A perfusion scan is a diagnostic imaging test that measures blood flow to a specific organ, most commonly the heart, lungs, or brain. A small amount of radioactive tracer is injected into your bloodstream, and a specialized camera tracks where that tracer travels, revealing areas that are getting good blood supply and areas that aren’t. The test helps doctors spot blockages, blood clots, and tissue damage that might not show up on standard imaging.
How Perfusion Scans Work
The basic principle is straightforward: a radioactive tracer injected into a vein travels wherever your blood flows. A camera then captures where the tracer ends up. Areas with healthy blood flow light up evenly, while areas with poor flow appear darker. These darker regions, sometimes called “cold spots” or “defects,” tell doctors that something is restricting blood from reaching that tissue.
The type of camera used depends on the scan. SPECT (single-photon emission computed tomography) is the most common and creates 3D images by rotating around your body. PET (positron emission tomography) scanners offer higher resolution and deliver less radiation but are more expensive and less widely available.
Heart Perfusion Scans
A myocardial perfusion scan, often called a nuclear stress test, is the most frequently ordered type. It evaluates whether your heart muscle is getting enough blood, particularly during exertion. The test is primarily used to detect coronary artery disease and to assess damage after a heart attack.
The scan involves two rounds of imaging: one while your heart is under stress and one while you’re resting. For the stress portion, you’ll typically exercise on a treadmill or stationary bike. If you can’t exercise, you’ll receive a medication that mimics the effect by increasing your heart rate or widening your blood vessels. The tracer is injected at peak stress, and images are taken shortly after. You’ll then return three to six hours later for the resting images, which don’t require exercise.
Doctors compare the two sets of images side by side. If a region of the heart shows reduced tracer uptake during stress but looks normal at rest, that suggests a narrowed artery is limiting blood flow when the heart works harder. If a region shows reduced uptake during both stress and rest (a “fixed defect”), that typically indicates scar tissue from a previous heart attack. In some cases, additional images taken 18 to 24 hours later can determine whether damaged-looking tissue is actually still alive and potentially recoverable.
Lung Perfusion Scans
A lung perfusion scan is almost always paired with a ventilation scan, which is why you’ll hear it called a V/Q scan (ventilation/perfusion). The perfusion half measures blood flow through the lungs, while the ventilation half measures airflow. The combination reveals mismatches: areas where air is reaching the lungs but blood isn’t, which is the hallmark of a pulmonary embolism (blood clot in the lungs).
V/Q scans are most commonly ordered when a pulmonary embolism is suspected. They’re also used to evaluate chronic clot disease, abnormal circulation in the pulmonary vessels, and regional lung function in people with advanced COPD or other chronic lung conditions. For detecting vascular obstructions at the segmental level, V/Q scans are more sensitive than CT angiography (the other main option), catching about 83 to 85% of blockages compared to CT’s 67%.
Brain Perfusion Scans
Brain perfusion scans map blood flow across different regions of the brain. Because blood flow in the brain is tightly linked to metabolic activity, these scans can reveal areas that look structurally normal on a standard MRI or CT but aren’t functioning properly.
The main clinical uses include evaluating suspected dementia, localizing seizure origins before epilepsy surgery, detecting cerebrovascular disease, and predicting outcomes after a stroke. For diagnosing Alzheimer’s disease specifically, brain SPECT scanning has a sensitivity of 92% and a specificity of 100%, making it a valuable tool when the diagnosis is uncertain from clinical evaluation alone.
What to Expect During the Test
For a heart perfusion scan, plan to be at the facility for a significant portion of your day. The stress portion, including exercise, injection, and imaging, takes roughly one to two hours. You’ll then have a gap of three to six hours before returning for rest images, which take another 30 to 60 minutes. Some facilities use protocols that complete both phases in a single visit with shorter gaps.
Lung and brain perfusion scans are generally shorter, often wrapping up within an hour or two. For a V/Q scan, you’ll breathe in a small amount of radioactive gas or aerosol for the ventilation portion and receive an injection for the perfusion portion. Brain SPECT involves a single injection followed by a waiting period and then imaging.
During imaging itself, you’ll lie still on a table while the camera rotates around you. The injection feels like a standard blood draw. The tracers themselves don’t cause side effects in the vast majority of patients. If you’re having a pharmacologic stress test (medication instead of exercise), you may feel flushing, a rapid heartbeat, or mild chest tightness, which typically resolves within minutes.
Preparation Before the Scan
Heart perfusion scans have the strictest preparation requirements. You’ll need to fast for at least four to six hours beforehand, though water is fine. Caffeine must be avoided for at least 12 to 24 hours before the test, and that includes coffee, tea, soda, chocolate, and caffeine-containing medications like some headache remedies. Alcohol and tobacco products should also be avoided for 24 hours. If you take nitrate medications, your doctor will likely ask you to skip them the day before and the day of the exam.
Bring a complete list of your medications, including supplements. Some drugs can interfere with the test results, and your doctor may adjust specific ones ahead of time. Lung and brain perfusion scans generally have fewer restrictions, but you’ll receive specific instructions when the test is scheduled.
Radiation Exposure
Perfusion scans use small amounts of radioactive material, and the radiation dose varies depending on the type of scan and the tracer used. For heart SPECT scans, the median effective dose in the United States is about 12 to 14 millisieverts (mSv). To put that in context, the average American receives about 3 mSv per year from natural background radiation, so a heart SPECT scan is roughly equivalent to four years of everyday exposure compressed into one test. PET heart scans deliver considerably less radiation, with a median dose of about 3.7 mSv.
Lung perfusion scans deliver lower doses than cardiac SPECT, typically in the range of 1 to 2 mSv. The tracers are cleared from the body relatively quickly, mostly through natural radioactive decay and normal kidney function. For any single scan ordered with a clear clinical question, the diagnostic benefit far outweighs the small radiation risk.
Understanding Your Results
Results are typically read by a nuclear medicine specialist and sent to the doctor who ordered the test, often within a day or two. The images are color-coded maps showing tracer distribution. Uniform, bright uptake indicates healthy blood flow. Areas with less uptake point to reduced perfusion.
For heart scans, the key distinction is between reversible defects (reduced blood flow only during stress, suggesting a blockage that could benefit from treatment) and fixed defects (reduced flow at both stress and rest, suggesting permanent damage). For lung scans, a “mismatch” where ventilation is normal but perfusion is reduced strongly suggests a blood clot. A “matched” defect, where both air and blood flow are reduced in the same area, points more toward lung disease like COPD or pneumonia rather than a clot.
Brain perfusion results are compared against expected patterns of blood flow for specific conditions. Alzheimer’s disease, for instance, produces a characteristic pattern of reduced flow in the temporal and parietal regions, while other types of dementia affect different areas. Epilepsy evaluations look for a focal zone of increased perfusion during a seizure and decreased perfusion between seizures.