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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4399_Библиотеки_им_академика_М_И_Перельмана
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CHAPTER 3
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Radiology Techniques for Optimal Computed
Tomography and Magnetic Resonance Images
Sujana S. Chandrasekhar
CT SCANS
Computed tomography (CT) images have become indispensible in the imaging of the temporal bone, for inflammatory disorders, ossicular pathologies, and conductive and sensorineural hearing loss. In order to be
useful, temporal bone CT images should be targeted
and thin cut to demonstrate as much of the pertinent
anatomy as is useful.
Imaging techniques for temporal bone CT scans
are very different than those for head/brain CT scans.
The most important difference is that image slice thickness is not 5 mm or 10 mm as it is for head CT but,
rather, 1.5 to 2 mm, usually overlapping slices. Additionally, the window setting for temporal bone CT scans
is ideally at a bone window, with soft tissue imaging being very secondary, if at all useful. The modifications to temporal bone CT imaging techniques allow a
large number of structures previously either not visualized or poorly visualized. These include: the tympanic
membrane, the ossicles with tendons and ligaments,
the Eustachian tube and tensor tympani semicanal, the
cochleariform process, the round window niche, sinus
tympani, facial canal, and the cochlear and vestibular
aqueducts.
Techniques are explained more fully in the radiology literature,1 but they will be summarized here.
Common CT scanners contain detector arrays of 30 calcium fluoride crystals, each 2.5 × 3.5 mm. For temporal
bone scans, the detectors are collimated so that only
the central 1.5 × 1.5 mm are open to the x-ray beam.
The x-ray beam width is narrowed to 2 mm by a manual slide, and the slice thickness is collimated to 2 mm
by a removable stainless-steel tube-side collimator.
The head CT scanning algorithm is modified by
increasing the sampling rate by a factor of two and decreasing the translation arm speed to about 40 seconds.
These two software modifications as well as the decrease in detector size improve geometric resolution allowing for visualization of objects as small as 0.75 mm
in test algorithms. The image is then back-projected
onto 0.3 mm pixels and recorded.
Single–detector row CT with high spatial resolu tion
is a well-established imaging technique for exam ination of temporal bone abnormalities. To get diagnosticquality images in two different planes with single–
detector row CT, however, a second coronal acquisition
must be performed in addition to the transverse acquisition, which is highly dependent on the patient’s
mobility. Multi–detector row CT offers the potential to
overcome this obstacle because reformatted coronal images have sufficient quality. The high image quality is a
result of the thinner section thickness (0.5 mm instead
of 1 mm in single–detector row CT) and the smaller reconstruction increment. Multi-slice CT (MSCT) images
are acquired in a single imaging plane using a multislice detector scanner, with the patient on their back
and with no tilt to the gantry of the scanner. The raw
dataset images are reconstructed using an ultra high
resolution reconstruction mode, in a plane parallel to
the lateral semicircular canal. This is the axial imaging
set. The coronal imaging set is reconstructed exactly
perpendicular to it.
Cone beam CT (CBCT) uses a rotating gantry on
which an x-ray tube detector is attached. One single
360 degree gantry rotation is sufficient to acquire a 3Dvolumetric data set, and then from that data reconstructions can be made in any plane. Axial plane reconstructions can be as thin as 0.3 mm and coronal and sagittal
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Temporal Bone Histology and Radiology Atlas28
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Figure 3–1. Schematic representation of MSCT (fan-shaped beam) compared to CBCT (cone-shaped beam) image acquisition. (From http://image.slidesharecdn.com/cbct-160504143756/95/cone-beam-computed-tomography-in-dentistry-7-638.jpg
?cb=1462372724.)
images, 1 mm. CBCT has shorter examination time,
higher spatial resolution, and lower radiation dose than
does MSCT, but has a higher sensitivity for motion artifacts. For CBCT, the patient has to remain perfectly still
for 40 seconds acquisition time (Figure 3–1).
Routine CT imaging technique
This section is adapted from work by Hugh Curtin,
MD et al.2 The patient is placed supine in the gantry
and positioned to place the lens of the eye as far as pos-
sible out of the pathway of the x-ray beam to minimize
exposure to the lens. A lateral topogram is then performed. The scan excursion is plotted from the arcuate
eminence (the summit of the temporal bone) through
the mastoid tip.
Collimation is of optimal importance to achieve
high resolution. Specialized centers routinely use a collimator of 0.6 mm and most commercially available units
can be collimated to at least 1 mm. Collimation wider
than 1 mm is not usually used, as the resolution is often
insufficient for the delicate structures in question.
Milliamperage (mA) is a major factor in determining the quantity of x-rays produced and is, therefore,
3. Radiology Techniques for Optimal Computed Tomography and Magnetic Resonance Images 29
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a good indication of the type of examination that can
be performed with a machine. The mAs factor (time ×
milliamperes) affects film density by governing the
amount of x-ray photons that reach the film emulsion.
