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Also accepted by ARDMS, CAMRT, NMTCB & Florida (00-Technical)
Authors: Karen F. Vieira, PhD, MSM & Takeesha R. Jenkins, MS
Abstract: Stroke is the fifth leading cause of death in the United States and one of the primary causes of long-term disability. Prompt diagnosis and treatment are often critical for the preservation of essential brain function. Imaging technology is typically necessary for the accurate diagnosis and prognosis of a stroke. Commonly used methodologies include: computed tomography (CT), magnetic resonance imaging (MRI), and functional magnetic resonance imaging (fMRI); with CT scans being the most frequently used device for acute stroke diagnosis. Radiographic advancements are consistently introduced and it is essential that medical professionals who use such devices be able to efficiently perform the techniques that are necessary for an expedited diagnosis. This article provides a continuing education activity for registered radiologic technologists (RT) with emphasis being placed on imaging considerations for suspected stroke patients.
This article was designed as a continuing education (CE) activity with focus being placed on the topic of imaging considerations for stroke diagnoses. The participants will be able to describe different types of strokes, the symptoms, standard treatments, and imaging techniques that are frequently used for the diagnosis of strokes. Participants will also be able to define the subdivisions of an ischemic stroke: embolic, lacunar, and thrombotic as well as the subdivisions of a hemorrhagic stroke: intracerebral, subarachnoid, hypotensive, and arteriovenous malformations (AVMs). A technologist may benefit from an improved understanding of the main features and signs of strokes that can be observed through CT, MRI, or fMRI scans. This may enhance the data acquisition, interpretation, and post-processing procedures.
Stroke is the fifth leading cause of death in the United States1. Women tend to suffer from and die of strokes more often than men, presumably due to age-related complications and higher numbers of elderly women within the population 1. The incidence of strokes is also two times higher for African Americans than it is for Caucasian, Hispanic, Asian, and Native Americans, due to a higher prevalence of risk factors and genetic diseases (e.g., sickle cell anemia, diabetes) that are associated with stroke among African Americans 2.
There are two general categories of risk factors that increase an individual’s chances of having a stroke. These are known as modifiable and unmodifiable. Modifiable risk factors include: drug abuse, smoking, consuming alcohol, hypertension, vitamin B deficiencies, obesity, unhealthy cholesterol levels, and high blood levels of an amino acid called homocysteine 3. Unmodifiable risk factors include: gender, age, ethnicity/race, and a family history of stroke, heart disease (e.g., atrial fibrillation), or diabetes (unrelated to obesity), and migraines 4,5. The combination of modifiable and unmodifiable risk factors has been viewed as one of the reasons that people in the United States have such high rates of strokes. In particular, several studies show that the excessive stroke prevalence is mainly attributed to risk factors such as smoking and obesity which contribute to the occurrence of chronic conditions that are associated with strokes (e.g., diabetes, heart disease, and hypertension), along with differences in socioeconomic status (e.g., income and education) 4,5.
Radiographic imaging is essential for the expeditious and proper diagnosis of individuals who are suspected of having a stroke as this helps increase the probability that critical brain function can be preserved before permanent damage occurs 6. There are two main types of strokes, ischemic and hemorrhagic, that are further categorized into different subdivisions depending on where and how the stroke develops. Computed tomography (CT) is the most commonly used diagnostic tool for suspected stroke cases and advancements such as CT perfusion have dramatically improved the resolution of images while also permitting a registered technologist (R.T.) to create graphs which chart blood flow and volume 7. Physicians are better able to make fast and accurate treatment decisions due to imaging technology that permits both the observation and quantification of blood perfusion in the brain. Furthermore, the combination of CT perfusion, CT angiography (CTA), and non-contrast CT provides a more comprehensive diagnostic evaluation of an acute stroke by displaying additional features such as regional hemodynamics, vascular anatomy, and arterial blood supply to the brain 8. Therefore, it is of the utmost importance that an R.T. be able to perform the techniques that are necessary for understanding a stroke diagnosis including data acquisition, post-processing, and appropriate anatomy demonstration
Magnetic resonance imaging (MRI) is another commonly used device for the diagnosis of strokes that allows subtle alterations of brain tissue to be observed 9. An MRI is usually more accurate than a CT scan and provides an earlier diagnosis of an infarction (the death of brain tissue due to a loss of blood flow), especially in regard to mini strokes. It also demonstrates the same accuracy in regard to displaying whether a hemorrhagic stroke has occurred. MRI scans are being increasingly used for the emergency diagnosis of strokes, but may not always be available at any hour in a hospital. An MRI also takes longer to conduct than a CT scan and may not be used for a suspected stroke if it would result in a significant delay in treatment. Therefore, CT is typically performed for acute stroke diagnoses.
