When it comes to the treatment of stroke, there are three words emergency healthcare providers live by: Time is brain.
The time it takes to respond with the most appropriate care for a stroke patient particularly within the first three hours after a stroke has occurred is one of the most crucial factors in determining whether a person lives or dies, recovers or is disabled.
Stroke care management requires immediate assessment to determine both the cause and the extent of damage to the neurological system. In the vast majority of medical centers, CT has become the primary medical imaging tool to evaluate patients who exhibit symptoms of a stroke.
A stroke is defined as a sudden partial loss of brain function associated with diminished blood flow to the brain. Disruption in the delivery of oxygen and nutrients causes brain cells to die.
There are basically three types of stroke:
Thrombotic strokes where fatty deposits constrict and finally block arteries that carry oxygen to the brain;
Embolic or ischemic strokes where a blood clot formed somewhere else in the body breaks loose and travels to block an artery that supplies blood and oxygen the brain; and
Hemorrhagic strokes when an artery in the brain ruptures and bleeds into the brain.
When it began
CTs role in stroke treatment began to gain momentum in late 1995 with the release of a study by the National Institute for Neurological Disease and Stroke (NINDS of Washington, D.C.). Research showed that if certain patients with stroke-like symptoms were treated with a thrombolytic agent such as Tissue Plasminogen Activator, or tPA those patients outcomes improve.
At Duke University, James D. Eastwood, M.D., uses a new investigational technique utilizing a bolus of intravenous contrast on a multi-detector CT scanner to map cerebral blood flow and cerebral blood volume to learn the pathophysiologies present in stroke. |
On the other hand, if a patient suffers a hemorrhagic stroke, administering anti-coagulant medications such as tPA, the only anti-coagulant approved by the FDA for acute stroke would induce additional bleeding and produce disastrous results.
CT was the imaging modality used in the study to help select which patients might benefit from the drug.
CT has become critical in stroke management, because you need to know acutely whether a patient has bleeding or not, says Allan Bernstein, M.D., chief of neurology at Kaiser Permanente (Santa Rosa, Calif.). You cant give someone a clot-dissolving drug, if they have bleeding in the brain. You need to know that instantly, because the newer treatments are only good within the first one to three hours of the acute event.
There also are clinical exclusions to using tPA, so physicians do not base their final treatment decisions solely on the CT scan results.
The main role for CT is exclusionary, says Douglas J. Quint, M.D., a neuroradiologist at the University of Michigan Medical Center (Ann Arbor, Mich.). We want to rule out other things that might be giving the [stroke] symptoms or that may be contraindicate using a thrombolytic agent.
Stroke-like symptoms also may be triggered by a tumor in the brain, infection or bleeding into the head. Those conditions also can be detected by a CT scan.
If you have seen that a large area of the brain more than half of a certain blood vessel territory is already affected by the stroke, those patients will not do well, so you cant give them tPA, says Quint. Most of the time, the scans are pretty normal and thats how you want to clear someone for tPA.
tPA timeframe
When tPA is used appropriately, it has proven very effective in clot-dissolving and the relief of stroke symptoms. The drawback is the narrow one- to three-hour timeframe in which the anti-coagulant must be administered. The window of opportunity is based on when the stroke occurs, not when the patient arrives at the hospital.
According to the March 1 issue of the Journal of the American Medical Association, very few patients qualify under the specified criteria to receive tPA. Less than 5 percent of people ever hit the time window to qualify [for tPA], estimates Kaisers Bernstein.
Thats the problem today, adds June Shea, manager of the Ellis/Sunnyview Stroke Center (Schnectady, N.Y.). The majority of people come to the hospital after three hours. Denial is very strong with stroke.
This non-contrast CT scan demonstrates subtle early changes of left-sided cerebral infarction. Straight arrows show decreased attenuation of the left insular cortex, as opposed to normal (wavy arrows).
The Ellis/Sunnyview Stroke Center treats approximately 300 stroke cases per year. The center was created by the 368-bed, acute-care Ellis Hospital (Schnectady) and Sunnyview Hospital (Schnectady), a neighboring rehabilitation facility with 104 beds.
