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Archived Issues of Radiology Rounds
MGH Department of Radiology Website
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CT and MR Imaging for Evaluation of Acute Stroke
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- Non-contrast CT detects hemorrhage or other possible causes of symptoms, if present
- WCTA (CT angiography) detects site of circle-of-Willis occlusion, carotid stenosis, or other vascular
abnormalities
- Regions of profound ischemia, likely to be irreversibly infarcted, are detected by CTA parenchymal
“source” images and, with greater sensitivity, by DWI (diffusion weighted MR imaging)
- “Potentially salvageable” ischemic regions that may be targets for thrombolysis are detected by
CTP (CT perfusion) and, with greater coverage, by PWI (perfusion weighted MR imaging)
- The data from these advanced CT and MR techniques is often complimentary, and helps triage
patients to appropriate therapy and management
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When
a patient arrives with symptoms of stroke, prompt imaging is a vital
part of the work-up. For patients with ischemic stroke, thrombolytic
therapy with intravenous tissue plasminogen activator (t-PA) must be
given within the first 3 hours and intra-arterial thrombolytic therapy
must be administered within 6 hours after the onset of symptoms to be
effective. Late therapy limits the efficacy of treatment and increases
the likelihood of brain edema and hemorrhage, which may be life
threatening. Therefore, the potential benefits of recanalization must
be weighed against the risks of increased morbidity and mortality. CT
and MR imaging now play a significant role not only for diagnosis but
also for assessing the volume of ischemic damage and amount of residual
perfusion. This data, although still under investigation, plays a role
in decision making on whether it is appropriate to give the patient
thrombolytic treatment.
CT Scans and CTA for Acute Stroke
The first diagnostic scan in an emergency setting is typically a
non-contrast CT to determine whether the symptoms are due to
intracranial hemorrhage or there are other lesions, such as a tumor
that are causing the symptoms. CT is very rapid and, unlike MRI, is not
contraindicated by implants such as pacemakers, clips, or other metal
objects, whose presence may not be established before emergent imaging.
If the non-contrast CT image appears normal, the next step at MGH is to
administer a non-ionic iodinated contrast agent and perform a CTA scan
of the entire neurovascular system, from the top of the head to the
aortic arch. High resolution (pixel size, 0.4 mm) CTA images can be
rapidly post-processed at the CT scanner console into 3-D images, which
are used to determine whether the symptoms are due to partial or
complete occlusion, dissection, trauma damage, arteriovenous
malformation, or aneurysm. CTA images can also show calcification and
some arterial wall thickening due to atherosclerotic plaque. |
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3-D CTA image of intracranial circulation showing occlusion of middle carotid artery (arrow).
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However, dense circumferential calcification may obscure a stenosis,
and CTA does not have sufficient resolution to detect vascular
occlusions at distal sites.
Information about tissue level, parenchymal brain perfusion may be
obtained by rapid injection of a small additional dose of contrast for
dynamic first-pass bolus imaging. Although, currently, coverage is
limited by the width of the CT detectors to a 2 cm thick “slab” of
brain per contrast bolus, future generations of CT scanners, available
soon, will have at least double that coverage. From the sequential
images acquired as contrast passes through the tissue capillary bed, it
is possible to calculate quantitative cerebral blood flow (CBF), blood
volume (CBV), and the mean transit time (MTT), on a pixel-by-pixel
basis. Blood volume weighted lesions – like MR-DWI lesions – tend to be
present in regions of severe ischemia, likely to be irreversibly
infarcted. CTA source images have the advantage of providing blood
volume weighted imaging of the entire brain, although they have the
disadvantage of being weighted by their unenhanced CT components as
well.
