for showing disc pathology. Specific uses
in the spine would be to show disc desiccation,
hyperintensity zones, and Modic end plate findings. The fat suppression increases the dynamic
range, although there is a loss in anatomic detail
Imaging of Lumbar Degenerative Disc Disease
363
Table 1. Lumbar Spine MRI Protocol
Section
Plane
Weighting Sequence
Repetition
time (ms)
Echo
time (ms)
Sagittal
T1
CSE
500-800
Sagittal
T2
FSE
3,000-4,000
60-120
Axial
oblique
T2
FSE
2,000-4,000
Axial
T1
CSE
500-800
Echo
Section Section
Train
Fat
Thickness Gap
Length Suppression
(mm)
(mm)
8-20
None
(minimum)
No
3-5
0.3-1
8-16
Yes
3-5
0.3-1
60-120
None
No
3-4
0-1
8-20
(minimum)
8-16
No
3-4
0-1
Miscellaneous
Prescription
should
cover
lateral
extraforaminal
aspect of
pedicles.
Fat
suppression
is variably
used.*
Prescribed
parallel to
the
intervertebral
discs.
Prescribed as
a stack
through
the lower
lumbar
spinal
canal.
*Fat suppression is variably used for sagittal T2-weighted images of the spine for routine imaging depending on site and magnet.
If fat suppression is used, then the echo time should be reduced (closer to 60 milliseconds) so that the background architecture
signal is preserved. Without fat suppression, a long echo time is needed to maintain fluid sensitivity of an FSE sequence.
because of the loss of cleavage planes. This effect
can be somewhat compensated for by maintaining a moderate echo time (approximately 60 to
80 milliseconds). This modification to the echo
time still allows for a fluid sensitive sequence, but
improves the anatomic background detail.
On any field strength scanner, fat suppression
and fluid sensitive sequences can also be obtained
using STIR, chemical shift imaging (in and out of
phase imaging), and Dixon techniques. On STIR
imaging, fluid is bright because the contrast is a
combination of T1 and T2-weighted images.
Therefore, structures that have a long T1 and a
long T2 are particularly bright, such as fluid. Fat
is always suppressed. STIR has poor intrinsic signal-to-noise characteristics when compared with
other pulse sequences. However, it is the most
robust fat-suppression technique that is readily
available on all scanners. It is very sensitive for
detection of edema, and works well as a screening
sequence for many neoplastic, infectious, and
traumatic pathologies. However, it is not as useful for degenerative conditions due to a high
degree of noise and low resolution that characterize this sequence. For this reason, STIR has
been referred to as the “bone scan of MRI.”
Gradient-recalled echo (GRE or GE) sequences use a very short repetition time and echo
time, and the signal is strongly influenced by
another parameter called the “flip angle.” A low
flip angle (eg, 5° to 20°) results in an image with
bright fluid, while a larger flip angle (eg, 40° to
90°) results in a T1-weighted like image with
bright fat. Unlike all the other sequences mentioned previously, a GE sequence is acquired
without a “refocusing pulse”; signals from the
patient (ie, “echoes”) are generated by rapidly
altering the magnetic gradients. The refocusing
pulse accounts for magnetic heterogeneity in the
patient’s body, so GE images are very susceptible
to artifact. For example, a metal plate or screws
will “black out” much more surrounding anatomy
than a similar spin echo sequence (CSE or FSE).
However, calcium and blood products also result
in a mild degree of low signal artifact, which can
be used to an advantage. For example, a disc
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Imaging of Lumbar Degenerative Disc Disease
herniation is bright (like the adjacent disc) compared with a calcium-containing spur. Also, in the
setting of trauma, GE images are useful to detect
small amounts of cord hemorrhage. These applications are especially useful in the cervical spine.
Because of the rapid nature of this technique,
variations of it are used for imaging blood flow of
the vertebral arteries (MR angiography). Another advantage of GE sequences is that they can
be acquired using very thin slices with high spatial resolution, and can even be acquired as a
3-dimensional dataset that can potentially be reconstructed in different planes.
In summary, regarding the MR pulse sequences, the following points should be emphasized. On T1-weighted images, fluid is dark and
fat is bright unless fat-suppression is used, which
is only typically done when contrast administration is performed. Proton density or intermediate
weighted images have the highest signal-to-noise
ratio, and fluid is somewhat bright, depending on
the echo time. On T2-weighted images, fluid is
bright; fat signal depends on whether spectral
fat-suppression has been used or not. STIR images are the most fluid-sensitive. Fluid is bright,
and other structures are typically dark to intermediate signal intensity. GE sequences are used
mostly in the cervical spine to generate rapid,
high spatial resolution images over a smaller
area but are not commonly used for lumbar spine
imaging.
Imaging Findings of Degenerative Disc
Disease
The intervertebral disc is a composite structure
consisting of 3 distinct components: (1) the nucleus pulposus, (2) the anulus fibrosus, and (3)
the cartilaginous end plates. They are cartilaginous joints and, in this sense, reflect intervertebral symphysis. The anulus fibrosus is the limiting capsule of the nucleus pulposus, and is
365
attached superiorly and inferiorly to the vertebral
body ring apophysis by Sharpey fibers. It is confluent with the anterior longitudinal ligament
and posterior longitudinal ligament. The anulus
fibrosus is made predominantly of type 1 collagen
and, because of the absence of free protons and
dense lamellar structure, it is normally hypointense on all MRI pulse sequences (Fig 1).
In the lumbar spine, the anulus fibrosus tends
to be thicker ventrally than dorsally. The nucleus
pulposus is made up of glycosaminoglycans
(GAG) and has approximately 85% to 90% water
content under normal conditions. Its signal intensity is intermediate on T1-weighted image and
hyperintense on T2-weighted images, reflecting
the high water binding of the GAG (Fig 1B). The
intervertebral disc height reflects the status of
the nucleus pulposus and typically increases
gradually as one goes from cephalad to caudal,
with the exception of the lumbosacral junction,
which may be narrower than the remainder of
the lumbar intervertebral discs. The posterior
disc margin tends to be concave in the upper
lumbosacral spine (Fig 1D), and is straight or
slightly convex at L4-5 and L5-S1. The posterior
margin typically projects no more than 1 mm
beyond the end plate. Often, there is a horizontally oriented developmental cleft present, best
identified on the T2-weighted images. The bilocular appearance of the adult nucleus resulting
from the development of a central horizontal
band of fibrous tissue is considered a sign of
normal maturation. This cleft is also well shown
on discography. The end plates are covered by
hyaline cartilage, serving as the biomechanical
and metabolic interface between vertebral body
and nucleus pulposus.
