Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_6032_Библиотеки_им_академика_М_И_Перельмана
.pdf
the individual imaging modality, the risks and contraindications of the
imaging source, and understanding the evolving clinical guidelines in the
appropriate use of imaging.
Radiography
Although there are fewer indications than before for the use of spine
radiography because of the widespread availability and excellent capability of
CT to depict osseous structures, existing indications include neck or back
pain without or with trauma, need for preoperative planning, postoperative
assessment, and evaluation of scoliosis.
The typical radiographic views of the spine are AP and lateral. Additional
oblique views are used to evaluate the degree of neural foraminal narrowing
in the cervical spine or to evaluate the pars interarticularis in the lumbar
spine. A cone-down lateral view of the lumbosacral junction is also often
obtained. In the cervical spine, an odontoid view is added because the skull
base obscures the odontoid on standard AP views, and a swimmer’s view can
be added because the shoulder often overlaps the C7-T1 junction.
Flexion/extension views are used to evaluate subluxation from injury to the
cervical ligamentous structures, although this is not recommended in
individuals with acute trauma because of muscle spasms. These views are
usually obtained with maximum passive flexion and extension. Significant
observations are most frequently considered to be changes greater than 2 mm
in the amount of spondylolisthesis or an 11° change in angulation.
When evaluating the thoracic spine, labeling the correct thoracic level is
imperative before any surgical treatment because transitional anatomy often
exists at the L5 or S1 level. Counting the upper thoracic vertebrae is often
difficult on the lateral view because the shoulders obscure details. Therefore,
obtaining AP views to localize the first and last ribs often is the best method
to determine the correct level for surgical treatment.
Many national medical societies publish guidelines for spine imaging.
The American College of Radiology (ACR) publishes Appropriate Use
Criteria (AUC) for multiple clinical variations in the use of imaging.1 In
trauma, radiographs are considered appropriate with a strong clinical
indication but not appropriate without it. For chronic back pain, radiographs
are appropriate and often the initial imaging study of choice. In patients with
uncomplicated low back pain, radiographs of the lumbar spine are usually not
appropriate and imaging is often not warranted. However, if low back pain is
complicated by conditions such as low-velocity trauma, advanced age,
osteoporosis, or chronic steroid use, the recommendation to obtain
radiographs of the spine becomes appropriate.
For evaluating the degenerative lumbar spine, the North American Spine
Society suggests lateral radiographs as the most appropriate noninvasive test,
and when possible, the lateral radiograph should be obtained while the patient
is bearing weight.2 For patients with isthmic spondylolisthesis, weightbearing radiographs with or without oblique views or dynamic view
radiographs are recommended as the most appropriate imaging test. For
situations in which the radiographic results are indeterminate, CT is
suggested as the next appropriate imaging study.
3
Computed Tomography
CT uses the same principles as radiography, but with the addition of multiple
projections to create cross-sectional images of the body. Photons generated
by an x-ray tube are sent through the body. The photons are absorbed
(attenuated) more by dense structures (such as metal, bone, and calcium).
Detectors measure the remaining photons after they pass through the body
and the information is used to re-create images from many different rotational
angles. The CT number (Hounsfield unit [HU]) is calculated by normalizing
the attenuation of each voxel to that of water (water measures at 0 HU). The
Hounsfield unit values range from −1,000 to 3,000. The images can be
“windowed” to emphasize different features by altering the center of the
values and the range of values being displayed. The typical bone window has
a center of 300 HU and a range of 2,000 HU. A soft-tissue window has a
center of 40 HU and a range of 400 HU. CT can now be performed quickly
using multidetector CT scanners. Multiplanar (sagittal and coronal) and threedimensional reconstructions are created easily and quickly.
CT is ideal for evaluation of the bones, in particular for fractures,
degenerative arthropathy, postoperative osseous fusion, and destructive
osseous lesions. Fractures are well characterized with excellent detail
regarding the number of fracture fragments and the presence of fragment
displacement, angulation, distraction, and rotation. Degenerative arthropathy
is demonstrated with osseous sclerosis, osteophytosis, loss of joint space, and
subchondral cystic change (Figure 1). The degree of osseous narrowing of
the neural foramina and the spinal canal also can be evaluated, noting that the
soft-tissue (disk or ligamentous) contribution to narrowing is not as clearly
defined as with MRI, particularly in the cervical spine. Bone union following
fusion is shown by solid bone mass bridging the disk space, facet joints, or
other sites of osseous fusion. As tumors replace bone marrow, either sclerotic
expansion or soft-tissue replacement of the normal bony trabeculae or cortical
bone occurs and is shown better on CT (Figure 2). Radiographs may not
show lytic lesions until 30% of the trabeculae are eroded and/or replaced. CT
also is a good modality for preoperative planning. The size of pedicles is well
demonstrated to help plan for screw placement. Three-dimensional
reconstructions can help visualize complex anatomy.
