Role of Diagnostic Imaging in Otology
Balasubramanian M.S. D.L.O.
Introduction: The past decade has
ushered in a revolution in imaging techniques, the fall out of which
is the ease with which we are able to see the inside of the
body from varying planes and angles. It is a quantum leap
when compared to the only radiological tool of yester years i.e.
plain xray both mastoids Lateral oblique view (LAW'S
view). The Law's view helped us only to compare the
pneumatisation of the temporal bone on both sides, inaddition to
identifying the presence of cavity in the temporal bone. The
advent of CT scan and MRI scan has helped us to pick
up acoustic schwannomas at a very early stage.
Available imaging tools:
1. CT scan
3. Functional imaging
Synonyms: Computer assisted tomography, CAT
British engineer Godfrey Hounsfield of EMI Laboratories, England,
and independently by South African born physicist Allan Cormack of Tufts University,
imaging modality includes a sectional tomography, where in the body
is radiographed in various sections. These sections are
processed by a computer to generate three dimensional
pictures. The data produced by a CT scanner can be manipulated
through a process known as windowing. The process of windowing
demonstrates the various structures based on their
ability to block the X ray beams. The images obtained
can be reformatted using a computer to provide saggital views
of the structure being examined.
scanning: Use of contrast agents help in assessing the nature
of the tumor by assessing the vascularity of the mass. To
study the vasculature of the mass intravenous contrast agents are
injected just before the scanning process is to start.
This produces . This is called contrast enhancement and is
extremely useful for identifying vascular
lesions. The intravenous contrast
material is on delayed CT scans.
Commonly used contrast preparations are iodine
based, hence is contraindicated in patients with known allergy to
iodine based drugs. Common drug used as contrast agent
is Ultra vist, Angiograffin.
of a CT scanner
Advantages of CT scanning in
1. It clearly demonstrates the intra cranial
complications of suppurative otitis media.
2. It clearly
delineates intracranial extension of tumors such as glomus jugulare
/ acoustic neuromas.
Routine CT views for petro mastiod area
is axial with 1 - 2 mm cuts. These cuts are viewed with
a wide window settings i.e. 3000 - 4000 H.U. (Bone
Great care must be taken to limit
radiation dose to the eye lens, cornea etc. This can be
achieved if the scan is performed in a plane 30 degrees to the
orbito meatal line. In this plane the globe is mostly
below the part sectioned.
CT temporal bone
showing atresia of left external auditory
High resolution CT scan (HRCT):
High-resolution computed tomography (HRCT) provides
excellent contrast between osseous structures, air and soft tissue
in conjunction with high spatial resolution. Therefore, thin-section
HRCT with bone window setting is the method of choice for the
examination of the middle ear structures.
It is indicated in
cases of acute / chronic inflammatory conditions of middle ear,
cholesteatoma / tumors of middle ear. It clearly delineates
bony walls and gives excellent resolution resulting in even minor
erosions of ossicles, scutum to be clearly seen. It also
depicts clearly the middle ear ossicles, and hence can be used to
assess the position of ossicles postopertively after
HRCT can be used to identify congenital
malformations involving cochlea. This will be helpful before
performing cochlear implants in these patients.
It is also useful
in identifying high jugular bulb.
showing erosion of scutum
Mondini deformity of cochlea (Cochlea with less than 2 1/2
A common condition labyrinthitis
ossificans (ossification of the cochlea) can cause problems during
cochlear implant procedures. These can be easily identified
during a routine HTCT examination. Ossification of cochlea
makes the procedure of cochlear implant a little difficult because
the round window through which the implant is normally introduced is
totally ossified, hence a third window will have to be drilled for
optimal positioning of the electrode.
image showing labyrinthitis ossificans
The MRI scan uses
magnetic and radio waves for imaging. The patient is not
exposed to X rays. The patient is made to lie inside a large
cylinder shaped magnet. Radio waves 10,000 to 30,000 times
stronger than the earth's magnetic field are sent through the
body. These radio waves affects the body's atoms forcing the
nuclei into a different position. When these nuclei move back
into their original positions they send out radio waves of their own
which is picked up by the scanner, which is then converted by the
computer into images.
body mostly is made up of water, the nucleus of hydrogen atom which
is a constituent of water is made use of in generating images of
body. Using MRI scanner it is possible to image any part of
the human body. The tissue which has very little hydrogen
atoms (Bone) appear dark while tissues which have large number of
hydrogen ions like fat
appear much brighter. By changing the frequency of the radio waves
it is possible to gain information about different types of
MRI scan will give
clear images of tissues surrounded by bone i.e. brain and spinal
cord. Traditional MR imaging comprises of non enhanced T1 and
T2 weighted images, followed by gadolinum enhanced T1 weighted
T1 weighted images: The term T1 indicates
the time constant. It is the time taken by the body's atom to
realign itself after being disturbed by radiowaves applied in a
longitudinal axis. Non enhanced T1 weighted images in axial
and coronal planes, identifies bone marrow, fat, and subacute
hemorrhage as high (bright) signal areas.
