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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_6032_Библиотеки_им_академика_М_И_Перельмана
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short distance into the foramen. The spinal cord can be assessed for focal or
diffuse thickening of caliber if concern exists for active myelitis or cord
tumor or for cord thinning in the setting of myelomalacia. The degree of
narrowing of the thecal sac and spinal canal, deformity of the cord, and the
presence of obstruction of CSF flow are well shown. However, the internal
architecture of the cord is not well assessed using myelography. Nerve roots
can be evaluated for thickening and/or compression with myelography. MRI
findings may underestimate the degree of nerve root compression in the
lateral recess compared with CT myelograms and may underestimate the
width of the spinal canal and neural foramina. Myelography and CT
myelography also allow dynamic imaging, both temporally for the evaluation
of thecal sac filling and to assess for site of leakage in patients with
intracranial hypotension.
Relative contraindications to myelography include the presence of a
coagulopathy or anticoagulation medicine. Use of the anticoagulant usually
needs to be withheld for a variable length of time before the procedure. The
presence of iodinated contrast material in the subarachnoid spaces also can
reduce the threshold for seizure, especially in the presence of certain
medications. These medications (mostly antidepressant and antipsychotic
medications) are often withheld for 48 hours before and 24 hours following
the procedure.9 It is important to consult with the performing radiologist
and/or the prescribing clinician to answer any questions about whether a
medication needs to be or can safely be withheld for the procedure.
The same precautions used with CT also need to be used with
myelography, including the risks of radiation to pregnant and pediatric
patients or in patients who may have an allergy to iodinated contrast.
10
Magnetic Resonance Imaging
MRI uses a strong magnetic field along with a series of radiofrequency waves
to render representations of the contents of the body. The magnetic field
strength of the current units most commonly used are 1.5 Tesla (15,000
gauss) and 3.0 Tesla (30,000 gauss), with lesser strengths available for open
and office-based MRI systems. Because of the high magnetic field strength,
the installation and use of the machines is highly regulated because patients
or staff with implanted ferromagnetic materials and certain devices cannot be
Figure 4
near the unit.
Magnetic resonance images of the cervical spine
demonstrate a spinal canal dermoid. T1-weighted (A) and
fast T2-weighted (B) images show fat with bright signal intensity in the
dermoid. C, Short tau inversion recovery sequence nulls the fat signal
in the dermoid. D, Contrast material–enhanced T1-weighted image
with fat saturation nulls the fat content signal in the dermoid and
subcutaneous tissues.
A major benefit of MRI over CT and radiography is that no ionizing
radiation is used. Therefore, MRI is an excellent modality for imaging in
patients who will need to undergo multiple follow-up examinations; for
imaging in children, who are more sensitive to the effects of ionizing
radiation; and for imaging in pregnant patients. MRI is ideal for imaging the
paraspinal soft tissues, disks, epidural space, neural foramina, and contents of
the thecal sac, including the spinal cord and nerve roots.
MRI Sequences
When imaging the spine, several different sequences are commonly used.
The two most well known and used are T1- and T2-weighted sequences
(most clinical protocols use fast T2-weighted sequences). The T1-weighted
sequences show fat as appearing bright (hyperintense) and water as dark
(hypointense); T2-weighted sequences (fast spin-echo) show fat as bright and
water also as being bright. A fat saturation method can be used to null the fat
signal intensity (or turn it hypointense) on the T1-weighted and (fast) T2weighted sequences. Alternatively, a short tau inversion recovery (STIR)
sequence can be used to obtain similar results by changing the bright T2 fat
signal intensity to a dark signal. Using either T2-weighting with fat saturation
or a STIR sequence, the bright fat is removed to accentuate the edema/fluid in
the T2-weighted images. This helps identify pathologic processes in the bone
marrow, spinal canal, and paraspinal soft tissues (Figure 4).
