Overview of Nuclear Medicine CNS Assessment, Protocols, and Interpretation
Computed tomography (CT) scans and magnetic resonance imaging (MRI) scans are the two main tools that doctors use to look at conditions that affect the brain and spinal cord, collectively known as the central nervous system (CNS). These scans provide an inside look at the structure of your brain and spinal cord. Nuclear medicine exams add another layer of analysis. They look at how the CNS functions and its metabolic activity, meaning how it uses energy.
The CNS, which includes your brain and spinal cord, can be examined using special types of scans in nuclear medicine. These scans use things called radiotracers which are substances that emit radiation and can be followed inside your body. They help to map the brain’s metabolic activity, or how your brain uses energy. You may hear these scans referred to as fludeoxyglucose F 18 (F 18 FDG) positron emission tomography (PET) or CT scans. They can also look at blood flow in the brain, detect the presence or non-presence of a type of brain cell called a dopaminergic neuron through dopaminergic transporter scans (DaTscans), or check the flow of cerebrospinal fluid (the fluid around your brain and spinal cord) using a substance called Indium-111 or Technetium 99m (Tc 99m).
Anatomy and Physiology of Nuclear Medicine CNS Assessment, Protocols, and Interpretation
The brain mainly uses glucose, a type of sugar, for energy. A tracer often used in medical imaging, called 18-fluorine deoxyglucose or 18F FDG, moves from the bloodstream into brain cells. Inside these cells, 18F FDG is chemically changed, a process that stops it from leaving the cell again. This allows doctors to create images of the brain and pinpoint areas where glucose use might not be normal.
The more 18F FDG a brain area uses, the more active its cells are. So, this imaging technique helps doctors identify parts of the brain where activity levels are higher or lower than usual. It’s also used to find the origin of seizures, examine brain shrinkage in diseases where brain cells break down, and recognize whether a tumor is dead or growing again.
Parkinson’s disease happens when cells that make a chemical called dopamine die off in a brain area called the substantia nigra. This disease can cause symptoms like slow movement, shaking when at rest, and other movement problems. It’s usually identified by these symptoms, but a scan called DaTscan can also help. It uses a chemical called ioflupane-123 to attach to the remaining dopamine-making cells, showing the difference between Parkinson’s disease and other conditions that cause shaking.
The blood-brain barrier is like a security gate, stopping most substances from moving from the bloodstream into the brain. However, some types of molecules can get through. Two of these, hexamethyl propylene amine (HMPAO) and ethyl cysteinate dimer (ECD), are used in brain scans to assess how well blood is flowing through different brain areas. These are helpful for checking blood flow in conditions like damaged blood vessels, blocked arteries, mini-strokes, or when testing for brain death. Additionally, a compound named diethylene triamine penta-acetic acid (DTPA) is used similarly to detect brain death.
The term ‘brain death’ defines a state where all brain activity, including brainstem, has permanently stopped. This diagnosis includes several factors, and a brain scan is used to support the diagnosis in specific cases, mainly when physical examination results are hard to interpret, such as in severe face or eye injuries. During brain death, the entire brain swells and blood starts to bypass the arteries inside the brain.
Cerebrospinal fluid (CSF) is a clear liquid that surrounds and cushions the brain and spinal cord. It is produced in the brain and flows throughout the central nervous system. Radiotracers such as Indium-111 DTPA and Tc 99m DTPA are used to check for conditions related to CSF, like normal pressure hydrocephalus (a buildup of fluid in the brain) and CSF leaks. They are also used to check if there’s a blockage in the tubes used to drain excess brain fluid in certain medical conditions.
Why do People Need Nuclear Medicine CNS Assessment, Protocols, and Interpretation
Nuclear medicine technology can be crucial in diagnosing many different types of brain disorders. These tools provide doctors with real-time images of what’s happening inside your body, as well as insight into your body’s metabolism. Here are some specific reasons why a doctor might use nuclear medicine to evaluate your condition:
* Looking for the origin of a seizure
* Checking if the brain is functioning (or “brain death”)
* Looking for signs of inadequate blood flow to the brain, like in the case of a stroke
* Telling the difference between the growth of a brain tumor and damage from radiation therapy
* Checking for dementia and distinguishing between the different types of dementia
* Looking for a condition called “normal pressure hydrocephalus,” which is when fluid builds up in the brain
* Checking for a cerebrospinal fluid (CSF) leak, which happens when the fluid around your brain and spinal cord leaks out of the protective compartment that fuels it
* Checking the patency of a ventricular shunt, which is a medical device that drains extra fluid from the brain
* Evaluating different types of Parkinson’s disease
When a Person Should Avoid Nuclear Medicine CNS Assessment, Protocols, and Interpretation
There are a few situations in which certain medical procedures should not be done because they could potentially be harmful. For example, if you’ve recently been taking blood-thinning medication, it could be risky to inject radiotracers into the spinal canal. Radiotracers are substances used in imaging tests that help doctors see how your organs are working.
