Overview of Nuclear Medicine Cerebral Perfusion Scan
Cerebral perfusion imaging is a type of test that looks at the blood flow in your brain. This can be done using various kinds of scans, such as MRI (magnetic resonance imaging), CT (computed tomography) scans, ultrasound, and nuclear imaging.
This piece will pay more attention to the role of nuclear medicine in cerebral perfusion imaging, focusing on SPECT (single-photon emission CT) and PET (positron emission tomography) scans. These two types of scans, PET and SPECT, are crucial for looking at conditions like stroke, long-term blood vessel disease in the brain, and epilepsy, among other conditions. They offer valuable data about the blood flow in the brain, both in terms of quality and quantity.
Anatomy and Physiology of Nuclear Medicine Cerebral Perfusion Scan
The brain uses certain drugs, called radiopharmaceuticals, to help create images of what’s happening inside it. These drugs include technetium-99m ethyl cysteinate dimer (Tc99m-ECD), technetium-99m hexamethylpropylene amine oxime (Tc99m-HMPAO), and I-123 Iodoamphetamine (I-123 IMP). These drugs are special because they can easily enter the brain and then stay there for a long time, giving doctors a good look at the blood flow in the brain.
These drugs are especially useful for patients with epilepsy because they can be used any time, even during a seizure. Once inside the brain, Tc99m-ECD tends to concentrate more in the gray matter, the part of the brain responsible for processing information, making the images clearer.
Xenon-133 (Xe-133) is another drug used to measure the amount of blood flowing to different parts of the brain. This drug works a bit differently, by comparing the amount of Xe-133 in a blood sample to the Xe-133 in the brain.
PET scan, another form of imaging, in addition to looking at blood flow in the brain, also checks the brain’s consumption of oxygen and sugar. However, simple comparisons and visual analysis typically provide the most useful information in clinical practice. For this, various radiopharmaceuticals like oxygen 15-labeled water (H215O), inhaled 15O2, and C15O are used, along with F-18 fluorodeoxyglucose (F-18 FDG) to measure the brain’s sugar activity.
The amount of blood flow in different regions of the brain can be measured with PET scanning by using oxygen-15 labeled water. Some methods don’t even need direct access to the arteries and can deduce the uptake of the drug in the brain based on the images.
Some drugs used in PET scanning require a machine called a cyclotron to produce, and have short lives. For example, compounds marked with oxygen-15 live between 2 to 20 minutes, while those with F-18 live up to 110 minutes. Therefore, hospitals and drug suppliers must work efficiently to use these compounds in time for the scans.
In addition, a drug called acetazolamide can be used both in PET and SPECT, another type of brain imaging, to estimate the brain’s reserve blood flow capacity. It works by increasing the production of CO2 in healthy brain tissue, making the blood vessels larger and increasing the blood flow to the brain. However, in areas where the brain is not getting enough blood, these vessels are already dilated as much as possible and may not respond as much to acetazolamide, indicating the severity of the issue.
Why do People Need Nuclear Medicine Cerebral Perfusion Scan
A nuclear medicine cerebral perfusion scan is a type of brain scan often used by doctors to evaluate blood flow to your brain. This scan can be helpful in several situations:
Firstly, it’s a tool that can identify the focal point of seizure activity in the brain. This is especially crucial for doctors who are planning a surgical intervention for epilepsy.
Furthermore, this scan can assess stroke risk in individuals who have long-term conditions that affect their brain blood vessels. Likewise, it can be used to evaluate stroke risk in patients who are potentially undergoing a procedure involving the carotid artery—the large blood vessel in the neck that supplies blood to the brain.
Additionally, during an acute stroke—a medical condition where blood flow to an area of the brain is cut off—, the cerebral perfusion scan can help identify areas of the brain that are lacking blood supply and those areas that have already suffered permanent damage.
This scan can also aid in distinguishing between different types of dementia—an overall term for diseases and conditions characterized by a decline in memory, language, problem-solving, and other thinking skills.
In cases where a patient is suspected to be brain-dead, such as after a severe brain injury, this scan can be used to confirm the diagnosis.
