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DOI: 10.1148/rg.26si065501
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State-of-the-Art Imaging of Acute Stroke1

Ashok Srinivasan, MD, Mayank Goyal, MD, Faisal Al Azri, MD and Cheemun Lum, MD

1 From the Department of Diagnostic Imaging, University of Ottawa, Ottawa Hospital, Ottawa, Ontario, Canada. Recipient of a Certificate of Merit award for an education exhibit at the 2005 RSNA Annual Meeting. Received January 13, 2006; revision requested May 9 and received June 13; accepted June 28. All authors have no financial relationships to disclose.

Figure 1
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  		Figure 1a.  (a) Schematic of brain involvement in acute stroke shows a core of irreversibly infarcted tissue surrounded by a peripheral region of ischemic but salvageable tissue referred to as a penumbra. Without early recanalization, the infarction gradually expands to include the penumbra. (b) Diagram shows the evolution of events at a microscopic level with decreasing cerebral perfusion (from right to left). Irreversible cell death generally occurs when cerebral blood flow decreases to less than 10 mL/100 g/min.

 

Figure 1
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  		Figure 1b.  (a) Schematic of brain involvement in acute stroke shows a core of irreversibly infarcted tissue surrounded by a peripheral region of ischemic but salvageable tissue referred to as a penumbra. Without early recanalization, the infarction gradually expands to include the penumbra. (b) Diagram shows the evolution of events at a microscopic level with decreasing cerebral perfusion (from right to left). Irreversible cell death generally occurs when cerebral blood flow decreases to less than 10 mL/100 g/min.

 

Figure 2
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  		Figure 2a.  Axial unenhanced CT images in a proximal segment of the left MCA in a 53-year-old man (a) and a distal segment of the left MCA in a 62-year-old woman (b), obtained 2 hours after the onset of right hemiparesis and aphasia, show areas of hyperattenuation (arrow) suggestive of intravascular thrombi.

 

Figure 2
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  		Figure 2b.  Axial unenhanced CT images in a proximal segment of the left MCA in a 53-year-old man (a) and a distal segment of the left MCA in a 62-year-old woman (b), obtained 2 hours after the onset of right hemiparesis and aphasia, show areas of hyperattenuation (arrow) suggestive of intravascular thrombi.

 

Figure 3
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  		Figure 3.  Axial unenhanced CT image obtained in a 53-year-old man (same patient as in Fig 2a) shows hypoattenuation and obscuration of the left lentiform nucleus (arrows), which, because of acute ischemia in the lenticulostriate distribution, appears abnormal in comparison with the right lentiform nucleus.

 

Figure 4
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  		Figure 4.  Axial unenhanced CT image, obtained in a 73-year-old woman 21/2 hours after the onset of left hemiparesis, shows hypoattenuation and obscuration of the posterior part of the right lentiform nucleus (white arrow) and a loss of gray matter–white matter definition in the lateral margins of the right insula (black arrows). The latter feature is known as the insular ribbon sign.

 

Figure 5
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  		Figure 5a.  Axial unenhanced CT images, obtained in a 45-year-old man 2 hours after the onset of left hemiparesis, show obscuration of the right lentiform nucleus (arrow in b). This feature is less visible with the routine brain imaging window used for a (window width, 80 HU; center, 35 HU) than with the narrower window used for b (window width, 10 HU; center, 28 HU).

 

Figure 5
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  		Figure 5b.  Axial unenhanced CT images, obtained in a 45-year-old man 2 hours after the onset of left hemiparesis, show obscuration of the right lentiform nucleus (arrow in b). This feature is less visible with the routine brain imaging window used for a (window width, 80 HU; center, 35 HU) than with the narrower window used for b (window width, 10 HU; center, 28 HU).

 

Figure 6
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  		Figure 6.  Schematic shows the 10 regions of the MCA distribution, each of which accounts for one point in the ASPECTS system: M1, M2, M3, M4, M5, M6, the caudate nucleus (C), the lentiform nucleus (L), the internal capsule (IC), and the insular cortex (I). For each area involved in ischemia depicted at unenhanced CT, one point is subtracted from the total score of 10.

 

Figure 7
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  		Figure 7a.  Unenhanced CT images in a 56-year-old man with right hemiparesis (a at a lower level than b) demonstrate involvement of the M1 region, insular cortex (I), and lentiform nucleus (L). Thus, three points are subtracted from the 10-point ASPECTS, and the final score is seven points. C = caudate nucleus, IC = internal capsule.

 

Figure 7
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  		Figure 7b.  Unenhanced CT images in a 56-year-old man with right hemiparesis (a at a lower level than b) demonstrate involvement of the M1 region, insular cortex (I), and lentiform nucleus (L). Thus, three points are subtracted from the 10-point ASPECTS, and the final score is seven points. C = caudate nucleus, IC = internal capsule.

