| Emergency Craniotomy | ||||||||||||||||||||||||||||||||||||||||||||||
Imaging
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Introduction
Following resuscitation, imaging of the head trauma victim’s head and neck should be done as quickly as possible. Intracranial imaging is essential not only for identification of surgical lesions but also for choosing, planning, and setting up the optimal craniotomy. Imaging
provides information complementary to that obtained from clinical
history and physical examination.
Without imaging, the physical examination and Glasgow Coma Scale (GCS) score may have to be
the sole basis for surgical decision making (with or without
placement of exploratory burr holes). Computerized
tomography of the head (“head CT”) is the current standard for
intracranial imaging following head trauma, although alternatives
(such as MR or angiography), should be available in the event that
CT scanning is not. Once the surgeon understands the principles underlying the acquisition, processing, display, and reproduction of head CT scans, he can learn how to read them. CT scan reading requires not only detection of pathology but also interpretation and diagnosis of abnormalities based on knowledge of the normal anatomic disposition of intracranial structures as well as imaging characteristics of injured brain tissue and traumatic hematomas. Principles
The
principles of intracranial CT scanning important to reading and
interpretation of these studies in head trauma victims are
technical and related to the image characteristics of injured
intracranial contents. (i)
Technical aspects Technical
aspects of head CT scanning include those of image acquisition,
processing, display, and reproduction.
The technical features that must be understood to
accurately interpret a CT scan are: 1.
Greyscale, 2. Windows, 3.Contrast enhancement, 4. Axis of
acquisition, 5. Interval between slices, 5. Three dimensional
reconstruction, and 6. Artifact.
Sections “Tomo”-
comes from the Latin for “to cut” (craniotome, osteotome, …). Xray
information on a single plane across an object is translated by
computer calculation into an image of the transected object along
the plane. Every
object that lies in the plane of x-ray beams at that level appears
on this tomogram. One
of the limitations of CT scans is that they are images in only the
axial plane. The
resolution of so-called reformats-coronal or sagittal images
created from serial axial image data-is only as good as the
distance between axial cuts.
The computer can reconstruct Sagittal and coronal views but
they are far less accurate and far less clear than the original
axial images. Even at
5 mm between cuts, the resulting reformatted image resolution is
less than that of the axial images from which it was derived,
which attain 1 to 2 mm resolution.
The axial cuts of the CT scan are taken 20 degrees off the
orbitomeatal line to avoid catching too many air-filled sinuses in
one cut. Also,
structures or lesions running parallel to but at a distance from
the axis of the x-rays may be missed by a CT image.
In addition, linear fractures without displacement of bone
may be thinner than the resolution of a given scanner. (b)
Greyscale The
display of a CT scan is simple: two dimensional, black and white.
CT scan looks inside intracranial tissues with a display “greyscale” (degrees of black and white) based not on optical
properties of a tissue surface but rather on the density
characteristics of the tissue volume. Plain
x-rays and CT scans are images resulting from differential
penetration of x-rays through tissues of different densities.
As on a plain x-ray, denser objects on CT are lighter, and
less dense objects darker. On
a cranial CT, bone is white, air is black, and brain tissue is
intermediate shades of gray.
The
Hounsfield unit (named after one of the inventors of CT)
quantitates the gradations from black to white on a density gray
scale. Individual tissue types, such as gray matter, white matter,
and fat, each have their characteristic Hounsfield value, which
can be used to determine density (and, therefore, type) of tissue.
Fat
is very low density, even less than CSF: on CT it is closest in
density of all tissues to air.
Within the parenchyma, the darker areas are less dense,
thus the “gray” matter of gross specimens appear lighter on CT
than the corresponding gross “white” matter. FIGURE:
Hounsfield units (c)
Windows Windows
are portions of the grayscale selected for attention and analysis.
Windows vary contrast characteristics within the image to
highlight or obscure tissues within certain selected density
ranges. The
brightness and contrast of the image can be adjusted to highlight
tissue of any selected density while de-emphasizing surrounding
tissue of other densities. There
are three brightness-contrast tissue settings, or “windows,” which are programmed into the computer for most head scans: The
“bone window” focuses on tissue of the uppermost
limits of the Hounsfield scale-the densest structures- with
virtual dropout of all others. This setting enables visualization of most fractures of
cranial vault and base. Images
focused on low-to-intermediate Hounsfield numbers highlight the
soft tissues of the brain parenchyma as well as extracranial soft
tissue (muscle, skin, and subgaleal space) and make up the “soft
tissue window”. A
separate “subdural window” focuses on recently
coagulated blood that might otherwise be difficult to distinguish
from underlying brain tissue, using the wider “soft tissue” settings. FIGURE:
Bone, soft tissue, and subdural windows (d)
Contrast Contrast
is used to demonstrate intracranial structures that have an absent
or deficient blood brain barrier.
