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Eur J Cardiothorac Surg 2007;32:521-526. doi:10.1016/j.ejcts.2007.05.024
Copyright © 2007, European Association for Cardio-Thoracic Surgery. Published by Elsevier B.V. All rights reserved
a Department of Radiology, Johann Wolfgang Goethe University Hospital, Frankfurt, Germany
b Department of Cardiothoracic Surgery, Johann Wolfgang Goethe University Hospital, Frankfurt, Germany
Received 27 December 2006; received in revised form 30 May 2007; accepted 31 May 2007.
* Corresponding author. Address: Department of Radiology, Johann Wolfgang Goethe University Hospital, Theodor Stern Kai 7, D-60590 Frankfurt, Germany. Tel.: +49 69 6301 7260; fax: +49 69 6301 5252. (Email: s.bisdas{at}med.uni-frankfurt.de).
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Abstract |
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 Top Abstract 1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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Key Words: CT • Perfusion CT • Intensive care unit • Stroke
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1. Introduction |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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Non-contrast CT (NCCT) imaging of the head is still the most widely accepted imaging modality in the world for the emergency setting of an acute stroke. However, the sensitivity of the NCCT in identifying the ischaemic regions and predicting the infarct extent which correlate with the clinical outcome is not high [2]. Perfusion CT (PCT) helps in this respect by allowing non-invasively the judgment of the brain parenchyma affection by means of quantitative perfusion values, including cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). Several studies have shown that PCT correlates well with other vascular imaging modalities [3]. The entire investigation requires only a few minutes and provides a similar amount of information as stroke MRI helps defining the infarcted region, the penumbra, and predicting the infarct extension, at least in a short follow-up [4].
Although PCT is widely accepted for the evaluation of acute stroke patients admitted to the emergency room, there is no data about the feasibility of PCT imaging and the advantages, which may offer, in the assessment of the intensive care unit (ICU) patients, who have undergone cardiac surgery, and present with an unknown or unclear cerebral pathology underlying their serious clinical condition. We sought to address this influence of PCT by determining the confirmation or alteration of the preliminary/working diagnosis (based on NCCT findings).
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2. Materials and methods |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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2.1 Imaging protocol
Standard baseline and follow-up NCCT scanning was performed in a 16-row multislice unit (Sensation 16, Siemens, Erlangen, Germany) using the following parameters: 120 kV, 180 mA, 512 x 512 image matrix, a 22-cm DFOV (Displayed Field of View) and 5 mm (posterior fossa/infratentorial)/10 mm (supratentorial) slice thickness.
PCT consisted of a 45-s series performed in four 6-mm thick adjacent brain sections during the intravenous administration (45 ml) of a 400 mg/dl iodinated non-ionic contrast material (Imeron 400, Altana, Germany). The most caudal section of the perfusion CT series was at the slice of the NCCT at the level of the third ventricle and the basal ganglia, positioned above the orbits to protect the lenses. There was no movement of the patient in the time interval between NCCT and PCT and the gantry tilt was the same.
The contrast agent was administered into an antecubital vein by using a power injector (Medrad, Indianola, PA) at an injection rate of 6 ml/s. The acquisition parameters were 80 kVp and 120 mAs. CT scanning was initiated 6 s after the start of the injection. The time delay before contrast material reached the brain parenchyma allowed the acquisition of non-enhanced baseline images, required for the post-processing of the perfusion CT data.
2.2 Data analysis
The patient data was blinded and two radiologists performed the readings in consensus and were blinded to the results of the clinical examinations. The NCCT studies (including those obtained preoperatively) were evaluated for conventional signs of stroke: hemispheric hypoattenuation or swelling, loss of the cortical ribbon (including the insular ribbon), hypoattenuation in the basal ganglia, and the hyperdense artery sign. Furthermore, the reviewers assessed hypoattenuation in the arterial territories. If a hypoattenuated brain area was found, the extent of the hypoattenuation was documented.