For 40 to 64 detector scanners, the effective mAs (defined as the mA × the gantry cycle time/helical pitch)
is adjusted according to the age and head size, attempting to maximize resolution while minimizing radiation
dose exposure, measured in milligray (mGy). Usually,
it is 150 effective mAs (CTDI
dex] 34 mGy) for neonates, 200 effective mAs (CTDI
[volume CT dose in-
vol
vol
45 mGy) for children ages 1 to 10 years, 250 effective
mAs (CTDI
57 mGy) for adolescents, and 320 (CTDI
vol
vol
72 mGy) for adults. The gantry cycle time is set at 1 cycle or gantry rotation/second. The kilovolt peak (kVp)
is usually 120.
Helical acquisitions allow for clearer coronal or
ob lique reformats and decrease susceptibility to motion
ar tifact. Intravenous (IV) contrast is rarely used. When it
is, which is for evaluation of vascular pathology, abscesses, or certain types of infection such as coalescent
mastoiditis, it is usually of the low osmolar type; it is
administered by power injector at standard doses of
1 mL/lb to a maximum of 80 to 100 mL for adults.
The raw data from each ear are separated and reconstructed into 0.6 mm (slice thickness) axial images in
bone algorithm at a dual field of view (DFOV) of 100 mm
that effectively magnifies the images. Then the 0.6 mm
images for each ear are brought up on the CT scanner
console, where the raw data are displayed in three orthogonal planes. The technologist scrolls through the
sagittal data to find an image where the anterior and
posterior limbs of the lateral semicircular canal are displayed in cross-section (Figure 3–2).
An axial dataset is then made in a plane parallel to
the lateral semicircular canal (LSCC). The technologist
“connects the two dots” of the LSCC and makes a
0.6 mm (image thickness) × 0.5 (distance between images) axial dataset in this plane parallel to the LSCC;
0.6 × 0.5 mm coronal images are made in a plane perpendicular to the axial images (Figure 3–3).
Pöschl (or sagittal) reformats are made by tracing a line parallel to the long axis of the summit of the
LSSC at six-tenths 0.5-mm intervals. The line must be
made as parallel as possible to the axis of the summit
of the LSSC, as a slight obliquity may spuriously obscure a dehiscence by volume averaging with the temporal bone on either side of the summit (Figure 3–4).
A study of 100 CT scans (50 single-detector and 50
multi-detector row images) evaluated by two radiologists, one of whom was specialized in head and neck
imaging and the other who was not, was undertaken
to evaluate image quality with respect to the detection
of subtle anatomic landmarks on transverse images by
using single– and multi–detector row CT techniques
and to compare the visibility of anatomic landmarks
on coronal single–detector row CT images with reformatted coronal multi–detector row CT images.3 A similar study was undertaken between two similarly experienced radiologists.4 There was high interobserver
agreement between transverse single– and multi–
detector row CT images. However, agreement between
both observers was higher for the transverse multi–
detector row CT images than for the transverse single–
detector row CT images. A similar result was obtained
for reformatted coronal multi–detector row images and
for coronal single–detector row images, and similarly,
interobserver agreement was higher for multi–detector
row CT. The overall findings suggested that detection
of temporal bone anatomy is less dependent on the experience of the radiologist with multi–detector row CT
than with single–detector row CT. This is important for
the otolaryngologist to keep in mind when evaluating
scans performed outside of academic or specialty head
and neck imaging centers.
Specic Ear Anatomy and Abnormalities
Identiable on Computed Tomography Scan
When using multi–detector row CT with a short scanner rotation time, the visibility of the ossicular ligaments was even higher than that found by others when
using a single–detector row CT technique with a relatively long rotation time of 2 seconds.
The tegmen tympani, a thin bony boundary between the tympanic cavity and the middle cranial fossa,
is commonly involved when cholesteatomas, tumors,
or fractures are present and shows dehiscence in 20%
of these cases. For these reasons, it is crucial to be able
to delineate this approximately 1-mm-thick bony structure, and this was found to be slightly, but not significantly, superior using MSCT.
In the temporal bone, the incidence of facial nerve
canal dehiscence varies between 25% and 74%. MSCT
detected FN dehiscence in approximately 60% of cases.
A histopathologic study of 1000 temporal bones
yielded a frequency of 0.5% of superior semicircular
canal dehiscence (SSCD) and a frequency of 1.4% of
severe bone thinning (bone thickness of only 0.1 mm
or less) of the superior semicircular canal.5 In the CT
study referenced above, MSCT identified SSCD five
times more than did single–detector row CT—5 of
Figure 3–2. Making the standard axial computed tomography dataset. (A) Once the source data are brought up in the three-
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dimensional (3D) viewer on the scanner console, the sagittal images are scrolled through until a sagittal image through the
anterior and posterior limbs (short white arrows) of the lateral semicircular canal (LSCC) are found.(B)A set of axial images is then generated with 0.1 mm overlap parallel to this plane. When correctly done, this should produce images such as
(C), where the entire lateral semicircular canal is displayed and (D), where the cochlea, modiolus, and stapes footplate are
clearly delineated. (Reproduced, with permission, from Caruso PA, Smullen JL, Liu R, Cunnane MB, Curtin HG. Temporal bone
imaging technique. http://radiologykey.com/temporal-bone-imaging-technique/.)
