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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4455_Библиотеки_им_академика_М_И_Перельмана
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100 Disorders of the Auditory System
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A
B
Figure 3–16. A multiplanar reconstruction (MPR) computed tomography (CT) image of the
temporal bone showing part of the middle ear ossicles and the cochlea (A) and true coronal image of
the same structures (B).
the radiation dose, as each scan must be
performed separately. As such, most imaging centers opt for a single axial acquisition and MPRs in the coronal or sagittal
planes. This not only limits the radiation
CT with contrast may be preferable to an
MRI with contrast (i.e., when evaluating
patients for small glomus tumors or bone
erosion or destruction, which are not seen
on an MRI scan).
exposure for the patient, but it also keeps
imaging time to a minimum, with scan
times generally lasting 60 sec or less.
Magnetic Resonance Imaging
The axial images (see Figure 3–16A)
should be obtained or reconstructed as
MPRs along a plane 30
o
superior to the
anthropologic baseline (i.e., the line intersecting the infraorbital rim and the external auditory canal) (Figure 3–17), and the
coronal images should be obtained at a 90
perpendicular angle to the axial images
(Chakeres & Augustyn, 2003). Both the
axial and coronal images should be reconstructed at 1- to 2-mm slice thickness. Supplemental intravenous contrast administration may be added to the protocol,
particularly when evaluating for soft tissue masses/tumors, but this should probably be reserved for patients with a contraindication to MRI. It should be noted,
however, that there are instances where a
Magnetic resonance imaging provides
true multiplanar imaging, with superb
soft tissue delineation, all without the
use of ionizing radiation. The procedure
is based on the principle that hydrogen
o
nuclei (protons) tend to align along a magnetic field. This principle is similar to how
a dial on a compass aligns with the magnetic field of the earth. When a patient is
placed within the MRI scanner (magnet),
the protons within the individual’s body
align along the magnetic field of the scanner. Various “pulses” can then be applied
to the field, and the time for the protons to
realign within the magnetic field is called
the relaxation time. The concentration of
protons within various parts of the body
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Figu re 3–17. CT scout image (axial cut) in the sagittal plane showing the
30° plane (heavy line) on which MPRs should be constructed in reference
to the anthropologic baseline (see text).
(voxels) will determine the net relaxation
times (e.g., T1, T2) for that particular
voxel. Based on the relaxation times, various gray-scale assignments can be given
to the voxels to generate an image.
Tesla (T) is the unit of measurement
used to define the strength of the magnetic
field within an MRI scanner. Most clinical
MRI scanners have magnetic strengths
ranging from 0.5 to 3.0 tesla. For reference, the earth’s magnetic field is around
30 microtesla, and the large magnets utilized in “junk yards” to transport scrap
metal and cars are only 0.3 tesla. Higherfield strength magnets (e.g., 3 T) provide
increased signal-to-noise ratios (and thus
reduced scan times) and superior resolution relative to midfield strength magnets
(e.g., 1.5 T). As they become more available commercially, the demand and utility
of imaging the auditory system with high-
field scanners (e.g., 3 T and higher) assuredly will increase owing to the increased
resolution ability of these more powerful
magnetic fields.
Most MRI protocols consist of a combination of T1-weighted and T2-weighted
images in multiplanar acquisition. In routine MRI scans of the brain, T1-weighted
images are considered best at defining
anatomy and any distortion of normal
structures owing to its superior soft tissue discrimination. T1-weighted images
are also useful after the administration
of intravenous contrast agents, as areas
of abnormal enhancement become bright
after contrast administration. Currently,
gadolinium-based contrast agents are
most widely used clinically and traditionally have been thought to have an excellent safety profile with an extremely low
incidence of anaphylactic allergic reaction.
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However, recently, an association between
nephrogenic systemic fibrosis (NSF) and
gadolinium-contrast agents used for MRI
scans has become a concern in patients
with preexisting renal insufficiency. T2weighted images are more sensitive to
changes in water concentration, and thus
are best for delineating areas of edema. In
contrast to CT, with which coronal and
sagittal imaging generally requires an
MPR, MRI scans have the advantage of
being able to acquire images in any plane
(e.g., axial, coronal, sagittal, oblique).
However, the added resolution and multiplanar capability comes at a time cost,
with each sequence (e.g., T1, T2) requiring several minutes for acquisition. Thus,
most MRI examinations require around 30
to 60 min of scan time, depending on the
complexity of the examination. As such,
patient compliance and cooperation can
be significant limitations of MRI scans, as
the patient must remain motionless during the several minutes needed to acquire
each sequence acquisition. For this reason,
some patients will require some form of
sedation to complete their examination.
