Assessing Brain Tissue Viability on Nonenhanced Computed Tomography After Ischemic Stroke

Awad Alzahrani, Xinyu Zhang, Adel Albukhari, Joanna M Wardlaw, Grant Mair, Awad Alzahrani, Xinyu Zhang, Adel Albukhari, Joanna M Wardlaw, Grant Mair

Abstract

Background: Treatment for ischemic stroke can be offered beyond conventional time limits for patients with favorable computed tomography perfusion (CTP), but this is not universally available. We sought a threshold for brain attenuation on nonenhanced computed tomography (NECT) to differentiate CTP-defined penumbra vs core, and correlated NECT features with CTP.

Methods: We retrospectively assessed consecutive patients presenting to King Abdulaziz University Hospital with ischemic stroke (2017-2020), baseline NECT, and a visible defect on concurrent CTP. Using CTP as the reference standard, we measured the attenuation of ischemic and healthy contralateral brain on NECT to produce attenuation ratios (ischemic/normal) for penumbra and core. We used area under the receiver operating characteristic curve to estimate the optimal computed tomography (CT) attenuation ratio for penumbra. Per patient, we qualitatively assessed 8 regions within the affected cerebral hemisphere: on NECT as normal, hypoattenuating (with/out swelling), or isolated swelling and on CTP as normal, penumbra, or core. We sought associations between isolated swelling and penumbra, and between hypoattenuation and core.

Results: We include 142 patients (86 male), mean age 61±14 years. Median 261 minutes (interquartile range, 173-382) to NECT. We measured 206 ischemic lesions (124 penumbra, 82 core). Optimal CT attenuation ratio for identifying penumbra was >0.87, with 86% sensitivity 91% specificity (area under the receiver operating characteristic curve, 0.95 [95% CI, 0.92-0.98]; P<0.0001). We qualitatively assessed 976 cerebral regions (72 isolated swelling, 254 hypoattenuation). On NECT, isolated swelling usually corresponded to CTP penumbra (70/72, 97%), whereas visible NECT hypoattenuation was found with core (141/254, 56%) and penumbra (109/254, 43%). CTP core lesions were rarely normal on NECT (13/155, 8%).

Conclusions: After ischemic stroke, brain tissue viability can be assessed using NECT. Isolated swelling is highly specific to penumbra. Visible hypoattenuation does not always represent core, nearly half of such lesions were penumbral on concurrent CTP and can be differentiated by measuring lesion attenuation.

Keywords: brain; ischemia; perfusion; stroke; tomography.

Figures

Figure 1.
Figure 1.
Measuring nonenhanced computed tomography (NECT) attenuation ratio for core and penumbral lesions in a 60-year-old patient with left-sided ischemia. NECT with measurements (A) and coregistered concurrently acquired computed tomography (CT) perfusion maps (B–D) demonstrate areas of lesion core (decreased cerebral blood flow [CBF; B] and cerebral blood volume (CBV; C) with increased mean transit time (MTT; D) and penumbra (decreased CBF with maintained or increased CBV and increased MTT). On NECT (A): left upper ROI measured CT attenuation of core lesion (32 Hounsfield units [HU]) mirrored to corresponding normal right hemisphere (39 HU), attenuation ratio 0.82; left lower ROI measured CT attenuation of penumbral lesion (40 HU) mirrored to corresponding normal right hemisphere (41 HU), attenuation ratio 0.98.
Figure 2.
Figure 2.
Example computed tomography (CT) scan assessment of 2 brain regions from the IST-3 trial (Third International Stroke Trial) template for the same patient. A, (1) Posterior half of peripheral middle cerebral artery (MCA) territory, (2) isolated focal swelling on nonenhanced CT (NECT), (3) decreased cerebral blood flow (CBF), (4) normal cerebral blood volume (CBV), and (5) increased mean transit time (MTT). B, (1) most of peripheral MCA territory + lateral part of basal ganglia, (2) subtle parenchymal hypoattenuation on NECT, (3) decreased CBF, (4) decreased CBV, (5) increased MTT.
Figure 3.
Figure 3.
Flowchart for patient selection. CTP indicates computed tomography perfusion; KAUH, King Abdulaziz University Hospital; and NECT, nonenhanced computed tomography.
Figure 4.
Figure 4.
Receiver operating characteristic analysis testing the expected discriminative ability of the optimal attenuation ratio (>0.87) to distinguish penumbra from core on baseline nonenhanced computed tomography. Area under curve 0.955.

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