Overview of SPECT Imaging
Single-photon emission computed tomography (SPECT) is a type of medical scan often done to help doctors diagnose certain conditions. Simply put, SPECT involves creating a 3D image of how a special type of medicine, known as a radioactive tracer, moves in your body when it’s put into your bloodstream. Certain parts of your body pick up this tracer and this special type of camera captures this process.
The value of a SPECT scan lies in its ability to show how blood flows to tissues and organs. This information is vital to determine how the tissues or organs are working. This sets a SPECT scan apart from other scans, such as computed tomography (CT scans), magnetic resonance imaging (MRI) or X-rays, which only show what these parts of the body look like, not how they function.
Anatomy and Physiology of SPECT Imaging
A SPECT scan is a special type of test that gives doctors detailed information about what’s happening inside the body. It does this by using special substances called radioactive tracers – specifically, these tracers are made up of a radioactive isotope (which a special camera can detect), and chemicals that interact with the body’s tissues. The tracers are given to the patient and they interact with the body’s cells, helping to identify what’s going on in those specific areas.
Three common radioactive isotopes used in these scans are technetium-99m (also known as Tc), iodine-123, and thallium-201. Which one is used depends on what the doctor is trying to find out, how safe it is for the patient, and how much it costs. Tc is often favored because it works faster and exposes the patient to less radiation, allowing clearer images to be created without posing a significantly increased risk to the patient.
On the other hand, iodine-123 has a slower pace of safe decay, which means it tends to accumulate in higher concentrations in the organ being inspected but can expose the organ to more radiation. Because of this, iodine isotopes are usually reserved for studies of organs that naturally use iodine, like the thyroid gland.
However, the choice of isotope is only part of the story. For a SPECT scan to work, the chosen isotope needs to be paired with a specific chemical that can interact with the body’s cells and carry the radioactive isotope to the right place. For instance, Tc can be linked with a substance called hexamethylpropyleneamine oxime (HMPAO), which can penetrate the blood-brain barrier and get taken up by brain cells. Because HMPAO is chosen and used by the brain based on the cell’s activity levels, it can reveal areas of the brain that are unusually active, as well as areas that seem normal but aren’t working correctly. This makes SPECT scans a valuable tool in diagnosing and treating disorders that affect blood flow in the brain, as well as dementia.
SPECT scans are also very useful in spotting problems with blood flow in the heart, particularly during stress tests. Here, Tc is combined with several MIBI groups to create a tracer called sestamibi, which can enter the cells of the heart at rates corresponding to blood flow. This allows the doctor to identify and assess any problems with the way blood is flowing through the heart. The scan can even be timed with the patient’s heartbeat, a type of test known as a multi-gated acquisition scan (MUGA), which gives detailed insights into how the heart is working at each stage of its beat. Thallium used to be commonly used for this kind of heart scan, but it’s been largely replaced by Tc.
Why do People Need SPECT Imaging
SPECT imaging, a type of scan used by doctors, is often recommended for many different health conditions. This decision is guided by the recommendations from medical imaging organizations. Here are some reasons why your doctor might order a SPECT scan for you:
Your doctor might suspect you have dementia, which is a condition affecting memory and other thinking skills. They may also use it to locate the exact area in the brain that causes seizures if you’re planning to have surgery for epilepsy. If they believe you have encephalitis, which is inflammation of the brain, this scan may be used for diagnosis.
SPECT imaging can be used after you’ve had a type of stroke known as a subarachnoid hemorrhage, to check how your blood vessels are recovering. It’s also useful during some surgeries as it can supply a map of blood flow in your brain. If your doctor’s trying to find out whether you have cerebrovascular disease, which affects blood flow to the brain, this imaging technique can also be useful. It can similarly be used to predict how well you might recover from a stroke.
If there’s a doubt whether someone’s brain is still functioning, a SPECT scan can give more information. Furthermore, it can be used for several purposes related to cancer.
The American Society of Nuclear Cardiology also suggests using SPECT scans for heart-related issues. It may be used to check if you have coronary artery disease, which is a condition where the blood vessels supplying your heart are blocked or narrowed. If you already know you have coronary artery disease, heart failure, or a condition known as cardiomyopathy which affects the heart muscle, SPECT scans might be used to see if a treatment is working or to plan future treatments. It can also be used if you can’t perform a standard exercise stress test but your doctor wants to diagnose coronary artery disease. Before any surgery, if your doctor suspects you have coronary artery disease, they might order a SPECT scan.
