Collaterals are so hot right now, especially after they played a prominent role in some of the most important and strikingly positive treatmenttrials the stroke community has seen in decades. Better collaterals as measured by catheter angiography (reference diagnostic), CTA, CTP and arterial spin labeling have been associated with less severe stroke, better results with acute stroke treatment (tPA and/or endovascular reperfusion), more favorable multiparametric imaging (e.g., smaller core infarct and larger “penumbra”) and smaller final infarct volume. Particularly in light of the aforementioned trials, collateral flow score (CFS) is being looked at as an important diagnostic variable in triaging acute stroke patients to the angiography as well as a therapeutic target itself.
These investigators provide us with a timely description of a CTP parameter that, in brief, correlates well with direct subtraction angiography (DSA)-graded collaterals and, in their small retrospective cohort, good outcome. The design of this study was relatively simple in that they included patients with acute ischemic stroke with demonstrated M1 MCA occlusions who underwent noncontrast CT, CTA and catheter angiography for endovascular reperfusion. Collaterals were measured with CTA (maximum intensity projection, or MIP) and DSA using previously published criteria and correlated to a parameter derived from standard clinical CTP that the investigators call volume transfer constant (Ktrans). Inter- and intra-rater reliability was measured amongst these various modalities, and collateral grade was compared to clinical parameters such as 90 day outcome. Again, in brief, Ktrans had excellent agreement with the reference standard DSA (k >0.8) and higher scores of any method was correlated with better outcome.
Although Ktrans is not at this time a mainstream clinical variable, these data are timely and important as those of us who care for acute stroke patients want to find some way to identify patients who might best benefit from endovascular reperfusion therapies in light of the sterling treatment trial results presented just days ago. What will be our best (e.g., most sensitive, fastest, most widely available and cheapest in descending order of importance) means of estimating collateral flow around a large artery occlusion? CT ASPECTS, CTA, CTP (+/- Ktrans), or MRA? We may end up with more than one answer to this important question, and it is good that we have options to study.