Gadolinum is used as a
contrast agent to enhance images in MRI.
Gadolinium-enhanced T1-weighted imaging in the same planes
demonstrates enhancement when a neoplastic, vascular, or
inflammatory process is present.
weighted images: This is again another time constant. Here the
radio waves are introduced in a transverse plane, and the time taken
by a certain percentage of tissue nuclei to realign and get back
into position is expressed as T2 weighted images. Standard
T2-weighted studies identify intra-axial disease, such as brainstem
tumor, stroke, and multiple sclerosis.
Fat is brighter on T1- weighted imaging,
infection or inflammation intensifies from T1- to T2- weighted
imaging, and cerebrospinal fluid (CSF) turns from black to white
from T1- to T2-weighted imaging. Bone appears dark on MR
Almost all the ossicular prosthesis used in
the middle ear are non ferromagnetic except for Mcgee's stapes
prosthesis. It is safe to perform MRI in patients with middle
ear implants except Mcgee's prosthesis.
Contraindications to MR imaging include pacemakers and aneurysm
clips that are ferromagnetic.
MR imaging in patients with
cochlear implants: The safety of patients who have undergone
cochlear implant depends on the type of implant used. If
imagining is done under low frequency i.e. 1 tesla imager,
then it is safe. As a rule of thumb any MR imagining in a
patient who has undergone a cochlear implant must be deferred unless
and until it is absolutely essential, then too the manufacturer of
the implant must be contacted regarding the safety of the procedure
High resolution magnetic resonance
imaging: is a recent innovation in imaging technique. This
technique produces a heavily T2 weighted image. This helps in
better visualisation of membranous labyrinth, and internal acoustic
meatus. The fluid signal of the membranous labyrinth also
appears bright, allowing for similar high-resolution examination of
labyrinthine anatomy and abnormalities.
HRMR imaging has replaced HRCT as the
preferred preoperative imaging study before cochlear
implantation. It can verify the patency of cochlea, as well as
to confirm the presence of cochlear nerve and its size.
Without the presence of cochlear nerve surgery is not going to be
successful. This study is
especially beneficial if there is a history of meningitis and
subsequent cochlear scarring is suspected.
Three dimensional Fourier transformation constructive interference
in steady state is one of the newest high resolution magnetic
resonance imaging applications. Eventhough this study
generates images (thin sections) in a single plane they can be
reformatted with the help of computers into various planes.
This imaging modality helps in identifying each and every nerve
inside the internal acoustic meatus. Infact the origin of even
the smallest acoustic neuroma can be identified.
This type of imaging is superior to the
conventional MR imaging because:
1. The scanning time is
2. Gadolinum need not be injected for
3. Cheaper than conventinal MR imaging
Demonstrates clearly the membranous labyrinth, hence could be used
to study patients with sensori neural hearing
1. The screening field is
2. It is unable to demonstrate inflammatory
This type of imaging
allows for measurement of cortical acitivity in response to a
specific stimuli. The imaging modalities include:
Functional MRI (fMRI)
2. PET scan
3. SPECT scan
Functional MRI scanning: Is the recently
developing scanning modality. MRI scanner is used to measure
the hemodynamic activity related to the neural activity in brain and
spinal cord. This procedure utilises the
unique feature of hemoglobin where in it is diamagnetic
when oxygenated and para magnetic when deoxygenated. These
signals can be recorded as blood oxygen level dependent contrast
(BOLD). Higher signal intensities can be recorded if
the concentration of deoxyhemoglobin decreases. This
leads to a higher BOLD value. The higher the tissue activity
lower is the concentration of oxyhemoglobin, lesser is the
value of BOLD.
Advantages of fMRI in otology
1. It can be successfully used in patient screening
before cochlear implant surgery
2. It provides better spatial
3. There is no need for contrast enhancement
This can be easily performed by stimulating the auditory
PET scan: Positron emission
This scanning modality produces a three
dimensional map of the functional part of the body. To
conduct the scan, a short-lived radioactive tracer isotope,
which decays by emitting a positron, which also has been chemically
incorporated into a metabolically active molecule, is injected into
the living subject (usually into blood circulation). There is a
waiting period while the metabolically active molecule becomes
concentrated in tissues of interest; then the research subject or
patient is placed in the imaging scanner. The molecule most commonly
used for this purpose is flurodeoxyglucose (FDG), a sugar, for
which the waiting period is typically an hour. This scan is
hot when the tissue is highly active
SPECT: Single photon emission computed
tomography. This imaging is performed using gamma rays.