Functional magnetic resonance imaging (fMRI) is an additional type of MRI device that can be used to predict the risk of having a stroke and diagnose cerebrovascular disease 10. The fMRI scan involves the uses of magnets to detect cerebral signal changes from oxygenated blood and displays brain activity through alterations in blood flow. This continuing education activity is meant to improve the understanding of the various stroke types as well as the main signs and features that are visible on a CT, MRI, and fMRI scan that may be used in the diagnosis of a stroke.
Stroke Types, Symptoms, and Treatment
There are two main categories of strokes: ischemic and hemorrhagic. An ischemic stroke is the result of a blocked blood vessel in the brain. A hemorrhagic stroke is the result of a ruptured blood vessel that caused blood to accumulate within and around the brain. Approximately, 85% of all strokes are ischemic and the remaining 15% are hemorrhagic 11. Strokes may also be completed or progressive in nature. A completed stroke is one that has stopped after the intracerebral event has occurred. A progressive stroke is one that is still occurring at the time of diagnosis, causing a continuous deterioration of a patient’s condition. If a progressive stroke affects the flow of oxygen into the vertebrobasilar blood supply, the intracerebral event may last for about 72 hours, but if it affects the carotid artery, the event usually only lasts for 24 hours 12.
An ischemic stroke is also referred to as cerebral ischemia as there is an abrupt decrease or complete disruption of blood flow in the brain due to an artery becoming blocked. Blood clots are typically the cause of the blockage and subsequent brain infarction. There are three categories of an ischemic stroke which are embolic, lacunar, and thrombotic 13. An embolic stroke occurs when a circulating blood clot called an embolus becomes wedged in an artery in the brain, thereby blocking blood flow. A thrombotic stroke occurs when a blood clot known as a thrombus forms in a cerebral artery and continues to increase in size until it blocks blood flow. A lacunar stroke occurs when a small, deeply penetrating artery in the brain becomes blocked. This form of stroke is most often attributed to chronic hypertension as this condition leads to the narrowing of veins, which increases the probability that blood clots will block blood flow 13. Ischemic strokes may also be caused by the buildup of plaque along the walls of the arteries (atherosclerosis), within the carotid arteries (carotid artery disease), or by an irregular heartbeat (atrial fibrillation); all of which may result in blood clots 13. Plaque, which is comprised of cholesterol and fat, reduces the amount of oxygen and blood that flows to the brain, thereby compromising the supplying artery and causing an ischemic stroke. Atherosclerosis within the carotid arteries is the most common reason that strokes occur 13.
Symptoms of an Ischemic Stroke
A transient ischemic attack (TIA), also known as a mini stroke, may occur due to the partial blockage of an artery from an embolus or plaque 3,12. A TIA typically does not cause permanent damage and the symptoms tend to diminish or discontinue altogether because the blockage usually dislodges or dissolves. However, a TIA strongly indicates that a more serious ischemic stroke will occur. Some individuals who experience a TIA have an ischemic stroke within the same month and without intervention, many others who have had a TIA will experience an ischemic stroke within five years 13. A TIA may occur in a carotid artery or a basilar artery and the symptoms differ for these two types of attacks.
The carotid arteries supply the retinal arteries in the eyes with blood, and a partial blockage of the carotid artery may affect one of the cerebral hemispheres or decrease the supply of oxygen to the eyes which results in visual impairments such as significantly darkened vision and poor night vision 3. If a cerebral hemisphere is affected by the TIA, speech problems may develop as well as temporary paralysis, numbness, and tingling on one side of the body. If a TIA occurs from a partial blockage in the basilar artery, both hemispheres in the brain may become affected. Symptoms associated with this form of TIA include: a loss of vision in both eyes, gray, blurry, or dim vision, numbness or tingling in the cheeks, gums, or mouth, a sharp pain in the back of the head, vomiting, confusion, nausea, dizziness, trouble talking and swallowing, weakness in the arms and legs, and falling down suddenly 3,12.