Disc degeneration begins early in life. The
etiologies may be related to normal aging, genetic predisposition, or environmental factors.
Component changes can occur in the nucleus
4
Figure 1. Normal magnetic resonance imaging (MRI) appearance of the lumbar intervertebral disc. (A) Sagittal
T1-weighted. (B) Sagittal T2-weighted without fat suppression. (C) Sagittal T2-weighted with fat suppression. (D)
Axial T2-weighted through the intervertebral disc level. Note that on T1-weighted images, the disc is hypointense
to the lumbar vertebral body, while on T2-weighted images it is hyperintense, reflecting normal water content of
the nucleus pulposus. Small intervertebral clefts may be present (arrow). On axial imaging, the posterior margin
should have a concavity (arrowhead), with the exception of the lumbosacral junction, which may normally have a
slight convexity. The disc margins should project no more than 1 or 2 mm beyond the vertebral end plate. Note
that the marrow is slightly hyperintense on the nonfat suppressed images and dark on the fat suppressed pulse
sequences.
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Carrino and Morrison
pulposus, anulus fibrosus, cartilaginous end
plates, and subjacent marrow. The nucleus pulposus typically shows desiccation, fibrosis, or a
vacuum phenomenon, while the anulus fibrosus
undergoes mucinous degeneration. Fissuring may
occur from radial tearing in the vertical or transverse direction (ie, rupture of the Sharpey fibers
near the ring apophysis). The cartilaginous end
plates show marginal osteophytes and subarticular marrow signal alteration.
There is some confusion over the terminology
of degenerative joint disease in general. Osteoarthritis or osteoarthrosis is a process of synovial
joints. Therefore, in the spine, it is appropriately
applied to the zygoapophysial (Z-joint, facet), atlantoaxial, costovertebral, and sacroiliac joints.
Degenerative disc disease is a term applied specifically to intervertebral disc degeneration. The
term “spondylosis” is often used in general as
synonymous with “degeneration,” which would
include both nucleus pulposus and anulus fibrosus processes, but such usage is confusing. It is
best that “degeneration” be the general term and
“spondylosis deformans” be a specifically defined
subclassification of degeneration characterized
by marginal osteophytosis without substantial
disc height loss that reflects predominantly anulus fibrosus disease. “Intervertebral osteochondrosis” is the term applied to the condition of
mainly nucleus pulposus and the vertebral body
end-plates disease, including anular fissuring (ie,
tearing).
There is a widely endorsed nomenclature supported by many subspecialty groups, which
should be the basis for describing disc related
pathology between different types of providers.1
It is important to recognize that the definitions of
diagnoses should not define or imply external
etiologic events, such as trauma, should not imply
relationship to symptoms, and do imply need for
specific treatment. The terminology used in this
article is supported by the “Nomenclature and
Classification of Lumbar Disc Pathology” document available on the Internet.2
The disc derives its structural properties
largely through its ability to attract and retain
water. Internal disc disruption is a term that was
coined in the 1970s to describe pathologic
changes of the internal structure of the disc.
Decreased tissue cellularity and altered matrix
architecture characterize intervertebral disc degeneration. The physiochemical change of dimin-
ished water binding capacity in the GAG is heralded on MRI by the loss of T2 signal and has
been called the “desiccated disc.” Thus, some
refer to this condition as “dark disc disease” or
“black disc disease” (Fig 2).
Osteophytosis is a hallmark of degenerative
disc disease and should be differentiated from
paravertebral calcification/ossification, syndesmophytes, and longitudinal ligament calcification/ossification. Marginal osteophytes tend to be
horizontal and parallel to the disc margin, as if to
be creating additional articular surface. However, they can be bridging, from one level to the
next. Anterior and lateral marginal osteophytes
have been found in 100% of skeletons of individuals over 40 years old and are thought to be
consequences of normal aging, while posterior
osteophytes have been found in only a minority of
skeletons of individuals over 80 years old, so are
not inevitable consequences of aging.3 The claw
osteophyte of McNabb is defined as the bony
outgrowth occurring very close to the disc margin
from the vertebral body apophysis, directed with
a sweeping configuration, towards the corresponding part of the vertebral body opposite the
disc. It is said to be associated with instability.
Paravertebral calcification/ossification tends
to come off the mid portion of the vertebral body
and can be seen in HLA B27 seronegative spondyloarthropathies, such as psoriasis and reactive
arthritis, formerly known as Reiter disease.
There is often a paucity of degenerative disc
disease, which can be helpful in the differential
diagnosis. Syndesmophytes are calcifications
along the outer margin of the anulus fibrosus and
have a thin vertical orientation from one disc
margin to the next. This is a hallmark of ankylosing spondylitis and occurs in the context of
young men with only minimal disc disease. Calcification may occur in the anterior or posterior
longitudinal ligament. Ossification of the posterior longitudinal ligament is a degenerative related condition typically seen in the cervical spine
and not often seen in the lumbar spine. Anterior
longitudinal ligament mineralization is predominantly seen in the thoracolumbar spine. This is
thought to be a senescent related condition, usually with only minimal disc height loss, and, when
it involves more than 4 contiguous segments, it is
referred to as diffuse idiopathic skeletal hyperostosis.
Imaging of Lumbar Degenerative Disc Disease
367
Figure 2. Disc desiccation. Magnetic resonance image (MRI) shows loss of the normal intervertebral disc
hydration. (A) Sagittal T1-weighted image. (B) Sagittal T2-weighted image. This result is manifested by low
nucleus pulposus signal intensity on the T2-weighted images at the L4-5 and L5-S1 level (arrows). There is an
associated disc herniation at L5-S1.
Schmorl nodes are intervertebral disc herniations and may be considered a transosseous disc
extrusion. Herniation of the nucleus pulposus
occurs through the cartilaginous end plate into
the vertebral marrow space. They often have a
characteristic round or lobulated appearance.
They may enhance after contrast administration,
with ring-like enhancement being most common.
