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Utility of CT Perfusion Scanning in Patient Selection for Acute Stroke

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Utility of CT Perfusion Scanning in Patient Selection for Acute Stroke Neurosurg Focus 30 (6):E4, 2011 Utility of CT perfusion scanning in patient selection for acute stroke intervention: experience at University at Buffalo Neurosurgery–Millard Fillmore Gates Circle Hospital PETER T. KAN, M.D., M.P.H.,1,4 KENNETH V. SNYDER, M.D., PH.D.,1,4 PARHAM YAshAR, M.D.,1,4 ADNAN H. SIddIQUI, M.D., PH.D.,1–4 L. NElsON HOpkINS, M.D.,1–4 AND ELAD I. LEVY, M.D.1–4 Departments of 1Neurosurgery and 2Radiology, and 3Toshiba Stroke Research Center, School of Medicine and Biomedical Sciences, University at Buffalo, State University of New York; and 4Department of Neurosurgery, Millard Fillmore Gates Circle Hospital, Kaleida Health, Buffalo, New York Computed tomography perfusion scanning generates physiological flow parameters of the brain parenchyma, allowing differentiation of ischemic penumbra and core infarct. Perfusion maps, along with the National Institutes of Health Stroke Scale score, are used as the bases for endovascular stroke intervention at the authors’ institute, regardless of the time interval from stroke onset. With case examples, the authors illustrate their perfusion-based imaging guide- lines in patient selection for endovascular treatment in the setting of acute stroke. (DOI: 10.3171/2011.2.FOCUS1130) KEY WORDS • computed tomography perfusion • acute stroke • stroke intervention ESPITE advances in pharmacological and mechani- tionale is that by limiting recanalization to patients with cal thrombolytic therapy, acute ischemic stroke large areas of ischemic penumbra, neuronal function may treatment remains challenging because of the lim- be restored without increasing the risk of hemorrhage.1,7 itedD time window during which therapy must be adminis- Compared with MR perfusion imaging, CT perfusion tered.3,4,17 Intravenous tPA, for example, is only given to scanning has the advantages of practicality, speed, lower between 3% and 5% of patients admitted with ischemic cost, availability, quantitative parameters, and ease of pa- stroke, largely due to a delay in presentation outside the tient monitoring.18 We review the clinical application of treatment window2 and patient ineligibility. Recent stud- CT perfusion imaging in the setting of acute stroke and ies suggest that clinical improvement is noted even in the present our CT perfusion–based institutional protocol for setting of late reperfusion if patients are selected care- patient selection in acute stroke intervention, along with fully using advanced imaging techniques. With the appli- illustrative cases. cation of CT perfusion–based selection for endovascular therapy, 20% of patients presenting to our institution 8 or more hours postictus improved to a modified Rankin Techniques and Parameters Scale score of 2 or less at 3 months.16 As a result, there Computed tomography perfusion imaging involves has been growing interest in developing CT and MR the injection of a single bolus of iodinated contrast mate- imaging techniques to identify patients in whom a large rial and subsequent spiral CT scanning when the contrast area of ischemic penumbra is present even if the patients bolus is passing through the cerebral vasculature.8 The present beyond the traditional treatment window. The ra- commonly available parameters include CBF, CBV, TTP, and MTT. Cerebral blood flow is measured in milliliters of blood per 100 g of brain tissue per minute (normal is Abbreviations used in this paper: CBF = cerebral blood flow; CBV = cerebral blood volume; MCA = middle cerebral artery; approximately 50 ml/100 g/min), and CBV is measured MTT = mean transit time; NIHSS = National Institutes of Health in milliliters of blood per 100 g of brain (normal is ap- 14,19 Stroke Scale; rCBF = relative CBF; rCBV = relative CBV; TIMI = proximately 5 ml/100 g). The MTT, the average time Thrombolysis in Myocardial Infarction; tPA = tissue plasminogen for blood to travel through a given volume of brain, is activator; TTP = time to peak. measured in seconds and reflects the time required for Neurosurg Focus / Volume 30 / June 2011 1 Unauthenticated | Downloaded 10/11/21 05:15 AM UTC P. T. Kan et al. the contrast bolus to travel from the arterial to the venous Normal Perfusion, Core Infarct, circulation. The TTP is the delay between the first arrival Ischemic Penumbra, Penumbra Mimics, of contrast intracranially and the time when the contrast and Clinical Implications medium reaches its peak concentration, which is mea- sured in seconds. Both CBF and CBV are derived from With normal brain perfusion, there is symmetric per- the time-density curve generated from the preprocessing fusion with higher CBF and CBV in gray matter than in source data using the deconvolution technique;10 the MTT white matter, reflecting the metabolic differences between is then calculated through the central volume principle: these 2 regions (Fig. 1). Regions of irreversible core infarct MTT = CBV/CBF.12 exhibit matched areas of significant decrease in CBF