Head trauma does not diffusely disrupt the blood brain
barrier and is therefore not an indication for the use of contrast
enhancement. FIGURE:
With and without contract (e)
Axis The
axis of the plane of two dimensional cross sections on standard CT
scans throughout the world is 20 degrees elevated with respect to
the inion-glabella line. The
axis of cross section in a head trauma CT scan is usually set to
20 degrees ____. Because the plane of the axial cuts is 20 degrees
off parallel to the plane of the line from familiar external
landmarks, such as the glabella and inion, the structures at each
level are not those of axial cuts orthogonal to the rostrocaudal
axis of the head. With
each successively higher view, the structures visualized are more
posterior than those that would have been seen on slice, parallel
to the glabella-inion line. FIGURE:
Angulation of axial CT scans (CT axis angle.tif) The
uppermost cut on the CT scan does not correspond to the uppermost
part of the patient’s head (i.e., the vertex).
Brain structures that appear far anterior on successive CT
scan slices are actually more posterior as the image series
progresses from bottom to top.
The scout view is useful in remembering the axis
orientation with respect to the cranium and extra-and intracranial
structures. (f)
Interval The
usual distance of 5 mm between respective axial sections provides
a resolution that is usually greater than that required evaluating
an extra-axial hematoma. When
time is limited, greater slice width, for example 10 mm or greater
gives fewer scans more quickly. (g)
3D The
computer can generate reconstructed images of the sagittal and
coronal planes but these take a long time and provide no
information that the surgeon does not already have from the axial
images. (h)
Artifact Technical
artifacts can result in display image abnormalities that can
erroneously be interpreted as pathologic and requiring
intervention. Image
artifacts are distortions of the CT image due to the technique of
processing the image rather than any factor intrinsic to the
patient’s anatomy. Computer-processing artifact increases directly and
dramatically at areas where plain x-ray beams incompletely
penetrate tissues such as thick bone (e.g., that anterior in the
posterior fossa). For this reason, CT is not a good technique for visualizing
the brainstem, fourth ventricle, and cerebellum. Artifacts
are spots, lines, shapes, densities generated by the computer and
displayed on the CT image that have no physical reality.
Computer-processing artifact increases directly and
dramatically at areas where plain x-ray beams incompletely
penetrate tissues such as thick bone (e.g., that anterior in the
posterior fossa). For
this reason, CT is not a good technique for visualizing the
brainstem, fourth ventricle, and cerebellum. FIGURE:
Posterior fossa artifact (ii)
Tissue imaging characteristics Brain,
CSF, and blood are the tissue types whose imaging characteristics
must be understood in order to accurately make pathologic and
monitor lesion evolution. (a)
Brain tissue The
low cellularity and high fat (myelin) content of the “white
matter” as seen in gross sections corresponds to darker areas on
CT. Heavily
cellular “gray” matter appears lighter on CT than the “white” matter. Gray
and white matter can be differentiated on CT scans because gray
matter is more cellular (and thus denser) than myelin-rich white matter.
Normally there is a distinct border between the gray and
white matter on the CT soft tissue windows (latter appears lighter
than former). Loss of
the gray-white boundary is suggestive of diffuse cerebral edema. Cerebral edema is associated with an increase in the water
content of the gray matter, which reduces its density (and
Hounsfield value), making it darker (closer in appearance to white
matter). Increased
intercellular water in the grey matter, as with post-traumatic
edema decreases the overall density of the tissue towards that of
the adjacent white matter. As their densities become closer, the contrast between
grey and white matter on CT decreases – an important finding
suggestive of cerebral swelling. FIGURE:
Grey-white demarcation Evaluation
of the brain surface includes looking for subarachnoid blood,
which is frequently present in quantities that are insufficient to
layer out enough for detection of a CT scan. (b)
CSF Cerebrospinal
fluid (CSF), like water, is less dense than brain parenchyma and
bone, so it appears darker on CT than these.
