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Cardiac magnetic resonance imaging for stroke assessment

Stroke is the fifth leading cause of death in the U.S. which isn't surprising considering that it is estimated that someone dies from stroke every four minutes. In fact, nearly 800,000 people experience a new or recurrent stroke every year. There are two main classifications for stroke: hemorrhagic and ischemic. Because of its prevalence, doctors and researchers have been searching for the causes of stroke and warning people of the risks. In some cases, they may use imaging tests, such as magnetic resonance imaging, to help them understand the cause of stroke in a specific patient.

What is the difference between hemorrhagic and ischemic stroke?1

The two types of stroke have different prevalence and causes. Often, the causes of the two types of stroke are completely different. The less common type of stroke is hemorrhagic, which, unfortunately, is most often the one that results in death. Ischemic stroke is more common, but it frequently has a large number of risk factors.

  • Hemorrhagic stroke occurs when a brain aneurysm bursts or a weakened blood vessel leaks into the brain. This is often referred to as a brain bleed. An aneurysm is a bulge in the wall of an artery, which is a small blood vessel that carries blood from the heart outward. Within the category of hemorrhagic stroke, there are two subcategories, which are intracerebral and subarachnoid stroke.
    • Intracerebral stroke is the most common of the two hemorrhagic strokes.1 It occurs when a blood vessel bursts or leaks blood into the surrounding brain tissue. The bleeding can cause brain cells to die, which, in turn, can cause the affected part of the brain to stop working as it should.
    • Subarachnoid stroke results from bleeding in the subarachnoid space, the area between the brain and the tissue covering the brain. This type of stroke is typically caused by a burst aneurysm, though there are other risk factors.
  • Ischemic stroke occurs when a blood vessel carrying blood to the brain is blocked by a blood clot. There are also two types of ischemic stroke that can occur: embolic and thrombotic. About 87% of all strokes are ischemic in nature.
    • Embolic stroke occurs when a blood clot or plaque (a substance that can form within the arteries) fragment forms somewhere in the body and travels to the brain. This usually originates in the heart and can block the blood's passage through small blood vessels. If the blood clot or plaque fragment lodges in the blood vessel, blocking blood flow. 
    • Thrombotic stroke is caused by a blood clot forming inside of one of the arteries supplying blood to the brain. This can also block blood flow to the brain. This blood clot can be one of two types of blood clot. Large vessel blood clots occur in the brains larger arteries, which is usually caused by atherosclerosis in combination with rapid blood clot formation. Small vessel disease can also cause this sort of stroke. It occurs when blood flow is blocked to a small arterial vessel and is also called lacunar infarction. Little is known about stroke caused by small vessel disease.

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Risk factors for stroke

  • Some medical conditions conditions, such as atherosclerosis or atrial fibrillation (Afib), may increase a patient's risk of stroke. About 15% of embollic strokes occur in people with Afib.1 Bleeding disorders and head injuries can also cause stroke.
  • High cholesterol levels may increase a patient's chance of having large vessel thrombosis.1 This may be because of the increased risk of atherosclerosis, which can cause large vessel stroke.
  • High blood pressure, or hypertension, can increase the risk of strokes.2 Hypertension can cause blood vessel walls to give out due to the constant pressure and cause a bleed in the brain.
  • Certain medications can cause patients to have more issues with stroke, especially blood thinners.1 In cases where a young woman has one or more risk factors, the risk for stroke can increase with birth control.3
  • Smoking can lead to a higher risk for stroke.4 It can also increase the risk for other cardiovascular diseases.
  • Patients with cardiovascular disorders and diseases, such as those with muscle issues in the heart or atypical ejection fractures, may be at higher risk of a stroke. In some cases, patients who are having a heart attack may have a stroke if they are down long enough.

What does cardiac MRI tell physicians about stroke?

Cardiac magnetic resonance (CMR) is a type of magnetic resonance imaging (MRI) scan that focuses on the heart. The images produced can lead to insights into the heart and the nearby cardiovascular system's features. Certain techniques used in CMR, such as delayed enhancement (DE) CMR, allow physicians and radiologists to see even more information about the heart.

One study of 106 patients, 85 of which had an ischemic stroke and the other 21 had had a transient ischemic attack, found that a combination of CMR and DE-CMR decreased the rate of cryptogenic stroke by 26.1% compared to their routine diagnostic workup.5 Cryptogenic stroke is a stroke that is considered to be one with an undetermined origin. The routine diagnostic workup includes brain diffusion-weighted imaging, telemetry, magnetic resonance angiography (CT and MR), carotid duplex ultrasonography, laboratory studies and transthoracic echocardiography.

Stroke can be assessed in a number of ways, but cardiac magnetic resonance may detect something that other imaging tests missed. CMR aids physicians and radiologists in treatment decisions in these cases. Its ability to show the details of the heart and cardiovascular system are astounding. Unfortunately, there is a sizable opportunity to test this with the nearly 800,000 people who have a stroke in a year. Hopefully, CMR of stroke can lead to a better understanding of this disease.

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References

1. American Heart Association. "What is stroke?" Stroke.org. Web. 14 June 2019. <https://www.stroke.org/understand-stroke/what-is-stroke/>.

2. "How High Blood Pressure Can Lead to Stroke." Heart.org. Web. 14 June 2019. <https://www.heart.org/en/health-topics/high-blood-pressure/health-threats-from-high-blood-pressure/how-high-blood-pressure-can-lead-to-stroke>.

3. Loyola University Health System. "Birth control pills increase risk of ischemic stroke: But risk is very small for women without other stroke risk factors."ScienceDaily.com. 5 March 2018. Web. 14 June 2019. <https://www.sciencedaily.com/releases/2018/03/180305092949.htm>.

4. "Be Smoke Free: Smoking and Stroke." Stroke.org. Web. 14 June 2019. <https://www.stroke.org/wp-content/uploads/2018/12/Be-Smoke-Free_Fact-Sheet_0.pdf>.

5. A. Baher, et al. "Cardiac MRI Improves Identification of Etiology of Acute Ischemic Stroke." Cerebrovascular Diseases. 2014; 37:277-284. Web. 17 June 2019. doi: 10.1159/000360073.