PCT data were analysed using commercially available PCT software (TeraRecon Inc., USA) based on the central volume principle, which is the most accurate for low injection rates of iodinated contrast material. Motion correction was not necessary for any patient. The applied deconvolution operation requires a reference arterial input function (AIF), which was selected by the software in a region of interest (ROI) that the user placed in the anterior cerebral artery (ACA). The venous outflow function ROI was placed in the superior sagittal sinus. CBF, CBV and MTT colour maps were generated by the software. The same two radiologists, with experience in PCT imaging, evaluated in consensus the coloured perfusion maps for abnormalities. Firstly, the readers evaluated the MTT map for time delays of the arrival of the contrast agent that are indicative of an arterial occlusion. The MTT maps are considered to be most sensitive in detecting acute stroke. Secondly, the CBV maps were evaluated for any abnormalities, which were correlated if possible with the MTT findings. Finally, the CBF maps were observed for any hypoperfused or hyperperfused areas that indicated ischaemic or luxury hyperperfused areas, respectively. Special care was taken not to include cerebrospinal fluid (CSF), or large vessels in the defined ROI. In case of unclear findings in PCT maps, the ROI with the suspected lesion was mirrored on the contralateral hemisphere in order to compare the perfusion values with the corresponding presumably healthy regions. If any perfusion value in the suspected lesion was different from the corresponding value in the healthy hemisphere for at least 2 SD, then it was considered pathologic.
The diagnostic contribution of the PCT to patient management was established by one radiologist and one cardiovascular surgeon in consensus, and was classified according to three categories:
As the goal of our study was to demonstrate the usefulness of PCT imaging in cardiovascular ICU patients, no correlation analysis between location and/or size of the lesions and clinical scores was attempted. The outcome diagnosis in our patients was defined as Type I neurologic outcomes, which included stroke, transient ischaemic attack (TIA), coma at discharge, or death due to stroke or hypoxic encephalopathy or Type II neurologic deficits such as deterioration in intellectual function, confusion, agitation, disorientation, memory deficit or seizure. The reporting period included the time from the admission in the ICU to discharge from the ICU or from the hospital.
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3. Results |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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4. Discussion |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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The limited spatial coverage of dynamic PCT might represent a major drawback and jeopardise its sensitivity. With multidetector-row CT technology used in our study, the spatial coverage of dynamic PCT can reach 20–24 mm for each bolus of contrast material. When we consider the amount of the contrast agent and the risk of nephrotoxicity, a maximum of two 40–50-ml boluses can be used; therefore, total coverage up to 40–48 mm can be reached. Furthermore, modern CT units can cover 40 mm in one slab and two successive injections can cover the entire hemispheres. To increase coverage with PCT during a single bolus of contrast agent, certain technical manipulations, such as the toggling-table technique, have been proposed [8]. The limitation of PCT in coverage can also be efficiently solved if the examined level is through the basal ganglia. A previous study reported a sensitivity of 95% in the detection of territorial perfusion deficits [5]. In our study, detection of the ischaemic region was possible in all patients. Nevertheless, calculation of the entire volume of ischaemia remains limited in cases in which the lesion extends beyond the covered area. The injection rate of 6 ml/s, which was used in our study in order to acquire a bolus arrival of the contrast agent, may be high and, thus, may be associated with some risk of contrast leakage. We think that the software calculations used in this study may be achieved with lower injection rates, i.e. 4 ml/s, compared to the maximum slope model analysis which requires very high injection rates [10].
The utilisation and possible benefits of PCT in patients of the ICU after cardiac surgery has not yet been investigated. This group of patients suffers from an increased risk for a postoperative ischaemic brain injury which may increase the hospital stay as well as deteriorate the long-term outcome. The incidence of clinically obvious strokes after CABG is reported to be between 0.8 and 5.2% [11]. The typically poor postoperative course of patients who develop stroke after cardiac surgery underlines the need for timely recognition, prevention/modification of factors that predispose to stroke and it implies certain clinical, ethical and socio-economic issues. In our study, findings on PCT studies contributed significant information in 24 of 33 (72%) patients. The PCT findings revealed acute infarction (contrary to the NCCT findings) and, thus, changed the initial diagnosis in nine patients. This group of patients benefited mostly from PCT, while in 10 other patients PCT revealed more lesions than NCCT. Though, it may be assumed that these cerebrovascular lesions would remain silent, it is possible that some of them were responsible for the disparity between the NCCT findings and clinical status. Finally, in five patients PCT demonstrated the real extent of abnormality, which was different from that shown in NCCT. The agreement between normal NCCT and PCT findings in nine patients, despite their clinical condition, does not necessarily show a pitfall of the PCT imaging. The occurrence of clinically obvious stroke, which was the outcome measure of our studies, is not easily identified. Thus, postoperative cognitive impairment may or may not be classified as ‘stroke’, and these patients may be classified in other categories, such as delirium, depression and dementia, or vice versa. Therefore, the patients referred for imaging may not reflect the true incidence of postoperative strokes.