In the spine, T1-weighted sequences are primarily used to evaluate the
signal intensity of bone marrow. Bone marrow is typically hyperintense on
T1-weighted sequences because of the high fat content within the marrow in
adults (children with a greater concentration of cellular hematopoietically
active marrow would show relatively dark signal intensity on a T1-weighted
sequence because of the reduced composition of fat). The fatty content of the
marrow can be assessed by comparing it with the intervertebral disks and
paraspinal muscles. If marrow appears darker than the paraspinal muscles in
adults, then a pathologic process should be considered. This is true for both
diffuse (lymphoma, leukemia) and focal (myeloma, metastasis, edema)
processes. Benign processes that cause diffuse low signal intensity in the
marrow are a result of the conversion to red marrow, such as in the setting of
anemia or rebound activation of the marrow following chemotherapy. Focal
marrow lesions that are bright on T1-weighted sequences are almost always
benign and usually represent hemangiomas (which have a high fat content),
with exceptions including hemorrhagic blood products (Figure 5). Fractures
are often seen well on T1-weighted sequences, showing up as a hypointense
signal line through the fatty marrow, with varying degrees of confluent
surrounding hypointense signal intensity representing edema.
T1-weighted sequences are also well suited for the evaluation of the
epidural space and of the contents of the neural foramina because both have
high adipose content. Any pathologic process that results in filling in or
replacement of the normal fat in these locations should result in close
attention given to these regions on the other sequences.
Figure 5
Sagittal spine magnetic resonance images show
hemangiomas at L1 and L5 (arrows). A, T1-weighted image
shows the bright signal intensity of the fat content in the lesion. B, T2weighted image also shows the bright fat in the hemangioma. C, Short
tau inversion recovery sequence shows the fat signal nulled, with a
small amount of fluid of the hemangioma in L1.
The disks and paraspinal muscles show up as intermediate to dark signal
intensity on T1-weighted sequences, with ligaments appearing dark on all
sequences because of their relative lack of fluid. T1-weighted sequences are
good for the evaluation of ligaments in the setting of trauma to assess for
discontinuity. Most other pathologies, including infectious and inflammatory
processes, are better evaluated on the T2-weighted and STIR sequences.
On T2-weighted sequences, marrow also shows up as bright, although
slightly less so than on T1 sequences. The cartilaginous end plates show up
as dark signal. The disks are hyperintense centrally in the nucleus pulposus,
with a thin peripheral rim of hypointense signal intensity arising from the
anulus fibrosus. Most pathology will show up as bright signal intensity on
T2-weighted images, including edema related to fracture as well as infectious
and inflammatory processes. CSF appears bright on T2-weighted images,
resulting in excellent contrast with the spinal cord and nerve roots, which
show up as dark. T2-weighted sequences are well suited for the evaluation of
spinal cord signal and to evaluate for nerve root compression of the
intrathecal portions of the nerves. T2-weighted images also show good
delineation of the nerve roots in the neural foramina because they are
surrounded by hyperintense fat.
Disk pathology is well seen on T2-weighted sequences, including loss of
disk height and disk desiccation (appearing as darkening of the disk on T2weighted images because of fluid loss). Annular tears (linear or irregular
bright signal), disk protrusions, disk extrusions, and resultant thecal sac and
neural foraminal encroachment are well evaluated on T2-weighted sequences.
Gadolinium contrast agent administration may help when concern exists
for neoplasm (Figure 6) or infection (Figure 7) or for the evaluation of the
postoperative lumbar spine. Gadolinium results in shortening of the T1 signal
because of paramagnetic effects of the heavy metal, resulting in increased
signal intensity. In the postoperative lumbar spine, recurrent or residual disk
protrusions/extrusions and granulation tissue have similar signal intensity on
the non–contrast enhanced sequences, and differentiation between the two is
difficult. With the administration of gadolinium agents, granulation tissue
should be enhanced, whereas the disk will remain dark (but often with a thin
rim of surrounding enhancement).
