CT perfusion in acute ischemic stroke

Parameters
A mathematical operation called deconvolution is used to minimize the effects of factors that otherwise may affect the arterial flow, such as impaired cardiac output, carotid stenosis, or variations in the injection rate or saline chase. However, there are different methods to do this, which is a source of variability in the studies.
 * Time to maximum (Tmax, delay time): time from the start of the scan until the maximum density of contrast arrives in the voxel, in seconds.
 * Tmax > 6 seconds (used by RAPID software) correlate well with penumbral tissue, destined for infarction in the absence of reperfusion.
 * Delay time > 3 seconds (used by MIStar and Olea software) also correlate well with penumbral tissue, destined for infarction in the absence of reperfusion.
 * Mean transit time (MTT): average time required for contrast to transverse the voxel, in seconds.
 * Cerebral blood flow (CBF): volume of blood flowing in a unit of brain mass per unit time, in mL/100 g/min.
 * Relative CBF (rCBF) < 30% is 95% specific for ichemic core
 * rCBF < 38% is 87% specific for predicting final DWI infarct, but may correspond best with final infarct volume
 * Cerebral blood volume (CBV): the fraction of a voxel containing blood vessels, in mL/100 g.

Things to check

 * 1) Head positioning: look to ensure head is symmetric without tilt, to appropriately compare hemispheres.
 * 2) Motion: Look at motion profile in the automated report and consider repeating study if there is significant motion.
 * 3) Contrast bolus:
 * 4) Look at input and output functions and ensure they return to baseline and are not truncated, which can happen because of inadequate IV access, proximal large vessel occlusion, or low cardiac output states.
 * 5) Check the arterial input location (usually A2 segment of ACA) and venous output location (usual superior sagittal sinus) for accuracy
 * 6) Core volume measurement: Look all the images, not just the calculated maps. Look at the NCHCT and evaluated for clear hypodensity which suggests infarcted tissue.
 * 7) Core volume may be underestimated if:
 * 8) The reduced rCBF has not yet reached a <30% threshold in an individual patient, as infarction may occur in regions with <38% perfusion as well.
 * 9) In the late time window, leptomeningeal collaterals have resulted in partial reperfusion of completely infarcted tissue.
 * 10) Core volume may be overestimated if:
 * 11) The stroke is very recent after onset.  CBF may be very low but the tissue may not yet be infarcted if reperfusion can be achieved quickly.  Optimal rCBF thresholds may be dependent on time from stroke onset.
 * 12) Ischemic penumbra measurement: Look at the Tmax map to see if there is skull base and posterior fossa artifact.
 * 13) Tmax may erroneously appear prolonged near the skull base and posterior fossa, and these will automatically be included in the calculation of penumbral volume
 * 14) Low flow states such as chronic carotid stenosis, low cardiac output, and cardiac arrhythmias may lead to an erroneously increased ischemic penumbra measurement
 * 15) Stroke mimics: think about these possibilities
 * 16) Seizures: can show ipsilateral hyperperfusion with increased CBF and CBV and decreased MTT or TTP. However, in the postictal phase they can look more like an ischemic stroke pattern,  Look for cortical ribboning, sparing of the basal ganglia, and nonvascular distributions of the abnormalities.
 * 17) Hemiplegic migraine: can result in region or hemispheric CBF reduction
 * 18) PRES can cause mixed patterns with reduced CBF but variable MTT and CBV
 * 19) Posterior fossa and lacunar infarcts: think about these possibilities, which do not reliably show up on perfusion image.