By Francesco Arba

Back to Basics: Non-Contrast CT in Acute Stroke Care

Chairs were Dr. Barbara Casolla from Nice, France, and Prof. Andrew Demchuk, from Calgary, Canada.

The first speaker was Dr. Uta Hanning (Hamburg, Germany) with “Semiquantitive vs automated evaluation of infarct volume on non-contrast CT”.

After a brief overview of epidemiological data of stroke (hemorrhagic, ischemic) and of the ischemic subtypes (lacunar, hemodynamic in the borderzone infarcts, territorial – thrombo-emobolic), dr. Hanning went into pathophysiology of stroke and the relation of cellular mehanisms (BBB, cytotoxic and vasogenic edema) with macroscopic changes as seen in CT, pointing out the evolution of infarct lesion with time and the relation with cerebral edema, the so-called Net Water uptake. She then moved to early ischemic changes in CT and their importance in acute assessment, introducing the ASPECTS score evaluation. Although ASPECTS has a low interrater agreement is still useful in clinical practice and there is available software for automated quantification of ASPECTS. The score has been applied in clinical trials, for example those on low ASPECTS trials, as a imaging selection criteria, supporting its utility in clinical trials and practice. ASPECTS has also importance in rt-PA administration, as stated by the American Heart Association guidelines. At the end of he presentation, she advanced an intriguing hypothesis for future research: could CT based evaluation of edema/early ischemic changes be adopted for wake-up strokes?  

The second speaker, Dr. Aad Van Der Lugt (Rotterdam, Netherlands), gave an overview about “Technical advances in non-contrast CT imaging”. He explained the new insights on CT reconstruction, artificial intelligence and applications, mobile CT for stroke assessment, dual energy CT to differentiate iodine contrast agents from hemorrhage, photon counting CT. A really intriguing advance is Spectral imaging analysis (DECT), where two different detectors receiving different energy contribute to reconstruct a more detailed CT imaging with more information about the material in the voxel (iodine concentration, virtual monocromatic changes). The detector used for this technique (PCCT) works with a direct conversion of the energy received: every single photon is converted into energy according to his attenuation through the brain parenchyma. The advantage of the spectral analysis is that energy given by single photon translates in better resolution. The applications are: differentiation between iodine and hemorrhage, more sensitive ischemia detection, reduction of technical artifacts, particularly those in posterior fossa, those due to implants (e.g. after aneurysms treatment), identitification of calcium vs hemorrhage. It has also application on the CTA (better resolution) for detecting athermoslceorosis of intracranial vessels, identificaiont of distal occlusions, resolution of carotid stenosis, carotid web identification. The conclusion of the talk was that vascular neuroradiology may benefit from DECT/PCCT. 

In the third talk, Dr. Than N. Ngueyn (Boston, USA) tackled the topic of multimodal imaging (“Reperfusion therapy selection: (when) do we need multimodal CT imaging?”). She started with an apparently blunt answer to this question: rarely. After a review of AHA and European guidelines, she provided a compelling overview of available evidence, illustrating original data already published by his group on the outcomes of patients with large vessel occlusion selected with different imaging (CT, CTP, and MRI), showing that outcomes did not differ between the three groups. With an international survey involving neuroitnerventionists and non-neurointerventionists she demonstrated that 64% used perfusion technique in any case. She then introduced the topic of treating stroke in low-income countries such as Africa, where only few hospital have perfusion facilities, and argued that the goal of imaging would be to identify patients whi can benefit from treatment, but not to identify patients that may have better outcomes than others. Perhaps, at the moment, one of the strongest evidence available is the MR-CLEAN LATE trial, that demonstrated benefit of endovascular treatment in patients with large vessel occlusion selected with CT and angio-CT up to 24 hours from stroke onset. Also, evidence from low ASPECTS trials suggest that CT could be used for selection (e.g. TENSION trial). 

The fourth talk was about measurements: Dr. Jelle Demeestere (Leuven, Belgium) talk, “NCCT vs multimodal CT: are we measuring the same thing?”, was about differences in what we identify as ischemic core with CT perfusion and what we consider irreversibly injured brain with ASPECT score. ASPECTS with NCCT and core detected with CTP are both associated with clinical outcomes, used for selection of patients for reperfusion therapies, and do not modify treatment effect. However, while early ischemic changes are related to net water intake as expression of impaired cellular homeostasis due to ischemia, CT perfusion core does not represent structural changes, rather uses changes in regional perfusion to detect areas supposed to evolve into established infarct. Hence, we are actually measuring different things. Then Dr. Demeestere tackled the concept of the ghost core infarct with supporting recent evidence on this interesting topic, and showed an anecdotical  case where CTP core was 0 ml and NCCT ASPECTS was 6, and the final infarct was exactly where the ASPECTS suggested. Arguably, he stated that disagreements between NCCT and CT perfusion are frequent and reflect stroke pathophysiology. The main concept to bear in mind is that CT perfusion is a snapshot at a certain time point reflecting cerebral perfusion, however, cerebral perfusion may change over time, so we basically rely on a snapshot of a dynamic pathophysiological process. He concluded that NCCT and CTP should be complementary: combination of both techniques increases the accuracy of definition of final lesion size, helping clinicians in taking decisions. 

The last talk, delivered by dr. Odysseas Kargiotis (Pireus, Greece), was about “Initial imaging in acute stroke: workflow organization and effect on outcome”.  

In the first part, dr. Kargiotis provided an overview from stroke guidelines. Afterwards, the concept that time matters was highlighted for the attendants, with an overview of EVT studies and the meta-analysis from the HERMES collaboration and data on patients selected only with NCCT. On the other hand, dr. Kargiotis showed studies that showed the potential feasibility of MR instead of CT for acute stroke assessment. In a study from Rapillo et al, recently published on Stroke, although MR reduced the number of stroke mimics treated with rt-PA, so reducing rt-PA rates, outcomes were not affected and door-to-needle time increased.  An interesting study from van Voorst et al., published on JNNP, showed that perfusion in early time window for endovascular treatment was not cost-effective and could potentially harm patients. In general, the use of NCCT  in acute stroke may improve workflow times of endovascular treatment, although there are areas of uncertainty. He concluded that protocols around the globe are various and heterogeneous. Some paradigms may reflect single regional and hospital organisations, see MR instead CT, but CT perfusion should be avoided in early time windows and according to guidelines in late time windows, although absence of mismatch may potentially exclude patients who can benefit from treatment. Selection based on collaterals may potentially be better than selection on perfusion. Again, CT perfusion may potentially facilitate detection of medium size occlusions, but we need more evidence about treatment in such cases and use of thrombolytic drug in extended time windows, where CT perfusion may have a potential role. 

This inspiring session provided an overview of potential evolution of stroke diagnostic assessment and treatment.