They are often incidental and likely to be developmental or post-traumatic rather than purely
degenerative or adaptive.4 There is now imaging
evidence of a significant genetic association
among the COL9A3 tryptophan allele (Trp3 allele), Scheuermann disease, and intervertebral
disc degeneration in patients who are symptomatic.5
Intradiscal calcification is most often incidental and can be seen in the pediatric population,
but it is also frequently seen in the setting of
degenerative disc disease or simply as senescent
changes.6 However, when associated with predominantly nucleus pulposus disease (ie, loss of
disc height and present at virtually every lumbar
segment), this is pathonomonic for alkaptonuria
(ochronosis). Ochronosis is a hereditary disorder
of amino acid metabolism consisting of the accumulation of a dark pigment (homogentisic acid)
in connective tissues. The imaging manifestations are marked height loss, vacuum, and sclerosis. There is minimal osteophytosis because
this is primarily a nucleus pulposus disease. Dystrophic calcification universally presents in all
discs is the radiographic hallmark.
Disc contour changes are part of the degenerative process, and have been broadly characterized as bulges and herniations. The following is a
summary of the accepted nomenclature for abnormal disc contours, typically referred to as
bulges and herniations. MRI is well suited to
show the severity and characterize contour ab-
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Carrino and Morrison
Figure 3. Disc contour abnormalities. (A-C) Axial T2weighted images at the level of the intervertebral disc.
(A) Annular bulge. Generalized displacement of more
than 180° of the disc margin beyond the normal margin of the intervertebral disc space is evident (arrowheads) and is the result of disc degeneration with an
intact anulus. (B) Disc protrusion. The base against
the parent disc margin is broader than any other diameter of the herniation. Extension of nucleus pulposus through a partial defect in the anulus is identified
(arrow). The herniated disc is contained by some intact
anular fibers. (C) Disc extrusion. The base against the
parent disc margin is narrower than any other diameter of the herniation. There may be extension of the
nucleus pulposus through a complete focal defect in
the anulus. Substantial mass effect is present, causing
moderate central canal and severe left lateral recess
stenosis (arrowhead).
normalities: (1) size, (2) morphology, and (3)
location. Mass effect on the spinal cord and nerve
roots can also be shown. However, this needs to
be put into the context of the clinical syndrome.
Hence, the following are pathoanatomical descriptors that do not imply a specific pathoetiology or syndrome.
An anular bulge is described as a generalized
displacement (more than 180°) of disc margin
beyond the normal margin of the intervertebral
disc (Fig 3A). The normal margin is defined by
the vertebral body ring apophysis exclusive of
osteophytes. It can be the result of disc degener-
ation with a grossly intact anulus. Disc margins
tend to be smooth, symmetric, or eccentric and
nonfocal, and may have a level-specific appearance in the lumbar spine.
Disc herniation is a localized displacement
(less than 180° of the circumference) of disc material beyond the normal margin of the intervertebral disc space (Fig 3B). This material may
consist of nucleus pulposus, cartilage, fragmented apophyseal bone, or fragmented anular
tissue. It is often the result of disc degeneration,
with some degree of focal anular disruption. The
types of disc herniation are designated as protru-
Imaging of Lumbar Degenerative Disc Disease
sion, extrusion, and free fragment (sequestration). Protrusion refers to a herniated disc in
which the greatest distance, in any plane, between the edges of the disc material beyond the
disc space is less than the distance between the
edges of the base in the same plane. Protrusions
are characterized by the following: (1) the base
against the parent disc margin is broader than
any other diameter of the herniation, and (2)
extension of nucleus pulposus may occur through
a partial defect in the anulus but is contained by
some intact outer anular fibers and the posterior
longitudinal ligament. The types of protrusions
may be broad-based (90° to 180° circumference)
or focal (less than 90° circumference).
Extrusion refers to a herniated disc in which,
in at least one plane, any one distance between
the edges of the disc material beyond the disc
space is more than the distance between the
edges of the base in the same plane, or when no
continuity exists between the disc material beyond the disc space and that within the disc space
(Fig 3C). An extrusion is characterized by the
following (1) the base against the parent disc
margin tends to be narrower than any other diameter of the herniation and (2) extension of the
nucleus pulposus through a complete focal defect
in the anulus fibrosus. Extruded discs in which all
continuity with the disc of origin is lost may be
further characterized as sequestrated. Disc material displaced away from the site of extrusion
may be characterized as migrated. It may stay
subligamentous, contained by the posterior longitudinal ligament or may migrate widely. A
chronic disc herniation may show a calcification,
ossification, or gas and vacuum phenomenon.
There are no formal staging systems for lumbar degenerative disc disease, and most observers
will report findings commonly using the designations of mild, moderate, and severe. However,
these designations will hold different meaning
among observers, especially regarding the degree
of disc degeneration. The following scheme is
used to define the degree of canal compromise
produced by disc displacement based on the goals
of being practical, objective, reasonably precise,
and clinically relevant. Measurements are typically taken from an axial section at the site of the
most severe compromise. Canal compromise of
less than one third of the canal at that section is
“mild,” between one and two thirds is “moderate,” and more than two thirds is “severe.” This
369
scheme may also be applied to foraminal (neural
canal) narrowing, with the sagittal images also
playing a very useful role for defining the degree
of narrowing. Observer interpretations are also
made with various degrees of confidence. The
statement of the degree of confidence is an important component of communication. The reporter should characterize the interpretation as
“Definite” if there is no doubt, “Probable” if
there is some doubt but the likelihood is more
than 50%, and “Possible” if there is reason to
consider but the likelihood is less than 50%.
Modic Changes
Modic and coworkers initially described vertebral
marrow end plate findings in association with degenerative disc disease, and this spectrum of findings is popularly referred to as “Modic changes.”7
Type 1 is “fluid-like,” and shows T1 hypointensity
and T2 hyperintensity (ie, follows fluid signal).
Type 1 Modic changes show bone marrow edema
(Fig 4), have mild enhancement that may involve
the disc, and are identified in 4% of patients
scanned. On contrast-enhanced MRI, the enhancement is proportional to reactive granulation tissue
present at the peripherally herniated nucleus pulposus, anular tear, or degenerated end plate. With
a degenerated end plate, this tends to be linear,
parallel with the disc being linear, spotty, or diffuse.
Type 2 Modic change is “fat-like” and follows
fat signal intensity on all pulse sequences (Fig 5).
