Year : 2004 | Volume
: 14 | Issue : 1 | Page : 11--12
CT perfusion : Does it have a role in acute stroke?
Joint Editor & Secretary, Indian Journal of Radiology and Imaging, India
Joint Editor & Secretary, Indian Journal of Radiology and Imaging
|How to cite this article:|
Kohli A. CT perfusion : Does it have a role in acute stroke?.Indian J Radiol Imaging 2004;14:11-12
|How to cite this URL:|
Kohli A. CT perfusion : Does it have a role in acute stroke?. Indian J Radiol Imaging [serial online] 2004 [cited 2019 Dec 13 ];14:11-12
Available from: http://www.ijri.org/text.asp?2004/14/1/11/28546
The last two decades have seen tremendous improvement in the management of coronary artery disease, resulting in increased longevity. With this increased longevity many other vascular problems are increasing in incidence. One important such condition is stroke, or now called brain attack as a simile to heart attack. To improve the management of stroke it is imperative to use thrombolysis, similar to as used in the heart. The problem similar to the heart is that thrombolysis must be performed early, ideally within 3 hours of the stroke for supratentorial ischemia and 6 hours for posterior fossa ischemic lesions. It is not enough for the patient to reach the hospital within this time, the patient has to reach earlier as a number of tests prior to thrombolysis need to be performed. Two important questions need to be answered prior to instituting thrombolysis. Firstly is there a hemorrhagic or nonhemorrhagic stroke. If there is hemorrhage, thrombolysis is contraindicated. To answer this question the best modality is a Plain CT scan. Once it is established that this is a nonhemorrhagic stroke then it needs to be established whether the brain tissue under risk has already infarcted or is there any ischemic tissue which may be viable and thus may be potentially salvaged. If the blood flow to the tissue at risk has reduced below a critical level then trying to increase the flow will not result in salvaging the tissue, in fact reperfusion of severely hypoperfused areas can result in hemorrhage with its associated complications. Therefore it is important to also obtain actually quantified values of cerebral blood flow, so as to make an accurate decision whether to thrombolise or not. To answer these questions four modalities may be used
MR Diffusion Perfusion
SPECT has the advantage that it provides a full 3D extent of cerebral blood flow impairment. It however has significant disadvantages, as it requires 15-20 minutes to perform. During this time a stroke patient will find it difficult to remain still resulting in substantial artifacts, which will mar the study. Further very few centers provide 24x7 coverage.
Xenon CT is an extremely cumbersome test where a patient needs to breathe a narcotic gas through a facemask.
MR diffusion is extremely sensitive in detecting infarction. MR perfusion is done following injection of 20 ml of gadolinium. 75% of hyper acute strokes will have a perfusion defect, which is larger than the diffusion defect. This mismatched area, i.e. area of perfusion beyond the diffusion defect is tissue, which is potentially salvageable. This tissue would most likely infarct. Though it must be remembered that occasionally perfusion MR may overestimate the size of the final infarct.
CT Perfusion is similar to MR perfusion; 50ml of contrast is injected at a rate of 5ml/sec into a peripheral vein. Images are obtained dynamically over the next 30-40 seconds. A CT Angio also can be obtained at the same time to provide images of the carotid arteries and the circle of willis. The time taken with CT for examination and processing is ten minutes.
The crux of perfusion imaging is to demonstrate regional cerebral perfusion. This is measured by CBF, MTT and CBF. CBV represents cerebral blood volume. This is blood volume per 100 gm of brain tissue. MTT is mean transit time, the time taken for contrast to reach the region of cerebrum under evaluation. . A time difference of 6 seconds in MTT between the two sides would indicate infarction. CBF is cerebral blood flow per 100gm of brain tissue. CBF is essentially derived by dividing CBV by MTT.
In ischemic tissue CBV is maintained or slightly increased due to autonomic autoregulatory mechanisms. MTT is prolonged. This is as the contrast reaches the ischemic tissue via collaterals, so the time taken to reach ischemic tissue is longer. In infarcted tissue, CBV drops, MTT increases therefore CBF is low. A major advantage of CT Perfusion is that all these values can be quantified. Normally CBF is 55ml/100gm/min. Less than 10ml/100mg/min is a critical level, these areas definitely infarct. The ideal modality to be used to answer these questions is CT Perfusion. The initial modality to answer the basic question of a hemorrhagic stroke versus a nonhemorrhagic stroke is a plain CT. If we were to use MRI for answering the question of if there is viable brain tissue which can be salvaged, then the patient would need to be shifted to the MRI suite. This would mean loss of precious time. Not only will time be lost taking the patient from one modality and repositioning the patient in the other modality but also if the other modality MRI is not free after the CT is over, the patient will need to wait till they can be accommodated. As a result there is significant time loss in the imaging department. This cuts into the 3 hour golden period to thrombolyse and salvage brain tissue. CT Perfusion has additional advantages, it is cheaper, at least a saving of Rs 1000 on contrast.
Quantified values are available for CBV, MTT, CBF which are not possible with MRI. Above all CT perfusion can be performed on any contemporary CT machine. This obviates the need to use two modalities saving time by performing the exam in one modality.
Plain CT, CT Perfusion and CT Angio provide comprehensive information, effectively answering all questions regarding diagnosis and further management of acute stroke.