If you’re allergic to drugs that contain sulfa, you should not be given acetazolamide, a medicine commonly used to treat glaucoma and some other conditions.
If you’re pregnant or breastfeeding, you should generally avoid undergoing any nuclear medicine tests. Nuclear medicine tests use small amounts of radioactive substances to create detailed images of the inside of the body. If you’re pregnant, these tests are usually not recommended due to potential risks to the unborn baby, especially for procedures that involve imaging the central nervous system (CNS), which includes the brain and spinal cord.
Equipment used for Nuclear Medicine CNS Assessment, Protocols, and Interpretation
When you get a nuclear medicine exam, there are certain types of special medical equipment that your doctor will use during this process. For instance, for certain types of scans that involve things like PET/CT scans, F 18 FDG scans, and CSF exams, the doctors use a machine called a PET/CT scanner. Gamma cameras also play a crucial role in these types of exams as they are necessary for brain death examination among other things.
Moreover, another scanner used often is the Single-photon emission computed tomography (SPECT)/CT scanner. This scanner helps with evaluating the blood supply to your brain and detecting certain neurological disorders using a process called DaTscan.
In certain exams, like the F 18 FDG, it’s important to keep a close eye on your blood sugar levels. This is why a glucometer, which is a device to measure blood glucose, is used in these types of examinations. High sugar levels in your blood can affect the scan results.
If your doctor suspects you have a CSF leak or normal pressure hydrocephalus, they will make use of a fluoroscopy machine. This machine helps guide the doctor to the right spot on your spine during an examination. In addition, the progress of this type of suspected spinal leak is followed with cotton balls or small pieces of cotton, labelled as pledgets, and a well counter, which is a device that detects radiation.
Who is needed to perform Nuclear Medicine CNS Assessment, Protocols, and Interpretation?
Getting good results from nuclear medicine imaging (a type of scan that gives doctors info about how your body is working) needs a team of experts with different roles. The goal of their work is to make sure the scan is accurate, you are safe, and that the scan is as effective as possible.
A doctor who specializes in radiology or nuclear medicine decides the best way to do the scan based on what you specifically need.
A nuclear medicine technologist (a professional trained in using the scanner) talks directly with you and does the scan.
A nuclear medicine pharmacist (an expert in the right type of radioactive substances for scans) is a key player in making sure the right imaging drug (radiotracer) is prepared and given in the correct amount, following set rules. This expert helps ensure that the scan is both safe and effective.
Preparing for Nuclear Medicine CNS Assessment, Protocols, and Interpretation
When doctors are checking if a brain tumor has returned or for damage from radiation, dementia, or the origin of a seizure, there are some important steps patients need to follow. They need to avoid eating for at least 4 hours before the brain PET/CT scan. This is done to ensure that their blood sugar levels are below 200 mg/dL. For patients with diabetes, they need to take long acting insulin the night before and regular insulin in the morning. The scan is usually scheduled for the afternoon for optimal results.
In some cases, doctors need to look at how blood is flowing in the brain or brain ‘perfusion’. To do this, patients are kept in a quiet, dark room with their eyes open for several minutes before given a special compound that can be tracked by a scanner. This helps get accurate results and prevents any decrease in blood flow to the visual part of the brain. If acetazolamide, a medication, is used, it should be given 20 minutes before the scan for best results.
For patients who are in a coma and are suspected to be brain dead, the cause of coma is essential to know. Any sedatives should be stopped and enough time, at least 5 half-lives of the medication, should pass before the brain death assessment procedure. This makes sure the results are accurate and trustworthy.
In case of Cerebrospinal Fluid (CSF) leaks, a compound is administered in the fluid around the spinal cord and brain. The flow of the fluid can be slowed down by acetazolamide, and this medication should be stopped a few days before the checkup. The patients are then taken for the procedure and the area for the lumbar puncture is thoroughly sterilized.
If there’s a suspected obstruction in the ventriculoperitoneal or ventriculoatrial shunts (devices used to drain excess fluid in the brain), the area is prepared in a sterile manner and the shunt tubing is injected with a tracer substance to see if it’s blocked.
For a DaTscan Test, which is a brain scan to check for certain brain disorders, a measure is taken to protect the thyroid gland from absorbing a radioactive tracer. This is done by administering potassium iodide or a solution containing iodide 1 hour before the test.
How is Nuclear Medicine CNS Assessment, Protocols, and Interpretation performed
When you go in for imaging tests, like a PET/CT scan or a SPECT/CT scan, a liquid known as a radiotracer is injected into your veins. This liquid acts as a highlighter for your body, lighting up certain areas so they can be seen more clearly on the scanner. This can take about an hour to spread through your body. During the scan, the machine looks at your whole head to get a complete picture of what’s happening inside.
There are a few different types of imaging tests.
“Brain perfusion imaging” is a scan that looks at how well blood is flowing through your brain. This helps doctors to see any abnormal patterns or problems that might be happening.