The symptoms of a more serious ischemic stroke also depend on the region where the event occurred. If a large embolus traveled to the brain and became lodged in an artery, the onset of the symptoms is abrupt. In particular, dizziness and headaches typically begin seconds after the blockage has occurred and the headache may be severe. If a thrombus forms in a narrow artery, the symptoms tend to develop more gradually, with the full onset occurring within minutes to hours or days to weeks in some cases 3. When a blockage develops on one side of the brain, it often affects the opposite side of the body, causing unilateral paralysis, weakness, or even blindness in one eye. A loss of coordination or balance may occur as well. Speech becomes impaired if the left side is affected by the blockage. Furthermore, the patient’s speech may become incoherent due to sudden confusion and the person may begin to struggle to express thoughts properly. Ischemic strokes also cause some people to experience seizures or a coma 3.
A hemorrhagic stroke is the result of an artery that bursts and allows blood to flow into brain tissue or the surrounding regions of the brain. In a healthy brain, nutrients and oxygen are supplied to neurons by crossing through the thin walls of capillaries. In addition, a blood-brain barrier that is formed by nervous system cells called glia also prevents neurons from coming in contact with blood. In doing so, this barrier protects and supports neurons by regulating which components of the blood can be transported to the neurons 14. When a hemorrhagic stroke occurs, blood that may come in contact with neurons can hinder proper function, the normal blood supply becomes altered, chemical balance in the brain becomes disrupted, and these events lead to an infarction.
Hemorrhagic strokes are further classified into the following subdivisions: intracerebral, subarachnoid, hypotensive, and arteriovenous malformations (AVMs). The most commonly occurring form of a hemorrhagic stroke is intracerebral, where a ruptured artery allows blood to leak into the brain 14. High blood pressure (hypertension), which puts arterial walls that may already be weak due to atherosclerosis under excessive pressure, causes the majority of intracerebral hemorrhagic strokes 14. The hemorrhage or ruptured artery causes an accumulation of blood, known as a hematoma, to put additional pressure on the brain. The risk of suffering from an intracerebral stroke further increases for patients with a history of heart attack, who have been prescribed blood thinners to reduce the occurrence of clots 14.
A subarachnoid hemorrhage develops due to blood that pools along the outer membranes of the brain (meninges) and leaks into the fluid-filled space surrounding the brain. A ruptured aneurysm that occurs spontaneously or from a head injury typically causes this kind of hemorrhagic stroke, but the incidence is minimal 14. In about half of the cases, subarachnoid hemorrhages become fatal and those who survive often have cognitive or neurological impairments.
Individuals who have AVMs, which are abnormal capillaries and blood vessels that tend to tangle and rupture, have an increased risk of suffering from a hemorrhagic stroke 14. Brain tissue that is directly within the region of an AVM usually receives microscopic damage due to previous minor hemorrhages that typically only cause headaches, leaving the patient unaware of the pre-existing condition. The headaches appear to develop because the AVM shunts blood from arteries directly to veins, bypassing capillaries that would normally transfer nutrients and oxygen to vital brain cells. The passing of blood through the capillaries before entering the veins usually results in normal blood pressure. The disruption of proper blood circulation due to AVMs causes the pressure of the blood that enters the veins to be too high and this increases the probability that one of the blood vessels will rupture and cause a hemorrhagic stroke 14.
A hypotensive hemorrhage is a rare form of stroke that is the result of abnormally low blood pressure which causes a dramatic reduction in blood flow and the brain’s oxygen supply. Proper chemical reactions and brain function cannot occur when blood flow is very low and this can lead to a stroke 14.
