Traditionally, stroke had been considered a fateful incident with no existing therapeutic options. However, with the introduction of recombinant tissue plasminogen activator (rt-PA) in the US in 1997 ² and in Germany in 1998,³ a causal therapeutic option has become available for the treatment of patients with acute ischemic stroke. Three major randomized, placebo-controlled trials have demonstrated an improved outcome of acute stroke patients, when rt-PA is given within the first three hours of the onset of stroke symptoms.⁴ However, with the present criteria, only a few per cent of acute stroke patients ever receive tissue plasminogen activator (t-PA) therapy. Several factors contribute to the low percentage of patients treated acutely. These factors include lack of public awareness, deficits in hospital organization and differences in medical infrastructure in rural compared with metropolitan areas.⁵

According to the findings of a recently published analysis, thrombolytic treatment exerts a potential benefit on stroke patients even beyond the three-hour time-window. ⁶ There is also probably neither an absolute viability threshold nor an absolute timewindow of tissue viability. The intricate model of cerebral tissue viability includes four factors: a time factor, a hemodynamic factor, a tissue factor, and an intervention factor.⁷ With time, however, the relative risk of major cerebral bleeding statistically outweighs any potential benefit.⁶ As a consequence, an individual ‘brain clock’ should replace the standard statistical ‘time clock’ currently used to select stroke patients for acute treatment. According to this concept, individual, patient-based imaging assessment of brain perfusion has been suggested as a possible selection criterion for treatment. ⁸ Indeed, thrombolysis achieved in cases of large cerebral infarcts with limited penumbra would not only be of little benefit, but would also potentially increase the risk of intracranial hemorrhage, even in the very early phase. On the contrary, thrombolysis may still be useful six to seven hours after symptom onset in patients with significant, persistent penumbra.

Since the 1970s, computed tomography (CT) has been the gold standard for diagnosing intracranial hemorrhage and is a mandatory imaging test for patients with stroke-like symptoms to rule out other etiologies such as intracranial tumor. CT has the major advantage, compared with other imaging modalities such as magnetic resonance imaging (MRI), of being readily available and accessible at all hospitals in an emergency setting. A new generation of advanced multislice CT (MSCT) scanners introduced in 1998 has enabled the clinical application of contrast-enhanced dynamic perfusion CT to quickly and easily map brain perfusion.⁹ Perfusion CT relies upon the extraction of functional rather than morphological information from the CT data. First conceived in the 1980s, dynamic CT brain perfusion was never accepted into mainstream clinical practice due to limited sampling rates and anatomic coverage of 10mm or less.¹⁰ Currently, dynamic CT perfusion relies on the speed and coverage of MSCT scanners to image at a rate of one rotation per second. The amount of brain tissue imaged during this rotation has increased from the first MSCT scanners which covered 20mm to the latest generation of 64-slice scanners that have the ability to cover 80mm of brain at a fast enough rate to trace the entry and washout of a bolus of intravenous (IV) iodinated contrast injected at approximately 4cc/second for a 40cc volume.

Dynamic perfusion CT has the major advantage of being able to assess and quantify both cerebral blood flow and cerebral blood volume.¹¹ It therefore allows direct insight into cerebral vascular autoregulation and can potentially describe the salvageable penumbra territory. In both the penumbra and the infarct, the mean transit time is prolonged and the regional cerebral blood flow is lowered. However, in the penumbra, regional cerebral blood volume is increased or preserved as a result of a local vasodilatation and recruitment of collaterals, in an attempt by the autoregulation processes to compensate for lowered cerebral blood flow. In the infarct core, autoregulation reflexes are compromised and the regional cerebral blood volume is lowered. This hemodynamic definition of penumbra, applied to the dynamic perfusion CT technique, has been demonstrated as being accurate in comparison with acute and delayed diffusion-weighted and perfusion-weighted magnetic resonance imaging (MRI).^12,13

Since CT is already part of an acute stroke patient’s work-up, perfusion CT studies can be performed easily and quickly in an emergency medicine setting, providing rapid assessment (‘time is brain’) and reliable information that attending physicians can utilize in order to base treatment decisions. Dynamic brain perfusion CT imaging with maps of the infarct core and penumbra provides a potential tool to move beyond the limiting established guidelines to provide individual imaging assessment of acute stroke patients.¹⁴