Computerized Tomography (CT) is a diagnostic imaging technology that produces 3-dimensional representations of any body part that has been scanned. The use of head CT scans in stroke diagnosis is routine.
CT scans visualize internal structures utilizing the same principles as ordinary X-rays: different tissues absorb X-rays differently and appear lighter or darker in the images. Head CT scans consist of multiple images of thin cross-sections of the brain. Computers are used to fuse the images together to construct a 3D computer model of the scanned area which the physician can rotate and examine from all angles.
Procedures for CT Scan in Stroke
When a CT scan for stroke is being performed, the patient lays down inside of a tunnel-like machine. A scanning head in the machine swivels around the patient and a beam of X-rays is sent through the brain to a device on the other side of the scanner which detects the different strengths of the X-rays. The CT scanner takes images of a single cross section of the head from 360 degrees. The patient is inched slowly through the machine and images of multiple sections of the brain are taken. In order to increase the quality and resolution of the image or to examine blood flow through the blood vessels supplying the brain, a contrast medium, a dye, is often injected when using a CT scan in stroke diagnosis.
Risks from CT Scan in Stroke
The specific risks of CT scan in stroke diagnosis are the same as for any other condition. Despite taking multiple images, CT scans use very little x-ray radiation. There are few side effects to a CT scan itself though if a contrast dye is used, there is a chance of experiencing a allergic reaction. CT scans are painless but because the patient in put into a narrow tunnel, patients with claustrophobia can experience some degree of anxiety, some people are unable to complete a scan.
Role of CT in Stroke Diagnosis
The early diagnosis of a stroke is critical due to the limited window of time in which treatment will be effective. In the emergency room a CT scan has been a reliable diagnostic test for cardio vascular accidents (CVAs). CT is not perfect, the brain will not appear abnormal in scans done at the onset of a stroke and may not show any abnormalities for several hours. CT scans are also apt to miss small infarcts or those deep in the brain such as the brain stem or the cerebellum.
A CT scan for stroke is one of the first tests a physician will order when evaluating a patient for an acute stroke; they are widely accessible in most hospitals and can acquire images rapidly enough to be useful in early diagnosis.
There are three primary benefits to a CT scan in stroke diagnosis:
• CT scans are able differentiate between irreversibly damaged tissue (an infarct) and reversibly affected tissue (the penumbra).
• CT scans will be able to exclude any other pathologies that can mimic the symptoms of a stroke,
• CT scans can differentiate between an intracranial hemorrhage or an ischemia(1).
There are two kinds of strokes, ischemic and hemorrhagic. In an ischemic stroke, a blood clot will form in the brain (a thrombosis) or be dislodged from elsewhere and travel to the brain (an embolism). The brain cells in the affected area are deprived of blood and begin to die. In hemorrhagic strokes, an artery bursts, interrupts blood supply to the brain and causes local bleeding. This kills brain cells in the area.
A CT scan will be able to differentiate between these two kinds of strokes. This is vital in guiding early treatment. Thrombolytic treatment is important in the early treatment of ischemic strokes but is counter indicated (should not be used) if a it is a suspected hemorrhaging stroke, they exacerbate the bleeding.
CT scans of hemorrhagic strokes show blood on brain and can be distinguished from an ischemia. CT scans of ischemic strokes appear darker than surrounding normal brain tissues because they are less dense (due to loss of blood supply). CT scans of hemorrhagic stroke looks white on CT images: the intracranial hemorrhage is denser that normal brain tissues. CT scans can rule out hemorrhagic strokes by not revealing any areas of blood on the brain.
A CT scan of an ischemic stroke will allow the physician to differentiate between an infarct and still viable brain tissue in the penumbra. The penumbra is an area of brain tissue that is affected by the ischemia but receives some blood supply from other unaffected arteries. This secondary blood supply can sustain the brain cells in the penumbra for a period allowing for treatment that restores the blood supply. Cells in the penumbra can fully recover with quick intervention. By injecting a radiographic contrast medium into an artery and visualizing the flow of blood through the brain, a blocked artery can be identified. This is known as a CT angiography.
A CT scan of either an ischemic stroke or a hemorrhagic stroke have specific and distinct characteristics that identify the infarct of hemorrhage, a CT scan can be used to differentiate a stroke from other conditions with similar clinical presentations, such as tumours.
Alternatives to CT scans in Stoke Diagnosis
A CT scan is one of two major kinds of CVA diagnostic tests used when a stroke is suspected, magnetic resonance imaging (MRI) being the other.
CT scans in stroke diagnosis are regularly performed due to their efficacy and the ubiquitous access ER physicians have to the scanners.
A CT scan for stroke is generally quicker than an MRI scan, the images take less time to be processed and are much less expensive. However, MRI technology is improving and becoming more readily available. Research is beginning to show that in some cases an MRI is superior to a CT scan in stroke diagnosis.
During the early acute phase of ischemic stroke, CT scans can appear normal. Using diffusion-weighted MRI (DWI) can reveal ischemic lesions during this phase more reliably that CT scans(2).
Approximately 5% of Patients presenting with stroke-like neurological deficits will have other conditions such as tumors, multiple sclerosis, or even hypoglycemia. Using an MRI is more revealing than CT(3)(4).
1. Allmendinger AM, Tang ER, Lui YW, Spektor V. Imaging of stroke: Part 1, perfusion CT – Overview of imaging technique, interpretation pearls, and common pitfalls. American Journal of Roentgenology. 2012. p. 52–62.
2. Bahn MM, Oser a B, Cross DT. CT and MRI of stroke. J Magn Reson Imaging [Internet]. 1996;6(5):833–45. Available from: http://www.ncbi.nlm.nih.gov/pubmed/8890023
3. Hakimelahi R, González RG. Neuroimaging of ischemic stroke with CT and MRI: advancing towards physiology-based diagnosis and therapy. Expert Rev Cardiovasc Ther [Internet]. 2009;7(1):29–48. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19105765
4. Chalela JA, Kidwell CS, Nentwich LM, Luby M, Butman JA, Demchuk AM, et al. Magnetic resonance imaging and computed tomography in emergency assessment of patients with suspected acute stroke: a prospective comparison. Lancet. 2007;369(9558):293–8.