Introduction to Clinical Neuroradiology Flashcards
Computerized tomography (CT) was developed directly
from conventional X-ray technology and therefore shares
many of the same principles. Like conventional X-ray radiographs,
CT scans measure density of the tissues being
studied. There are really only two differences from
conventional X-rays:
- Rather than taking one view, the X-ray beam is
rotated around the patient to take many different
views of a single slice of the patient; hence the
term “tomography” (from the Greek tomos, meaning
“section”). - The X-ray data acquired in this way are then
reconstructed by a computer to obtain a detailed
image of all the structures in the slice (including
soft tissues, liquid, and air, as well as bone); hence
the term “computerized.”
spiral (helical) CT scanners have been developed that can acquire data
continuously as the patient moves through the scanner ring, without requiring
stops. In addition, instead of acquiring single slices, up to
256 rows of
detectors are now being used so that multiple overlapping slices can be obtained.
These technical advances reduce patient radiation exposure and also
greatly improve the resolution and speed of CT scanning
As in conventional X-rays, dense structures like bone or other calcifications
appear
white in CT scans, and less dense materials such as air appear black
hyperdense used to
refer to
brighter areas on CT scans
hypodense used to
refer to
darker areas on CT scans
Structures of intermediate
density similar to that of brain tissue appear gray and are called
isodense
Cerebrospinal fluid (CSF) is dark gray, and fat tissue (seen subcutaneously just outside the skull) appears
nearly black. Since fat is less dense
than water, white matter (which has a high myelin content) appears slightly
darker than the cellular gray matter (which has a high water content).
Density in CT scans is often expressed in
Hounsfield units (HU). The HU scale is based on the following values: HU = 0 for water and HU = –1000 for air.
TISSUE HU Air \_\_\_\_\_\_\_\_ Fat \_\_\_\_\_\_\_\_ Water \_\_\_\_\_\_\_\_ CSF \_\_\_\_\_\_\_\_ White matter \_\_\_\_\_\_\_\_ Gray matter \_\_\_\_\_\_\_\_ Freshly congealed blood \_\_\_\_\_\_\_\_ Bone \_\_\_\_\_\_\_\_
TISSUE HU Air –1000 to –600 Fat –100 to –60 Water 0 CSF 8–18 White matter 30–34 Gray matter 37–41 Freshly congealed blood 50–100 Bone 600–2000
The appearance of hemorrhage on CT depends on how recently it occurred. Fresh intracranial hemorrhage
coagulates nearly immediately
and therefore shows up on CT scans as hyperdense areas relative to
brain. With typical image display settings, fresh hemorrhage may appear about
as white as bone, although the actual HU is significantly lower.
As the clot is broken down, after about a week it becomes isodense with brain
tissue, and after 2 to 3 weeks it becomes hypodense.
Acute cerebral infarctions often cannot be seen by CT scanning in the
first 6
to 12 hours after the event. Subsequently, cell death and edema lead to an area
of hypodensity seen in the distribution of the artery that has been occluded,
along with some distortion of the local anatomy due to the edema. Over weeks to months, the brain tissue surrounding
the infarct may shrink, producing a local area of prominent sulci or enlarged
ventricles. Persistent areas of hypodensity in the brain tissue may be present
as a result of gliosis and of brain necrosis with replacement by CSF.
Neoplasms may appear
hypodense, hyperdense, or isodense depending
on the type and stage. They may contain
areas of calcification, hemorrhage, or fluid-filled cysts. Neoplasms may
produce surrounding edema that is hypodense. Intravenous contrast dye is often helpful in visualizing neoplasms.
Mass effect is anything that distorts the brain’s usual anatomy by displacement.
This can occur with edema, neoplasm, hemorrhage, and other
conditions. It can be detected on CT by
localized compression of the ventricles,
effacement of sulci, or distortion of other brain structures seen, for example,
in subfalcine herniation of brain structures across the midline
Intravenous contrast material is sometimes used in CT scanning, especially
to facilitate visualization of suspected neoplasms or brain abscess. The
contrast material contains iodine, which is denser than brain and will therefore
appear hyperdense (white) in areas of increased vascularity or breakdown
of the blood–brain barrier. The structures
that normally take up contrast by comparison with the noncontrast images.
These structures include
arteries, venous sinuses, the choroid plexus,
and dura. In suspected intracranial hemorrhage, it is very important to obtain
a noncontrast CT scan. This is because small hemorrhages often appear
on CT as whitish areas at the base of the brain, which could be masked by
the normal hyperdense contrast material in blood vessels and meninges at
the brain base. Another important application of intravenous contrast is in
CT angiography (CTA).
CT scanning is combined with another form of contrast enhancement in
myelography. In this technique, a needle is introduced into the CSF, usually
by lumbar puncture, and an iodinated contrast dye is introduced
into the CSF. This allows better radiological visualization of
nerve
roots and of abnormal impingements on the spinal CSF space—caused, for
example, by a herniated intervertebral disc. In conventional myelography, plain films are made. In CT myelography, a CT scan of the
spine is performed as well, providing very clear visualization of the vertebral
bones and the contents of the spinal canal.
Images produced by CT can be adjusted to improve the contrast for tissues
over a particular density range. To optimize contrast we adjust two parameters—
the window and level, which determine the conversion between
the calculated density values and the gray scale used for display. For example,
images displayed with bone windows
are used to carefully inspect the
skull for fractures, while soft tissue windows are used to
look at brain structures. In addition to contrast windows, the reconstruction
algorithm (filter kernel) used to generate CT images can also be selected to
provide the best results for particular tissues.
Magnetic resonance imaging (MRI) provides high-contrast imaging of the nervous
system in striking anatomical detail. It is therefore
the imaging method of choice for detecting
low-contrast or small lesions such
as multiple sclerosis plaques, low-grade astrocytomas, acoustic neuromas, and
so on. In addition, unlike CT scanning, in which the dense bones at the base of
the skull obscure the adjacent areas with “shadowing” artifact, MRI provides remarkably clear images of crucial basilar structures such
as the brainstem, cerebellum, and pituitary fossa. The spinal cord is also more
clearly visible on MRI for similar reasons.
MRI has its disadvantages as well. The main drawbacks
are time, cost, and
inferior performance in imaging fresh hemorrhage and bony structures. Also,
MRI cannot be done in patients with cardiac pacemakers, certain other devices, or metallic fragments in the heart or eye. A typical MRI
scan takes about 20 to 45 minutes to complete, while a quick CT
scan in an unstable patient can be done in 5 to 10 minutes. A CT
scan usually costs about two-thirds as much as an MRI. Finally,
CT images depend on overall tissue density, while MRI depends
on proton density and proton environment (see the next section).
Therefore, bone (high overall density but low proton density) and
fresh hemorrhage (high overall density due to fibrinogen but proton
density and environment similar to CSF) are imaged better
with CT than with MRI.
CT is the preferred technique for patients
with
head trauma or suspected intracranial hemorrhage
and as a first screening method to detect most intracranial lesions,
especially in the emergency setting. MRI is better for patients
who, on the basis of the clinical story, are suspected of having
low-contrast lesions or brainstem or skull-base lesions, or as a
secondary technique when a lesion is suspected that was not visible
on CT. In non-urgent situations, in which a single, more definitive
imaging method is desired, MRI is often the test of choice.
