A 57-year-old male patient was admitted at the emergency room in a state of confusion, aphasia and dysarthria of acute onset (<4 hours), therefore an acute ischaemic stroke was suspected. He underwent an unenhanced CT, followed by perfusion CT (pCT).
Unenhanced CT revealed hyperdensity of the left middle cerebral artery, which is a sign of acute vessel thrombosis (Fig. 1). In our protocol, patients suitable for systemic thrombolysis undergo pCT to evaluate the extension and the “penumbra” zone in the damaged brain tissue. In this case pCT revealed an area with normal cerebral blood volume (CBV) in the left frontal and parietal lobe, moderately decreased cerebral blood flow (CBF) and increased mean transit time (MTT), thus identifying the area of "penumbra"; and a smaller area with a severely decreased CBV, CBF and an increased MTT corresponding to the infarcted tissue (Fig 2). Considering the patient's age, the absence of contraindications and also the large area of "penumbra" identified with pCT, the patient was treated with systemic thrombolysis with an almost complete recovery of symptoms. An unenhanced CT after 24 hours showed only a small hypodensity at the left frontal lobe (Fig. 3) that coincided with the infarcted tissue demonstrated with pCT.
pCT, in association with CT angiography and unenhanced CT is becoming more widespread and available in emergency departments, to identify patients eligible for revascularization in the context of acute stroke [1, 2]. pCT can be quickly acquired with multidetector CT and the perfusion maps can be easily obtained from workstations with the appropriate software installed. With this examination, a fast qualitative and quantitative information on cerebral perfusion is obtained, which allows a distinction between normal tissue, ischaemic tissue but still savable ("penumbra") and infarcted tissue . The concept of "penumbra" describes the cerebral tissue that can still be saved in spite of the reduced perfusion, which is the target of the recanalization therapies, both systemic and locoregional. Dedicated softwares supply pCT maps that can be used to evaluate the vitality of the ischaemic tissue; while the infarcted core typically shows a reduction of both CBF and CBV, the "penumbra" is identified by a decrease of CBF with a conserved or even increased CBV, due to the dilation of the precapillary arteries and venous obstruction. With the introduction of CT equipment with more slice capacity, the whole brain can be studied, identifying both supra and infratentorial perfusion abnormalities . In this case, a 256 multi-detector CT device (Brilliance iCT 256 slices, Philips Medical Systems, Best, The Netherlands) with a dedicated acquisition module (Jog-scan) was employed. This system enables the evaluation of 16 cm of brain tissue, sufficient for an almost complete evaluation of the brain in all cases of suspected ischaemic stroke.
In the setting of acute stroke, pCT allows to identify ischaemic penumbra, which is an indication that thrombolytic therapy, both systemic or mechanical, may be beneficial, even though the decision to treat depends mainly on the clinical status of the patient and the estimated risk-to-benefit ratio of the therapy. pCT imaging is performed as an extension of the standard unenhanced CT protocol, with the goal of helping to determine whether thrombolysis is useful .
It has been demonstrated that the thrombolytic therapy is safe within 4.5 hours, therefore pCT is particularly helpful in patients who awake with symptoms or that are admitted in the emergency department within 4.5 hrs from their onset and are evaluated to detect any penumbra and to tailor the most adequate therapy. Patients in whom no penumbra is seen are not usually treated with thrombolytic drugs, since the risk-to-benefit ratio is unfavourable .
 Michel P, Bogousslavsky J (2005) Penumbra is brain: no excuse not to perfuse. Ann Neurol 58:661-663 (PMID: 16240337)
 Gonzalez RG (2006) Imaging-guided acute ischemic stroke therapy: From \"time is brain\" to \"physiology is brain\". AJNR Am J Neuroradiol 27:728-735 (PMID: 16611754)
 Zhu G, Michel P, Aghaebrahim A et al (2013) Computed tomography workup of patients suspected of acute ischemic stroke: perfusion computed tomography adds value compared with clinical evaluation, noncontrast computed tomography, and computed tomography angiogram in terms of predicting outcome. Stroke 44:1049-1055 (PMID: 23404718)
 Soares BP, Dankbaar JW, Bredno J et al (2009) Automated versus manual post-processing of perfusion-CT data in patients with acute cerebral ischemia: influence on interobserver variability. Neuroradiology 51:445-451 (PMID: 19274457)
 de Lucas EM, Sánchez E, Gutiérrez A, Mandly AG, Ruiz E, Flórez AF, Izquierdo J, Arnáiz J, Piedra T, Valle N, Bañales I, Quintana F (2008) CT protocol for acute stroke: tips and tricks for general radiologists. Radiographics 28(6):1673-87 (PMID: 18936029)
 Srinivasan A, Goyal M, Al Azri F, Lum C (2006) State-of-the-art imaging of acute stroke. Radiographics 26 Suppl 1:S75-95 (PMID: 1705052)