MRI and Spinal Fixation Hardware
Although MRI is generally considered to be safe in the presence of spinal
fixation hardware, the makeup of the material (iron, cobalt, nickel, stainless
steel) can affect the images by substantial artifact degradation attributable to
susceptibility effects of ferromagnetic materials, which could render the
images nondiagnostic. Some techniques can be used with MRI to reduce the
effects of metal artifact degradation. This degradation is becoming less of an
issue as titanium implants are more commonly used.
11
Figure 6
Sagittal T1-weighted spine magnetic resonance images
obtained before (A) and after (B) contrast material
administration show diffuse enhancement of the vertebral body
infiltrated by a tumor (arrow).
In general, magnetic susceptibility effects are less at lower field strengths,
so 1.5 Tesla is preferred to 3.0 Tesla or higher field strength. Most imaging
centers have set sequences available called metal artifact reduction
sequences. Adjusting technical imaging parameters of voxels, matrix size,
and bandwidth can minimize the artifact.
11
MRI Safety
MRI can result in a safety hazard to a subset of patients and staff. Therefore,
an extensive screening process needs to be implemented before allowing
anyone to enter the environment of the magnetic field. For patients who will
be in the imager, the screening process involves filling out a form about
Figure 7
history of surgery, implanted devices, and the possibility of foreign metallic
bodies, including any history of working with metal, for which concern exists
for the presence of metal fragments within the orbit. Any patient who
answers positively to any question will require further evaluation to assess
the exact nature of the foreign device or material to determine if MRI is safe
to perform. The materials that can cause a safety hazard are composed of
ferromagnetic material, which can result in heating/burning or
dislodgement/torque that can injure the surrounding tissues.
Sagittal short tau inversion recovery sequence magnetic
resonance images obtained from a patient with discitis
show edema in the disk space and adjacent bone marrow and diffuse
enhancement (arrow) before (A) and after (B) contrast material
administration.
Implanted devices are designated as MRI safe, MRI conditional, or MRI
unsafe. MRI-safe devices should not cause any adverse effects when exposed
to the magnetic field or the radiofrequency pulses within the unit. MRIconditional devices have several requirements as designated by the
manufacturer that need to be followed to safely perform imaging. As long as
these recommendations are adhered to, MRI poses no known hazards for
patients with MRI-conditional devices.
8,12
It is important to keep in mind that
some devices are only suitable for imaging certain parts of the body (such as
only the brain or extremities), and torso or spine imaging may be excluded.
Clear identification of the manufacturer and device model number is
mandatory before permitting the patient to undergo MRI.
All facilities that contain an MRI unit are required to have zones set up
that are clearly demarcated and allow safe movement of people within and
around the MRI environment. Zone 1 includes all areas that are freely
accessible by the public. Zone 2 is still a public area but serves as an interface
between public zone 1 and strictly controlled zone 3. Patient screening often
occurs in zone 2. Zone 3, which usually consists of the control room where
the technologist works, needs to be separated by a locked door that allows
ready access only by prescreened staff. Zone 4 is the imaging room. Any time
zone 3 or 4 is entered, it is important to adhere to the technologist’s requests
to ensure everyone’s safety. This adherence is critical especially in urgent
situations of patient decompensation or cardiopulmonary arrest, which
requires multiple members of the code team who may be unfamiliar with the
risks of the magnetic field.
Gadolinium agents also can result in a safety hazard to patients. The
major concerns associated with gadolinium are the possibility of an allergictype reaction and of nephrogenic systemic fibrosis, a disease that can result in
fibrosis of the skin and internal organs related to the administration of
gadolinium-containing agents in patients with renal insufficiency.
Gadolinium-based contrast agents are not considered nephrotoxic at the
routinely administered doses.