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Advantages |
Limitations |
| CT |
| Fast |
| Widely available |
| High resolution |
| CTA sensitivity for large vessel occlusion, 98% |
| CTA images show thickening and calcification of arterial wall |
| CTA source images show region with low blood volume (ischemic core) |
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| Non-contrast CT has an approximately 50% accuracy for acute stroke detection |
| CTA may miss distal occlusions |
| Exposure to radiation |
| Iodinated contrast administration with potential allergy or toxicity* |
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| MRI |
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DWI abnormalities visible within minutes of stroke onset with sensitivity, 94%; specificity, 96%
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| DWI more accurate for detecting brain stem and lacunar infarction |
| DWI abnormalities highly likely to progress to infarction (ischemic core) |
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Contraindicated for patients with pacemakers, clips, or other metal objects
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| Subject to movement artifacts |
| MRA images have lower resolution and sensitivity than CTA |
Difficult to perform during intensive patient monitoring, such as that required for the administration of IV-tPA
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* Radiology Rounds, October 2003, http://www.massgeneralimaging.org/newsletter/october_2003
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MRI and MRA for Acute Ischemic Stroke
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Perfusion Imaging
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Diffusion
weighted MR imaging detects alterations in the normal pattern of
movement of cellular water due to stroke. Diffusion is restricted,
within minutes of stroke onset, due to cell swelling, caused by severe
restrictions of blood flow. DWI is highly sensitive and specific for
detecting severely ischemic lesions in the brain, regions with a high
probability of irreversible infarction. Multiple DWI lesions in
different vascular territories, if present, provide evidence that the
stroke is caused by emboli, which are often associated with cardiac
arrhythmia. Abnormalities on T2 weighted images do not typically appear until at least 6 hours after symptom onset and, therefore, a DWI abnormality with no T2 abnormality gives some indication of the timing of the stroke when this is unknown (as in “wake up” strokes).
Gadolinium contrast injection can also be used to generate MR
angiography (MRA) images of the neck and great vessel origins, although
this is not typically part of the acute stroke imaging protocol.
However, although MRA images are inferior to CTA with respect to
resolution and artifacts, they may be useful if there is a significant
degree of atherosclerotic calcification.
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Information
about tissue level, parenchymal brain perfusion may be obtained with CT
by rapid injection of a small additional dose of contrast for dynamic
first-pass bolus imaging. Although, currently, coverage is limited by
the width of the CT detectors to a 2 cm thick “slab” of brain per
contrast bolus, future generations of CT scanners, available soon, will
have at least double that coverage. From the sequential images acquired
as contrast passes through the tissue capillary bed, it is possible to
calculate quantitative cerebral blood flow (CBF), blood volume (CBV),
and the mean transit time (MTT), on a pixel-by-pixel basis.
MR perfusion weighted images (PWI) are acquired by serial imaging of
the whole brain as a bolus of gadolinium contrast agent passes through
the vasculature. As in perfusion CT, computer processing provides
data about CBF, CBV, and MTT.
If there are perfusion deficits that extend beyond the DWI or CT-CBV
deficit, then that patient may be a candidate for treatment with
intravenous t-PA if symptom onset was within 3 hours or by
intra-arterial t-PA if symptom onset was within 6 hours. However, if no
MR DWI-PWI or CT CBV-CBF mismatch is present to define “tissue at
risk”, thrombolytic treatment may not be appropriate. |
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MR IMAGING (A and B)
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A. DWI showing bright region of abnormalities due to severe ischemia.
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B.
MTT image of same patient, showing bright region of perfusion deficit
that is larger than DWI lesion and indicating that this patient may
benefit from thrombolytic treatment if stroke onset was within past 3-6
hours.
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Further Information
Neuroradiologists
are available at all times, both day and night, to conduct and
interpret CT and MR studies. For further questions on stroke
imaging, please contact Michael Lev, M.D., Director of Emergency Neuroradiology, or Pamela Schaefer, M.D., Director of Clinical MRI and the Associate Director of the Neuroradiology Division, both at 617-726-8320. |
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This article provided useful information about the appropriate use of imaging studies:
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References
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Cullen, SP, Symons, SP, Hunter, G, Hamberg, L, et al. (2002) Dynamic contrast-enhanced computed tomography of acute ischemic stroke: CTA and CTP. Semin Roentgenol 37: 192-205
Ezzeddine, MA, Lev, MH, McDonald, CT, Rordorf, G, et al. (2002) CT angiography with whole brain perfused blood volume imaging: added clinical value in the assessment of acute stroke. Stroke 33: 959-66
Latchaw, RE, Yonas, H, Hunter, GJ, Yuh, WT, et al. (2003) Guidelines
and recommendations for perfusion imaging in cerebral ischemia: A
scientific statement for healthcare professionals by the writing group
on perfusion imaging, from the Council on Cardiovascular Radiology of
the American Heart Association. Stroke 34: 1084-104
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Lev MH, Farkas J, Rodriguez VR, Schwamm LH, et al. (2001) CT angiography in the rapid triage of patients with hyperacute stroke: Accuracy in the detection of large vessel thrombus. JCAT 2001;25:520-8
Mullins, ME, Schaefer, PW, Sorensen, AG, Halpern, EF, et al. (2002) CT
and conventional and diffusion-weighted MR imaging in acute stroke:
study in 691 patients at presentation to the emergency department. Radiology 224: 353-60
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