 

Figure 8
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  		Figure 8a.  (a) Unenhanced CT image in a 72-year-old woman with acute right hemiplegia shows hyperattenuation in a proximal segment of the left MCA (arrows). (b, c) Axial (b) and coronal (c) reformatted images from CT angiography show the apparent absence of the same vessel segment (arrows). The presence of an intravascular thrombus in this location was confirmed by comparing the reformatted images with the CT source images (not shown).

 

Figure 8
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  		Figure 8b.  (a) Unenhanced CT image in a 72-year-old woman with acute right hemiplegia shows hyperattenuation in a proximal segment of the left MCA (arrows). (b, c) Axial (b) and coronal (c) reformatted images from CT angiography show the apparent absence of the same vessel segment (arrows). The presence of an intravascular thrombus in this location was confirmed by comparing the reformatted images with the CT source images (not shown).

 

Figure 8
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  		Figure 8c.  (a) Unenhanced CT image in a 72-year-old woman with acute right hemiplegia shows hyperattenuation in a proximal segment of the left MCA (arrows). (b, c) Axial (b) and coronal (c) reformatted images from CT angiography show the apparent absence of the same vessel segment (arrows). The presence of an intravascular thrombus in this location was confirmed by comparing the reformatted images with the CT source images (not shown).

 

Figure 9
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  		Figure 9a.  CT perfusion maps of cerebral blood volume (a) and cerebral blood flow (b) show, in the left hemisphere, a region of decreased blood volume (white oval) that corresponds to the ischemic core and a larger region of decreased blood flow (black oval in b) that includes the ischemic core and a peripheral region of salvageable tissue. The difference between the two maps (black oval = white oval) is the penumbra.

 

Figure 9
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  		Figure 9b.  CT perfusion maps of cerebral blood volume (a) and cerebral blood flow (b) show, in the left hemisphere, a region of decreased blood volume (white oval) that corresponds to the ischemic core and a larger region of decreased blood flow (black oval in b) that includes the ischemic core and a peripheral region of salvageable tissue. The difference between the two maps (black oval = white oval) is the penumbra.

 

Figure 10
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  		Figure 10a.  The steps involved in the postprocessing of CT perfusion data include the placement of ROIs in an arterial pixel (arrow in a) and a venous pixel (arrow in b) and the generation of a time-attenuation curve (c) for each ROI. The curves are then used to formulate color-coded CT perfusion maps.

 

Figure 10
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  		Figure 10b.  The steps involved in the postprocessing of CT perfusion data include the placement of ROIs in an arterial pixel (arrow in a) and a venous pixel (arrow in b) and the generation of a time-attenuation curve (c) for each ROI. The curves are then used to formulate color-coded CT perfusion maps.

 

Figure 10
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  		Figure 10c.  The steps involved in the postprocessing of CT perfusion data include the placement of ROIs in an arterial pixel (arrow in a) and a venous pixel (arrow in b) and the generation of a time-attenuation curve (c) for each ROI. The curves are then used to formulate color-coded CT perfusion maps.

 

Figure 11
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  		Figure 11a.  Acute stroke in a 65-year-old man with left hemiparesis. CT perfusion maps of cerebral blood volume (a), cerebral blood flow (b), and mean transit time (c) show mismatched abnormalities (arrows) that imply the presence of a penumbra. The area with decreased blood volume represents the ischemic core, and that with normal blood volume but decreased blood flow and increased mean transit time is the penumbra.

 

Figure 11
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  		Figure 11b.  Acute stroke in a 65-year-old man with left hemiparesis. CT perfusion maps of cerebral blood volume (a), cerebral blood flow (b), and mean transit time (c) show mismatched abnormalities (arrows) that imply the presence of a penumbra. The area with decreased blood volume represents the ischemic core, and that with normal blood volume but decreased blood flow and increased mean transit time is the penumbra.

 

Figure 11
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  		Figure 11c.  Acute stroke in a 65-year-old man with left hemiparesis. CT perfusion maps of cerebral blood volume (a), cerebral blood flow (b), and mean transit time (c) show mismatched abnormalities (arrows) that imply the presence of a penumbra. The area with decreased blood volume represents the ischemic core, and that with normal blood volume but decreased blood flow and increased mean transit time is the penumbra.

 

Figure 12
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  		Figure 12a.  Acute stroke in the left medial temporal lobe in a 44-year-old man. (a, b) Axial T2-weighted (a) and fluid-attenuated inversion recovery (b) images show areas with increased signal intensity. (c) Gradient-echo image shows abnormal low signal intensity in the same areas. These findings are suggestive of hemorrhage. (Courtesy of Ellen Hoeffner, MD, University of Michigan Health System, Ann Arbor, Michigan.)

 

Figure 12
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  		Figure 12b.  Acute stroke in the left medial temporal lobe in a 44-year-old man. (a, b) Axial T2-weighted (a) and fluid-attenuated inversion recovery (b) images show areas with increased signal intensity. (c) Gradient-echo image shows abnormal low signal intensity in the same areas. These findings are suggestive of hemorrhage. (Courtesy of Ellen Hoeffner, MD, University of Michigan Health System, Ann Arbor, Michigan.)