Symptoms of a Hemorrhagic Stroke
Similar to an ischemic stroke, the symptoms of a hemorrhagic stroke depend on where the event occurs. The symptoms, which start abruptly and usually continue for several hours, include: confusion, nausea, vomiting, and a headache. An early symptom of a subarachnoid hemorrhage is sensitivity to light that is the result of blood vessel leakage which may occur days or even up to one month before an aneurysm forms in the vessel and ruptures 14. After the aneurysm ruptures, a terrible headache, problems concentrating, neck stiffness, vision loss, or the fixation of the eyes in one direction may develop 3. The symptoms of a hypotensive hemorrhage include: weakness mainly in the hands, shoulders, and thighs, a loss of vision, and decreased alertness 3.
A CT scan is the most common technique that is used to diagnose strokes 15. The reason being is that a CT scan can display bleeding in the brain immediately, allowing for the differentiation between an ischemic and hemorrhagic stroke and expedited treatment. In particular, a standard method to remember the necessary steps during imaging and diagnosis entails measuring the 4 Ps 16: parenchyma, pipes, perfusion, and penumbra. These measurements can be performed in minutes through a CT or MRI scan 17.
Noncontrast CT or diffusion-weighted MRI can be used to obtain an image of the parenchyma, which refers to the nervous tissue in the brain, as both modalities provide a clear distinction between an ischemic stroke or a hemorrhagic stroke. This is a critical step toward administering the appropriate treatment. Pipes refer to the vascular structures where the stroke begins such as the large veins and arteries that are involved in an infarction or hemorrhage. The vessels that can be viewed are typically 0.5 mm or larger and include: the vertebral and carotid vessels, the aortic arch, the proximal cortical branches, and the major branches of the circle of Willis16. CTA or MR angiography can be used to identify lesions in these structures, with CTA currently being the preferred method of choice3. In addition, CTA is an effective means of assessing stenosis, the blockage of the vertebral or carotid arteries in the neck, which may predispose some patients to having a stroke.
Perfusion refers to the total cerebral blood flow in a certain region of the brain at any given point in time. Measuring perfusion rates provides an additional indication of the degree to which a vessel has been blocked as well as the potential for brain tissue survival or further infarction. Both CT perfusion and MR perfusion offer a noninvasive assessment option for this measurement 16. The penumbra, which refers to darkened areas where the brain has become infarcted, is the most critical factor for the diagnosis of an ischemic stroke.
A penumbra may not become visible until a few hours after a stroke has occurred. Therefore, images from CT scans should not be used as the only diagnostic data for a suspected stroke. Furthermore, the images from a CT scan for about 40% of patients who present with physical symptoms of a stroke may display normal physiology during the first several hours following the stroke.18.
Therefore, a noncontrast CT may be used to detect the presence of an embolus or thrombus in a blood vessel in combination with CTA to assess the state of the arteries and veins affected, and CT perfusion to evaluate the extent of brain infarction and salvageable tissue; all of which can help a physician make a decision regarding treatment including whether or not to perform surgery 19. This combination is referred to as multimodal CT. Furthermore, multimodal CT is safe, fast, and generally adds no more than five minutes to the time that it takes to perform a noncontrast CT scan 20, 21.
The general advantages of CT imaging include:
- Produces less motion artifacts than MR imaging
- Can be completed in 5 to 10 minutes
- Provides good visualization of major vascular structures
- Readily available at most centers and at various times
- Identifies acute symptoms during an emergency
- Can be performed on patients who have defibrillators, pacemakers, or are claustrophobic
If MR imaging will be performed for the diagnosis of a suspected stroke, the following techniques are commonly used 15,17:
- T1-weighted imaging (T1-WI), where cerebrospinal fluid (CSF) is displayed through a low-signal intensity in relation to the region of brain tissue being examined.
- T2-weighted imaging (T2-WI), where CSF is displayed through a high-signal intensity in relation to area of brain tissue being assessed.
- Spin density–weighted imaging, where CSF has a density that is similar to brain tissue being examined.
- Gradient echo imaging, which demonstrates the highest sensitivity in relation to detecting hemorrhagic changes early on.
- Diffusion-weighted imaging (DWI), where the images display the random motion of microscopic water molecules.
- Perfusion-weighted imaging (PWI), where hemodynamically-weighted MR sequences depend upon the ability of the MR contrast agent to pass through brain tissue.