Figure 1
Sagittal lumbar spine CT scan shows the narrowed disk
space and end plate sclerosis (arrow) resulting from
degenerative disease.
A disadvantage of CT regarding evaluation of the spine is that
demonstration of soft-tissue edema and/or infection and inflammation of soft
tissue is limited. Although inflammation or edema can be seen as fluid
density and stranding within the paraspinal fat, these processes are shown
much better on MRI, particularly when the muscles, spinal canal, or bone
marrow is involved. For example, discitis and osteomyelitis can be seen on
CT as narrowing of the disk space with end plate irregularity and potential
loss of vertebral body height, possibly with edema in the surrounding
paraspinal soft tissues. The additional findings of bone marrow edema,
epidural abscess, and soft-tissue abscess are seen much better on magnetic
resonance images. Abscesses can have viscous contents that show density
similar to soft tissue on CT images and often can be missed, particularly in
the psoas muscle, which is a common location for edema and abscess with
spinal column infection. The spinal cord and nerve roots are not well seen
because of their relatively small sizes, attenuations similar to cerebrospinal
fluid (CSF), and poor penetration of the spinal canal by x-ray photons, which
is attributable to the dense surrounding bone of the spinal column (resulting
in poor definition of the contents of the canal).
Figure 2
Sagittal thoracic and lumbar spine CT scan shows the
destructive, lytic process caused by the tumor at T9 and
T10 (arrow).
Iodinated Contrast Material: Use and Risks
The use of iodinated contrast material often is not necessary for indications
for which CT is requested, especially in the spine. The most common setting
for which contrast material is used is if concern exists for an infectious
process or tumor and MRI is contraindicated in the patient. Contrast material
also can help evaluate for abscess by showing a distinct rim of enhancement
around a fluid density collection (Figure 3). Soft-tissue tumors can show
solid, heterogeneous, or peripheral enhancement.
Allergic-like reactions can occur when iodinated contrast material is
administered. The mechanism of the reaction is not clearly understood, but an
antigen-antibody response is not always found. The reactions are considered
idiosyncratic. An individual who already has experienced an allergic-like
reaction to contrast material is at higher risk of having another reaction (10%
to 35% risk in the future if not premedicated), usually to a similar degree as
any prior reaction. Occasionally, a more severe reaction will occur.
Irrespective of this, the reactions are treated similarly to true allergic
reactions and the patient should be premedicated before future studies with
steroids and with or without an antihistamine. Using a different contrast agent
for future injections also can be considered. Prior allergic-like reactions to
iodinated contrast material should not preclude injections with gadoliniumbased contrast agents for MRI because no cross-reactivity has been shown to
occur, but patients with a history of atopy or reactions to other agents may
have a greater tendency to have reactions to either.
4
Patients with acute or chronic renal failure also may be at risk for the
development of worsening renal function following the administration of
iodinated contrast agents. This theory has been questioned because many
patients whose creatinine levels increase after contrast administration have
other comorbidities, and many previous studies involved patients who
received intra-arterial contrast material for cardiac angiography (which
administers a more concentrated dose of contrast material to the kidneys than
intravenous administration).5 Therefore, a distinction is now made between
contrast material–induced nephropathy and postcontrast acute kidney injury,
Figure 3
which avoids faulting the contrast material. No cutoff value of creatinine or
estimated glomerular filtration rate (eGFR) has been agreed on for which
iodinated contrast media is contraindicated. A risk-benefit analysis must be
performed before administering contrast agents to any patient, accounting for
all risks, benefits, and alternatives. A cutoff eGFR value of 30 mL/min/1.73
m2 can be used in patients with chronic renal insufficiency and alternatives
considered in the setting of acute kidney injury.6 Patients with end-stage renal
disease who are anuric and undergoing routine hemodialysis are not at risk
for postcontrast kidney injury because their kidneys are nonfunctioning.
A, Contrast material–enhanced axial CT scan of spinal level
L5 shows abscess fluid with trapped air (arrow). Note signal
enhancement and edema in adjacent muscle. B, Axial postcontrast
T1-weighted magnetic resonance image at the same level as panel A
shows enhancement around the abscess (arrow).
