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BACKGROUND AND PURPOSE: PCT has emerged as an alternative to MR imaging for the assessment of patients with suspected acute stroke. However, 1 disadvantage of PCT is its limited anatomic coverage, which may impact the characterization of hemispheric ischemic strokes. The purpose of this study was to determine the optimal brain CT coverage required to accurately estimate the size of the infarct core relative to the MCA territory and the infarct-penumbra mismatch, by using a criterion standard of these parameters measured on PCT with 80-mm z-axis coverage. MATERIALS AND METHODS: Fifty-one patients with acute ischemic hemispheric stroke underwent PCT scanning (2 boluses, total coverage of 80 mm, 16 × 5 mm sections) within the first 24 hours of symptom onset and a follow-up NCCT of the brain between 3 days and 3 months after the initial stroke CT study. The volumes of PCT infarct and penumbra for each possible extent of z-axis coverage derived from the individual PCT sections were recorded (beginning with 5 mm of z-axis coverage above the orbits and then increasing the coverage in 5-mm increments in the z-axis up to 80 mm above the orbits). The infarct-penumbra mismatch and the size of the infarction relative to the MCA territory were calculated for each extent of z-axis coverage. Using the 80-mm z-axis coverage as the criterion standard, we calculated the accuracy of the values of the relative PCT infarct size and mismatch that were obtained by using more limited z-axis coverage. The impact of different levels of PCT z-axis coverage on the eligibility for reperfusion treatment was assessed. RESULTS: On the admission PCT, by using 80-mm of z-axis coverage, the mean perfusion infarct core volume was 45.9 ± 44.0 cm(3) (range, 0–170 cm(3)) and the mean penumbra volume was 64.5 ± 64.4 cm(3) (range, 0–226 cm(3)). The mean perfusion infarct core/MCA territory ratio was 19.6% ± 16.2% (range, 0.1%–56%). The penumbra / (infarct + penumbra) ratio was 68.6% ± 23.6% (range, 16.4%–100%). The final infarct volume on follow-up NCCT was 115.4 ± 157.3 cm(3) (range, 1.79–647.4 cm(3)). The minimal z-axis PCT coverage required to obtain values similar to those obtained with 80-mm z-axis coverage was 75 mm for a mismatch of 0.5, fifty millimeters for a mismatch of 0.2, and 55 mm for a size of PCT infarct relative to the MCA territory. CONCLUSIONS: Seventy-five millimeters is the minimal PCT coverage required to use PCT as a tool to select patients with acute stroke for reperfusion therapy by using a mismatch of 0.5. A z-axis coverage of 50 mm was sufficient for a mismatch of 0.2; and 55 mm, for the size of PCT infarct relative to MCA territory (one-third or more).