SCPDI 2001 CYCLOPSSTROKEQUANTIFIER - ISCHAEMIC STROKE DETECTION SYSTEM USING DYNAMIC CT RAFAEL CHARNOVSCKI1, RONIE C.F. CARDOSO2, LUIZ FELIPE DE SOUZA NOBRE3; ALDO VON WANGENHEIM1 1
Projeto Cyclops, Laboratório de Integração Software Hardware (LISHA), Departamento de Informática e Estatística (INE), Centro Tecnológico (CTC), Universidade Federal de Santa Catarina (UFSC) Campus Universitário - Trindade. CEP 88040-900. Florianópolis - SC – BRASIL. {charnovs,awangenh}@inf.ufsc.br http://www.cyclops.lisha.ufsc.br 2 Laboratório de Telemedicina, Hospital Universitário, Universidade Federal de Santa Catarina (UFSC)
[email protected] 3 Centro de Ciências da Saúde (CCS), Departamento de Medicina, Universidade Federal de Santa Catarina (UFSC)
[email protected] Abstract Ischaemic stroke is a very common disease that affects blood vessels in the brain causing cerebral tissue damage. Despite several studies on this subject, the early detection of acute ischaemic stroke is difficult as well its treatment. In this work we present some features and initial results of our research which includes not only a new image acquisition protocol, but also a software tool in order to help physicians improve the accuracy of the therapy. Keywords acute ischaemic stroke, ischaemic stroke detection, penumbra zone, dynamic CT, DICOM.
1. Problem Description The term ischaemic stroke refers to sudden endangering of the cerebral functions due to a variety of histopathological alterations involving one or more intra or extra-cranial blood vessels. Although we dont't have precise statistics for Brasil, it is known that the mortality rate amongst patients with stroke is 0.5 to 1 within 1,000 individuals. Moreover, stroke is the third main cause of death related to clinical pathologies and the second more frequent cause of neurological morbidity. In the USA stroke provokes 150,000 deaths and invalidity of 200,000 people annually. Yet there are estimates about 2 billion people suffering from stroke's after-effects and expenses of 30 billion each year because of this disease (Matchar et al. 1994). Despite those numbers and the knowledge spreading , physicians in general have little interest in the disease and, as a consequence, little knowledge about it. Most times doctors are not skilled to provide the correct attention (Toole, 1990). About 80% of the stroke cases are caused by insufficient blood flow (ischaemic stroke) (Benett & Plum et al. 1996), and of these, 75% are due thrombosis and blockage (Fieschi et al. 1989). When the blood flow, in a specific cerebral region, falls bellow of 20ml/100g/min, the cerebral electrical activity fails and the symptoms appear. If the blood flow keeps falling to 10ml/100g/min, an ischaemic central area arises resulting in the death of the neurons. Around this central there is a region where the blood flow is reduced, however the perfusion is kept by the collateral circulation. These neurons that don’t function but are alive form the penumbra zone. The penumbra has no metabolic activity but this area may be recovered with medical intervention. If the therapy is not started as soon as possible, the cerebral infarct in the central area extends to the margins within a time not know for the human being.
The axial CT (computerized tomography) is an important examination and the first one to be requested. It allow us to differentiate an infarct from an hemorrhage (Hoggard, 2001). Once the ischaemic stroke is identified, the therapy can be performed with the utilization of neuroprotectors agents or thrombolytics as an attempt to restore the blood flow in the obstructed region. This type of therapy has the purpose to restore the flow as soon as possible in order to limit the neuronal losses in the penumbra area. Currently, these are some of the limitations for adopting this therapy with thrombolytics: Difficulty for radiologists to identify ischaemic stroke within six hours after onset using only CT (Wardlaw et al. 1999); The visualization of signs of ischaemia within this window of time is itself an independent factor of badly prognostic (Wardlaw et al. 1998); Difficulty for identification and measuring of the penumbra zone, which would be the potentially recoverable region. There are several studies to develop early ischaemic stroke detection methods (Hoggard et al. 2001). Although those methods which use magnetic resonance imaging (MRI) are more sensitive, they are difficult to perform in large scale due constraints like examination time and high cost. The specification of a methodology to perform CT examination joined to a software tool that could help physicians to detect early ischaemic stroke and to identify penumbra zones, will benefit the therapy, since several works point towards better results whenever the thrombolytic therapy is started within three hours after the first symptoms. 2. Objectives This work is the first stage of a whole project for development of a radiological protocol and a decision support software to help physicians decide whether the