Therefore, type 2 changes show T1 and T2 hyperintensity without fat suppression (Fig 5B) or
T2 hypointense with fat suppression (Fig 5C).
Type 2 Modic changes are identified in 16% of
patients scanned for lumbar disease. Type 3
Modic change is “sclerosis-like” and shows hypointensity on all pulse sequences (Fig 6). Type 3
can also be identified on radiography as a
rounded area of sclerotic opacity abutting the
end plate and is known as discogenic vertebral
sclerosis (Fig 6C). The characteristic findings for
Modic changes are that they are related to the
end plate. They can be round or hemispherical
but do not have to be. The disc shows degeneration, meaning that there is at least some desiccation of the nucleus pulposus. The differential
diagnosis may include infection and, one way to
distinguish this, is that infection tends to have
intradiscal fluid-like signal and end plate erosions. Modic findings are thought to be along a
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Carrino and Morrison
Figure 4. Vertebral marrow signal alteration (Modic type 1 change). (A) Sagittal T1-weighted magnetic resonance
imaging (MRI). (B) Sagittal T2-weighted MRI. Disc height loss and desiccation at multiple levels is evident. At the
L3-4 level, this is associated with rounded areas of signal alteration that abut the end plate and follow fluid-like
signal with T1 hypointensity and T2 hypointensity (arrows).
spectrum from type 1 through type 3. However,
mixed end plate findings are often present and
are typically associated with more severe degenerative disc disease.
The significance of Modic end plate findings
for predicting a clinical syndrome beyond being
simply a marker for degenerative disc disease
(painful or painless) is indeterminate. There is
conflicting evidence in terms of predicting a positive response to provocative discography. One
investigation showed no significant relationship
between vertebral end plate signal changes at
MRI and discography.8 Another investigation
showed that moderate and severe end plate abnormalities of the Modic type 1 and type 2 varieties are useful for predicting discography positive pain response in patients with symptomatic
low back pain.9 In support of this, others have
found that Modic changes are relatively specific
but an insensitive sign of a painful lumbar disc in
patients with discogenic low back pain.10
High Intensity Zone
High intensity zone (HIZ) is the term coined to
denote the finding of an area of hyperintense
signal without the periphery of the disc in the
region of the anulus fibrosus on T2-weighted MRI
(Fig 7A). Posterior tends to be more common
than anterior. In the patient population having
MRI for lumbar back pain, this finding may be
noted in approximately 25%. The presence of a
HIZ correlates with an anular tear and about
an 85% chance that there will be concordant
pain reproduction at discography.12 A follow-up
investigation found similar results with only
one HIZ found in control subjects. Therefore, the
initial understanding was that for patients with
symptomatic low back pain, the HIZ was a reliable
marker of painful outer anular disruption.13 Others
have also concluded that the lumbar disc HIZ in
patients with low back pain is likely to represent
painful internal disc disruption.14
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371
Figure 5. Vertebral marrow signal alteration (Modic
type 2 change). Magnetic resonance images (MRI)
show disc desiccation at multiple levels. (A) Sagittal
T1-weighted. (B) Sagittal T2-weighted without fat suppression. (C) Sagittal T2-weighted with fat suppression. At the L5-S1 level, this is associated with a
rounded area of signal abnormality in the anteroinferior aspect of L5 abutting the end plate. This follows
fat signal on all pulse sequences, and is hyperintense
on T1-weighted and T2-weighted images without fat
suppression. On the fat suppression image (C), the
area is signal void. This predominantly consists of fat.
The normal marrow usually has some hematopoietic
elements and, thus, is not as hypointense as the Modic
type 2 changes on fat suppressed images (arrows).
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Carrino and Morrison
Figure 6. Vertebral marrow signal alteration (Modic
type 3 change). (A) Sagittal T1-weighted. (B) Sagittal
T2-weighted with fat suppression magnetic resonance
imaging (MRI). Marked degenerative disc disease with
disc osteophyte complex formation and a prominent
bulge at the lumbosacral junction are evident. The
anterior aspects of L5 and S1 show areas of T1 and T2
hypointensity abutting the end plate. (C) A characteristic radiographic pattern is identified with a rounded
area of sclerotic opacity involving the L4 vertebral body
abutting the end plate at a disc level where there is
narrowing and vacuum phenomena. This has been referred to as discogenic vertebral sclerosis (arrowhead).
Also note sclerotic findings in the inferior aspect of L3.
Imaging of Lumbar Degenerative Disc Disease
373
Figure 7. Hyperintense zoned (HIZ). (A) Sagittal T2weighted image shows a small focus of hyperintensity
(arrow) within the posterior anulus fibrosus. (B) It is
inconspicuous on the sagittal T1-weighted image without contrast. (C) Intravenous contrast enhanced sagittal T1-weighted image shows enhancement within
the posterior anulus fibrosus (arrow) corresponding
the HIZ identified on the T2-weighted image. This
phenomenon of enhancement is thought to reflect the
ingrowth of fibrovascular tissue to the area. Reprinted
with permission.11
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However, disagreement exists in the literature
as to the significance of the HIZ shown on MRI as
a potential pain indicator in patients with low
back pain. Although the HIZ is present within the
posterior anulus of some abnormal discs, it is not
necessarily associated with a concordant pain response at provocative discography.15 So, although
several investigations confirm that the HIZ is a
marker of a posterior anular tear, the usefulness
of this as a prediscography predictor of pain is
limited by low sensitivity.16
Studies comparing symptomatic to asymptomatic people having both MRI and discography
have revealed that as in other disc related MRI
findings, asymptomatic HIZ may also be encountered. The presence of a HIZ does not reliably
indicate the presence of symptomatic internal
disc disruption; it is a marker of pathoanatomy
and not a specific painful syndrome. Although a
higher percentage of HIZ exists in symptomatic patients, the prevalence in asymptomatic
individuals with degenerative disc disease
(25%) is too high for meaningful clinical use.