The general advantages of MR imaging include:
- The visualization of deep and superficial vascular systems
- Allows brain parenchyma to be visualized
- Provides early detection of ischemic changes
- Helps detect deep vein and cortical thromboses
- Helps demonstrate the onset of acute or subacute symptoms when time is not critical
- Identifies small ischemic or hemorrhagic strokes more efficiently than CT
The main goal of the treatment for an ischemic stroke is to restore blood flow. This is typically achieved through the administration of thrombolytics. Recombinant tissue plasminogen activator (tPA) is a thrombolytic agent that is an effective form of treatment for progressive, acute ischemic stroke 22,23. A thrombolytic substance such as tPA stops the progression of the stroke by dissolving the clot that is blocking the flow of blood through the brain. The use of tPA can reduce the extent of brain infarction and even improve a patient’s outcome after having a stroke. In particular, tPA treatment has led to a 30% decrease in the probability of suffering from a neurologic disability in numerous stroke survivors22. Unfortunately, due to the strict clinical guidelines that govern the administration of tPA, only approximately 4% of acute stroke patients receive this form of treatment 22. In addition, not all hospitals are capable of becoming certified stroke centers that can quickly assess whether suspected stroke patients are eligible for consideration of tPA treatment 22.
The guidelines for tPA treatment ineligibility include 23:
An acute bleeding disorder
Active internal bleeding
Major surgery 14 days prior to the stroke
Patients currently taking blood thinners
A previous stroke or serious head trauma within the preceding 3 months
A history of uncontrolled hypertension and intracranial hemorrhage
A history of intraspinal or intracranial surgery in the past three months
An arterial puncture that occurred within the past seven days
Noncontrast CT scan that demonstrates a multilobar infarction that has spread to more than one-third of cerebral hemisphere
The diagnostic confirmation that bleeding (hemorrhage) has not occurred and will not occur is also critical as there is a risk of post-thrombolytic hemorrhage after a stroke. Furthermore, if more than one-third of the middle cerebral artery is affected by the stroke, the risk of experiencing an intracerebral hemorrhage increases 3.5-fold 19. Bleeding can increase after tPA is administered and would make a hemorrhagic stroke worse 21. Previously, it was recommended that there be a time lapse of no more than 60 minutes from the time the patient arrives at the emergency department until tPA treatment begins, but only a very small number of patients were able to be properly diagnosed and treated within this time frame 22, 23. More recent studies have shown that tPA must be administered to a patient that has been accurately diagnosed with having an ischemic stroke within 3 to 4.5 hours of the onset of the stroke symptoms 22-24.
For ischemic stroke patients who are ineligible for tPA treatment due to health reasons or being seen outside of the treatment time window, a surgical procedure called carotid endarterectomy may be performed 25. The surgery entails the removal of plaque from the blocked carotid arteries (stenosis), especially when a patient’s imaging results display more than 50% stenosis of the carotid arteries. Clinical research indicates that this form of treatment has helped improve visual, speech, and neurocognitive functions in individuals who had minor strokes caused by a blocked carotid artery 25.
Extracranial/intracranial bypass (EC/IC) is a surgical procedure that may be performed to treat a blocked artery (ischemia) or aneurysms that would lead to a hemorrhagic stroke 26. The goal of this surgery is to restore blood flow to the area of the brain that is being deprived by redirecting the flow through a healthy artery located in the scalp to the brain tissue that has been affected by the stroke. Clipping is an additional surgical technique that is used to circumvent subarachnoid hemorrhagic strokes that may develop due to brain aneurysms 27. During the procedure, the aneurysm is clamped shut from the blood vessel in order to reduce the probability that it will rupture and bleed. A detachable coil technique may be used to treat intracranial aneurysms as well 28. This process involves inserting a small coil, usually made out of platinum, through an artery located in the thigh and then threading it through until it reaches the aneurysm site. The coil is then placed into the aneurysm where it stimulates a response from the immune system that results in the formation of a blood clot within the aneurysm. The clot is believed to strengthen the artery wall, thereby reducing the risk that it will rupture. The aneurysm may then be clipped as the chance of it rupturing during surgery decreases due to the detachable coil.
