Diagnostics for Stroke

GENERAL DIAGNOSTIC TESTING CONSIDERATIONS

Indications

Diagnostic testing is indicated when it contributes to the diagnosis or management of the patient. Specific to the management of stroke, diagnostics may assist the interprofessional team with determining the time of stroke onset, the stroke subtype, as well as the location and distribution of the stroke. Even the best clinicians cannot distinguish between an ischemic stroke and intracerebral hemorrhagic (ICH) stroke or subarachnoid hemorrhage (SAH) by physical assessment and patient history alone. Diagnostic imaging is necessary to verify the stroke pathology and determine the next steps in managing the patient (Jauch et al., 2013). Test results may also assist the caregivers in ruling out common mimics of a stroke and may provide and contribute valuable information about the underlying pathophysiology of the stroke. Diagnostic imaging studies can assess the status of intracranial vessels and may be useful when conducted at any time along the stroke care continuum by influencing decision making for primary or secondary prevention options as well as acute intervention treatment options (Jauch et al., 2013). Advanced neuroimaging may also reveal the degree of reversibility of the stroke, thereby immediately impacting treatment decisions and directly impacting the course of care for the patient with stroke. Finally, diagnostic testing assists the interprofessional care team in the monitoring of stroke evolution and complications during the inpatient and posthospital care period.

Testing Considerations

The need for information to guide patient diagnosis and management and other advantages of testing must be weighed against the potential risks associated with testing (Dillon, 2012). All tests carry a degree of risk, and the risk should be weighted with the benefit in any clinical case. The clinician must consider all factors, including the patient’s condition prior to performing a diagnostic test, how well the patient is expected to tolerate the diagnostic test, and the contribution of knowledge gained from the test results to the overall care of the patient. If the patient is not hemodynamically stable at the time of diagnostic testing, collaboration between the care team members should occur to evaluate the need and appropriate timing for the test.

General Interprofessional Team Testing Considerations

The patient’s understanding of the need for the diagnostic test, risks or side effects associated with testing, and alternatives to testing are central to informed decision making and the responsibility of all team members. Patients with cognitive deficits as a result of their stroke may require surrogate decision makers and will need frequent reinforcement of information for continued comprehension. Some diagnostic tests contain procedural components and are invasive in nature, necessitating additional consent and more complex care during and after the procedure. Informed and written consent should be obtained when indicated. To address patient safety issues associated with invasive procedures, organizations have embraced a Universal Protocol (UP) or a bundle of patient safety interventions aimed to reduce the risk of harm associated with procedures. A time-out or pause before a procedure to verify patient identity, procedure to be conducted, and potential complications has become a critical component of UP and should be completed with any invasive diagnostic procedure according to organizational protocol.

When the administration of contrast medium is necessary to complete a diagnostic test, an allergy screen should be conducted, and the patient should be monitored for any signs of distress. Patients receiving iodine contrast and have history of iodine allergy may require a steroid prophylaxis preparation due to 5% incidence of allergic reaction to contrast (Hickey & Murphy, 2010). Preprocedure evaluation should include a baseline assessment of liver and kidney function because the liver metabolizes most contrast media and the kidneys secrete most contrast media. In patients with underlying liver or kidney disease, the benefit of information afforded by the test should be weighed against potential worsening of the liver or kidney disease. For patients with underlying renal insufficiency, periprocedural hydration should be administered when certain intravenous (IV) contrast media are administered. Although administration of Mucomyst or sodium bicarbonate to provide renal protection is a common practice, evidence does not show that this practice provides a benefit beyond additional hydration.

NONCONTRAST COMPUTED AXIAL TOMOGRAPHY OR COMPUTED TOMOGRAPHY SCAN

Technique

A computed tomography (CT) scan of the brain uses attenuated x-ray imaging of tissues in a 360-degree rotation. X-ray attenuation determines tissue density from many different angles and is processed by a computer to create cross-sectional slices of brain and spine. CT imaging is conducted in a helical sequence; modern CT devices are able to obtain helical slices at 0.5 to 1 mm thick. All tissues in the brain and spine have different densities and are represented on CT imaging according to their density and are quantified in Hounsfield units (Deshmukh & Yafai, 2008). Tissue density varies from hyperdense, or the most dense; isodense; to hypodense, or the least dense. The CT image represents different densities with gradations of white to black (Dillon, 2012). Hyperdensity appears white in color and may represent bone, calcium deposits, or fresh blood. Isodensity appears gray in color and may represent cerebral tissue or subacute blood, about 1 week after the initial bleeding. Hypodensity is dark gray in color and may represent cerebral edema, fat, or chronic blood older than 2 weeks. Black seen on CT scan indicates cerebrospinal fluid (CSF) or air. CT imaging of the brain may also be obtained with contrast, providing a pictorial representation of vessels in the brain, as well as perfusion imaging, providing a color map representing the perfusion of blood to and away from the tissue of the brain (Dillon, 2012). The sophistication of CT imaging devices as well as software for image processing has improved significantly in recent years, resulting in improved images and increased use in stroke diagnosis and management.

Indication

CT imaging of the brain is a fast and noninvasive test to examine brain and spine anatomic structures (Dillon, 2012). CT of the brain without contrast is the preferred test to provide quick anatomic imaging of the brain for any patient with suspected stroke (Jauch et al., 2013). Noncontrast CT of the brain is generally the first imaging test done on any patient with suspected stroke and may be repeated to monitor the progress or complications associated with the stroke event. Hemorrhage may be seen on CT imaging within moments of it occurring. Ischemia is not immediately apparent and may take 6 hours or longer to be visible on a noncontrast CT of the brain (Jauch et al., 2013). The addition of IV contrast allows superior imaging of blood flow within cerebral arteries as well as areas of increased blood flow or capillary leakage. Cerebral edema or brain tissue herniation may also be detected on CT imaging of the brain. Therefore, noncontrast CT of the brain is used to detect a hemorrhagic stroke or SAH regardless of onset, ischemic stroke that is older than at least 6 hours, as well as cerebral edema, hydrocephalus, shift in brain structures from edema or hemorrhage, and herniation associated with any type of stroke. CT of the brain is less reliable in posterior fossa injury due to skull bone artifact.