thrombolytics therapy is suitable or not. Specific objectives are as follows: • Calculation and graphical representation of the blood flow in the brain based on dynamic CT; • Using computer analysis for early detection of ischaemic stroke; • Identifying low blood flow regions (penumbra zone) that could be potentially recoverable; • Measuring the size of the penumbra zone; • Validating the software prototype with help of physicians; • Integrating this software with Cerebral Atlas System (Wagner, 2001) 3. Methods To develop the first software prototype a group of 4 patients have been used. Three patients were suspected to have ischaemic stroke and one patient was healthy. The patients have been submitted previously to an axial CT with cuts of 1 cm thick in order to exclude the cases with haemorrhagic stroke. The brain area chosen to acquire the images is the region of the medium cerebral artery, since it is important for the blood flow supply. Immediately after the patients have taken the contrast agent, dynamic CT images with 5 mm thick have been acquired (the first cut is considered as a native cut). The results obtained with the software analysis were compared to the patients’ anamnese and to the blood flow quantification from the healthy patient. Firstly, we develop a DICOM-compatible software module to load the CTs from a radiological image database. This software has an interface which allow us to visualize and to start all the calculations related to signal variation and perfusion maps. For the calculation of the signal variation and construction of the perfusion maps it is initially calculated the percentual signal raise curve relative to the same point on the native slice (slice number 1) for each image region corresponding to the cerebral parenchima. The image regions showing cerebral parenchima are automatically chosen by the software depending on both, its radiological density and characteristics of its neighbourhood. The contrast agent absorption curves are stored to generate perfusion maps on each slice and they can be visualized by simply moving (dragging) the mouse on any slice. The perfusion maps for each slice are then further calculated through the comparison of the relative contrast agent absorption variations, previously calculated for each point of that slice, with the mean contrast agent absorption in a volume of reference (VOR) manually selected by the user with the mouse. After the manual selection of a VOR region, the system calculates a curve that represents the relative mean contrast agent absorption passing through the VOR for all slices. This curve, called the curve of reference (COR) will finally be used as a reference for the calculation of the perfusion maps for each slice. This means, that the perfusion map shows the relative difference between the signal raise in the VOR
(represented by the COR) and the signal raise of the selected slice. 4. Results The results obtained so far are promising since the tests for the 4 patients are consistent. The software hasn’t found any suspicious area in the healthy patient examination. However, the system has found areas with low or no perfusion in the patients suspected to have ischaemic stroke. For these patients the software marked low perfusion areas, which are related to the symptoms presented in the anamnese. At this moment there is a first version of the software and a large-scale study has been planed. The software's graphical user interface (GUI) can be viewed in figure 1. That image is from a patient suspected to have ischaemic stroke isquêmico with a) tomography slice nr.15, b) signal variation curve related to the VOR (green) compared to the other volume chosen (yellow), c) perfusion map on slice nr 16 showing higher perfusion areas than the VOR mean turning to purple and lower perfusion areas turning to red, and d) VOR limits (green). 5. Conclusion A treatment for the ischaemic stroke has been difficult due to the lack of suitable techniques that permit precocious identification. The accuracy of the treatment depends on the penumbra zone identification, however there hasn’t been an adequate protocol to locate and measure it. Nowadays the patients with potentially recoverable cerebral areas aren’t treated, so that they may have a permanent cerebral lesion. Besides providing a diagnostic tool with utmost importance, this work aims to add important and substantial information in the clinical routine associated to the manipulation of patients with acute ischaemic stroke. Furthermore, it will be possible to personalize the treatment if we can differentiate reversible ischaemic regions from those not reversible (Klotz & König, 1999) so that we may recommend or not the therapy. References [1] BARBER P.A.; DARBY, D.G.; DESMOND, P.M.; et al. Prediction of strke outcome with echoplanar perfusionand diffusion-weighted magnetic resonance imaging. Neurology, 51:418-426, 1998. [2] BENNETT, J.C.; PLUM, F.; et al. Cecil Textbook of Medicine, 20th edition. Bennett & Plum, Philadelphia, pp 2271 e 2272, 1996. [3] CENIC, C.; NABAVI, D.G.; CRAEN, R.A.; GELB, A.W.; LEE, T. Dynamic CT Measurement of Cerebral Bloob Flow: A Validation Study. Am. J. Neuroradiol., 20:63-73, 1999.
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Fig. 1: System's user interface. Notice in the perfusion map (c) that the areas turning to purple show higher perfusion areas than the VOR marked as green (d). Yellow, orange and red areas indicate lower perfusion.
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