CSF is denser, and therefore slightly less black, than air.
When blood mixes with CSF it changes its imaging
characteristics making it brighter (increased density) on CT.
Blood cells and heavier debris are bright and layer out (“hematocrit” effect) in dependent (in the patient supine during CT scanning)
CSF-filled spaces such as the posterior (occipital) horns of the
lateral ventricles. Replacement
of CSF density cisterns by brain parenchymal density tissue (“obliteration
of the cistern”) indicates that brain tissue, either globally or
focally in the region adjacent to that cistern, is swollen. If
the cistern is next to or surrounds the brainstem, its
obliteration is a sign that brain tissue from another compartment
(i.e., herniated) is pressing on the brainstem at that site. (c)
Hematoma Blood
density depends on the concentration of cellular elements and the
balance between clot formation and lysis.
Clotted blood has numerous fibrin and other protein
coagulation substances that form a tight meshwork to stop the flow
of blood. Blood cells
are more densely packed in this meshwork than freely floating in a
liquid. Blood can
appear very dense when it first clots, but over time the protein
meshwork breaks down, is dissociated by the activity of
fibrinolytic enzymes, and the blood gradually returns to a more
liquefied state. Concomitant
with the dissolution of the protein coagulant meshwork is lysis,
or breakup, of the cellular components of the blood that have
died. As these
cellular components break down and are resorbed, the fluid that
remains in less dense than fresh blood.
The
consistency of the hematoma correlates to density and is therefore
well visualized on CT. Low
density hematomas are fluid in consistency.
High density = solid.
Thus, blood at different stages in different conditions can
appear more dense than brain parenchyma, although always less
dense than bone. It
can appear of approximately the same density as the parenchyma and
thus be difficult to distinguish from it.
After a long period of time (possibly 2 weeks or longer),
blood can appear much less dense, sometimes even comparable in
density to that of CSF. However,
blood, when mixed with less dense CSF, tends to sink down, leaving
the CSF above it. When
the patient’s head is laid flat in the gurney, the blood in the
ventricles can be seen to layer out in a straight line, forming a
meniscus – the so-called “hematocrit effect.” Layering
out of blood in a low- density fluid collection is indicative of a
subdural hematoma. This
finding is important in planning treatment because it indicates
that the likelihood of complete clot evacuation through one or two
burr holes over the lesion will be sufficient for its complete
evacuation and that a full craniotomy will be required. FIGURE:
Hematocrit effect in hematoma (hematocrit effect.tif) Blood
vessels can be, but are not reliably, visualized by head CT.
Although best visualized following administration of
intravenous (i.e., intravascular) contrast, major vascular
structures can be visualized by their density and round shape. Active
bleeding into the subarachnoid or epidural space may be detected
on CT scan because of the different density of blood collections
of differing ages. Fresh
bleeding under pressure has a lower-density appearance than that
of coagulated blood into which it forces itself, thus, an actively
bleeding epidural hematoma may appear as a high-density “biconvex
lens” with low-density streaming bands.
Active or recent acute bleeding from the bridging veins
into a preexisting low-density chronic subdural hematoma (the
so-called subacute subdural hematoma) appears as a lower-density
area juxtaposed onto a higher-density layer. FIGURE:
Active bleeding into a hematoma (active bleeding.tif) (b)
Reading
The
reading of a CT scan involves four distinct stages:
1. Organization, 2. Orientation, 3. Diagnosis, 4. Decision
making. (i)
Organization Organized
reading of a CT scan begins with arranging the films, verifying
the identity of the patient, looking at the scout and, finally, at
the axial views. Head
CT reading is an assessment of normal and normal anatomy at each
layer on the three window views in the order of their display:
bone, soft tissue, and subdural hematoma.
Frequently, a striking finding is accompanied by other
findings that although subtle, are important for management of the
patient. For example,
if detected preoperatively, the skull fracture above an epidural
hematoma can be deliberately incorporated into the craniotomy bone
flap. The ipsilateral
temporal bone hairline fracture probably explains the isolated
facial weakness better than the 1-cm contralateral subdural
hematoma. (a)
Films If
a separate film has been made for each of multiple windows, the
films should be lined up for viewing -- left to right, top to
bottom. First
(from left) in the display sequence are bone windows, followed by
soft tissue, and subdural windows. CT
reading is organized to evaluate each layer on the three window
views in the order of their display: bone, soft tissue, and
subdural hematoma. (b)
Demographic The
demographic panel with the patient’s name, sex, and date of
birth should be compared with the information obtained from both
the patient’s chart and an attached name band. (c)
Scout The
scout view is a useful reference for the axis of orientation of
the cuts with respect to extra-and intracranial structures.
