The
non-contrast head CT scan remains the first-line imaging study in
suspected stroke patients due to its ubiquity and exquisite sensitivity
for the detection of blood. On the initial emergency CT scan, overt
nonstroke processes must be confidently excluded. These “stroke mimics”
include tumor, subdural or epidural hematoma, SAH, and intraparenchymal
hemorrhage.
Since thrombolytic therapy may
produce lethal bleeding in patients with intracranial hemorrhage,
exclusion of hemorrhage has been the key criterion in the major
thrombolytic trials. While detection of hemorrhage is the most
fundamental and critical step in the evaluation of the head CT, the
ECASS trial demonstrated that early signs of major cerebral infarction
(e.g. sulcal effacement, mass effect, edema, and loss of the insular
ribbon) are also important features to look for, since they are
associated with an increased risk for intracerebral hemorrhage in
patients who receive thrombolytic therapy. Some guidelines have
incorporated these more subtle signs into recommendations against
thrombolytics when present. The NINDs trial, however, did not use these
CT criteria for making decisions regarding thrombolysis.
There are three possible classes of CT diagnoses when a patient presents with an acute neurologic deficit:
I. Non-ischemic stroke lesion:
A: Hemorrhagic:
1. SDH
2. EDH
3. SAH
4. Intracerebral hemorrhage (cortical, subcortical, cerebellar, and brainstem)
B: Non-hemorrhagic:
1. Tumor
2. Abscess
II. Ischemic Stroke Signs:
A. Hyperdense Artery Sign
B. Loss of Insular Ribbon (“Insular ribbon sign”)
C. Loss of cortical gray-white differentiation
D. Mass effect
III. Normal CT
Non-ischemic stroke lesions
Fundamental
to cranial CT interpretation in potential stroke cases is the
identification of non-stroke lesions, particularly hemorrhage. Acute hemorrhage will usually appear hyperdense (whiter, like bone) to gray matter, and have a heterogeneous appearance. It also tends to have less mass effect than would be predicted for the size of the lesion.
All hyperdensities, however, are not necessarily acute hemorrhage.
Calcification, proteinaceous material, and lesions with a high
nucleocytoplasmic ratio (e.g. GBM) can all appear hyperdense to normal
brain.
Epidural hematoma (lens shaped/ do not
cross suture lines), subdural hematoma (falx shaped/ can cross suture
lines) and intraparenchymal hematoma (supra/infratentorial) are all
clinical possibilities which are usually readily apparent on CT
scanning. Subarachnoid hemorrhage can be more difficult to diagnose.
SAH typically appears as a hyperdense (white) collection, most commonly
in the basal cisterns (including circummesencephalic, suprasellar,
sylvian, and quadrigeminal). The ability to diagnose a SAH on CT scan
depends on the volume of blood present and the duration of its presence.
Typically, CT scan loses sensitivity for detecting subarachnoid
hemorrhage as time from the SAH ictus increases.
Tumor
and abscess can have similar appearances on non-contrasted scans. Both
can demonstrate mixed densities within them, and both are frequently
associated with edema in the surrounding brain matter.
Normal CT Scan
Even
with state-of-the-art 3rd and 4th generation scanners, most ischemic
strokes will go undetected for the first few hours. Hence, the “Normal
CT Scan” is perfectly compatible with acute ischemic stroke.
Unfortunately, a normal scan is also perfectly compatible with seizure,
metabolic disease (hypoglycemia, hyponatremia), and TIA as etiologies
for a neurologic deficit.
In general, gray
matter is more susceptible to ischemia than white matter, as it is more
metabolically active. Hence, loss of gray-white differentiation due to
the influx of edema into the gray matter is the earliest change to be
noted. Subtle edema has been detected as early as 46 minutes from
the ictus, but changes this early are the exception. By 6 hours from
the ictus, ¾ of patients with MCA strokes will show early edema in the
insular cortex. After 6-12 hours, additional edema is recruited into the
area, making the lesion more conspicuous on CT imaging. Early on, it is NOT POSSIBLE to distinguish between ischemia and frank infarction by imaging techniques.