It’s also worth noting that a SPECT scan can be useful for other conditions that aren’t related to the heart or brain. This includes osteomyelitis, a type of bone infection; spondylolysis, a condition affecting the spine; parathyroid disease, affecting glands near the thyroid; pulmonary embolism, a blood clot in the lungs; and to locate where an abscess, a pocket of pus, is in your body.
When a Person Should Avoid SPECT Imaging
In general, there are no absolute reasons why someone cannot have a SPECT imaging test. However, in rare cases, patients may have allergic reactions to the tracing substance used in the test. More often, reasons to avoid SPECT imaging are related to the specific procedure being done rather than the imaging test itself. So, if a heart stress test being done with SPECT imaging carries the same risks as a regular stress test.
Doctors should also consider the risks related to exposure to radiation when referring patients who are pregnant for SPECT imaging. Also, radioactive iodine substances should be avoided in these patients because it can be absorbed by the unborn baby.
Last but not least, some heavyset patients may exceed the weight limit of the scanner, which is another reason why they may not be able to have this type of imaging test performed.
Equipment used for SPECT Imaging
3D-SPECT imaging is a medical test that uses a special camera and a radioactive tracer to take detailed, three-dimensional images of a specific body tissue. The rotating multi-headed camera’s job is to capture signals called photons, which the tracer sends out from inside your body. These photons are emitted in all directions from your body.
A device called a collimator acts like a filter, only letting through photons that are directly in line with the camera. This helps the camera focus and get a clearer picture. The type of collimator chosen can influence how detailed the final image is and how sensitive the camera is in picking up signals.
The more heads or detectors a camera has, the clearer and more detailed the final image will be. It also means the scan will take less time. But don’t worry, even if the camera has one detector, trained health care professionals like medical physicists and nuclear radiologists can still produce high-quality images.
Who is needed to perform SPECT Imaging?
The American College of Radiology sets rules for the different team members involved in carrying out a SPECT study safely and effectively. A SPECT study is a type of scan that provides 3D images of how your internal organs are working. Different medical professionals have their own important jobs in this process.
The team generally includes a:
1. Clinician: This is a doctor who has contact with patients and looks after your overall health.
2. Nuclear medicine technologist: This is a professional who operates the machines that take the scan.
3. Nuclear pharmacist: This specialist prepares and assures the safety of the radioactive materials used in the scan.
4. Medical physicist: They make sure the machines and procedures are safe and giving out the right amount of radiation.
5. Radiation safety officer: This person is responsible for maintaining and ensuring safety from radiation in the hospital or clinic.
A nuclear cardiologist or nuclear radiologist, who are expert doctors in reading and interpreting these scans, should oversee the study to ensure everything goes smoothly and safely.
Preparing for SPECT Imaging
Patients should avoid drinks with caffeine, such as coffee or soda, for at least 12 hours before their medical test. This is because caffeine can interfere with, or get in the way of, certain medicines that may be given during the test. These medicines may help to open up blood vessels (a process called vasodilation). Plus, caffeine can change the flow of blood in the brain. Both of these effects from caffeine could cause difficulties in getting a clear picture during a brain imaging test known as a cerebral SPECT.
Patients are also advised to stop taking certain medicines like dipyridamole and other similar drugs at least 48 hours before the test. They work with the body to make blood vessels get wider, an effect that could overly lower your blood pressure and cause unsafe conditions during the test.
Also, patients shouldn’t eat or drink anything three hours before the test (a condition known as nil per oral) and they should use the bathroom right before the test begins. These steps help to make the patient more comfortable during the procedure.
How is SPECT Imaging performed
First, a special compound called a radio-labeled tracer is injected into the patient. This compound is carefully chosen by a professional called a nuclear pharmacist. For brain studies, the patient is asked to sit in a quiet, dimly lit room and not to read or talk for at least 10 minutes before the tracer is injected. If the patient needs to be calmed down or made drowsy (sedation), this happens after the tracer injection. Once injected, there’s a waiting period. This allows time for the tracer to travel around the body and reach the areas to be examined. The waiting time varies depending on the area being studied and the tracer used; it could be as short as 15 minutes for heart stress tests or as long as 90 minutes for brain studies.
