Not All Neuroimaging is Created Equal: The importance of perfusion scans for acute stroke identification.
There is the false belief that a CT scan (or CAT scan as its often wrongly referred to and needs to go away!) and non-contrast MRI can tell you whether someone is having a stroke. Although that can be halfway true because these basic scans may rule out bleeding in the brain, your basic head CT or MR (which is most often prescribed when patients arrive to the ED with stroke like symptoms) tells you very little about whether someone is actually having an ischemic stroke.
An ischemic stroke results from a loss of blood flow to an area of the brain most likely from a clot or blockage in a blood vessel. After time that area of the brain starts to change its density, shape and activity. If you did a basic head CT or MRI on someone who is just starting to have stroke symptoms (say anywhere between 0-4 hours) the scanning technology used is not strong enough to pick up subtle changes occurring at the level of the brain tissue. However, over time as brain cells die and swell (become filled with water), the basic head CT and MRI will likely be able to spot ischemic brain areas, but by that time it is probably too late to do anything to treat them. The reason why this happens is because, very simply, CT or MR scanning picks up the densities of tissues in the body based on the amount of water they have in them. CT’s emit small beams of radiation into the body and using sophisticated statistical algorithms turn the information the beams of radiation generate into pictures of the body. These pictures are sort of like shadows and reflect the outline of internal organs and other body structures once they are re-constructed into the pictures of images we are familiar with. MRI’s are similar, but produce a magnetic field that attracts and aligns all of the photons in your body at the same time and releases them, as the photons return to their normal state they release energy in the form of radio signals that are picked up by the MRI scanner and once again by statistical magic, are turned into images we are familiar with.
A basic CT image of the brain of someone with an ischemic stroke many hours after symptom onset may have a dark or hypodense area in the brain that corresponds to a lack or loss of blood flow to that area. If we were looking at an MRI at the same time, then that area would be white or hyperdense just because of the differences in CT and MR technology. However, if we looked a scan of that person within minutes to hours after onset, we would have a hard time finding something ‘treatable’ on the basic scan which severely limits its utility in acute stroke.
Physiological scanning or MR/CT scanning based on flow of contrast through the blood vessels, provides real-time information about how blood is flowing in and out of blood vessels, versus the basic CT/MR that only really gives you anatomic information. However, to conduct physiological scanning you need a special CT/MR scanner and clinical expertise to interpret the scans in addition to increasing the cost of the evaluation and hoping that your patient is not allergic to shellfish (because if they are they can’t get contrast or they made end up with a life threatening allergic reaction). These limitations are the primary reason why physiological scanning is only done in specialized centers and certainly not in every suspected stroke patient.
In some instances, MR perfusion technology does a better job of identifying ischemia (loss of blood flow) and provides greater resolution of small areas; however, CT perfusion is better at identifying blood in the subarachnoid space, so clinicians must way the pros and cons of each technology and make a quick decision as to what scan to conduct based on the patient’s symptoms and overall history. Physiological scanning (or scanning based on contrast administration) is really the only way to identify acute ischemic changes early enough to facilitate therapeutic treatment. Diffusion weighted MRI (a form of physiological perfusion scanning) is the best way we have to identify acute ischemic infarct, far surpassing CT perfusion and any other imaging techniques we have to date. However, very few hospitals have the capability of conducting diffusion weighted MRI scanning, thus the default is to either use CT perfusion (if available), a basic head CT/MR or nothing at all.
Clinical decisions are based on a level of confidence. That level of confidence is generally increased when a clinician’s suspicion is verified by a medical test. But when clinical suspicion and medical tests don’t line up, more often than not a good clinician is going to go with their intuition. MR and CT scans provide clinicians with additional confidence to either rule in or rule out a suspected stroke diagnosis. These scans are timely, costly, and depending on which technique you have available may or may not give you the information you need. Taking this into consideration and stepping outside of the stroke treatment facilities we are affiliated with, it becomes increasingly apparent that an unbiased, quick and easy to interpret blood test would be of benefit to any emergency room provider who does not practice in a stroke treatment facility when evaluating suspected stroke.