Patient claustrophobia issues, which may
occur in some patients due to the small confined area within the scanner, often can be
resolved with sedation or the use of “open”
magnets that deviate from the traditional
“tube” design, but the open magnets may
be of decreased field strength (0.5 to 1.0 T)
and thus have decreased resolution.
Dedicated imaging of the temporal
bone and auditory system with MRI scans
requires clinical input to ensure that the
proper study is performed. For example,
one might want to include an MRI angiogram (arterial or venous) when evaluating
a patient for pulsatile tinnitus, as vascular
anomalies may be best appreciated on
these examinations. Magnetic resonance
imaging of the auditory system is most
frequently employed in selected cases of
an acquired sensorineural hearing loss,
specifically when evaluating for tumors
(e.g., schwannomas) associated with the
vestibular and cochlear nerves. Dedicated
imaging of the auditory system, referred
to as an “internal auditory canal” protocol
at most institutions, consists of T1 imaging (both without and after intravenous
contrast administration) with thin section acquisition in the axial and coronal
planes. Additionally, imaging should
include “fluid sensitive” T2 images utilizing a high-resolution matrix and a small
field of view (Figure 3–18). The high resolution T2 images can be acquired in multiple planes, providing exquisite detail of
the vestibular and cochlear nerves, and
are often used as a “screening” technique
for vestibular schwannomas, obviating
the need for contrast. The vestibular and
cochlear nerves can be seen surrounded
by cerebral spinal fluid in the cerebellopontine angle and the internal auditory
canal. Fluid signal within the labyrinthine
structures can also be assessed on the T2
sequences, with the loss of the normal
bright fluid signal intensity indicating a
pathologic process. Similar approaches
can be applied to the imaging of various brain structures as well. Generally,
T1 imaging both without and following
intravenous contrast is utilized in the
evaluation of selected sensorineural hearing loss to avoid misinterpreting a congenital lesion (e.g., lipoma) as a neoplasm
with only postcontrast or T2 imaging.
Normal Anatomy and
Clinical Applications of CT
and MRI in the Evaluation
of the Auditory System
Sound is produced when vibrational
forces from the tympanic membrane are
transmitted to the ossicular chain and
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Figure 3–18. Magnetic resonance imaging (MRI) axial T2-weighted
sequence image at the level of the internal auditory canal showing the
cochlear nerve and fluid-filled cochlea.
then into the vestibule via the oval window. With modern imaging techniques,
the ossicles can generally be discriminated (Figure 3–19). The cochlea is a
shell-like structure composed of two-andone-half turns, which should be visible as
the apical, middle, and basal turns. The
vestibule represents the common chamber at the base of the semicircular canals.
The round window is situated at the basal
turn of the cochlea, with the oval window
situated at the vestibule. The vibrational
forces delivered to the cochlea are converted to energy “potentials” within the
labyrinthine structures that synapse with
the neural fibers that eventually make
up the cochlear nerve. The cochlear nerve
then courses through the internal auditory
canal, through the cerebellopontine angle,
to enter the brainstem (cochlear nuclei).
Computed tomography scans of the
“temporal bone” provide excellent detail
of the normal bony anatomy of the auditory system, including the internal auditory canal, the labyrinthine structures,
the ossicular chain, the middle ear cavity,
and the external auditory canal. Pathology is manifested mainly as alterations
in the bony morphology (e.g., destruction, erosion) of the temporal bone and its
components or in the presence of abnormal soft tissues or fluid in the middle ear
and/or mastoid.
Magnetic resonance imaging performed with an “internal auditory canal”
protocol will provide excellent visualization of the facial and the vestibular and
cochlear nerves (see Figure 3–18). Pathology is generally manifested as a soft tissue “lesion” of one of the nerves, with
increased conspicuity of the lesion after
administration of gadolinium-contrast
agents (Figure 3–20). Additionally, lesions
of the brainstem affecting the auditory
A
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Figure 3–19. Temporal bone
axial CT image showing the
vestibule, horizontal semicircular canal, and basal turn of the
cochlea (A). A more inferior axial
view demonstrating the internal
auditory canal, head of the malleus, short process of the incus,
and the second turn of the cochlea
(B). At the level of the apical turn
of the cochlea, the stapes can be
B
visualized (C).
C
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nuclei are best seen with MRI. Although
the labyrinthine structures are visible
with MRI, it currently does not afford the
same detail as CT.