When injected during discography, a similar
percentage of asymptomatic and symptomatic
individual discs with a HIZ were painful.17
Therefore, merely the presence of a HIZ does
not define a group of patients with particular
clinical features.18
The nature of the HIZ finding remains unknown, but it probably represents an area of
secondary inflammation as a result of an anular
tear. As has been well shown, HIZ correlates with
peripheral anular tears shown at discography
(painful or not). The focal T2 hyperintense areas
may indicate fragmentation of the outer collagenous anulus fibrosus. The preferred term for such
lesions is “fissures” rather than “tears” because
of the connotation of a traumatic etiology with
the term “tear.” However, “tear” is so entrenched in medical practice that it is likely to
persist. A HIZ may enhance after contrast administration, reflecting the fibrovascular tissue
ingrowth into the region of the anular fissure (Fig
7C). In addition, nerve tissue has also been seen
by histology in this lesion and is the purported
mechanism by which peripheral anular fissures
generate pain. Given the current data, the prognostic or therapeutic significance of this finding
has not yet been elucidated.
Role of CT Myelography
CT myelography continues to be requested extensively. MRI is not only limited in specificity
but, in some instances, accurately depicts the
pathoanatomic state. CT myelography is equally
accurate to MRI and can be more specific because
of the ability to distinguish bone osteophytes
from soft tissue. The advantages of MRI include
providing excellent visualization of regions proximal and distal to severe stenosis or a block. It
often avoids the need for contrast, although contrast improves conspicuity. The main reasons
cited for using CT myelography in conjunction
with or in lieu of MRI are improved visualization
of the definition of the extent of disc herniations,
showing of focal neural compression by small
herniations, and clarifying abnormalities of the
facets, including synovial cysts. However, there is
still an opportunity for refinement of the indications for CT myelography, given the wide range
in variability of use.
MR myelography can also be obtained using
heavily T2-weighted images with fat suppression.
The disadvantages can be poor ability to differentiate desiccated disc from osteophyte. MR myelography yields images that resemble conventional myelography and may be used to help
confirm abnormalities seen on conventional MR
in selected cases. However, there are a large
number of false-positive and false-negative findings.19 Although MR myelography does not significantly improve the diagnostic accuracy of
MRI, it allowed a better overall view of the dural
sac and root sleeves, therefore making it easier to
diagnose spinal stenosis and disc herniation in a
minority of cases.20 The development of better
3-dimensional pulse sequences with isotropic voxels combined with improved signal and spatial
resolution available on higher field strength systems (eg, 3 or 7 T) may make MRI competitive
with the spatial resolution and anatomic detail
that surgeons seem to favor in CT myelography.
These datasets may also allow the ability to develop a virtual “spinoscopy” application, allowing
an operator to navigate through the spinal canal
and its contents.
Provocative Discography
There is anatomic evidence and, hence, concept
validity that the disc can be a source of pain
Imaging of Lumbar Degenerative Disc Disease
(nociceptor) because of the innervation from the
ventral nerve roots that provide branches anteriorly and posteriorly.21 Although the concept of
“discogenic pain” represents a reasonable paradigm, poorly performed discography can assuage
the importance of making this diagnosis. There
are also concerns regarding whether intradiscal
injection, which produces a tensile load, is comparable pathophysiologically to the compressive
load that is exerted by virtue of humans’ bipedal
existence. The primary purpose for discography
is for documentation of the disc as a pain source.
For patients who have chronic predominately axial and nonmyelopathic and nonradicular pain,
imaging may be insufficient or equivocal for determining the nature, location, and extent of
symptomatic pathology.
A position statement regarding lumbar discography from the North American Spine Society
(NASS) was published in 1995.22 Specific indications include patients with persistent pain in
whom noninvasive imaging and other tests have
not provided sufficient diagnostic information. In
preoperative patients who are to undergo fusion,
discography can be used to determine if discs
within the proposed fusion segment are symptomatic and if the adjacent discs are normal.
Surgeons concerned with limiting the extent of
fusion are interested in obtaining more evidence
beyond MRI abnormalities to document what intervertebral disc levels are contributing to the
painful syndrome. In postoperative patients who
continue to have significant pain, discography can
be used to assist in differentiating between postoperative scar and recurrent disc herniation
(when MRI or CT is equivocal); or to evaluate
segments adjacent to the arthrodesis. Discography can also be used to confirm a contained disc
herniation or internal disc disruption as a prelude to minimally invasive intradiscal therapy.
Discography is also being used as part of the
selection criteria for many clinical trials assessing
lumbar interbody fusion devices or percutaneous
intradiscal treatments.
Interpretation of a discogram includes a morphologic and functional evaluation. The fundamental tenet of discography is that injection into
the discs and subsequent increased intradiscal
pressure will elicit a concordant pain response
(ie, one that mimics the patient’s typical pain) if
that disc is a significant nociceptor. A scale of
subjective pain severity from 0 (no pain) to 10
375
(maximal pain) can be determined during the
procedure by asking the patient to relate what
his/her level of pain is during each injection. The
patient is also asked whether the pain mimics
his/her typical pain (ie, is “concordant”) or a
component thereof. To evaluate the patient’s
pain response more “objectively,” multiple vertebral levels around the suspected pain generator
are injected during the procedure; the patient is
not told which level is being injected or when the
injection is starting. Before the procedure, patients are instructed regarding the reporting of
pain and monitoring for spontaneous pain elicited during the examination. It is important to
establish a “reference level” or relatively painfree level with injection. For discography to be
considered positive, there should be at least one
reference level, which is defined by the absence of
pain or lack of concordant symptoms on injection.
An unequivocally positive discogram consists of a
single concordantly symptomatic intervertebral
disc, with control discs above and below that level
if it is not the lumbosacral junction.
Manometric measurement of intradiscal pressure is an attempted refinement recently applied
to lumbar discography. There is an interest in
characterizing and segmenting patients based on
the results of pressure-controlled manometric
discography. This technique may help stratify
patients into categories who are more likely to
improve from interbody fusion.23 It is believed
that with the use of pressure-controlled manometric discography, improved and more specific
diagnostic categorization is possible. Some have
advocated that pressure-controlled, provocative
discography should be considered for athletes
with chronic constant lumbar discogenic pain.24
The goal is to categorize precisely and prospectively positive discographic diagnoses to predict
outcomes from treatment, surgical, or otherwise.
Although retrospective analysis has shown promise, there is no validation of this schema, and the
use of pressure-controlled manometric discography is variably used in clinical practice.
Detailed technical descriptions of lumbar discography are available elsewhere.11,25,26 However,
a few technical points are worth emphasizing.
