DIAGNOSIS AND PROGNOSIS OF
ACUTE ISCHEMIC STROKE
Jennifer L. Apon
Magnetic resonance imaging (MRI) is a noninvasive method of studying the body, in this case the brain, in vivo. MRI utilizes the physics theory of nuclear magnetic resonance (NMR) in which magnetic fields and radio waves cause atoms to give off radio signals. In the case of MRI, signals from hydrogen nuclei in brain water are viewed. The response signal emitted by the hydrogen atom can vary in length, depending on the type of tissue. For example, cancerous tissue emits response signals that last longer than non-cancerous tissue. These response times are called relaxation times. There are two, referred to as T1 and T2. Every tissue in the body has its own T1 and T2 values (i.e. white brain tissue has different values than blood). A T1 controlled image will display low values as bright and high values as dark picture elements, while a T2 controlled image will display low values as dark and high values as bright picture elements. It is the variation of relaxation times that allows each tissue to be distinguishable. The result is a two-dimensional image that looks like a picture of a brain slice in black, white, and many shades of gray. The shade of gray depends on the relaxation times of the tissue and thus a radiologist will usually want to see both a T1 and T2 controlled image.
Sample Image
Magnetic Resonance Imaging and Computed Tomography
Many people think a MR image is a picture like a computed tomography (CT) scan. A MR image is an image created by magnetic fields and radio waves, while a CT scan is created by x-ray. In the 1980’s MRI gained much popularity because of its ability to visualize abnormalities in the posterior fossa of the brain and in the upper cervical spine around bony structures. A CT scan is unable to produce pictures of these areas because x-rays photograph these bony structures. A MR image also offers more flexibility than a CT scan in that it can provide a cross-sectional image taken at any plane in the body, while a CT scan is limited to the angles from which a x-ray can photograph.
Uses of Magnetic Resonance Imaging
Magnetic resonance imaging umbrellas many techniques that offer invaluable uses for clinical diagnosis, prognosis, and treatment. Because of its superiority in soft tissue differentiation, it is excellent for the study of brain matter, including functional research on treatment and management of neuropathologies and trauma (Neurospectroscopy). MRI is especially useful in cardiology, as MR angiography (MRA) is used for congenital heart disease evaluation, the viewing of intracranial and peripheral blood vessels, and CSF studies. MRI is also able to demonstrate changes in bony structures of the muskuloskeletal system, essential for evaluating bone tumors for surgery and management. It is also replacing arthography for studying joint disease and periarticular soft tissue abnormalities.
Diffusion Weighted Imaging and Perfusion Imaging
MRI can unequivocally detect hemorrhage and evaluate the location and extent of a subacute infarction one to seven days after onset. The problem occurs during the critical first 12 to 24 hours after onset of ischemic stroke as this technology is poor at detecting the location and extent of infarction at this time. Fortunately, there are two up and coming techniques that will enable radiologists to better evaluate ischemic stroke within hours after onset. Diffusion weighted imaging (DWI) and perfusion imaging (PI) allow for the measuring of the extent and location of the acute focal brain ischemia and the disturbed flow of the cerebral vasculature associated with acute ischemic stroke.
Diffusion weighted imaging maps water motions in the brain. When the brain experiences a stroke, the water mobility in the ischemic tissue slows rapidly by a factor of two. On the images, substances such as CSF, where there is greater water movement, appear darker than ischemic tissue where there is less movement and enable differentiation.
Sample Image Apparent diffusion coefficients (ADC) of water can be calculated for each tissue type allowing the investigator to evaluate water movement in any plane. ADC values rapidly decline after stroke onset causing the ischemic region to appear hyperintense and enable its visualization. ADC decline can be detected within minutes after arterial occlusion. This allows the investigator to follow the vascular disturbance from where it begins to where it spreads. Lesions have been detected as early as two hours after onset with diameters as small as four mm. ADC values normalize after three to four days. This information allows the investigator to distinguish old ischemic tissue from a new stroke.
Perfusion imaging can evaluate blood volume, blood transit times, and blood flow as relative measures. Contrast agent bolus tracking can be used to image effects on the brain or on the arterial blood. An intravenous bolus of a MRI contrast agent containing gadolinium is given to the patient and effects on the brain are observed by ultrafast imaging. MRI contrast agents containing gadolinium cause distortions of the magnetic field in the tissue surrounding the vessels, causing a loss of signal intensity, resulting in a darkening of the images.
Sample Image The amount of signal loss can be quantified and is proportional to cerebral blood volume in normal brain. Arterial blood transit time is observed by acquired an image every one to two seconds following contrast injection enabling tracking of the stroke. Passage of contrast over time is monitored quantitatively or qualitatively. Cerebral blood flow is derived from the ratio of cerebral blood volume to mean transit time of blood through tissue. Contrast agent bolus tracking reveals well-demarcated decreases in blood flow, and perfusion imaging has allowed these defects to be visualized within two hours after onset. The combination of DWI and PI may be able to distinguish stroke patients who will improve spontaneously from those who will not enabling a more appropriate prognosis and treatment.
Advantages and Disadvantages of Magnetic Resonance Imaging
As with every advance in technology, there are advantages and disadvantages. One advantage of using MRI is superior soft tissue differentiation. MR images are excellent for demonstrating display and boundary contrast between anatomical structures. MRI’s high sensitivity to early pathological changes makes early detection possible. Its multiplanar display allows for flexibility in viewing certain regions and MRI does not give off harmful radiation, as do x-rays. These advantages establish MRI as an invaluable tool for diagnosis and treatment of most diseases in the body. Disadvantages of MRI include a varied group of people who could face serious injury or death from its use. Because of the strength of the magnetic field created, people with pacemakers, implanted insulin pumps, aneurysm clips, vascular coils and filters, heart valves, ear implants, surgical staples and wires, shrapnel, bone or joint replacements, metal plates, rods, pins, screws, contraceptive diaphragms or coils, penile implants, and permanent dentures should not have a MRI scan. Patients with claustrophobia simply cannot lie still in a small, enclosed space for 20 to 60 minutes and must often forgo a much-needed diagnostic. MRI is also a very expensive procedure and it is often recommended to explore other options first.
Magnetic resonance imaging has revolutionized neuroimaging by providing increased strengths in efficiency, safety, and validity. DWI is excellent for initial lesion assessment and monitoring of treatment efficacy as it provides information about location, extent, severity, and responsiveness of ischemic infarcts. Additional information about cerebral blood flow information from PI gives more detail about the stroke. These techniques, DWI and PI, allow for early intervention so that precise characterization of the age and treatment potential of stroke in the individual patient appears to be within reach.
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