Feature article

MRI of the Brain to Diagnose and Monitor Stroke

When a person shows signs of having a stroke, time is the most critical factor for ensuring their survival and minimizing the extent of brain damage. Therefore, the speed at which a radiologist can determine which type of stroke has occurred is essential for administering the proper treatment. With speed and time as significant factors for saving a patient’s life, the standard test to assess a stroke has usually been computed tomography (CT) scans. CT scans vary in length, but they are generally very quick, and some may be completed within five minutes.2 However, In 2010 The American Academy of Neurology revealed that magnetic resonance imaging (MRI) scans provide a better image to allow radiologists to detect ischemic stroke damage compared to CT scans.1

The Danger of Stroke

In the United States, stroke is the third leading cause of death and it is the leading cause of serious, long-term disability. Stroke causes over 140,000 people to die every year.3 A stroke occurs when blood flow to a part of the brain is interrupted by either a blocked or broken blood vessel. Cells in the brain no longer receive a constant supply of oxygenated blood and may die, causing permanent brain damage.4

There are two different types of strokes that occur, and they each require different treatment. The types of stroke include:

  1. Hemorrhagic stroke occurs when a blood vessel in the brain ruptures, causing blood to leak into the brain.4
  2. Ischemic stroke occurs when a blood vessel to the brain is blocked by either severely narrowed arteries or a blood clot. This type of stoke is the most common, accounting for approximately 87% of all strokes.5

Depending on what part of the brain has been affected by the stroke, different symptoms will occur. In most cases, symptoms develop suddenly and without warning. Symptoms of stroke include:

  • Severe headache
  • Numbness or weakness of the face, arm or leg
  • Trouble speaking or understanding speech
  • Problems seeing in one or both eyes
  • Dizziness, loss of balance, and coordination4

Medical researchers have identified many risk factors associated with stroke. Unfortunately, some stroke risk factors are uncontrollable and affect many people.

Examples of uncontrollable stroke risk factors include:

  • Age - Stroke more commonly effects the elderly. The risk of stroke doubles for every ten years of life over age 55.
  • Sex - Stroke occurs more often in men.
  • Family history - If a close relative has had a stroke it increases your odds of having one at an early age.

Examples of controllable risk factors include:

  • High blood pressure - Regular blood pressure check-ups and medication to treat high blood pressure can reduce this risk factor.
  • Smoking - Stroke is associated with the health problems caused by smoking.
  • High cholesterol - High levels of LDL cholesterol leads to atherosclerosis, which is the buildup of plaque in arteries. Clogged arteries are the leading cause of stroke.

The nature of ischemic and hemorrhagic strokes differ greatly, therefore they must also be treated uniquely. For an ischemic stroke, doctors must restore blood flow to the brain as quickly as possible. This is often accomplished with clot-busting drugs. Treatment of hemorrhagic stroke focuses on controlling the bleeding and reducing pressure in the brain.6 It is imperative that physicians are certain about what type of stroke has occurred before starting therapy. To accomplish this, a medical imaging exam is necessary.

Why use MRI for a Stroke?

An MRI is a medical imaging test that uses powerful radio waves and a magnet to create a highly detailed, contrasted image of the brain. This machine can detect a wide variety of brain and blood vessel abnormalities and can visualize minute differences between tissues that are unclear on other modalities such as x-ray and CT scanners. In many cases, MRI can display tissue abnormalities that are too small or located in regions of the brain that cannot be detected by CT.

The first step in assessing a stroke patient is to determine whether the patient has experienced an ischemic or hemorrhagic stroke. An MRI of the head is often the first test performed. MRI can detect brain tissue that has been damaged by both an ischemic stroke and a brain hemorrhage. Also, an MRI is very sensitive and specific in distinguishing ischemic lesions and identifying pathologies that resemble stroke, known as “stroke mimics”. Furthermore, contrast-enhanced MRI can reveal cortical patterns of ischemic enhancement that provide information about the size and location of the stroke event.7

MRI images taken of the human brain are often created in three orientations in order to provide radiologists with a conclusive, 3D view of the body part. The three orientation displays are:

  1. Coronal orientation: a slice dividing the head into front and back halves
  2. Sagittal orientation: a slice dividing the head into left and right halves
  3. Axial orientation: a slice dividing the head into upper and lower halves3

There are several different MRI sequences used during one scan, and each sequence highlights different aspects of brain tissue and can be used to answer specific questions. Certain sequences are particularly useful for detecting abnormalities in the first few hours after ischemic stroke, such as diffusion-weighted MR.3 The following are commonly used MRI techniques when imaging the brain:

  • T1-weighted imaging in which cerebrospinal fluid (CSF) has a low signal intensity in relation to brain tissue
  • T2-weigthed imaging in which CSF has a high signal intensity in relation to brain tissue
  • Spin density-weighted imaging in which CSF has a density similar to brain tissue
  • Gradient echo imaging is a sequence that is most able to detect early hemorrhagic changes
  • Diffusion-weighted imaging reflect the microscopic random motion of water molecules 8