The tip of the disc puncture needle should be
positioned as close as possible to the center of the
disc so that injection is into the nucleus pulposus
(Fig 8A) instead of the innervated anular fibers,
which can result in a false-positive pain response
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Carrino and Morrison
Figure 8. Lumbar spine discogram morphology. (A) A morphologically normal disc shows a central globule of
contrast collection and may have opacification around the horizontally oriented intervertebral cleft, which is
typically identified by magnetic resonance imaging (MRI). (B) Internal disc derangement (IDD) and degenerative
disc disease are indicated by the irregular linear distribution of contrast that continues posteriorly beyond the
vertebral body margin. Reprinted with permission.11
(Fig 9C). After all needles are placed, contrast
material is injected at each level, with fluoroscopic monitoring and evaluation of elicited pain,
if any. A morphologically normal disc shows a
central globule of contrast collection or “hamburger-bun” configuration (Fig 8A), and degeneration is indicated by a horizontal, linear distribution of contrast (Fig 8B). An anular tear is
diagnosed if contrast extends into the periphery
of the disc in the expected region of the anulus
fibrosus.
Transaxial CT is often used as a complementary study to fluoroscopy “spot” images or radiographs after injection (Fig 9A). CT provides useful additional information to confirm and
characterize anular pathology. The typical candidate lesion for intradiscal therapy (eg, nucleoplasty, electrothermal anuloplasty) is to identify
an intervertebral disc level that has a contained
anular fissure or contained protrusion (Fig 9B)
without substantial disc height loss and generated a concordant pain response at the time of
contrast injection (ie, a “positive” discogram).
Anular injections can be readily differentiated
from nuclear injections (Fig 9C).
There is a scheme for anular tear classification (Dallas Discogram Description) using
CT,27 which has undergone some modification.
The scheme goes from 0 to 5, with the following
grades: (1) 0 ⫽ contrast entirely within the
nucleus pulposus; (2) 1 ⫽ contrast within the
inner third of the anulus fibrosus; (3) 2 ⫽
contrast in the middle third of the anulus fibrosus; (4) 3 ⫽ contrast in the outer third of
the anulus fibrosus; (5) 4 ⫽ a radial dissection,
which means there are also some concentric
components; and (6) 5 ⫽ full thickness tear
with contrast extravasation through the outer
anulus fibrosus. Although this scheme is a useful morphologic construct, it can be difficult to
Imaging of Lumbar Degenerative Disc Disease
377
Figure 9. Computerized tomography (CT) characterization after intradiscal contrast injection. Transaxial
CT after discography. (A) A normal nucleogram characterized by central globule of contrast material that
remains within the expected confines of the nucleus
pulposus. (B) Anular fissure. Contrast material is
noted within the nucleus pulposus but also extends in
a radial fashion posteriorly beyond the expected confines of the nucleus pulposus into the region of the
anulus fibrosus (arrow). (C) The prior 2 patterns
should be compared with this collection of contrast
material, which roughly parallels the nucleus/anulus
junction without central collection of contrast material
(arrowheads). This pattern is indicative of an anular
injection, and may create a false-positive pain response. The CT appearance should not be confounded
for an anular tear.
apply consistently, and there are few data regarding prognostic information.
The demand for discography is increasing as a
diagnostic tool to determine levels of pain generation for patients who are being considered for
surgical treatment (eg, interbody arthrodesis) or
another type of procedure.28 Although the diagnostic use of discography is quite evident, the
treatment use based on the patient outcome is
paramount. Therefore, the value added feature
that discography should provide, is to identify
patients amenable to available therapies and not
to contribute to the treatment dilemma. Mean-
while, less invasive forms of intradiscal therapy
are also evolving, which may make discography
more relevant. Therefore, a “spine specialist”
who is considering instituting disc-specific therapy most often requests discography. This is not
considered a diagnostic test used in the primary
care provider setting. However, either for patient
driven or other reasons, it may be necessary to
establish the disc as a “nocicepter” despite no
change in therapeutic treatment.
Discography is performed on an outpatient
basis. Guidance for needle placement is preferably done with a C-arm, floating image intensifier,
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or with biplane fluoroscopy. Patients must be
informed ahead of time that the purpose of the
procedure is to generate a pain response, which,
in some circumstances, can be severe. Complications include persistent pain, infection, bleeding,
and injury to exiting nerve roots. To minimize
the risk of disc infection, the procedure should be
performed with a surgical-type preparation and
drape of the patient, and surgical scrub, gown,
mask, and gloves for the physician.
Discitis following discography is an uncommon
occurrence (ie, 1% to 4%),29,30. It can be debilitating for the patient and can pose a diagnostic
dilemma. Signs and symptoms are not always
apparent, and the diagnosis is often delayed secondary to inconclusive laboratory and imaging
studies early in the course of the illness. Preliminary data show that uncomplicated discography
does not produce MRI abnormalities following
intradiscal injection.31,32 Therefore, MRI is suitable for evaluation of potential complications after discography. The frequency of discitis after
discography is minimized by prophylactic antibiotic administration, either by intravenous or intradiscal administration. Intradiscal administration of antibiotic mixed with the contrast media
is widely used, however, this is not an approved
route of administration.
Imaging Strategies: Indications and
Guidelines
Currently, there are many options available for
spine imaging evaluation, which contribute to the
quandary of how to use them best. Radiography is
typically the first line imaging of the lumbar
spine and often is used as a “screening” test, in
part because it is readily available, has a rapid
acquisition time, and provides a reasonable
global assessment. CT is used predominately for
trauma, when MRI is not available or contraindicated, or for a specific problem solving application related to osseous integrity. Scintigraphy is
useful for a global physiologic assessment. MRI
has become the mainstay for advanced imaging
of the spine and offers complementary features
to radiography so most patients with chronic
symptoms will have these 2 imaging modalities.
Discography is a provocative examination performed under image guidance, and is most useful
for establishing a discogenic pain origin and confirming if there is an anular tear or contained
protrusion often as a prelude to intradiscal therapy or fusion. CT myelography is also predominantly used as a preoperative test to provide a
“roadmap” to surgical planning. MRI can underestimate root compression caused by degenerative changes in the lateral recess, while conventional and CT myelography are more accurate
when using surgery as the reference standard to
confirm degenerative root impingement in the
lateral recess as the cause of radiculopathy.33
The role of the scintigraphy in patients with
acute low back pain is limited. The bone scan is a
moderately sensitive test for detecting the presence of tumor, infection, or occult fractures of the
vertebrae but not for specifying the diagnosis.