Lastly, in some cases of brain injury or trauma, the damaging effects might not be clear with common scans like Magnetic Resonance Imaging (MRI) or a Computerized Tomography (CT) Scan. A nuclear medicine cerebral perfusion scan can potentially identify hidden brain lesions or injuries in these situations.
When a Person Should Avoid Nuclear Medicine Cerebral Perfusion Scan
There aren’t many reasons why a patient wouldn’t be able to have SPECT or PET scans, which are types of imaging tests. The main factors that could prevent these tests are if a patient can’t stay still during the test or if the patient can’t be placed safely on the scanning equipment. It’s incredibly rare, but in some instances, patients could have severe allergic reactions (also known as anaphylactic reactions) to the things that make these scans work, called radiotracers. In these rare cases, doctors can use different radiotracers.
There’s a medication called acetazolamide that is not recommended for use in certain patients. If a person is allergic to sulfa drugs, has serious imbalances in the levels of their body fluids and minerals (also known as severe electrolyte imbalances), or has severe liver or kidney problems, they shouldn’t take acetazolamide.
Equipment used for Nuclear Medicine Cerebral Perfusion Scan
SPECT and PET scanners are devices that capture 3D images by detecting tiny particles of light called photons. These photons are emitted by special radioactive substances known as radiopharmaceuticals. In the case of SPECT, the radiopharmaceuticals directly emit these photons which are detected by the SPECT scanners.
For PET, the radiopharmaceuticals, also known as radiotracers, emit a different type of particle called a positron. When this positron encounters an electron, it releases two photons that travel in opposite directions. The PET scanner detects these two photons simultaneously to distinguish the ‘true’ source of radiation from scattered radiation. This technique, called “coincidence detection,” eliminates the need for a physical device (known as a collimator) to block scatter radiation, which is required in SPECT.
This is a main reason why PET images tend to be clearer and more detailed than SPECT images because they essentially have less noise from scattered radiation. In addition, PET scanners typically collect more data counts while scanning which contributes to higher image resolution.
However, SPECT scanners do have a specific advantage – their ability to detect multiple different radiotracers at once. This is based on the differing energies of the photons they emit. PET scanners can’t do this because all positrons emit photons with the same energy. Another thing to note is that because the photons emitted by SPECT radiotracers are of slightly lower energy, the resulting images can be less detailed or sharp.
Today, many PET and SPECT scanners are combined with CT or MRI scanners to provide more detailed anatomical information. The majority of PET scanners in use today are these combined PET/CT scanners, whereas combined SPECT/CT scanners are less common. There are also SPECT scanners specifically designed for brain imaging, but more often, you’ll find general SPECT scanners that can image any organ in the body.
Combining PET or SPECT with CT scanners also has another benefit: it allows for standardizing measurements of radiotracer activity based on the density of the tissue it passes through. This is a process known as CT attenuation correction. It can also be done using a previous CT, but the results may not be as accurate because the patient’s position may change between scans.
Preparing for Nuclear Medicine Cerebral Perfusion Scan
Before an exam, patients are advised to not consume caffeine, alcohol, or nicotine at least 10 hours beforehand. These substances can change the way blood flows in the brain, which could interfere with the test results. The injections for the test should be done in a room with low lighting, and the patient should have time to relax before the injection of a special drug called a radiopharmaceutical.
A needle should be placed into a vein for the injection at least 10 minutes before the radiotracer, a special dye that helps show more detail in imaging tests, is given. The patient should also be asked not to talk or interact with the person doing the test for 5-10 minutes after the injection of the radiotracer. This ensures the clearest, most accurate possible results from the test.
How is Nuclear Medicine Cerebral Perfusion Scan performed
The Society of Nuclear Medicine & Molecular Imaging (SNMMI) and the European Association of Nuclear Medicine (EANM) have approved certain techniques for various nuclear medicine tests. When using a type of nuclear medicine referred to as “Tc99m-HMPAO” or “Tc99m-ECD” for brain death tests, these groups recommend taking moving pictures while the medicine (radiotracer) is being injected. Later, after 20 minutes, flat pictures of the front and sides of the brain should be taken.