Allergic-type reactions to gadolinium agents, although less frequent and
often less severe than the reactions that patients have to iodinated CT contrast
agents, are serious and treated with the same intensity. These are not true
allergic reactions because patients can have a reaction without ever having
been exposed to the agent before and often will not consistently have the
reaction with every subsequent exposure. An antigen-antibody response is
not always identified. Therefore, the responses are currently considered an
idiosyncratic reaction, and can consist of rash, hives, throat swelling,
difficulty breathing, or anaphylactoid reactions resulting in hypotension,
cardiorespiratory failure, and death. As with iodinated contrast reactions,
patients should be premedicated with a regimen of corticosteroids with or
without an antihistamine before injection to attempt to decrease the risk of
injection and a different contrast agent may be used. For more severe
reactions, consideration should be given to performing a noncontrast study. If
contrast is absolutely necessary, the study should be performed in the hospital
setting with trained staff and appropriate anesthesia support available.
4
Nephrogenic systemic fibrosis can result in fibrosis of the skin and
internal organs, similar to systemic scleroderma. Gadolinium has been found
in the affected tissues. The group most commonly affected is patients with
stage IV renal failure who are undergoing routine hemodialysis, but it has
also been seen, albeit less frequently, in patients with acute renal failure or
lesser degrees of chronic renal insufficiency. Gadolinium-based agents are
usually considered safe in patients with chronic renal failure who have an
eGFR greater than 30 mL/min/1.73 m2, but the agents should be used
cautiously in patients with acute renal failure or with an eGFR less than 30
mL/min/1.73 m2.
4,13
If contrast is deemed necessary, a risk-benefit
assessment should be performed and there should be direct discussion
between the referring clinician and approving radiologist. The patient also
should be made aware of the risks associated with the gadolinium-based
agent and informed consent should be obtained.
MRI Guidelines and Recommendations
For cervical trauma, the AUC of the ACR consider MRI an appropriate and
complementary imaging modality to CT.1 MRI also is the study of choice for
patients with neurologic symptoms concerning for spinal cord injury,
hematoma, or disk herniation, as well as ligamentous injury. Some
controversy exists regarding the use of MRI in patients with normal CT
examination findings. The ACR suggests using MRI in patients whose
neurologic status cannot be fully evaluated after 48 hours, even with normal
CT examination results.
1
For lumbar degenerative spondylolisthesis, MRI is suggested as the most
appropriate noninvasive test for imaging spinal stenosis by North American
Spine Society guidelines.2 In patients with clinical concern for disk
herniation, MRI is the recommended study of choice, but CT myelography
should be performed in patients for whom MRI is contraindicated. For
lumbar spinal stenosis, MRI is recommended as the most appropriate imaging
study, with CT myelography performed for patients in whom MRI is
contraindicated. In addition, workgroup consensus suggests MRI as the firstline diagnostic study for patients with spinal stenosis.
14
Imaging recommendations for low back pain are evolving. For
uncomplicated acute low back pain, the ACR suggests no imaging studies
because the pain is a benign self-limiting process. However, in patients with
red flags for serious injury or with prolonged pain after medical management,
MRI is recommended, although many payers will require radiographic
evaluation prior to approving MRI.1 Many patients have no correlative
abnormal imaging findings. Also, imaging abnormalities can exist in people
without back pain. As summarized by the AUC of the ACR, the challenge for
practicing physicians is to identify the small subgroup of patients within the
large population that requires spinal imaging in whom serious disease should
be suspected.
1
Nuclear Scintigraphy and PET
Bone scintigraphy and PET are the modalities most often used in the
diagnosis of spine disease. Bone scintigraphy (also called bone scanning) is
useful for bone lesions, including the spine, and used less for soft tissues. The
isotope technetium Tc-99 methylene diphosphonate (99Tc MDP) and planar
or multiplanar single-photon emission computed tomography images are
obtained a few hours after injection. 99Tc MDP binds to hydroxyapatite
crystals in proportion to local blood flow and osteoblastic activity, making
the agent a good marker for bone turnover and bone perfusion.
Fractures, osteomyelitis, and osteolytic metastatic tumors (Figure 8) all
show increased “hot” activity in the bone scan. Occasionally, areas that show
a lack of activity (“cold”) may exist in aggressive metastatic lesions, indolent
healing abscesses, plasmacytomas, or disruption of blood flow such as with
bone infarcts or prostheses.
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