The yield is very low in the presence of normal
radiographs and laboratory evaluation, and highest in known malignancy.34 High-resolution isotope imaging, including SPECT, may localize the
source of pain in patients with articular facet
osteoarthritis before therapeutic facet injection.35 Similar scans may be helpful for detecting
and localizing the site of painful pseudarthrosis
in patients following lumbar spinal fusion.36 The
isotope bone scan remains invaluable when a survey of the entire skeleton is needed.
Imaging costs have been cited as a major reason for increases in health care expenditures.
Actual cost information for delivering radiology
services is difficult to quantify accurately using
traditional methods. Activity-based costing focuses on processes that drive cost. By tracing
health care activities back to events that generate cost, a more accurate measurement of financial performance is possible. However, this is not
available for lumbar spine imaging. Charges by
institutions and reimbursements by insurers are
not true reflections of cost. However, to gain an
appreciation of how imaging modalities are valued by the US government, Medicare global reimbursement (circa 2000) was as follows (in US
dollars) (1) radiography ($36), (2) scinitigraphy
($204), (3) CT ($280), (4) discography ($335), (5)
myelography ($352), and (6) MRI ($542). These
dollar values have to be put into the context of
information gained, risk to the patient, and
downstream relevance to treatment. The value of
information to a provider or a patient, albeit
often negative or exclusionary, has not been emphasized but has likely been a substantial driving
force. Given the high incidence and prevalence of
back symptoms, a reduction in imaging expendi-
Imaging of Lumbar Degenerative Disc Disease
tures in this domain is an area that health care
payers, health services researchers, and evidenced based medical groups have focused on.
Low back pain is most frequently associated
with degenerative disc disease. Conversely, imaging reveals asymptomatic disc abnormalities in a
substantial proportion of patients. Unfortunately, this is the framework that spine providers
must contend with. The basic algorithm for low
back pain used by many providers traditionally
consisted of initial radiographs, followed by crosssectional imaging (CT or MR) if the radiographs
were not definitive. This paradigm assumes that
the different etiologies of back pain are of similar
consequence and ignores the fluctuation in symptoms characteristic of many chronic disorders.
The high prevalence of abnormal MRI or CT
findings in the asymptomatic population also
makes this approach problematic. Unfortunately,
there is no specific imaging biomarker for discogenic pain.
On MRI of the lumbar spine, about one-third
of asymptomatic subjects have a substantial abnormality.37 Many people without back pain have
disc bulges or protrusions but not extrusions.
Given the high prevalence of these findings and
of back pain, the discovery by MRI of bulges or
protrusions in people with low back pain may
frequently be coincidental.38 Findings on MRI in
asymptomatic people are not predictive of the
development or duration of low back pain. In a
longitudinal study of initially asymptomatic individuals, a poor correlation was found with the
development of back pain and the degree of anatomic abnormality on presymptomatic imaging.39 There is also evidence that abnormalities
should be correlated with age in addition to clinical signs and symptoms before operative treatment is contemplated. In patients younger than
50 years old, disc extrusion and sequestration,
nerve root compression, end plate abnormalities,
and osteoarthritis of the facet joints are less common and, therefore, may be predictive of low back
pain in symptomatic patients.40
Another difficulty is that for patients with nonspecific low back pain, a precise anatomically
based diagnosis is often impossible, which leads
to various imprecise diagnoses. Radiography is
useful for a specific diagnosis in only a minority of
patients. MRI and CT are more sensitive than
radiography for the detection of early spinal infections, cancer, herniated discs, and spinal ste-
379
nosis. The role of imaging in other situations is
limited because of the poor association between
low back pain symptoms and anatomic findings.41
In isolation, an imaging finding of disc degeneration may represent part of the aging process
and, in the absence of extrusion, is of only modest
value in diagnosis or treatment decisions. The
most common indication for the use of advanced
cross-sectional imaging procedures, such as MRI
or CT, is the clinical context of low back pain
complicated by radiating pain (radiculopathy, sciatica) or cauda equina syndrome (bilateral leg
weakness, urinary retention, saddle anesthesia),
usually caused by herniated disc and/or canal
stenosis. Some believe that the use of advanced
imaging should be reserved only for potential
candidates for surgery.
The Longitudinal Assessment of Imaging and
Disability of the Back (LaidBack) study baseline
data analysis highlights that a marker for more
significant pathology may be a history of multiple
episodes of back pain.42 Those patients who had 5
or more episodes of previous low back pain were
much more likely to have a disc extrusion than
those who had never had low back pain. The
prevalence of moderate or severe central stenosis
or nerve root compromise was also higher in
those patients with multiple previous episodes of
low back pain. Unlike the other MRI findings,
which were linked to aging, disc extrusions and
nerve root compromise were not significantly associated with age but were associated with previous low back pain. The 3-year follow-up results
from this large cohort of initially asymptomatic
patients has been recently presented. The incidence of new low back pain was 60%. Overall, the
incidence of new imaging findings was low (2% to
9%), and most patients with new imaging findings had no higher incidence of new back pain or
sciatica than those without new findings. However, all subjects with new extrusions, new nerve
root compression, or new central stenosis also
had new low back pain. Although the number of
subjects with new imaging findings is too small to
permit definitive conclusions, these results suggest that disc extrusions and nerve root compression are likely important imaging findings regarding low back pain. These results also
minimize the clinical importance of imaging findings such as anular tears (HIZ) and disc desiccation (T2 signal loss).43
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The differential diagnosis of back pain includes the broad categories of fracture, degeneration, neoplasm, inflammation (infectious and
noninfectious), and neurologic. Some back pain
causing etiologies are far more serious, requiring
an expedited diagnosis and prompt treatment,
but the vast majority of causes do not. The nonlife threatening causes can be treated conservatively for several months before embarking on an
imaging work-up. With this paradigm in mind,
the fist step is to decide whether the patient has
any signs of symptoms that fall into one or more
serious strata: (1) fracture, (2) cancer and/or
infection, or (3) cauda equina syndrome. These
signs and symptoms are often referred to as “red
flags.” The natural course of many cases of
chronic back pain is to wax and wane regardless
of what treatment is applied.