 

Figure 12
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  		Figure 12c.  Acute stroke in the left medial temporal lobe in a 44-year-old man. (a, b) Axial T2-weighted (a) and fluid-attenuated inversion recovery (b) images show areas with increased signal intensity. (c) Gradient-echo image shows abnormal low signal intensity in the same areas. These findings are suggestive of hemorrhage. (Courtesy of Ellen Hoeffner, MD, University of Michigan Health System, Ann Arbor, Michigan.)

 

Figure 13
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  		Figure 13a.  Intravascular thrombus. Time-of-flight MR angiograms in two patients with acute stroke symptoms reveal flow gaps in the left proximal middle cerebral artery (arrow in a) and the basilar artery (arrows in b). Both findings were due to intravascular thrombi, which were confirmed later at digital subtraction angiography. (Courtesy of Ellen Hoeffner, MD, University of Michigan Health System, Ann Arbor, Michigan.)

 

Figure 13
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  		Figure 13b.  Intravascular thrombus. Time-of-flight MR angiograms in two patients with acute stroke symptoms reveal flow gaps in the left proximal middle cerebral artery (arrow in a) and the basilar artery (arrows in b). Both findings were due to intravascular thrombi, which were confirmed later at digital subtraction angiography. (Courtesy of Ellen Hoeffner, MD, University of Michigan Health System, Ann Arbor, Michigan.)

 

Figure 14
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  		Figure 14.  Acute stroke–induced cytotoxic edema in the right cerebellar hemisphere. Diffusion-weighted MR image (b = 1000 sec/mm2) shows areas of signal intensity increase due to the restricted mobility of water molecules.

 

Figure 15
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  		Figure 15a.  Acute stroke of the posterior circulation in a 77-year-old man. (a) Diffusion-weighted MR image (b = 1000 sec/mm2) shows bilateral areas of increased signal intensity (arrows) in the thalami and occipital lobes. (b) ADC map shows decreased ADC values in the same areas (arrows). These findings are indicative of acute ischemia.

 

Figure 15
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  		Figure 15b.  Acute stroke of the posterior circulation in a 77-year-old man. (a) Diffusion-weighted MR image (b = 1000 sec/mm2) shows bilateral areas of increased signal intensity (arrows) in the thalami and occipital lobes. (b) ADC map shows decreased ADC values in the same areas (arrows). These findings are indicative of acute ischemia.

 

Figure 16
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  		Figure 16a.  Chronic infarcts in a 71-year-old man with a remote history of multiple strokes. (a) Diffusion-weighted MR image (b = 1000 sec/mm2) shows areas of decreased signal intensity in the left frontal lobe. (b) ADC map shows increased ADC values in the white matter of the right frontal lobe. These features are suggestive of chronic infarction.

 

Figure 16
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  		Figure 16b.  Chronic infarcts in a 71-year-old man with a remote history of multiple strokes. (a) Diffusion-weighted MR image (b = 1000 sec/mm2) shows areas of decreased signal intensity in the left frontal lobe. (b) ADC map shows increased ADC values in the white matter of the right frontal lobe. These features are suggestive of chronic infarction.

 

Figure 17
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  		Figure 17.  Time–signal intensity curve illustrates the decrease in signal intensity within an ROI after the administration of an MR contrast agent bolus. The signal intensity decrease is due to the T2* effect of the contrast agent, an effect that is exploited in dynamic susceptibility-weighted MR perfusion imaging to calculate perfusion parameters.

 

Figure 18
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  		Figure 18a.  Acute stroke in a 67-year-old woman with acute left hemiplegia 2 hours after carotid endarterectomy. (a) Diffusion-weighted MR image (b = 1000 sec/mm2) shows an area of mildly increased signal intensity in the right parietal lobe (arrows). The ADC values in this region were decreased. (b) Perfusion-weighted MR image shows a larger area with increased time to peak enhancement (arrows) in the right cerebral hemisphere. The mismatch between the perfusion and diffusion images is indicative of a large penumbra.

 

Figure 18
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  		Figure 18b.  Acute stroke in a 67-year-old woman with acute left hemiplegia 2 hours after carotid endarterectomy. (a) Diffusion-weighted MR image (b = 1000 sec/mm2) shows an area of mildly increased signal intensity in the right parietal lobe (arrows). The ADC values in this region were decreased. (b) Perfusion-weighted MR image shows a larger area with increased time to peak enhancement (arrows) in the right cerebral hemisphere. The mismatch between the perfusion and diffusion images is indicative of a large penumbra.

 

Figure 19
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  		Figure 19.  Flow chart shows an acute stroke imaging protocol. CTA = CT angiography, CTP = CT perfusion imaging, DWI = diffusion-weighted MR imaging, IA = intraarterial, IV = intravenous, MRA = MR angiography, MRI = conventional MR imaging, NECT = unenhanced CT, PWI = perfusion-weighted MR imaging.

 




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