Because the plane of the axial cuts is 20 degrees off
parallel to the plane of the line from familiar external
landmarks, such as the glabella and inion, the structures at each
level are not those of axial cuts orthogonal to the rostrocaudal
axis of the head With
each successively higher view, the structures visualized are more
posterior than those that would have been seen on slice, parallel
to the glabella-inion line. FIGURE:
Scout views CT. The
scout image is analyzed before the axial sections. Parallel lines
from the base of the skull to the vertex are displayed and
numbered on the scout view. Finding
the corresponding number on any given slice makes it possible to
determine the level of that slice with respect to external head
landmarks. External landmarks visible grossly and on CT provide
the surgeon with a stereotactic reference for surgical target
localization. The
uppermost cut on the CT scan does not correspond to the uppermost
part of the patient’s head (i.e., the vertex).
Instead, it corresponds to a few centimeters posterior.
Brain structures that appear far anterior on successive CT
scan slices are actually more posterior as the image series
progresses from bottom to top.
The scout view is useful in remembering the axis
orientation with respect to the cranium. (d)
Axial sections Read
the sequence of axial cuts from the base of skull to vertex (i.e.,
upper to lower; left to right).
Read each axial cut starting from the outside and moving
inward, identify asymmetries of structure and tissue density, and
inspect the midline for left or right shifts. FIGURE:
CT reading sequence CT
axial cuts should be read left to right, top to bottom.
Each slice is read starting from the outside and moving
inward, identify asymmetries of structure and tissue density.
Structures in the midline are identified and assessed for
left or right shifts. (ii)
Orientation Orientation
to an individual scan requires identification of certain basic
anatomic landmarks (usually bone) present on all, and relies
heavily on the symmetry of structures to the right and left of
midline. (a)
Geometry The
midline is the most important geometric feature of head CT scans.
It results from the right-left bilaterally of many of the
intracranial structures. Midline shift refers to displacement of
normally midline structures (e.g., the pineal body, the falx, the
third ventricle, the fourth ventricle, a large prepontine
vertebral artery) to the right or left. Even
to those without knowledge of intracranial anatomy the arrangement
of the middle cuts of a CT scan is reminiscent of a face: The
anterior horns of the lateral ventricles are the eyes, the midline
third ventricle the nose, and the quadrigeminal cistern, the
smiling mouth: the face results from the symmetry of its
constituent structures. Symmetry
of paired structures such as the globes, lateral ventricles
(frontal, occipital, and temporal horns), choroid plexus (often
calcified), cerebral peduncles, colliculi, Sylvian fissures, and
basal cisterns is the norm against which purported asymmetry of
any of these structures should be evaluated. “Mass
effect” refers to alterations in symmetry due to unilateral
compression of solid structures, effacement is a decrease in the
volume of those that are fluid-filled.
The
pineal body is the most easily recognized midline structure (that
can be displaced by pathologic swelling of mass effect).
The pineal is not only in the midline but is usually
calcified (thus visible without contrast).
The falx or portions of it can be identified if calcified
or overlain by a (subdural) hematoma. TABLE:
CT geometry
FIGURE:
Degrees of shift and mass effect
(b)
Anatomy Anatomic
analysis of axial views requires familiarity with the appearance
of the major intra-and extracranial structures on CT. (i)
Extracranial Anatomic
analysis of CT scan layers begins with the extracranial soft
tissues. Although the CT resolution is not fine enough to show the
individual layers of the scalp, certain extracranial pathology
will show up. Disruption
of the normally smooth contour of extracranial soft tissue layers
is an important CT finding.
Although the CT resolution is not fine enough to show the
individual layers of the scalp, certain extracranial pathology
will show up. Frequently,
based on this evaluation when the intracranial compartment is
normal, the diagnosis of head trauma is conclusively made.
