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Standardized Time to Peak in Ischemic and Regular Cerebral Tissue Measured with Perfusion MR Imaging

Christian Nasela, Nicole Kronsteinera, Erwin Schindlera, Sören Kreuzera and Stephan Gentzscha

a From the Division of Neuroradiology, Department of Radiology, University of Vienna, Austria



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  		FIG 1. Frequencies of stdTTP values in group 1 (infarct) and group 2 (reference). The predefined limits of the proposed model for stdTTP were as follows: stdTTP ≤3.5 seconds=regular range, stdTTP ≥7 seconds=critical range, and 3.5 seconds<stdTTP<7 seconds = tolerance range. A complementary distribution of stdTTP between infarcted and normal areas is depicted. In the tolerance range (about 13.5% of all voxels in both groups), interpretation of the perfusion measurement was possible depending on the context.



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  		FIG 2. Linear regression analysis.

A, Comparison of volumes derived from stdTTP measurement during acute ischemia, with only critically perfused voxels (stdTTP ≥ 7 seconds) taken into account, and DWI findings in the resulting lesion after 24–72 hours (linear regression through the origin; Pearson corrected r2=0.86, r=0.933). In larger (nearly complete hemispheric) infarcts, there may be a slight tendency to underestimate the resulting ischemic volume, as defined by DWI.

B, Comparison of volumes derived from stdTTP measurement during acute ischemia, with critically and tolerably perfused voxels (stdTTP > 3.5 seconds) taken into account, and DWI findings in the resulting lesion after 24–72 hours (linear regression through the origin; Pearson corrected r2=0.82, r=0.913). The stdTTP volumes contain voxels with stdTTP in the critical (≥7-second) and tolerable (3.5–7- second) ranges during acute ischemia. The results suggest a tendency to overestimate the resulting ischemic volume, as defined by DWI, especially in smaller infarcts.

C, Comparison of volumes derived from DWI during acute ischemia and from DWI of the resulting lesion after 24–72 hours (linear regression through the origin, Pearson corrected r2=0.45, r = 0.708). Cytotoxic edema develops over time; therefore, in acute ischemia, DWI cannot predict the possible size of the infarction. Nevertheless, DWI is essential to exclude ischemic injury in tissue at risk; the latter is defined, eg, by stdTTP measurements.



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  		FIG 3. Frequencies of stdTTP values in group 1 (infarct) and group 2 (reference) in the tolerance range. Within the range of 3.5–7 seconds, voxels with ischemic injury (group 1, 12.7%) are represented as often as voxels without ischemic injury (group 2, 14.8%). A tendency of stdTTP values in group 1 (median, 5.5 seconds) toward the critical limit of 7 seconds and in group 2 (median, 4.3 seconds) toward the regular limit of 3.5 seconds is shown. A clear cutoff value between critical and regular perfusion cannot be determined.



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  		FIG 4. StdTTP measurements as a tool for assessing acute ischemia.

A, StdTTP map (time resolution, 610 milliseconds; unit of scale, seconds) calculated within the first 6 hours (at 1.5 hours) after the onset of stroke symptoms. Although the patient was agitated during imaging, the study is interpretable. The depicted hemodynamic disorder gradually worsens from the margins to the center, which is an area with nonperfusion (black) in the central territory of the left middle cerebral artery. The surrounding tissue had stdTTP values of ≥7 seconds (gray), which indicate critical perfusion. At the margins, stdTTP is in the tolerance range (blue).

B, DWI map (isotropic b=1000 seconds/mm2) of the corresponding section of the stdTTP map in A. DWI was performed during the same examination as perfusion MR imaging but reveals an only a small area of ischemic injury.

C, DWI map (isotropic b=1000 seconds/mm2) of a section comparable to those in A and B obtained 24 hours after the initial event. This study reveals an ischemic injury (infarct) that now almost entirely matches the area indicated as critically perfused in A. Critical perfusion, depicted as stdTTP ≥7 seconds, seems to suggest a high risk of ischemic injury in brain tissue. Additionally, severe edematous swelling of the hemisphere is depicted in this large infarct.





Copyright © 2009 by the American Society of Neuroradiology. Print ISSN: 0195-6108 Online ISSN: 1936-959X