Introduction: There is evidence that CTP volumes are affected by scan duration, but the minimal scan duration needed to obtain reliable volumes has not been established.
Hypothesis: The minimal CTP scan duration for individual patients depends on the arrival time and width of the injected bolus.
Methods: We included CTP scans of patients who presented with an acute ischemic stroke. To identify the optimal CTP scan duration, we assessed how truncation of the scan (from 90s to 20s) impacts lesion volumes. We included only patients for whom truncation of the final 10.8 sec (from 90 to 79.2 sec) did not affect lesion volumes to ensure that the subjects had been scanned with sufficient duration. For each included patient, further truncation was used to identify a “stability point”, defined as the minimal scan duration needed to achieve stable estimates of the perfusion lesion. For each patient we also determined the arrival time (t0) and width (full width half max - FWHM) of the Venous Output function (VOF). We used a linear model to test if these VOF features could predict the stability point.
Results: We analyzed CTP scans of 70 patients with acute ischemic stroke. Of these, 59 (84.3 %) had sufficient scan duration and were included. Both t0 and VOF width were associated with the stability point (R2 = 0.49). Based on the regression model, minimum scan duration is predicted by “t0 + 1.61* FWHM” (p < 0.001) with an interquartile prediction interval from -5 to 7s.
Conclusions: The minimum CTP scan duration needed for accurate perfusion lesions are patient specific and depend on width and arrival of the injected bolus as expressed by the easy to measure VOF. This knowledge has important clinical implications. After acquisition, it can be used to rate if a scan is of sufficient duration for reliable lesion volume estimation. Prior to acquisition, a low contrast and radiation dose test bolus scan can be used to inform the optimal scan time, thereby limiting the radiation dose to the minimal necessary amount for accurate measurements. For example in our cohort, 52 of 70 patients could have been scanned for just 60 seconds (33% dose reduction) with the same results as a 90 second scan.