Irregularities
of the scalp include high-density swellings consistent with acute
blood, so-called cephalohematoma, between the galea and
periosteum. Radio-opaque
foreign bodies, such as bullet and bone fragments, can be seen in
cephalohematomas. The next layer in is low-density subcutaneous
tissue (fat). The
ears stick out from the scalp in the midlevel cuts. Radio-opaque
foreign bodies, such as bullet and bone fragments, can be seen in
cephalohematomas. In comatose patients “found down” (coma
etiology unknown) with intracranial CT negative for pathology,
cephalohematoma on extracranial CT is the frequently the basis for
the diagnosis of “head trauma”. The
next layer in from the skin surface is low-density subcutaneous
tissue (fat). The
ears stick out from the scalp in the midlevel cuts.
The external auditory canal and pinnae are readily
visualized and are useful reference landmarks for vertical
orientation. (ii)
Skull The
next extracranial to intracranial layer is the calvarium.
The internal and external contours of the skull are smooth.
Discontinuities and step-offs involving dense cortical bone
are readily identified. At
the skull base, look for any lines out of place or the distortion
of the normally circular or ovoid temporal and petrous bony
foramina. SKULL
BASE The
status of the bony structures of the skull base can be assessed
from careful review of the CT bone windows.
The petrous bone and its many foramina (holes), which are
the sites for cranial entry and exit of major blood vessels and
cranial nerves, can be seen on CT bone windows (FIGURE: 2-10).
The foramina rotundum (passageway for second division of
CN5). The ovale (the third division of CN5), and the spinosum
(which admits the middle meningeal artery) are all distinguished. The
internal auditory canal is seen if the axial cut intersects it.
Other easily identified skull base structures include the
orbits (which are filled with fat) and the aerated bony sinuses,
including the frontal, maxillary, ethmoid, and sphenoid sinuses as
well as the mastoid air cells.
Bleeding into the air-filled sinuses can appear as a
meniscus of fluid within the normally air-filled (black) space or
as its complete opacification and suggests a fracture of the basal
denomination frequently associated with Battle’s sign.
Maxillary sinuses are unaerated (gray) when filled with
blood, but are black when filled with air.
Maxillary sinus opacification is the CT correlate of “raccoon’s
eyes” seen on physical examination.
The middle ear is normally aerated; therefore,
opacification at this site may suggest an accompanying
hemotympanum. Look
for fractures of maxillary, frontal, and ethmoid bones.
Next, look at the intraorbital contents: The contents of
the bony orbits are black (fat).
The optic nerves originate at the back of the globe and
extend back towards the optic canal. Disruption of the optic nerve is sometimes discernible on a
cut through the orbit. However,
disruptions in the bone and fragment placement are also suggestive
of optic nerve compression, distortion, or transection. Although
the foramen magnum is well visualized on CT, herniation at this
level is difficult to visualize because of bone artifact in the
posterior fossa. FIGURE
Petrous foramina The
integrity of the bony structures of the skull base can be assessed
from careful review of the CT bone windows that show the petrous
bone and its many foramina (entry and exit holes) for cranial
blood vessels and nerves. The
foramina rotundum (passageways right and left for second division
of CN5), the ovale (for third division of CN5), and the spinosum
(admits middle meningeal artery) can all be seen on most
contemporary CT scans. The
internal auditory canal is seen if the axial cut intersects it and
is a guide to the location of temporal bone fractures. The middle
ear is normally aerated; therefore, opacification at this site may
suggest an accompanying hemotympanum. Other
easily identified skull base structures include the orbits (which
are filled with fat) and the aerated bony sinuses, including the
frontal, maxillary, ethmoid, and sphenoid sinuses as well as the
mastoid air cells. The
optic nerves originate at the back of the globe and extend back
towards the optic canal. Disruption of the optic nerve is sometimes discernible on a
cut through the orbit. However,
disruptions in the bone and fragment placement are also suggestive
of optic nerve compression, distortion, or transection. Bleeding
into the air-filled sinuses can appear as a meniscus of fluid
within the normally air-filled (black) space or as its complete
opacification and suggests a fracture of the basal denomination
frequently associated with Battle’s sign.
Maxillary sinuses are unaerated (gray) when filled with
blood, but are black when filled with air.
Maxillary sinus opacification is the CT correlate of “raccoon’s
eyes” seen on physical examination.
Although
the foramen magnum is well visualized on CT, herniation at this
level is difficult to visualize because of bone artifact in the
posterior fossa. CRANIAL
VAULT Fractures
of the cranial vault are identified by reference first to the
cranial sutures, which are symmetric.