For adults younger than 50 years old, with no
signs or symptoms of systemic disease, symptomatic therapy without imaging is appropriate. If
the patient’s symptoms resolve within 4 to 6
weeks, then they can return to normal activities,
and no imaging studies are needed. However, if
their symptoms persist despite conservative therapy, then further work-up can be pursued. For
patients older than 50 years, or those with “red
flags,” radiography and simple laboratory tests
can almost completely exclude underlying systemic diseases. Looking for “red flags” indicating
cancer or infection is a sensitive method, and the
use of biochemical markers (Erythrocyte Sedimentation Rate or C-reactive protein) can be
helpful. Advanced imaging should be reserved for
those patients considering surgery or those in
whom systemic disease is strongly suspected.
MRI is recommended over CT when the differential includes spinal stenosis, osteomyelitis, epidural abscess, tumor, or recent fracture.
A diagnosis of nonmechanical back pain (eg,
ankylosing spondylitis) is made only with a strong
clinical suspicion. The classic clinical context of
ankylosing spondylitis is a young male, with several months of insidious low back pain that is
worse predominantly in the morning and improves with exercise. Physical examination reveals tenderness to palpation over the sacroiliac
joint region. Treating these patients conservatively for a short time is thought to be appropriate. Compression fractures are a common and
possibly preventable cause of low back pain in the
elderly osteoporotic population, and should be
suspected in an elderly individual with an acute
onset of significant axial back pain possibly
caused by a minor trauma or mechanical event.
In terms of an algorithmic approach, there are
several resources available for the evidenced
based practitioner. The American College of Radiology (ACR) has developed clinical practice
guidelines using a consensus process intended to
direct imagers, referring providers, and patients
in making initial decisions about diagnostic imaging and therapeutic techniques. The ACR Appropriateness Criteria rank imaging examinations on an ordinal scale from 1 (least
appropriate) to 9 (most appropriate) for diagnosis and treatment of specified medical condition(s). There is a guideline for acute low back
pain (lumbosacral pain of less than 3 months
duration), with or without radiculopathy, with
several variants. The use of the ACR Appropriateness Criteria is free to the noncommercial
Internet health care community (www.acr.org).
The NASS is continuously developing clinical
guidelines related to the diagnosis and treatment
of spinal disorders. These guidelines are developed as educational tools for multidisciplinary
spine care professionals to improve patient care
by outlining reasonable information-gathering
and decision-making processes used in the treatment of low back pain in adults. Phases I and II
provide clinical algorithms on low back pain.
Phase III provides Clinical Guidelines for Multidisciplinary Spine Care Specialist (www.spine.
org). These documents are available for a nominal fee from NASS. The National Guideline
Clearinghouse is a public resource for evidence
based clinical practice guidelines sponsored by
the US Agency for Health Care Research and
Quality (formerly the US Agency for Health Care
Policy and Research) in partnership with the
American Medical Association and the American
Association of Health Plans. Information regarding spine imaging and treatments may be found
on the website (www.guideline.gov), and a subscription service is available. The National
Guideline Clearinghouse offers guideline abstracts from ACR, NASS, and other sources, links
to full-text and ordering information, comparison
use for comparing guidelines side by side, guideline syntheses, and annotated bibliographies.
The following is a synopsis of the current trend
in evidenced based imaging of the lumbar spine.
It is obvious from numerous studies and “expert”
Imaging of Lumbar Degenerative Disc Disease
panels that the majority of uncomplicated acute
low back pain is a benign, self-limited condition
that does not warrant imaging studies. It is expected that these patients return to their usual
activities within 30 days. The challenge for the
health care provider confronted with evaluation
of these patients is to distinguish the small segment within this larger population that should
obtain imaging because of a more serious condition. Indications of a more complicated status
(“red flags”) include recent trauma, unexplained
weight loss, unexplained fever, immunosuppression, history of cancer, intravenous drug use, risk
factors (eg, corticosteroid use) or documentation
of osteoporosis, and older than 70 years.44
Another medical decision making point is to
decide if the patient is having primarily low back
symptoms, or whether the pain is sciatic or radicular in nature (ie, mechanical versus neurologic
pain). In patients with sciatica, early imaging is
unnecessary because many patients will improve
with conservative therapy and even severe cases
may resolve over time. In patients with prolonged
or worsening radicular symptoms, MRI or CT can
define the lesion and confirm the site of nerve
root compression. For chronic (more than 3
months) primarily low back symptoms, lumbosacral radiograph (anteroposterior and lateral
views) is appropriate as the initial imaging test.
Additional views may not add substantial diagnostic information.
The issue of early MRI as a screening test
(reduced protocol) and a replacement to radiography has been studied.45 Radiographs are frequently used as the initial imaging study for low
back pain but are neither sensitive nor specific
for many causes of low back pain. Recently developed rapid MRI protocols provide more accurate
anatomic information. Furthermore, because of
reduced imaging time, rapid MRI costs may approach that of radiography. The Seattle Lumbar
Imaging Project (SLIP) is a randomized controlled trial measuring cost-effectiveness from
the societal perspective of rapid MRI versus radiography for patients with low back pain. This
study has completed the data collection portion
and is undergoing analyses. The preliminary results suggest that the extra cost of rapid MRI
does not result in improved functional status,
and, currently, it should not replace radiography
in clinical practice.46 Also in support of a “minimalist” imaging approach is a randomized, un-
381
blinded controlled trial performed in the United
Kingdom, showing that radiography of the lumbar spine for primary care patients with low back
pain of at least 6 weeks’ duration is not associated
with improved patient functioning, severity of
pain, or overall health status.47
Conclusion
Spine imaging can exquisitely provide information regarding pathoanatomy with respect to degenerative disc disease but often does not define
a specific painful clinical syndrome for a patient.
The more common imaging findings of disc degeneration and associated conditions have been
described in this article. However, abnormal imaging findings of the lumbar discs may be degenerative, adaptive, genetic, or a combination of
environmental and determined factors. Many
findings may simply represent senescent changes
that are the natural consequences of stress applied during the course of a lifetime. The imaging
appearance of lumbar spine degenerative disc
disease has a similar incidence between symptomatic and nonsymptomatic populations. Therefore, the appropriate use of imaging modalities
within a defined clinical context is paramount.
For some patients with complicated or recalcitrant symptoms, the most useful aspect for advanced imaging techniques may be in the exclusion of more serious causes of axial low back pain,
such as infection, neoplasm, or fracture, rather
than the inclusion of any specific degenerative
findings.
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