It is performed using a gamma camera to acquire multiple
2 D images also known as projections. These images
are fed in to a computer which reconstructs a 3 dimensional
image. This type of functional imaging is helpful in
evaluating patients after a cochlear implant. Since a gamma
camera is used, there is no adverse effect due to the presence of an
Role of imaging in studying cerebellopontine angle
and internal acoustic meatus:
Imaging of this area
is performed as a screening test to rule out acoustic neuroma in
patients with unilateral sensori neural hearing loss.
Gadolinum enhanced magnetic resonance imaging clearly
demonstrates even the smallest of neuromas in this area. High
resolution MRI can identify tumors of even 1mm size. If high
resolution MRI produces a negative result it can be safely assumed
that acoustic neuroma is not present. This scanning
modality also clearly visualises the site of origin of the
tumor. Since schwanomas are slow growing tumors, growth
rate of vestibular schwanomas can be assessed by serial high
definition MRI taken every 6 months after the initial
Post operative MRI
scans can be taken 6 months after surgery to rule out residual
HRCT can pick up acoustic neuromas of 2 cms
diameter. It can be performed if MRI scanning is
contraindicated in some patients.
neuromas should be differentiated from meningiomas in this
area. Meningiomas infact gives the same MRI imaging signal
characteristics as schwanomas. In meningioma there is
enhancement of adjacent dural trail, and the intracanalicular
component of the mass is generally absent. Calcifications
are commonly seen in meningiomas. Meningiomas commonly arise
from cerebello pontine angle.
present in the cerebello pontine angle provide a low (dark) signal
on T1 weighted MRI, and a high (bright) signal on T2 weighted
MRI. The signal intensity of the lesion is greater than that
of CSF. Arachnoid cysts of this area show a more or less
same signal intensity as epidermoid tumors, but they are difficult
to differentiate from CSF.
MR angiography will help in
the diagnosis of aberrent vessels in the CP angle area.
They also clearly show dilated vessels as seen in aneurisms.
Study of facial nerve:
The choice of imaging modality for study of facial nerve depends on
the suspected pathology and the segment of nerve involved.
When the entire course of the nerve needs to be imaged then a
conventional MRI will suite the bill. The complete examination
of the nerve should be done from the brain stem up to the level of
the parotid gland.
High resolution MR examination will
help in the study of facial nerve within the internal acoustic
meatus. But this type of scanning does not help
in identification of Bell's palsy. The intra
temporal portion of the facial nerve cannot be satisfactorily
examined by MRI, only a high resolution CT scan will give a clear
picture of the intra tympanic course of facial nerve. Temporal
bone fractures can be clearly shown only in a high resolution CT
scan. In cases of suspected tumors involving the facial nerve
HRCT and gadolinum enhanced contrast MRI will prove complimentary to
each other. MR imaging is not indicated routinely for Bell's
palsy. In the case of atypical Bell's palsy, then gadolinum
enhanced MRI should be performed to rule out tumors involving the
nerve. Normal MRI in patients with pure Bell's palsy will show
enhancement without focal enlargement of the nerve.
CSF leaks involving the temporal bone: can be studied using
HRCT. Contrast can be injected intrathecally before scanning
to clearly show the site of leak.
Imaging Petrous apex
area: This area is a very silent area. It provides very
few early symptoms, hence imaging is the only way to pick up early
lesions. Infact lesions of the petrous apex have been
incidentally identified on routine scanning of the area. HRCT
and gadolinum enhanced MR scanning help in identifying lesions of
this area. HRCT clearly shows bony destruction if present in
granuloma and epidermoid of the petrous apex can often be
differentiated by using T1 weighted Magnetic resonance
imaging. Cholesterol granuloma appears bright on T1 weighted
signal where as the epidermoid cyst appears dark on T1 weighted
Imaging jugular foramen area: To
examine jugular foramen area both HRCT and gadolinum enhanced MRI
scan should be performed. In glomus jugulare the bone
erosion seen in HRCT resembles a moth eaten appearance.
Examination of labyrinth: Congenital anamolies
involving the labyrith can be studied using HRCT in combination with
HRMRI. A perilymph gusher may be predicted before stapedectomy
by performing high resolution MRI. This is so when the CSF
signal is identical to the perilymphatic fluid signal at the
internal acoustic meatus.
Patients with complicated
middle ear inflammatory disease will have to under go MRI scanning
to rule out intra cranial complications. This scanning
modality also helps in the identification of sigmoid sinus
External auditory canal: Imaging of
external canal is performed only in cases of refractory otitis
externa. Scanning will help in identifying minor bone erosions
which could occur in patients with malignant otitis externa.
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