Depending on the severity of the stroke and the efficacy of the treatment, a stroke patient may need to undergo rehabilitation which may include: physical, occupational, speech, or psychological therapy. Research also indicates that patients who are treated at stroke centers have higher recovery and survival rates than those who are treated at hospitals that cannot provide specialized care 29, 30.
Primary Stroke Centers
In 2005, the American Stroke Association released a publication regarding recommendations for the establishment of stroke centers. In the report, it was explained that some states have implemented policies that better accommodate guidelines that have been made by stroke centers such as the recommendation that: first response medical personnel or hospitals should ensure that patients displaying the signs and symptoms of a stroke be transported to the closest stroke center or hospital with an equivalent unit 31. Subsequently, the Center for Disease Control and Prevention (CDC) collaborated with the National Association of Chronic Disease to assess stroke policies, including primary stroke center policies across the United States.
The findings from the study showed that in almost all states, primary stroke centers are designated through The Joint Commission, state-based policy and regulations or both mechanisms 32. The Joint Commission is a voluntary accreditation process that a stroke center can utilize to identify and correct problems regarding care as well as to improve the quality of services being offered. The criteria for accreditation include:
Compliance with the national standards
The implementation of performance measurements and improvement activities
The effective use of clinical practices and recommendations for managing stroke care
This process is reviewed continuously throughout the year and centers must apply for certification annually. The CDC’s study also revealed that by 2009, all of states had at least one primary stroke center that had been certified through The Joint Commission. Furthermore, by mid-2010 more than 750 hospitals across the United States received certification to function as primary stroke centers 32. In addition to The Joint Commission, some states have established regulations for the designation of a hospital as a stroke center. As of 2010, 19 states had established stroke center legislation, administrative policies, or pending legislation. During that time, the other states did not have stroke center designation policies in place, but were working toward passing supporting legislation 32. The information that was published by the CDC regarding stroke centers is the most comprehensive review that is available to date, as it involved assessing stroke centers in all of the states across the U.S.
Overall, stroke center policies vary considerably across all of the states in America, including the types of legislation that are passed and the governing bodies that are authorized to designate or certify stroke centers. At the time of the CDC’s study, well-established stroke center policies were most evident in the Northeast, while Western and Midwestern states, with the exception of Washington, had not established stroke center regulation and policies 32. Furthermore, some states adopt The Joint Commission criteria, but if a state agency or independent authority certifies a hospital, the criteria may not be based on that of The Joint Commission.
Discussion and Conclusions
Unwarranted delays in stroke diagnosis and treatment lead to increased damage of brain tissue and a higher rate of morbidity. This emphasizes the importance of utilizing efficient and effective diagnostic tools that can expedite this process. Evaluating a patient’s medical history, conducting a neurological examination, an electrocardiogram, and blood tests are among the first strategies that are used to detect a suspected stroke. In addition, verbal, physical, and mental assessments may also be performed in the emergency room, with the results being based on how the patient responds to the questions and physical symptoms that may be observed.
Although various types of stroke assessments are available, stroke imaging has become a standard because it effectively confirms a preliminary stroke diagnosis so that a treatment plan can be implemented and it provides an assessment of infarcted brain tissue as well as salvageable tissue in order to determine a patient’s prognosis 24. The continuous technologic advancements that have been made in relation to neurovascular imaging provides a number of options for medical institutions to incorporate specialized radiographic devices (e.g., CTA, CT perfusion, fMRI) into the acute stroke diagnosis and treatment procedures. An RT who is well-versed in the common practices involving such devices is better equipped to perform the necessary data acquisition, post-processing techniques and appropriate anatomy demonstration.
Both CT and MRI scans provide unique imaging data that is an important component of the comprehensive stroke diagnostic evaluation 15. Currently, CT is the most readily available diagnostic tool for most emergency departments, especially depending on the time of day, and this method produces images rapidly. Additional advancements in fMRI also provide the visualization of regional homogeneity, which demonstrates stroke-related changes in cerebral activity.
Appropriate treatment is dependent upon clearly distinguishing between an ischemic or hemorrhagic stroke as well as the subdivisions of these two stroke types. The application of multimodal CT is currently a fast and safe means of assessing suspected stroke patients and an RT who is able to effectively perform this evaluation through the use of CT can help physicians make the most accurate treatment decisions.
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