Fractures occur along cranial suture lines, but when they
do, the width of the lucent lines is greater than that of the
nonfractured suture line. These
suture lines include the coronal, lambdoid, and sagittal sutures,
any of which can be confused with a fracture. Disruptions
of the smooth internal and external contours of the cranial bones
(inner and outer tables) suggest fracture.
Low-density lines traversing the bone can represent
fractures. Facial
fractures, particularly those involving the zygoma, maxilla, and
orbits are visible on CT. The
air-filled sinuses provide good contrast for detection of
fractures through their surrounding high-density, white bony
walls. Fractures of the facial bones on CT can appear at disruptions
of cortical margins or lucent lines through bone. Fractures
of the cranial vault are identified by reference first to the
cranial sutures, which are symmetric.
Fractures occur along cranial suture lines (“diastatic”
(<???) fractures), but when they do, the width of the lucent
lines is greater than that of the nonfractured suture line.
These suture lines include the coronal, lambdoid, and
sagittal sutures, any of which can be confused with a fracture. Disruptions
of the smooth internal and external contours of the cranial bones
(inner and outer tables) suggest fracture.
Low-density lines traversing the bone can represent
fractures. Facial
fractures, particularly those involving the zygoma, maxilla, and
orbits are visible on CT. The
air-filled sinuses provide good contrast for detection of
fractures through their surrounding high-density, white bony
walls. Fractures of the facial bones on CT can appear at disruptions
of cortical margins or lucent lines through bone. (iii)
Dura The
dura, the next layer moving inward, can be visualized in some but
not all locations on CT. Dural
structures that can be evaluated on the CT scan include the
tentorium and the falx. Sinus thrombosis appears as a high density in the region of
the torcular Herophili. The
sagittal sinus is often seen as a lower-than-dural density
triangular structure just inside a higher density triangle of
dural leaves. Thrombosis
of the sinus can result in uniform density of the sinus triangle
and its content. Midline
shift and mass effect can displace dural structures.
Because it is only a few millimeters thick and in a plane
not parallel to the 20 degrees of the axial images, the tentorium
is not usually well visualized on CT.
The falx, denser than the adjacent parenchyma and
frequently calcified, can appear nearly as dense as bone. Air
in the subarachnoid space with low-density blackness in sulci is
the best sign that the dura has been perforated at some point.
In the event that subarachnoid air is seen, its source may
be found on review of the bone window film for facial sinus or
mastoid fracture. Subarachnoid air (frequently reported as “pneumocephalus”)
is not always associated with CSF leak but frequently is when
associated with mastoid or frontal fracture.
An
irregularity of the skull, with a portion of it placed interiorly,
may have been the site of a laceration of the dura.
There may be a high-density mass suggestive of contusion or
bleeding in brain substance just under the dura at the site of a
depressed fracture fragment. A
hematoma formed by high-pressure arterial blood tends to displace
the dura, dissecting it off of the overlying cranium and thereby
forming a hematoma that is shaped like a biconvex lens.
Although not absolutely pathognomonic, the “lens” configuration of the hematoma, especially in the presence of as
temporal fracture of the skull without prominent swelling of the
underlying brain, is suggestive of a local impact epidural
hematoma. A
subdural hematoma that is formed by transection of low-pressure
veins, spanning the space from the surface of the brain to the
sagittal or other dural sinuses, fills the space above the
arachnoid and below the dura.
This subdural hematoma follows a path of low resistance,
assuming a crescent shape. Because
the force required to tear the bridging veins is greater and
involves more displacement of the brain with respect to the skull,
small subdural hematomas can be associated with disproportionate
amounts of cerebral swelling with midline shift. Because
the dura is so thin compared with the other structures and the
resolution of the CT scanner is not good enough to detect
structures so thin, the dura may be identified only by default.
A hematoma formed by high-pressure arterial blood tends to
displace the dura, dissecting it off of the overlying cranium and
thereby forming a hematoma that is shaped like a biconvex lens.
Although not absolutely pathognomonic, the “lens” configuration of the hematoma, especially in the presence of as
temporal fracture of the skull without prominent swelling of the
underlying brain, is suggestive of a local impact epidural
hematoma. (iv)
Brain surface Evaluation
of the brain surface on a CT scan includes looking for
subarachnoid blood, which is frequently present in quantities that
are insufficient to layer out enough for detection on head CT. Blood
vessels can be, but are not reliably, visualized by head CT.