Diffusion-Weighted Imaging

For stroke patients, one of the most important MRI sequences they can undergo is diffusion-weighted imaging (DWI). DWI sequences can measure the speed at which water molecules self-diffuse, or move randomly, in different parts of the brain.5 This process shows where molecular water motion is restricted, and therefore where brain damage has occurred. With this technique, DWI MRI can detect ischemic stroke within mere minutes of stroke onset. This rate of speed is much faster in comparison to other imaging tests.5 One large study found that stroke was accurately detected 83% of the time by DWI MRI, compared to 26% of the time by CT.1

In addition, the guideline set forth by the American Academy of Neurology found that MRI scans more accurately detected lesions from stroke, and helped identify the severity of some types of stroke. This ultimately helps people get faster acute stroke diagnosis and treatment.1

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Monitoring Stroke Recovery

Once a stroke is diagnosed and treated, MRI is a valuable tool for evaluating and monitoring the patient’s recovery process. In the past, MRI has been primarily focused on the acute management of stroke and staging of ischemic damaged tissue with the goals of extending the treatment window and early detection and prediction of ischemic damage.9 MRI is a useful exam for noninvasively monitoring the temporal profiles of functional recovery and tissue remodeling after stroke.9

The traditional method of monitoring stroke recovery is with a functional MRI (fMRI) scan. An fMRI measures the small changes in blood flow that occur with brain activity. It is often used to determine which parts of the brain are handling critical functions such as speech and movement.10 An fMRI shows when neural activity increases in a certain area of the brain, because the MRI signal intensity also increases.

fMRI can be used to measure the progress a patient is making in stroke rehabilitation therapy. A study conducted by Mintzopoulos, Dionyssios, et al., monitored brain activation after stroke in 5 patients. This study evaluated 60 fMRI datasets on a 3.0T MR system from five right-handed patients with left-sided stroke less than six months prior, with mild to moderate hemiparesis. These patients trained the paretic right hand at about 75% of maximum strength for one hour a day, three days a week, for four weeks. fMRI data was acquired before, during, and upon completion of training. The fMRI results showed that the therapy training increased their number of activated sensorimotor cortical voxels, indicating functional cortical plasticity in chronic stroke patients.11 This indicated the potential of rehabilitation in inducing cortical plasticity in chronic stroke patients.

MRI is a vital tool for diagnosing, treating and monitoring stroke. As technology advances, there is continued interest for using MRI to monitor the structural substrates of spontaneous recovery and neurorestorative treatment in stroke patients.9 In addition, stronger magnets and faster scan times are making the modality more appealing in emergency situations. As the American Academy of Neurology discusses, studies have proven the importance of using MRI in emergency settings. Emerging technology hopes to remove doubt that MRI can be used to assess stroke in clinical settings.

 

 

References:

  1. “New Guideline: MRI Better Than CT Scans at Diagnosing Stroke.” American Academy of Neurology. 12 July 2010. Web. 6 September 6, 2018. <https://www.aan.com/PressRoom/Home/PressRelease/849>.
  2. McKenzie, Jason, Goergen, Stacy. “Computed Tomography (CT).” Insider Radiology. 31 August 2017. Web. 6 September 2018. < https://www.insideradiology.com.au/computed-tomography/>.
  3. “Stroke Statistics.” The Internet Stroke Center. 6 September 2018. < http://www.strokecenter.org/patients/about-stroke/stroke-statistics/>.
  4. “Stroke.” org. 30 January 2017. Web. 6 September 2018. < https://www.radiologyinfo.org/en/info.cfm?pg=stroke>.
  5. “Stroke.” American Society of Neuroradiology. 6 September. < https://www.asnr.org/patientinfo/conditions/stroke.shtml>.
  6. “Stroke.” Mayo Clinic. 16 May 2018. Web. 6 September 2018. < https://www.mayoclinic.org/diseases-conditions/stroke/diagnosis-treatment/drc-20350119>.
  7. Vymazal, Josef, et al. “Comparison of CT and MR imaging in ischemic stroke.” 29 September 2012. Web. 6 September 2018. < https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3505566/>.
  8. Sen, Souvik. “Magnetic Resonance Imaging in Acute Stroke.” 13 October 2017. Web. 6 September 2018. < https://emedicine.medscape.com/article/1155506-overview>.
  9. Jiang, Quan, et al. “MRI of Stroke Recovery.” 24 December 2009. Web. 6 September 2018. < https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2853777/#R5>.
  10. “Magnetic Resonance, Functional (fMRI)-Brain.” org. 25 February 2018. Web. 6 September 2018. < https://www.radiologyinfo.org/en/info.cfm?pg=fmribrain>.
  11. Mintzopoulos, Dionyssios, et al. “Funtctional MRI of Rehabilitation in Chronic Stroke Patients Using Novel MR Compatible Hand Robots.” 27 September 2008. Web. 6 September 2018. < https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2695624/>.