Computed tomography imaging has
advantages over MRI in that it is readily available, with scanners located in
nearly every hospital and most sizeable
outpatient clinics. As previously stated,
CT is fast; thus, scans of diagnostic quality can generally be obtained without
the need for sedation in agitated or confused patients (e.g., trauma) as well as in
infants/children. The procedure provides
excellent characterization of bony abnormalities, and thus is preferred for evalua-
Figure 3–20. An axial T1 MRI showing the
seventh and eighth cranial nerves.
tion of fractures and ossicular disruption
(Figure 3–21), bony destruction associated
Figure 3–21. An axial temporal bone CT scan image show-
ing the course of a fracture of the temporal bone and disarticulation of the ossicles.
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with soft tissue tumors (e.g., cholesteatoma, Figure 3–22), and bony abnormalities associated with congenital hearing
loss (e.g., vestibulocochlear dysplasia,
Figure 3–23). Relative to MRI, the cost of
CT imaging is significantly less.
The disadvantages of CT imaging are
most notable in its use of ionizing radiation, limited “true” multiplanar capability
(although this is becoming less of an issue
with high-quality MPRs), and poor evaluation of internal auditory canal pathology
(notably vestibular schwannomas) secondary to artifact from adjacent bone. If
intravenous contrast is indicated, the risk
of an anaphylactoid reaction is higher
with iodinated contrast agents relative to
gadolinium, and, in cases of renal insufficiency, care must be taken to avoid potentially adverse effects and worsening renal
function.
Advantages of MRI lie in its lack of
ionizing radiation, superior soft tissue discrimination, true multiplanar capability,
and use of a safer contrast agent. The superior soft tissue discrimination afforded by
MRI makes it ideal for the evaluation of
internal auditory canal pathology (e.g.,
schwannomas, Figure 3–24) and for soft
tissue tumors (e.g., paragangliomas, Figure 3–25). The addition of an MRI angiogram may help the evaluation of vascular
anomalies (e.g., aberrant internal carotid
artery, jugular diverticulum/dehiscence).
Disadvantages of MRI include longer
scan times requiring patient compliance
(and possibly sedation), more limited
availability than CT, and increased cost.
Once thought to be extremely safe, gadolinium-based contrast agents now have
been associated with adverse effects (e.g.,
nephrogenic systemic fibrosis) in many
Figure 3–22. An axial temporal bone CT scan showing a right-sided
cholesteatoma.
Figure 3–23. An axial temporal bone CT scan dem-
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onstrating right cochleovestibular dysplasia (arrow).
The vestibule is widely dilated with abnormal semicircular canals and the cochlea appears as a common
cavity instead of the normal 2½ turns.
A
Figure 3–24. Axial MRI images demonstrating a small intracanalicular vestibular schwannoma.
This lesion appears as a white-enhancing lesion on T-1 weighted images (A) and as a dark-filling
defect on T2-weighted images (B).
B
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A
B
Figure 3–25. Complementary axial CT (A) and contrasted T1-weighted MRI (B) images at the
same level demonstrating an enhancing lesion (possible paraganglioma) filling the middle ear space.
The MRI helps to differentiate between the enhancing lesion and fluid collection due to middle ear
obstruction.
patients with renal insufficiency. In addition to patients with renal insufficiency,
other patients also may have a contrain-
Table 3 – 6. Absolute and Relative
Contraindications to Magnetic Resonance
Imaging
dication to MRI. Although not a complete
listing of all of the contraindications for
MRI, Table 3–6 lists some of the major
contraindications.
In central auditory assessment, MRI
is an important tool to corroborate test
findings. Key individual auditory and
vestibular structures of the central ner-
Absolute
Cardiac defibrillator
Cardiac pacemaker, pacer
dependent
Metallic foreign body in
critical location (e.g., eye)
Certain cerebral aneurysm
clips
vous system can be visualized readily with
this imaging procedure (Figures 3–26 and
3–27). Figure 3–26 is an image of a normal
brain showing several key structures. In
Figure 3–27, the effects of a stroke can be
noted in Heschl’s gyrus. The patient from
whom this image was obtained demonstrated abnormal findings on behavioral
Relative
Cochlear implant
Pregnancy
Unstable patient
Combative patient
(consider general anesthesia
if scan is imperative)
Figure 3–26. A midline, sagittal view of an MRI of the brain showing
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some key structures such as the corpus callosum, the pons, and the cerebellum, as well as a clear depiction of the gyri and sulci at the brain’s surface.
Figure 3–27. MRI of a brain with left temporal lobe lesion positioned near
and including parts of Heschl’s gyrus (see text).
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