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Figure 3–3. Making the standard coronal computed tomography dataset. (A) The coronal reconstructions are made in a similar
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fashion. Starting with the sagittal image in Figure 3–2A, a plane perpendicular to the LSCC is established, and the coronal reformats are made along this plane. (B) This technique should yield a set of coronal images where the facial nerve canal (white
arrow) and scutum (lateral epitympanic wall) are clearly delineated. (Reproduced, with permission, from Caruso PA, Smullen JL,
Liu R, Cunnane MB, Curtin HG. Temporal bone imaging technique. http://radiologykey.com/temporal-bone-imaging-technique/.)
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Figure 3–4. How to make a Poschl reformat. (A) Once the source data are brought up in the three-dimensional viewer on
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the scanner console, the axial images are scrolled through until an image through the summit of the superior semicircular
canal (SSC) is found. (B) The Poschl reformats are made in a plane perpendicular to the long axis of the SSC, and (C) should
yield a view of the entire excursion of the SSC (short white arrows) from anterior to posterior. (Reproduced, with permission, from Caruso PA, Smullen JL, Liu R, Cunnane MB, Curtin HG. Temporal bone imaging technique. http://radiologykey.com
/temporal-bone-imaging-technique/.)
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3. Radiology Techniques for Optimal Computed Tomography and Magnetic Resonance Images 33
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50 patients versus 1 of 50 patients.3 Because the total
number of examined temporal bones in the radiology
study was small compared with that in prior histopathologic studies, those percentages are somewhat
inflated when considering the raw numbers.
Small ossifications of the basal turn of the cochlea,
which may be found after labyrinthitis or in cases of
otosclerosis, cause a bony obliteration of the lumen. It
is important to detect these bony changes before cochlear implant surgery to circumvent these problems.
In the CT study above, when the osseous spiral lamina,
which has a thickness of approximately 1 mm close to
the modiolus, is selected as an anatomic landmark to
be identified, multi–detector row CT, both transverse
and even reformatted coronal images, were significantly superior to the transverse and coronal single–
detector row CT images.
Imaging of the bony canals of the superior and inferior division of the vestibular nerve is possible with
single–detector row CT with high spatial resolution.
On single–detector row CT images, the canal of the
saccular nerve is detected as a lucency originating at
the fundus of the internal auditory canal and running
to the vestibule. However, the canal of the posterior
Figure 3–5. Axial CT image shows: 1, mastoid antrum; 2, aditus ad antrum; 3, epitympanum; 4, lateral semicircular canal;
5, vestibule; 6, labyrinthine segment of the facial nerve; 7, IAC; 8, posterior semicircular canal. (Reprinted, with permission,
from Juliano AF, Ginat DT, Moonis G. Imaging review of the temporal bone: Part I. Anatomy and inammatory and neoplastic
processes. Radiology 2013;269:1, 17–33.)
Temporal Bone Histology and Radiology Atlas34
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Figure 3–6. Axial CT image shows: 1, mastoid air cells; 2, incus (short process); 3, incudomalleal joint; 4, malleus (head);
5, epitympanum (anterior epitympanic recess); 6, basal turn of the cochlea; 7, middle turn of the cochlea; 8, otic capsule; 9, IAC;
10, modiolus; 11 , vestibule. (Reprinted, with permission, from Juliano AF, Ginat DT, Moonis G. Imaging review of the temporal
bone: Part I. Anatomy and inammatory and neoplastic processes. Radiology2013;269:1,17–33.)
ampullary nerve and the canal of the accessory nerve
of the posterior ampullary nerve, which has been variably described in the anatomy literature as existing in
5.6% to 100% of temporal bones, have not yet been imaged with single–detector row CT reliably. In 18% of
the temporal bones examined with multi–detector row
CT, a canal for the accessory nerve of the posterior ampullary nerve was found.
4
The superior performance of multi–detector row
CT in the delineation of anatomic landmarks on con-
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Figure 3–7. Axial CT image shows:1, stapes (head); 2, incus (long process); 3, malleus (neck); 4, tensor tympani; 5, carotid
canal; 6, apical turn of the cochlea; 7, middle turn of the cochlea; 8, basal turn of the cochlea; 9, interscalar septum; 10, round
window niche; 11, vestibular aqueduct; 12, sinus tympani; 13, stapedius; 14, pyramidal eminence; 15, mastoid portion of the
facial nerve; 16, facial recess. (Reprinted, with permission, from Juliano AF, Ginat DT, Moonis G. Imaging review of the temporal
bone: Part I. Anatomy and inammatory and neoplastic processes. Radiology 2013;269:1,17–33.)
ventional and reformatted images, the smaller dependency on the observer skills in image evaluation, and
the higher certainty in the delineation of anatomic landmarks in comparison to single–detector row CT supports the use of multi–detector row CT in the diagnosis
of temporal bone abnormalities.
For purposes of histologic comparison, the axial
images conform to the horizontal temporal bone sections, and the sagittal images correspond to vertical
temporal bone sections. Histologic specimens are not
obtained in the coronal plane. When comparing the
images side by side, it is important to remember that
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