As indicated earlier, ischemic stroke may be detected on noncontrast CT of the brain, although the appearance will differ according to the age of the infarct (Dillon, 2012). Age of the infarct is classified as acute (<24 hours), subacute (24 hours up to 5 days), and chronic (weeks) ischemic. An acute ischemic stroke that is less than 24 hours may be visualized on CT imaging because of the loss of gray-white matter differentiation in the insular ribbon area caused by vasogenic edema. Effacement of sulci can also be an early sign of ischemia causing edema. A hyperdensity noted in a large vessel may indicate an acute clot and is most often seen in the M1 or M2 branches of the middle cerebral artery (MCA) and the basilar artery. A subacute ischemic stroke that is 1 to 5 days old will appear as a defined area of infarcted tissue and may continue to have edema apparent on imaging. After 5 to 7 days, a chronic ischemic stroke is visualized on CT imaging of the brain, with clear territory margins and associated loss of brain tissue.

Blood is immediately apparent on noncontrast CT of the brain and is seen as hyperdense or white on imaging (Jauch et al., 2013). Associated edema may be visualized on imaging, with shifting of brain structures from the hemorrhage, edema, or both. Hydrocephalus or enlarged ventricles may be seen with any type of stroke and is more common with ICH, SAH, and posterior fossa ischemic stroke. Several different cerebral herniation syndromes are possible depending on the type, size, and location of the stroke and will be apparent on CT imaging.

Testing Considerations

CT scan is a noninvasive diagnostic test with minimal discomfort. The test has a short imaging time and is sensitive to motion artifact. The biggest risk associated with CT imaging is exposure to radiation, and this is of particular concern with repeated exposure for younger patients (Dillon, 2012). The radiation dose is higher with extended tests, such as CT angiography and/or perfusion CT, and risk of renal injury is higher due to IV contrast administration. Due to the radiation, there is a potential risk to fetal development if imaging is conducted while pregnant. Some patients experience claustrophobia or may be agitated during testing due to their stroke. These patients may require a short-acting sedative during the testing period. CT technologists and radiologists are involved in the planning and execution of CT imaging and should be consulted with any concerns related to radiation, indications, or contraindications to testing .

An additional consideration with higher acuity stroke patients is the safety of transport to CT imaging. A transport team may be necessary to safely transport the patient. The patient will have his or her head flat for at least several minutes. This may be a concern in patients with elevated intracranial pressure (ICP) and should be discussed by the care team when considering the test. If contrast medium is going to be administered, the patient should be assessed for allergies to iodine, shellfish, or contrast dyes. Liver and renal function should be assessed if contrast medium is administered, and the patient should be well hydrated before and immediately after the test.

Patient education prior to CT of the brain should include indication and testing procedure, including the need to remain still during the test to avoid motion artifact. During the test, the patient may hear clicking sounds. The test should be completed in anywhere from 5 to 30 minutes, depending on the number of images ordered. If the patient receives contrast medium, he or she may feel a warm or flushed feeling when it is administered.

There are no significant short-term concerns with CT imaging. Cumulative radiation doses may be a consideration with repeated CT imaging. If contrast medium is administered during the test, patients should be monitored for allergic reaction, and fluid intake should be increased for a period of 6 to 8 hours to reduce the toxic effects of contrast on the kidneys and liver (Hickey & Murphy, 2010).

COMPUTED TOMOGRAPHY ANGIOGRAM

Technique

A CT angiogram (CTA) is a CT scan as discussed earlier, with the addition of IV contrast (Dillon, 2012). The test is a quick and noninvasive means to visualize the details of the cerebral vessels and assess for large vessel occlusions. CTA offers multiplanar views of the cerebral vessels, and digital software allows three-dimensional (3D) reconstruction of the vessels. Slow or turbulent flow in a cerebral aneurysm can be visualized using the CTA. Plaque buildup, incomplete or complete occlusion of large- and medium-sized cerebral arteries, may also be visualized on CTA.

Testing Considerations

Testing considerations for CT are discussed at length earlier and are similar for a CTA. CTA may be added to a noncontrast CT of the brain to detect an ischemic stroke, cerebral aneurysm, or other vascular abnormality or to determine the extent of intracranial atherosclerosis.

COMPUTED TOMOGRAPHY PERFUSION

Technique

A CT perfusion (CTP) is a CT of the brain with administration of IV contrast and calculation of several values to create a color map representing tissue perfusion (Dillon, 2012). CTP is composed of several values, including cerebral blood flow, cerebral blood volume (CBV), mean transit time (MTT), and time to peak (TTP). The four values provide a picture of speed and extent of blood flow, providing a picture of decreased or absent blood flow representing an ischemic stroke or other vascular abnormality.

Testing Considerations

Testing considerations for CT are discussed at length earlier and similar for a CTP. CTP may be added to a noncontrast CT of the brain to detect an ischemic stroke. A CTP also allows for the quantification of the ischemic penumbra or damaged tissue that is not yet infarcted. Therefore, CTP may be helpful for patients when additional rescue therapies are under consideration, such as endovascular therapy. A low-flow or oliguric blood flow state, indicative of ischemic penumbra, will appear as blue in color on the color map. A high-flow state will appear as red color on the map.

MAGNETIC RESONANCE IMAGING