If a different type of nuclear medicine is used, Tc99m-DTPA, which doesn’t specifically target the brain, the approach is slightly different. Flat pictures should be taken of the front, right side, left side, and back of your head. The pictures are taken continuously until the counts of the radiotracer reach 500,000 to 1,000,000 per image.
In addition, even when using the non-brain-binding medicine, they recommend capturing moving pictures during the injection of the radiotracer. Also, in the case of Tc99m-HMPAO and Tc99m-ECD, 3D pictures can be taken. This can help to tell the difference between medicine activity inside the brain and that in the scalp.
Furthermore, both the SNMMI and EANM have approved some general instructions for using Tc99m medicines in blood flow imaging of the brain. This includes how to prepare the patient and handle the images after they’re taken.
Beyond tests for brain death, the SNMMI and EANM haven’t approved specific techniques for blood flow imaging of the brain. Techniques for these tests, which are not done for brain death, vary. But, they generally involve capturing moving pictures during the injection of the radiotracer. This is followed by 3D pictures at the time when the medicine activity is expected to be highest.
Possible Complications of Nuclear Medicine Cerebral Perfusion Scan
Studies on the negative side effects of commonly used drugs for brain blood flow imaging (called cerebral perfusion radiopharmaceuticals) are limited. However, we know that side effects could include a warm flush sensation, feeling sick to your stomach, and pain where you got the injection. Severe allergic reactions (known as anaphylaxis) to these drugs are extremely rare.
The side effects of drugs used to improve the effects of the radiopharmaceuticals, such as acetazolamide, happen more often. When given through an IV, the common side effects of acetazolamide can include feeling flushed, having tingling sensations (paresthesias), feeling numb around the mouth, and getting headaches. In rare cases, acetazolamide can also cause a severe allergic reaction.
What Else Should I Know About Nuclear Medicine Cerebral Perfusion Scan?
Brain death can be confirmed in a medical imaging technique known as nuclear cerebral perfusion imaging. This procedure uses a special kind of scan known as SPECT, along with a useful agent that binds to the brain like Tc99m-HMPAO or Tc99m-ECD. When you go through this type of scanning process, you’ll have different types of images taken of your brain – some that are dynamic (or moving), some that are static (or still), and some that are three-dimensional.
These images can show doctors if your brain has stopped working – in medicine, this is known as brain death. Dynamic images reveal if there is blood flow in important vessels in your brain, while the static planar images can show if there is significantly reduced or absent activity in the brain. A common sign seen in patients with brain death is the “hot nose” sign, which is an increased activity around the nose and face due to a flow of blood in those areas. PET scans could theoretically be used to confirm brain death, but they are less commonly used. This is because they require high level coordination and short half-lives of radiotracers.
When it comes to stroke, SPECT and PET scans can be very useful. Commonly, these scans use special brain-binding agents that help doctors to identify affected areas in the brain. This can inform the appropriate treatment path for you, such as whether or not you would benefit from a procedure called intravenous thrombolysis, which is a treatment to break down clots. However, this decision depends on the blood flow in your brain tissue.
Areas of the brain with good blood flow, even if there is some decreased function, tend to benefit from this treatment. However, brain regions that have less blood flow and are under-functioning may have a higher risk of bleeding in the brain if treated in the same way. These scanning techniques can also inform doctors about the risk of subsequent strokes and the benefits of a procedure called revascularization for patients with chronic blockages in the brain or neck arteries.
In those patients with chronic obstructions, SPECT and PET scans can evaluate the risk of future strokes and decide on the potential advantages of revascularizing the arteries. The exams can be further enhanced with an intravenous dose of acetazolamide, which causes the vessels in the brain to dilate. This dilation helps doctors to understand how much blood flow different regions of your brain can have.
Lastly, if you have epilepsy, SPECT scanning can be particularly helpful. This procedure uses special agents that have prolonged retention rates in the brain. This means doctors have more time for you to stabilize during a seizure before performing the scan.