Ischemic Stroke Signs
The Hyperdense Artery Sign:
When
an artery (typically MCA, PCA, or ACA) appears hyperdense, this is
indicative of a major occlusion of the vessel with thrombus formation. The
specificity of the hyperdense MCA (HMCAS) is 98%. False-positives can
occur in the case of unilateral calcification of the MCA trunk. The
sensitivity of the HMCAS, however, is only about 50%. Variability in blood volume and composition of the thrombus are responsible for the frequent false-negative findings. The hyperdense artery sign suggests that a major cerebral vessel is occluded
and that this vessel’s territory is at risk for hypoperfusion. ICA and
MCA trunk occlusions have more serious clinical implications than
occlusions of the MCA branches, the PCA, or the ACA because the
threatened territory is larger. Whether the affected territory will
undergo ischemic necrosis is a matter of collateral blood supply.
Therefore, this is not an infarct sign, rather it indicates volume of
tissue that will die if the collateral blood supply fails and
recanalization is not achieved. Clinically, ischemic stroke patients
with an HMCAS have a poorer prognosis than those without this
radiographic finding.
The Insular Ribbon Sign
The insular ribbon is an area of extreme gray-white differentiation that is readily examined on the CT scan. Located between the Sylvian fissure and the basal ganglia, it is supplied by small perforating branches of the MCA. Loss of the insular stripe is one of the more subtle early indications of MCA stroke. The normal insular cortex appears as a thin white line (gray-matter) adjacent to a darker gray subcortical area (white-matter). With ischemia, the metabolically active gray matter is effected first, resulting of intracellular edema, with resulting hypodensity.
Thus, the insular ribbon or stripe is lost, and a homogeneous
appearance is noted. This finding alone is not an exclusion criterion
for intravenous thrombolytic therapy.
Mass Effect
Brain
swelling is very subtle during the first hours after arterial
occlusion. Swelling of brain tissue is assessed on CT scans by looking
for compression of CSF spaces and asymmetry of cortical sulci.
Swelling visible within the first 6 hours indicates severe edema and
indicates a poor prognosis for the majority of patients.
A
retrospective evaluation of CTs from the ECASS study suggested that if
ischemic changes were present in greater than 1/3 of the MCA territory,
the patient was at increased risk of hemorrhage. Other studies have
supported this conclusion, but it has been demonstrated that the
interobserver variability for identifying greater that 1/3 of the MCA
distribution is very poor. For this reason, this criterion is difficult
to rely on for clinical decision making, even when the CT is
interpreted by a neuroradiologist.
Future Trends in Stroke Imaging
While
CT is the current brain-imaging method of choice for determining
qualification for thrombolysis, this may change in the future. MRI,
especially diffusion-weighted and perfusion-weighted MRI are exquisitely
sensitive to early pathologic changes of ischemic infarction and subtle
brain edema. These techniques are superior to CT in that they can
detect abnormalities much sooner than a conventional non-contrast CT
scan. The major limitation of MRI remains its relative insensitivity to
detecting hemorrhage, which is the key neuroimaging branch point in a
clinical protocol. PET scanning, Xenon CT scanning, and cranial doppler
are all being investigated in the stroke arena, but are all currently
considered to be experimental.16,18
Outcome of a case:
You
interpret the cranial CT correctly as demonstrating an early stroke in
the left MCA distribution with obscuration of the insular ribbon on that
side. Unfortunately for the patient, however, you also identify a
right frontal mass lesion with significant edema surrounding it. You
wisely withhold tPA in the presence of the presumed tumor. The patient
is transferred to a nearby tertiary care center with neurosurgical
expertise, where he undergoes craniotomy. A diagnosis of Astrocytoma is
made, and the patient is ultimately discharged to a rehabilitation
hospital.
Questions
1. NINDs CT scan criteria for exclusion from thrombolysis included:
A. Dense MCA sign
B. Hemorrhage
C. Edema in > 1/3 of MCA distribution
D. A&C
E. All of the above
2. Current treatment guidelines specify that the cranial CT must be read by a:
A. Neuroradiologist
B. Neurologist
C. Radiologist
D. Physician skilled in cranial CT interpretation
3. The normal cranial CT scan:
A. Is incompatible with acute stroke
B. Is pathognomonic of acute stroke
C. Is compatible with the diagnosis of acute stroke
Answers
1. Answer b.
2. Answer d.
3. Answer c.