If iodinated contrast material is administered in the setting of renal
insufficiency, hydration is the only prophylactic therapy consistently shown
to be effective. Isotonic solutions are preferred, with a protocol example of
0.9% saline at 100 mL/h administered for 6 to 12 hours before contrast
material administration and continuing for 4 to 12 hours after. Decreasing the
dose of contrast material at lower eGFR levels (theoretically decreasing the
risk to the kidneys because of reduced contrast material load) can be
considered as long as diagnostic information can be obtained with the smaller
dose.
4
CT Radiation Dose
With CT, radiation doses are calculated based on the imaging parameters
used, including the length of the scan, and are normalized to the expected
dose based on measurements using a phantom. Actual patient doses can vary
from the calculated dose for given imaging parameters, depending on patient
cross-sectional diameter and attenuation, with smaller patients receiving a
higher actual dose and larger patients receiving a smaller dose than estimated.
Radiation dose is measured in millisieverts (mSv). The average dose for a
spine CT scan is 5 to 6 mSv, which corresponds to an individual’s amount of
natural radiation exposure over approximately 2 years.
Adverse events associated with radiation exposure are stratified into those
that have threshold radiation exposures and those that result from cumulative
exposure without a set threshold level. One type of event that occurs with a
set threshold level is skin damage from direct radiation exposure. This
occurrence is uncommon at levels of radiation used for CT scanning, but is
more common with long interventional fluoroscopic studies. The primary
concern of cumulative radiation exposure over time is the induction of
cancer. This risk is greater in those who undergo multiple imaging studies
and in children, who are more radiosensitive than adults. In these settings, it
may be prudent to perform MRI rather than CT.7 In all cases, a common
principle in radiology is to keep doses “as low as reasonably achievable” by
optimizing each scan to obtain diagnostic information while keeping the
radiation dose at a reasonably low level and considering acceptable
alternatives that do not use ionizing radiation (such as ultrasonography or
MRI).
CT Guidelines and Recommendations
The most important recent change in imaging recommendations for trauma to
the spine is the replacement of radiography with CT. Radiography is reserved
for patients in whom suspicion for spine injury is low. The ACR’s AUC
recommend non–contrast-enhanced CT for the evaluation of patients with
trauma to the cervical, thoracic, and lumbar spine regions, as indicated by
clinical criteria.1 Using CT data, sagittal and coronal reconstruction images
should be obtained to improve the evaluation of fractures and subluxations.
Arterial injury including dissection is a concern in patients with cervical
spine injury. The evidence for obtaining a CT angiogram to evaluate for
dissection is minimal. Given the low concern for arterial injury in blunt
trauma, disagreement exists on the use of CT angiography for this group of
patients. For lumbar degenerative spondylolisthesis, the North American
Spine Society guidelines suggest CT myelography as the most appropriate
study for those patients in whom MRI is contraindicated.
2
CT Myelography
CT myelography is performed following injection of iodinated contrast
material into the thecal sac, usually by means of lumbar puncture. The use of
CT myelography has declined sharply over the past few decades as a result of
the widespread use and availability of MRI in the evaluation of the spinal
canal, cord, and nerve roots. CT myelography can be used when MRI is
contraindicated, such as in the setting of an implanted device for which MRI
would be unsafe, when the patient is too claustrophobic to withstand MRI,
with image degradation because of metallic implants, or if symptoms are not
explained by MRI findings. More implanted devices are being manufactured
undergo MRI under appropriate imaging conditions.8 Before determining a
patient cannot undergo MRI, the safety of a device must be verified by using
the device card or by the performing surgeon. Myelography also can be
useful in addition to MRI when a cystic collection/arachnoid cyst in the
spinal canal is suspected, to assess for continuity with the subarachnoid CSF
space and to delineate the margins of the cyst; to evaluate for a CSF leak; or
to define anatomy in the setting of suspected spinal cord herniation.
Fluoroscopic guidance usually is used for the procedural portion of the
examination. If lumbar puncture is difficult because of anatomy or extensive
osseous fusion, CT guidance or cervical puncture may be necessary. Cervical
puncture also can be used in the setting of active infection in the lumbar soft
tissues, which increases the risk of seeding the infection along the needle
tract, or in the setting of obstruction of cranial flow of contrast material
because of severe spinal canal stenosis in the thoracic or lumbar region
during evaluation of the cervical or thoracic canal.
The presence of contrast material in the thecal sac allows for evaluation
of the spinal cord and nerve roots on their course through the spinal canal and
into the neural foramina because the dural sheath follows the nerves for a
Соседние файлы в папке Библиотека им академика М.И. Перельмана