CT perfusion mean transit time maps optimally distinguish benign oligemia from true "at-risk" ischemic penumbra, but thresholds vary by postprocessing technique

Abstract

Background and purpose: Various CTP parameters have been used to identify ischemic penumbra. The purpose of this study was to determine the optimal CTP parameter and threshold to distinguish true "at-risk" penumbra from benign oligemia in acute stroke patients without reperfusion.

Materials and methods: Consecutive stroke patients were screened and 23 met the following criteria: 1) admission scanning within 9 hours of onset, 2) CTA confirmation of large vessel occlusion, 3) no late clinical or radiographic evidence of reperfusion, 4) no thrombolytic therapy, 5) DWI imaging within 3 hours of CTP, and 6) either CT or MR follow-up imaging. CTP was postprocessed with commercial software packages, using standard and delay-corrected deconvolution algorithms. Relative cerebral blood flow, volume, and mean transit time (rCBF, rCBV and rMTT) values were obtained by normalization to the uninvolved hemisphere. The admission DWI and final infarct were transposed onto the CTP maps and receiver operating characteristic curve analysis was performed to determine optimal thresholds for each perfusion parameter in defining penumbra destined to infarct.

Results: Relative and absolute MTT identified penumbra destined to infarct more accurately than CBF or CBV*CBF (P < .01). Absolute and relative MTT thresholds for defining penumbra were 12s and 249% for the standard and 13.5s and 150% for the delay-corrected algorithms, respectively.

Conclusions: Appropriately thresholded absolute and relative MTT-CTP maps optimally distinguish "at-risk" penumbra from benign oligemia in acute stroke patients with large-vessel occlusion and no reperfusion. The precise threshold values may vary, however, depending on the postprocessing technique used for CTP map construction.

Fig 1.

Expansion of core infarct size (area of selected axial section) between the admission DWI scan and the coregistered follow-up CT or MR imaging was an inclusion criterion, and was present in all patients (left). Sample ROC curves (right) showing the sensitivity/specificity of different CTP parameter thresholds used to define “at-risk” penumbra destined to infarct, comparing maps processed by using standard software. Green curves represent rMTT; blue curves, rCBF; orange, rCBV; and purple, the rCBF*rCBV voxel product value maps.

Fig 2.

Example of thresholded MTT map prediction of penumbra destined to infarct in a 70-year-old woman presenting with left hemispheric stroke symptoms. Ictus-to-CTP imaging time was 5 hours 33 minutes, admission NIHSS score was 6, and follow-up MR imaging was performed 44 hours after admission CTP scanning; NIHSS score was 12. Infarct core is segmented on the admission DWI scan (A, and red overlays on D, E), and final infarct volume is segmented on follow-up DWI scan (B). CT-MTT map shows blue/green regions with increased mean transit time (C). D and E, respectively, show the optimally thresholded absolute-MTT (12 seconds threshold) and relative-MTT (249% threshold) maps, both postprocessed by using standard algorithm, which distinguish benign oligemia (green overlays) from true “at-risk” ischemic penumbra (blue overlays).