When the waiting period is over, the patient is placed into a special scanner. If the patient is having a heart stress test, they are given heart stimulants like atropine following normal stress testing procedures. Scans of both the brain and heart could require medicines that widen blood vessels (vasodilators) to check blood flow to tissues. Once the patient is set and the needed medicines have been given, the scanner rotates around the patient, taking scans every 3 to 6 degrees. These scans are then combined to create a final 3D image. The exact scanning process can vary. It could involve one single scan (like in brain imaging) or multiple scans taken at specific intervals (like in stress/rest imaging for heart SPECT).
SPECT can provide detailed information about tissues in the body but it also has limitations. For example, small changes that may be identified by SPECT can be hard to pinpoint without additional imaging. To address such challenges, a combined SPECT/computed tomography protocol has been developed, which allows functional and anatomical abnormalities detected by the SPECT study to also be imaged simultaneously on computed tomography.
There are certain SPECT protocols that can be used to reduce radiation exposure and hazards. These include selecting the right patients for SPECT imaging based on a clear need, which helps to reduce unnecessary radiation exposure. Protocols that use technetium-99 m (such as sestamibi and tetrofosmin) are considered safer and preferred for evaluating common conditions such as chest pain and diagnosing ischemia, because they expose the patient to lower radiation levels than thallium-201 protocols. The amount of radiotracer should be based on the patient’s weight to optimize the required dose of radioactivity.
Further, the use of special detectors in the imaging camera, like cadmium zinc telluride detectors, can help in using the best radiation techniques. SPECT stress-only protocols, particularly with technetium-99m labeled radiotracers, can reduce radiation exposure by about 25% compared to typical rest/stress studies. Stress-first imaging is advisable for patients who don’t have a high chance of an abnormal study result. This is particularly useful and doable in younger patients, especially those with a low to moderate chance of having coronary artery disease.
Other ways to minimize radiation exposure include image acquisition practices such as lengthening the time for which images are captured, positioning the camera as close to the patient as possible, and using newly developed methods to maintain image quality even with a reduced dose of the radiotracer. For instance, a novel wide-beam reconstruction algorithm can provide superior image quality with 50% less radiation dose. In case a scanner uses a CT scan for attenuation correction (correcting alterations in the x-ray beam as it passes through the body), the settings can be optimized to use the lowest dose. Advances in software, such as resolution-recovery techniques, can further reduce radiation exposure significantly.
Possible Complications of SPECT Imaging
The main side effects from this test are due to the medications used to widen your blood vessels (vasodilators) and other drugs given during the procedure. These effects could include a flushed face, headache, stomach discomfort, and feeling lightheaded. More serious side effects may include low blood pressure, irregular heartbeat, chest discomfort, or a block in the main electrical pathway of the heart (AV block).
In rare cases, you might be allergic to the compound used to track blood flow in your body or other medicines used in the test. The medical team also needs to consider the risks of exposing you to radiation during the test, particularly if you are pregnant or intending to conceive soon.
For some perspective, a Tc stress/rest cardiac scan carries the highest radiation dose at 11.8 mSv, whereas a Tc brain scan carries a dose of 5.7 mSv. The lower limit of radiation exposure for most SPECT scans, excluding cardiac stress/rest studies, is usually less than 10 mSv. As a comparison, the radiation doses for a head CT scan, chest CT scan, and heart CT angiography (a test that uses dye to make the blood vessels in your heart visible on a CT scan) are 2.0, 7.0, and 16.0 mSv, respectively.
What Else Should I Know About SPECT Imaging?
Myocardial perfusion testing involves examining the blood flow to your heart muscles, to detect any abnormalities. SPECT – which stands for Single Photon Emission Computed Tomography – is a test often used in this examination. This test has been found to correctly identify heart artery disease 82% of the time (sensitivity) and accurately exclude those without the disease 76% of the time (specificity). Moreover, patients who get a normal result from this heart SPECT test have less than a 1% chance of having heart problems each year.
When it comes to testing for Alzheimer’s dementia, using the brain SPECT imaging is also beneficial. The test correctly identifies this condition 92% of the time (sensitivity) and is 100% accurate in confirming individuals without the disease (specificity). This test also has a 92% positive predictive value, meaning that 92% of those who test positive will indeed have Alzheimer’s. However, the negative predictive value – which means the proportion of patients with negative test results who are correctly diagnosed – is only 57%.