“Cerebrospinal fluid imaging” is a scan that looks at the fluid that surrounds your brain and spinal cord. This scan can help doctors to see if there is a leak in this fluid, or if there is a buildup of it, which could be causing pressure in your head. Depending on what doctors are looking for, they might take pictures at different times (right after the dye is injected, or 6, 24 or 48 hours later).
“DaTscan” is a specific type of brain scan that lets doctors see the area of the brain that controls movement. This test is often used if a doctor suspects a movement disorder, like Parkinson’s disease.
Each of these scans needs different preparation, and may also use different types of dye. The specifics will depend on what your doctor is looking for, and what kind of scanner is being used.
Some scans may also need follow-up tests to get more detailed information. This could involve using a special type of pledget (a small piece of cotton or another material) to soak up any fluid that might be leaking.
Each of these tests has its own purpose and helps doctors to better understand what’s going on in the brain, so they can properly diagnose and treat you.
Possible Complications of Nuclear Medicine CNS Assessment, Protocols, and Interpretation
When medical professionals administer radiotracers into your spinal cord, commonly referred to as intrathecal administration, there may be certain potential complications. These might include bleeding in the area around the spine (epidural or subdural hemorrhage), infection of the membranes around your brain or spinal cord (meningitis), or unintended leakage of the fluid that surrounds your brain and spinal cord (CSF leak).
When checking if a shunt, a device used to redirect fluid from one area to another, in your brain is working correctly, there is a small risk that dirt or bacteria may accidentally be introduced into the shunt system. This is because this procedure involves accessing the shunt, and despite all precautions, there’s a small chance of contamination.
Additionally, before some nuclear medicine exams, you might be given a drink containing potassium, such as potassium iodide or Lugol solution. This is done before an exam involving ioflupane, which contains iodine-123, a type of radioactive iodine. This drink helps to protect your thyroid gland from absorbing any radioactive iodine that might be used in the exam.
Though possible complications like the leaking of radiotracers into the surrounding tissues can occur during radiotracer injections into your veins, generally, nuclear medicine exams are considered low-risk and don’t often lead to significant problems.
What Else Should I Know About Nuclear Medicine CNS Assessment, Protocols, and Interpretation?
F 18 FDG PET/CT scans are used to support the diagnosis of dementia, especially when it’s not completely clear from the patient’s symptoms. These scans examine the brain and can help identify specific types of dementia because different types affect different areas of the brain. However, it’s important that these scans are only used when there’s already a high chance that the patient has dementia.
In a healthy brain, these scans typically show an even and balanced tracer uptake throughout the brain. However, with Alzheimer’s disease, the scans show a reduction in tracer uptake in specific areas of the brain, which worsens as the disease progresses. Similarly, in dementia cases with Lewy bodies, a decline in tracer uptake is noticed in particular areas of the brain. A key difference between Alzheimer’s and Lewy body dementia is that in Lewy body dementia, one particular area of the brain, the posterior cingulate cortex, is unaffected.
Patients with frontotemporal dementia typically show a decrease in tracer uptake in the frontal and temporal lobes of the brain. Vascular dementia, caused by multiple areas of the brain not getting enough blood, leads to a gradual decline in cognitive function, with symptoms corresponding to the affected areas of the brain. In cases where interpretation may be challenging due to overlappin patterns or potential confounders, various systematic approaches for evaluating F 18 FDG activity have been proposed and can be beneficial for potentially confounding cases.
This same scan can be used to distinguish between dead tissue within a tumor (necrosis) and a recurring tumor by evaluating tracer uptake. Dead tissue areas typically show a decrease in metabolism (hypometabolism), while recurrent tumors show an increase in metabolism (hypermetabolism).
A different radiotracer called Tc 99 DTPA can be used for brain death assessment as an angiographic examination. In a normal brain, this tracer can be seen in the arteries both inside and outside of the brain. In a brain-dead patient, it’s absent from the arteries inside the brain.
For patients who cannot undergo CT or MRI scans due to severe allergies or incompatible medical devices, brain blood flow can still be evaluated using a particular kind of scan (SPECT/CT) with Tc 99m HMPAO or ECD. Normal areas of the brain demonstrate consistent tracer uptake, but deficient or dead areas of the brain will show reduced or zero tracer uptake.
Nuclear medicinal examinations are particularly useful when symptoms are not clear, for example, for identifying the origin of seizures. Whether during the seizure or between them, different radiotracers are used to identify the active brain area during a seizure. This can, however, be challenging in some areas of the brain and require careful interpretation.
Finally, radiotracers can also be used to identify issues with the cerebrospinal fluid (CSF), like CSF leaks, shunt failure, or communicating hydrocephalus, when other diagnostic methods are inconclusive or not suitable. In a typical cisternogram to assess normal pressure hydrocephalus, the radiotracer progresses up the spinal canal.