Embolic phenomena have been previously implicated in the pathophysiology of stroke after On-pump CABG, whereas myocardial stunning and hypoperfusion (resulting in transient ischaemic attacks (TIAs)) may be possible mechanisms associated with delayed onset of stroke after Off-pump CABG [12]. It is not assured that multiple territory cerebral microinfarcts (as a result of extended embolic phenomena) or TIAs can be reliably identified with PCT. This implies that both the NCCT and PCT delivered false-negative results and their agreement is misleading. At this field, the combined use of DWI and PW may provide better results [13], though prospective studies are required and the clinical implications of TIAs imaging are not ‘evidence-based’ [14].
Some sceptics may argue for the therapeutic implications of the application of PCT as a straightforward and sensitive diagnostic tool for detecting cerebral ischaemia in patients after cardiac surgery. Seen from a descriptive point of view, the involvement of one or more large ischaemic territories in PCT was associated with an unfavourable outcome, while an isolated lesion in one more site seemed to result in Type II neurologic outcome. Although an aggressive lysis of the causative thrombus may be an inappropriate therapeutic option, basic acute management, including normalisation of blood glucose and body temperature, volume therapy, maintaining a high blood pressure and cardiac output to improve remaining cerebral perfusion in the presence of ischaemically impaired autoregulation, treating cerebral oedema, prophylaxis of thrombosis, and early mobilisation, arrhythmia therapy as well as management of contributory factors, like systemic inflammatory response, may in the early stage minimise the existing cerebral injury and avoid a new one.
The value of the use of preoperative characteristics to identify patients who are at greatest risk is the potential to alter care and give appropriate information to clinicians and patients in their treatment decisions. A stroke risk index including PCT, on the basis of preoperative characteristics may be developed to predict a patient's risk of perioperative stroke. PCT may identify certain patients with abnormal flow patterns and impaired cerebrovascular reserve capacity. In such cases, PCT maps before and after vasodilatator stimulation may be quantified and a side-to-side comparison of the cerebral hemispheres may reveal impaired vascular supply or vascular reactivity; thus, a modification of intraoperative algorithms, e.g. temperature or blood pressure management, may prevent injury [15].
Certain drawbacks of this study are the small patient population, the absence of a quantification of the perfusion parameters and their correlation with the clinical scores for admission as well as discharge. Thus, the known predictive role of PCT in the long-term outcome was not be verified in our patient population [4]. Nevertheless, we think that these drawbacks do not understate our findings which aim to highlight the primary diagnostic advantages of PCT in ICU patients.
In conclusion, the results of this work demonstrate the potential role of PCT in ICU patients (after cardiac surgery) in whom conventional imaging findings are not in line with the severity of the clinical condition. PCT has shown a greater sensitivity in detecting and mapping acute ischaemic stroke, and the implementation of PCT in the clinical routine CT surveys for ICU patients may be beneficial.
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References |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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This article has been cited by other articles:
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  L. K. von Segesser Post-cardiac surgery arrest: what to do in the ICU (intensive care unit) Eur. J. Cardiothorac. Surg., July 1, 2009; 36(1): 1 - 2. [Full Text] [PDF]
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 A.A. Konstas, G.V. Goldmakher, T.-Y. Lee, and M.H. Lev Theoretic Basis and Technical Implementations of CT Perfusion in Acute Ischemic Stroke, Part 1: Theoretic Basis AJNR Am. J. Neuroradiol., April 1, 2009; 30(4): 662 - 668. [Abstract] [Full Text] [PDF]
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