The centers services include stroke education in the community, healthcare provider education and research for stroke medication.
Even though you have the CT immediately, an ischemic stroke will not show on the CT scan, Shea explains. A hemorrhagic stroke will show immediately. It takes at least 24 hours to get a good CT scan reading of the location of the ischemic stroke, because thats when changes occur.
Older CT technology used to be more of a speed bump to treatment, rather than a direct route. CT scans took as long as an hour to complete in the past. Todays technology with scan times of less than 10 minutes make CT considerably more effective.
Today, Bernstein says, you can probably whip somebody in and out of there in about eight minutes.
A CT scanners open design also makes the technology more appropriate for stroke patients who need to be monitored during the scan. Those kinds of considerations are one reason why MRI systems generally are ruled out as an option to detect the cause of a stroke.
If someone has just had a stroke or is having an acute stroke, you want to watch them to see how well they are breathing or whatever, Bernstein says. In a CT, you can literally run a respirator next to the patient. If you need to ventilate someone, you can do that while you are running a CT.
tPA, by the way, is not the only clot-dissolving solution. Bernstein was conducting a research protocol at Kaiser on snake venom a few years ago. Snake venom also will dissolve blood clots. We did that about the same time tPA came out, he adds. The problem was tPA got there [FDA approval] first and the snake venom, although it worked, has not been able to establish in the market.
Xenon CT
CTs role in stroke detection and treatment also has been enhanced in recent years with the use of xenon, a non-radioactive, inert gas with an atomic number close to that of iodine. It was approved for use as a contrast agent in the late 1980s.
The xenon-CT connection laid somewhat dormant until recently. The technology on which it is very dependent CT scanners, computers and computer networking was rather primitive 10 years ago compared to todays standards.
Howard Yonas, M.D., chief of cerebral vascular surgery and director of the Stroke Institute at the University of Pittsburgh, was one of the physician/scientists who designed and developed the technique for the measurement of blood flow using a CT scanner and inhaling stable xenon for approximately 4 minutes to obtain a series of CT scans.
We can do that for every millimeter of the CT image, says Yonas. We do that 24,000 times per CT level and make a quantitative measurement of blood flow. It is real blood flow on an absolute scale.
Technology advances in the last couple of years have allowed physicians, among other things, to reduce the dose of xenon inhaled by a patient in order to produce a quality image. Today, CT imaging procedures require a concentration level of xenon in the 25 to 28 percent range. The mid-20 percent range is down from the mid-30s of past years, because todays newer CT machines have lower noise levels.
The arrows show large right middle cerebral arterial territory hemorrhagic cerebral infarction. The hemorrhagic component rules out the use of aggressive thrombolytic therapy.
It allows us to reduce the amount of signal we need to maintain the mathematics that are critical, says Yonas. In the mid-20s [percent range], it causes most folks almost nothing, maybe a little tingly feeling. Xenon concentrations of 80 percent can put a patient to sleep.
While the patient is on the table waiting for a CT scan, they can have a 4-1/2-minute inhalation of stable Xenon, adds Yonas. The calculation of the blood flow follows in less than 5 minutes, all the maps are completed and ready for review right in the CT suite. Now it can be integrated in the prospective decision-making.
Yonas estimates that there are 50 xenon-CT units operating in the U.S. That amount is approximately twice the number of units from a year ago. He says there are approximately 20 xenon-CTs in use in Europe. Japan is, by far, the leader with some 700 xenon-CT scanners up and running.
Stroke exploration
While most medical centers are using standard CT images of the head to evaluate the cause of a stroke, James D. Eastwood, M.D., assistant professor of radiology in the section of neuroradiology at Duke University Medical Center (Durham N.C.), describes a new investigational technique Duke is exploring. The CT method uses a bolus of intravenous contrast material.
We look at a single slice or series of slices in the case of a multi-detector CT, Eastwood says. Basically, we are able to track the bolus as it comes in and goes out of the slice. That allows us to make maps of the important parameters of cerebral blood flow (CBF), cerebral blood volume (CBV) or mean transit time (MTT). By analyzing these, were learning more about the kind of pathophysiologies that are present in stroke.
