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Acute ischemic stroke refers to the death of brain tissue caused by interruption of the blood flow to brain. This interruption is caused by occlusion of a cervical or cerebral artery, or cerebral vein (Uchino, Pary, & Grotta, 2011). It is among the leading causes of death in the world. According to the estimates of American Heart Association, nearly one stroke occurs in every 40 seconds in the U.S., approximating 795,000 cases of stroke per year (American Heart Association Stroke, 2015). Assessment and management of acute ischemic stroke patients need to be done as soon as patients arrive in the emergency department of hospital. Due to huge impact of stroke on public health, researchers are doing a lot of work on stroke (Dubey, Pandey, & Moonis, 2013).
Diagnosis of Stroke
One of the first tasks to deal with the problem of stroke is to make appropriate diagnosis that can be done with the help of a head computed tomography (CT) and magnetic resonance imaging (MRI) (Jauch et al., 2013). These diagnostic techniques can help in diagnosis of stroke, and in differentiating ischemic stroke from hemorrhagic stroke. Ischemic stroke is considered as brain injury caused by impairment or blockage of arterial blood supply (Hof & Mobbs, 2010), whereas in hemorrhagic stroke rupture of a blood vessel results in bleeding into the brain (Health at a Glance: Europe 2012, 2012). Detailed vascular and brain imaging studies can help in excluding stroke mimics (Uchino et al., 2011).
Thrombolytic and fibrinlytic agents can be used to improve the condition of stroke patients. However, it is important to know the presence of intracranial hemorrhage in stroke patients, in which those agents are contraindicated. Research shows that non–contrast-enhanced computed tomography (NECT) is sufficient for patients with ischemic stroke to get well-timed intravenous fibrinolytic therapy. It is important to obtain NECT within 25 minutes of the arrival of patient in the emergency department (Jauch et al., 2013).
Standard CT utilizes X-rays passing through the body at different angles. These rays are then processed by computer in the form of cross-sectional images, or parts of the internal structure of the body (NIH, 2011). NECT scanning of the brain helps in accurate identification in many cases of intracranial hemorrhage. It is also helpful in discriminating nonvascular causes of neurological symptoms. NECT is able to show slightly visible parenchymal damage in three hours. However, it is comparatively insensitive to detect acute as well as small cortical or subcortical infarctions, particularly in the posterior fossa. In spite of these limitations, it is more commonly used in ischemic stroke scanning due to immediate availability, comparatively easy interpretation, and acquisition speed (Jauch et al., 2013).
Interest has grown on the use of NECT in the identification of minor signs that appear in the early stages of ischemic brain injury, i.e. early infarct signs or arterial occlusion. One of the signs of cerebral ischemia that develops within few hours of the symptom onset, which usually appears on NECT, is decrease in gray-white differentiation. This sign can be considered as the combination of the densities of the cortex with underlying white matter present in the insula as well as over the convexities. Another sign of cerebral ischemia that appears on NECT is inflammation of the gyri that generates sulcal effacement. Rapid appearance of these signs shows the severity of the problem. However, the observers’ ability to find these early infarct signs with the help of NECT can change, and these signs usually occur in less than or equal to 67% of cases imaged within three hours. Identification of these signs is usually affected by the size of the infarct, severity of the problem, as well as the time between the development of symptoms and scanning. Detection of the problem can be enhanced by using structured scoring system as, for example, the Alberta Stroke Program Early CT Score (ASPECTS) and/or the CT Summit Criteria. Moreover, better CT “windowing and leveling” can also be used to distinguish normal tissues from abnormal tissues (Jauch et al., 2013).
Standard MRI utilizes radio waves generated by computer along with powerful magnet to develop detailed 3D images of different body parts and nerves (NIH, 2011). MRI of the brain can also help in the detection of acute ischemia. However, standard MRI sequences (fluid-attenuated inversion recovery [FLAIR], T2 weighted, and T1 weighted) are comparatively less sensitive to the variations developing in acute ischemia. In this case, diffusion-weighted imaging (DWI) is considered as highly sensitive (from 88% to 100%) as well as specific (from 95% to 100%) imaging technique for the detection of acute infarct. DWI is far better as compared to NECT or any other MRI technique. It can detect infracted areas in the very start, i.e. problems can be detected after some minutes of the development of symptoms (Jauch et al., 2013).
DWI can help in the identification of the lesion size, age, and site. It can detect comparatively little cortical lesions as well as small subcortical lesions. It can also help in scanning of those areas in the brain that may not be observed by NECT scanning techniques or standard MRI sequences. DWI can also recognize subclinical satellite ischemic lesions providing information on the mechanism of stroke (Jauch et al., 2013).
Figure 1 shows MRI images after onset of symptoms of ischemic stroke and after its recovery. In the figure, two upper images are images of the brain after one hour of the onset of stroke symptoms. Colored version of perfusion MRI shows an area of the brain that is deprived of blood due to the presence of clot in an artery, and black and white DWI shows the lit-up area of injured cells in the brain. On the other hand, two lower images show the condition of brain after the clot is dissolved by using clot-removing drugs. Colored version of MRI shows that the brain returns to almost normal condition as blood started nourishing the starved area of the brain, and black and white area also shows the normal condition in the lower DWI (NIH, 2011)
Figure 1: MRI images after the development of symptoms and when the brain recovers (NIH, 2011)
Determination of brain compartment volume with MRI
High-resolution MR images are required for the determination of brain compartment volume. Volume of brain components can be measured by using stereological methods in which the area of interest on the brain is superimposed on a grid of points, and then the points are estimated to determine the volume. Another method is tracing method in which an investigator traces the brain region of interest with a mouse driven cursor, count the pixels, summed up the transect area and multiply it by the distance present between the consecutive sections traced to find the volume. Some automated and semi-automated methods are also available in which manual estimation of the area of interest is not required (DeCarli et al., 2005; Keller & Roberts, 2009).
Infarct size in MRI scanning
Infarct volume can help in determining the effectiveness of drug treatment for stroke (Maiese, 2012), and MRI can help in the determination of the size of middle cerebral artery infarction. Studies show that with increase in infarct volume, outcomes would be worsened. Patients with a starting infarct volume of less than 70 to 80 cm3 are found to have better outcomes as compared to patients with larger infarct volumes (Šaňák et al., 2006; Saunders, Clifton, & Brown, 1995). Researchers also found that mean infarct volume varies in different groups of people. Mean infarct volume of independent patients was 35.7±29.7 cm3. Mean infarct volume of dependent patients was 88.3±71.3 cm3. Mean infarct volume for dead patients was highest, i.e. 166.5±65.9 cm3 (Saunders et al., 1995). Researchers have also reported that stroke lesions reach final infarct volume by 30 days, though they continue to evolve from five to 90 days (Gaudinski et al., 2008). However, another study shows that final infarct volume can be estimated after 7 days of stroke (Krongold et al., 2015).
Concluding Remarks and Future Directions
Acute ischemic stroke is among the leading causes of death in the world. With the advancement in medical field, several medicines and therapeutic procedures such as thrombolytic treatments have been developed. However, still it is important to diagnose the problem in the early stages for as much recovery as possible. Therefore, several diagnostic biomarkers and devices are used such as MRI and CT. MRI is found to have more benefits as compared to CT in the detection of acute ischemia. With the help of MRI, infarct volumes can also be established, thereby helping in knowing the severity of the problem. In order to deal with thrombolytic events, it is important to work on the sensitive and reproducible imaging biomarkers that help in accurate and efficient assessment of fast developing thrombolytic treatments.
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