Although best visualized following administration of
intravenous (i.e., intravascular) contrast, major vascular
structures can be visualized by their density and round shape. (v)
Gray and white matter Gray
and white matter can be differentiated on CT scans because gray
matter is more cellular (and
thus denser) than myelin-rich white matter.
Normally there is a distinct border between the gray and
white matter on the CT soft tissue windows (latter appears lighter
than former). Loss of
the gray-white boundary is suggestive of diffuse cerebral edema.
Cerebral edema is associated with an increase in the water
content of the gray matter, which reduces its density (and
Hounsfield value), making it darker (closer in appearance to white
matter). (vi)
CSF spaces Replacement
of CSF density cisterns by brain parenchymal density tissue (“obliteration
of the cistern”) indicates that brain tissue, either globally or
focally in the region adjacent to that cistern, is swollen. If
the cistern is next to or surrounds the brainstem, its
obliteration is a sign that brain tissue from another compartment
(i.e., herniated) is pressing on the brainstem at that site. Cisterns
that can be asymmetrically obliterated (and that should be
evaluated on every head CT scan) include the cisterna magna,
mesencephalic cistern, prepontine cistern, and all sulci and gyri.
Loss of the contour of the CSF space over gyri and within
sulci suggests tissue expansion and mass effect. Posterior
fossa cisterns that can be evaluated on CT and that are important
in detection of phenomena such as herniation include the cisterna
magna, which appears as a CSF density space posterior to the
cerebellum. The
perimesencephalic cistern (surrounding the midbrain and site of
exit from the brainstem of the third cranial nerve) is also
important in assessing brainstem compromise.
Herniation of the mesial portion of the temporal lobe (the
uncus) over the tentorium, into the perimesencephalic cistern,
with compression of the third nerve shortly after its exit from
the midbrain, results in the characteristic “blown” pupil..
Asymmetry of this posterior fossa cistern is most ominous in terms
of impending or evolving brainstem compression. The
interhemispheric cistern and that around the pineal body may be
asymmetric in the presence of hemispheric swelling.
The size, configuration, and location of the cerebral
ventricles can also be altered by head trauma.
Effacement refers to a decrease in the size of CSF-filled
spaces seen on CT and thus is applicable to ventricles as well as
cisterns and sulci. The
appearance of the anterior and posterior portions of the lateral
ventricles is mirror images on axial CT cuts.
Asymmetry of the ventricles usually means that asymmetric
intrahemispheric forces are compressing one or the other.
The third ventricle appears on CT as a vertically oriented
line between two lobes of the thalamus.
The fourth ventricle is located between the pons and the
cerebellum. Shift of
either normally midline CSF-filled cavity suggests the presence of
mass effect. FIGURE:
Cisterns and cerebral edema (c)
3D The
surgeon needs to be able to mentally construct a three-dimension
representation of intracranial lesions and nearby structures from
the series of axial CT images. With
other forms of x-ray, objects are localized in three-dimensional
space by determining the object’s location in the intersection
of two flat planes. With
multiple tomographic views, the three-dimensional plane is
mentally reconstructed by stacking axial views vertically with
intervening extrapolation, creating a virtual image of the head
and brain. The
x and y axes define the plane of the CT axial images.
The x axis moves left to right; the y axis moves up and
down. The z axis is
orthogonal to the xy plane. The CT scan provides explicit visual information about the x
and y axial locations of structures on different slices, with
respect to a z (vertical) axis orthogonal to the plane of the
axial cuts. The
surgeon must use anatomic clues, such as structures readily
recognized on the axial views, to determine the location of a
given slice with respect to the z axis of the patient’s head. The
pinnae of the ears, the glabella of the nose, and the coronal
suture are readily seen extracranial landmarks for localization
along the z axis. The pinnae are at the level of the floor of the
temporal fossa; the glabella is at the floor of the frontal fossa. Intracranial
landmarks, such as the orbital floor, the mastoid air cells,
pineal calcification, third ventricle, and cerebral peduncles, and
Sylvian fissures are also useful for determining vertical lesion
location. The
internal auditory canal is a divot in the petrous bone that is a
useful intracranial landmark for localizing the pons at the level
of the facial and vestibulocochlear nerve complex. FIGURE:
Internal auditory canal on CT (IAC on CT.tif) (iii)
Diagnosis Detection
of CT abnormalities and their correct pathologic attribution
depends on knowledge of the normal appearance and anatomic
anatomic arrangement of universally present structures.