Eastwood says that much of this study to date has employed different imaging modalities, such as PET (positron emission tomography) or xenon CT. Because CT currently is in use in so many institutions, extending its capability through use of this technique, he believes, offers much promise.
The ability to look at both CBV and CBF simultaneously is what is very new and exciting about this method, Eastwood adds.
The investigational work at Duke is enhanced through use of software developments from GE Medical Systems (GEMS of Waukesha, Wis.).
Eastwood says GEMS has developed what he describes as very sophisticated software that is able to analyze the data set that we collect in a way that allows us to make these very accurate quantitative measurements and compare them between the different parameters. It is a very accurate method, but previously was very slow and took long calculation times.
Using these techniques permits the visualization of the affected area and provides a quantitative analysis of the data that proves useful in diagnostic and treatment decision-making.
When you consider that most people are going for the non-enhanced CT anyway, Eastwood says, it makes sense to try to develop a CT-based technique where you could find all the information you need prior to potential thrombolysis.
Saving time
To determine how effective the use of CT and time-saving procedures can be in detecting and treating stroke in the emergency room, 40 hospitals affiliated with VHA Inc. (Irvine, Texas) collaborated on such a project as part of the group purchasing organizations Clinical Advantage program.
The initiative focused on improving and reorganizing the process healthcare providers use to evaluate and treat stroke patients. By more efficiently coordinating personnel and procedures in the ER, the lab and the CT suite, the hospitals over a six-month period reduced by almost 50 percent 219 minutes to 115 minutes the time from a stroke patients arrival to the interpretation of the CT scan.
Marilyn Rymer, M.D., is the national chairperson of VHAs Clinical Advantage stroke initiative and medical director of the Stroke Center for Saint Lukes Hospital (Kansas City, Mo.). Saint Lukes is a 650-bed, tertiary care community teaching hospital affiliated with the University of Missouri-Kansas City School of Medicine.
Unlike thrombolytic therapy for a heart attack, you have to have a CT head scan before you can safely treat the patient for stroke to rule out any hemorrhage. It adds a step and that step can be a barrier [to quick treatment], Rymer says. The idea is to get the CT done and the scan interpreted within 40 to 45 minutes after arrival, so the door to treatment time can be within one hour.
The process begins, she says, by getting the emergency room doctor to the patients side within five to 10 minutes, so the physician can order a CT scan, if necessary. Some facilities have trained nurses to make that decision after the patient is evaluated and stabilized, if the ER physician is busy.
In our facility, if we have a stroke patient who is within the [three-hour tPA] timeframe, he or she is admitted like a trauma case, Rymer says. If we dont have all the demographic information, the patient comes in as a John Doe and then we fill in the blanks later.
With the CT located in a room next to the ER, Saint Lukes ER personnel can have the scan performed and interpreted within five minutes. CT scanners can be located closer to ERs than MRI systems, simply because the systems dont need to be situated in leaded, shielded rooms as MRI units do.
Rymer, however, does not discount the use of MRI later in the stroke treatment process, when the patient is beyond the three-hour tPA window. An MRI could help determine if the patient has a blocked artery and whether the brain has maintained good collateral circulation.
CT vs. MRI
How long CT will continue as the gold standard in evaluating stroke is debatable.
The University of Michigans Quint expects MRI to play a larger role in stroke detection in the future, as MRI units become faster and allow radiologists to see and gather more information, perhaps on the molecular level through functional MRI.
The future is MR and it should replace CT in the upcoming decade, Quint predicts. It is a question of cost and getting the patients in the machines fast enough. The speed of MRI will get better, the cost will come down and people will do studies which will define who will benefit from therapies.
There are things you can see with MRI that you cant see with CT, which may be even better for defining stroke, adds Quint. However, there are no studies saying that you base your tPA decision on MRI findings yet.
Very few facilities, however, currently are planning on siting MRI machines in or near ERs, partly because of shielding issues and the havoc MRIs strong magnets could cause for ER equipment.
So, when it comes to stroke, CT still reigns supreme and time is brain.