and Both CBF and CBV maps created using the deconvo- CBV due to the loss of cerebral autoregulation, with in- lution technique are sensitive to contrast bolus delay and creased TTP and MTT demonstrated on perfusion maps can overestimate the core infarct in patients with severe (Fig. 2). Specifically, rCBV of < 30%–40% (compared with extracranial hemodynamic compromise caused by chronic the normal contralateral side) and rCBF of < 30% (again, disease states, such as extracranial internal carotid artery compared with the normal contralateral side) are indica- stenosis, atrial fibrillation, and congestive heart failure. tive of core infarct.9,13,21 A significant reduction in CBV is On such occasions, algorithms that correct for the delay particularly sensitive and specific for core infarct.24,25 It is by shifting the tissue time-density baseline curve to gener- widely believed that reperfusion of such necrotic core is in- ate a delay map can reduce the occurrence of overestima- effective and would likely increase the risk of hemorrhage.6 tion.20 The core infarct that can be corrected by the delay- By contrast, ischemic penumbra shows normal or correction algorithm is referred to as pseudoreversible. By mildly decreased CBF, increased TTP and MTT, and a rel- correcting the underestimation of cerebral perfusion and atively spared CBV (with early ischemia, vasodilation, and the overestimation of core infarct, delay maps can improve recruitment occurring as a result of autoregulation) (Fig. our ability to differentiate irreversible core infarct from 3A). In areas of oligemia, rCBF and rCBV are > 60% and salvageable penumbra in patients with low-flow states. 80%, respectively, whereas in areas of ischemic penumbra The appropriate selection of arterial input function (tissue at risk), rCBF and rCBV are > 30% and 60%, re- and venous output function from CT perfusion source im- spectively. 9,13,21 ages is important to generate representative arterial input Several disease states mimic the perfusion patterns and venous outflow time-attenuation curves from which seen in acute ischemia. In extracranial carotid artery ste- CT perfusion parameters are calculated. The arterial input nosis, proximal intracranial stenosis, atrial fibrillation, and function is usually obtained from the A2 segment of the congestive heart failure, MTT prolongation is frequently anterior cerebral artery because it is orthogonal to the axial seen,22,23 mimicking or overestimating the ischemic pen- plane and can be found easily on multiple images. Venous umbra. This highlights the importance of obtaining a con- outflow is typically obtained from one of the dural venous current CT angiogram to assess the extra- and intracranial sinuses. Inaccurate arterial input function and venous out- vasculature in conjunction with the interpretation of the put function placements can affect both qualitative and delay maps to help distinguish these chronic disease states quantitative assessment of CT perfusion scans.20 from acute ischemia. FIG. 1. Computed tomography scan (upper left) and normal perfusion maps with symmetric perfusion: CBF (lower left), CBV (upper center), TTP (upper right), MTT (lower center), and delay map (lower right). Originally published in Kan et al: Computed tomography (CT) perfusion in the treatment of acute stroke. World Neurosurg 74:550–551, 2010. 2 Neurosurg Focus / Volume 30 / June 2011 Unauthenticated | Downloaded 10/11/21 05:15 AM UTC Computed tomography perfusion in acute stroke treatment FIG. 2. Left: Matched reduction in CBF and CBV and increase in TTP/MTT indicating an area of core infarct. Originally published in Kan et al: Computed tomography (CT) perfusion in the treatment of acute stroke. World Neurosurg 74:550–551, 2010. Right: Quantitative CBV map (same patient) showing the area of core infarct with a relative CBV of < 30%. Thresholds for Penumbra and Infarcts ischemic penumbra. Ischemic penumbra is defined as the area of mismatch between increased TTP/MTT and the Currently, no consensus exists on the thresholds of loss of CBV on perfusion maps; core infarct is defined by a CBF, CBV, and TTP used to define core infarcts and isch- significant loss in CBV (rCBV < 30%).6,9,13,21,24,25 11 < emic penumbra. According to Konstas et al., a 50% Time of onset, NIHSS score, and CT perfusion pat- reduction of CBF on CT perfusion imaging suggests sal- > terns are used to select patients for therapy. Eligible pa- vageable penumbra, whereas tissue with a 66% reduction tients presenting within the intravenous tPA window (3– of CBF would likely progress to infarction. Wintermark et 4.5 hours from symptom onset) are treated with intrave- al.24,25 selected a threshold CBV of 2.0 ml/100 g to define nous tPA. Patients with proximal vessel occlusion (M2 or core infarcts. These investigators believe that the relative more proximal) in whom intravenous tPA failed to effect increase in MTT (145% over baseline) or a product of CBF an improvement are eligible for mechanical intervention and CBV most accurately defines the ischemic penum- 15,24 after CT exclusion of a hemorrhage. Eligible patients treat- bra.
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