Fractures, hematomas, edema, and mass effect are the
pathologic abnormalities with whose CT appearance the head trauma
surgeon must be familiar in order to intelligently and effectively
diagnose and plan treatment. (a)
Fracture (i)
Hematomas The
head CT provides information about an intracranial
hematoma-location, size, and consistency-that is important in
planning a craniotomy. The
location must be determined in relation to the skull, the dura,
and the brain parenchyma, as well as to both intra- and
extracranial lobes and surface landmarks. Structures contiguous to a traumatic hematoma, potentially
injured in evacuation of same, should be identified on the CT
preoperatively so that they are approached expectantly rather than
stumbled on or into at surgery. The
size of the hematoma is measured with reference to the vertical
line with centimeter interval tick marks seen on each axial image.
The volume is calculated as the length and depth of the
hematoma, multiplied by the number of slices, then divided by the
centimeter interval between the slices. Active
bleeding can be identified by the tissue characteristics described
above. (ii)
Edema (iii)
Mass effect (iv)
Planning (a)
Exposure (b)
Craniotomy (c)
Hazards TABLE:
Head CT abnormalities in the head injured patient (2)
Angiography
Some
trauma centers do not have CT scanning available.
Vascular displacements seen on angiogram were used pre-CT
to determine if an extra-axial or intraparenchymal clot is
present. Angiography
may also be helpful if a patient lost consciousness before head
trauma, suggesting a possible bleed from an intracranial vascular
malformation as the cause of a fall, motor vehicle accident, etc.
The unwary surgeon who approaches the clot as if it were
nothing more than a traumatic intracerebral hematoma may, within
seconds, be inundated with arterial blood on entry into
arteriovenous malformation. An
unusual contusion location (namely, above the floor of the
anterior of middle cranial fossae) is a clue that a hematoma is
caused by something other than head trauma, such as an anterior
communicating artery rupture.
When hemorrhages occur in locations other than at the bases
of the frontal or temporal lobes typical for traumatic contusions,
a preoperative angiogram should be considered to rule out a
vascular malformation. Some
trauma centers do not have CT scanning available. These centers must rely on examination of cerebral
vasculature through use of an angiogram to determine if an
extra-axial or intraparenchymal clot is present. Angiography
may also be helpful if a patient lost consciousness before head
trauma, suggesting a possible bleed from an intracranial vascular
malformation. The
unwary surgeon who approaches the clot as if it were nothing more
than a traumatic intracerebral hematoma may, within seconds, be
inundated with arterial blood on entry into arteriovenous
malformation. An
unusual contusion location (namely, above the floor of the
anterior of middle cranial fossae) is a clue that a hematoma is
caused by something other than head trauma, such as an anterior
communicating artery rupture.
When hemorrhages occur in locations other than at the bases
of the frontal or temporal lobes typical for traumatic contusions,
a preoperative angiogram should be considered to rule out a
vascular malformation. c)
Social
If
historical, physical examination, and imaging data consistently
indicate surgical intervention, social (including legal)
considerations must be factored into the decision as to whether or
not to operate to remove a traumatic intracranial hematoma.
Family members know about living wills or other advance
directives which should be respected during the decision-making
process. Very
old patients, patients with terminal diseases (cancer, end stage
HIV infection,… may not be surgical candidates regardless of
their GCS or the size of an intracranial hematoma.
While the social and ethical controversies surrounding the
concept and management of brain death are many, there is universal
agreement that patients actually or iminently brain dead, there is
no indication for surgical intervention.
The
neurotrauma surgical decision with the greatest social and ethical
overlay is whether or not to operate on patients with GCS of 3 and
4. Although the
surgeon may have strong personal and religious reasons for
opposing surgical intervention, the family’s are precedent.
Surgical informed consent conversations can drag on for
hours while families agonize over the quality-versus-preservation
of life decision. TABLE
Consent for and refusal of treatment
d)
Grade
A
Surgical Candidacy Grading system quantitates findings and can be
the basis for surgical decision making. TABLE
Surgical Candidacy Grade
2. Implementation< |