Presentation
Stroke code, urgent cerebral CT perfusion performed.
Patient Data
Red curve represents arterial input function (AIF) ROI; blue represents venous output function (VOF). AIF placed orthogonal to the slice in distal ICA to reduce volume averaging. VOF placed over torcula heterophili.
Long delay after injection of contrast before the intensity curves start rising. The venous peak and decline are not reached within the 50 second perfusion scan window. The arterial peak is also not sharp. Appearances consistent with poor cardiac output.
Apparent raised MTT in both hemispheres (left > right) but likely inaccurate due to poor cardiac function.
Cardiomegaly with small pleural effusions.
CT four days later (patient had not been thrombolysed) shows limited acute infarct in left MCA (M2 and M5 areas), far smaller than the apparent MTT elevated areas. This confirms that the MTT abnormality is not accurate.
Case Discussion
In a patient with good cardiac output, the initial plateau, upstroke and downstroke are all reached within 30 seconds. In this case, the upstroke only starts at about 30 seconds and as a result the VOF peak is not reached within the 50 second scan window. The VOF peak is important as it serves as a reference for normalization of the perfusion parameters and correction for volume averaging of arterial HU.
In patients with poor cardiac output, atrial fibrillation or severe vascular stenosis, the AIF will lag behind the true tissue density. Also, in these cases, the bolus forming the AIF is dispersed over multiple pathways proximal to the ROI (in CT perfusion, an assumption is that the ROI represents the only input to the tissue of interest). These factors lead to overestimated MTT (ie erroneous diagnosis of extensive ischemia or global hypoperfusion) and underestimated CBF.
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