What Will Light Up on a PET Scan?

A PET scan lights up anything that actively absorbs the radioactive tracer injected before the scan, and the most commonly used tracer is a sugar molecule tagged with a tiny amount of radioactive fluorine. Because cancer cells, inflamed tissue, active brain regions, and even hardworking muscles all consume glucose at high rates, all of them can glow on the resulting images. That breadth is both the scan’s greatest strength and its trickiest limitation, since “lighting up” does not automatically mean cancer, and failing to light up does not always mean a tissue is healthy.

Why Certain Tissues Glow

The standard PET tracer, called FDG (short for fluorodeoxyglucose), is essentially a glucose molecule with a radioactive tag. Your body treats it like regular sugar: cells pull it inside using the same transport proteins they use for normal glucose, and enzymes lock it in place. The key difference is that FDG cannot be fully broken down for energy, so it gets trapped inside cells that grabbed it. The more metabolically hungry a cell is, the more FDG it accumulates, and the brighter it appears on the scan.¹

This is why cancerous tissue tends to stand out so dramatically. Tumor cells burn through glucose at a much faster rate than most normal cells, a metabolic quirk researchers have known about for nearly a century. They over-produce the transporter proteins that pull sugar across the cell membrane and ramp up the enzymes that trap it inside.² The shift toward rapid, inefficient glucose burning even when oxygen is available is sometimes called the Warburg effect, and it is one of the hallmarks of cancer biology.³

But cancer is far from the only thing with a sweet tooth. Your brain, for instance, is one of the biggest glucose consumers in the body, so it always shows bright FDG uptake. Your heart muscle, kidneys filtering the tracer into urine, and your liver all show some degree of background glow that radiologists learn to read around.

Cancer and Its Exceptions

Oncology is the dominant reason PET scans are ordered. Doctors use them to find tumors, figure out whether cancer has spread, check whether treatment is working, and watch for recurrence. Most aggressive solid tumors light up reliably: lung cancers, lymphomas, melanomas, head-and-neck cancers, esophageal cancers, and colorectal cancers are among the types where FDG PET performs well.

Not every cancer cooperates, though. Slow-growing, well-differentiated tumors can have glucose metabolism that is only modestly higher than surrounding tissue, making them harder to spot. A good example is certain neuroendocrine tumors, which may barely register on an FDG scan because they are not dividing fast enough to burn through sugar at high rates. Even among a single cancer type, there is variation. In a study of invasive mucinous adenocarcinoma of the lung, about 60% of the nodular-type tumors showed significant FDG uptake, while every one of the pneumonic-type tumors did.⁴ So even within one cancer diagnosis, the subtype matters for whether the scan will be informative.

Prostate cancer is another notorious exception. Many prostate tumors have relatively low glucose metabolism and do not light up well on standard FDG PET. This is why an entirely different tracer was developed for prostate cancer, which we will get to shortly.

The Brain on PET

Neurological imaging is the second major arena for PET scanning, though the questions being asked are quite different from oncology. Instead of hunting for a bright spot against a dark background, brain PET often looks for areas that are dimmer than expected, signaling reduced function.

In Alzheimer’s disease, specialized PET tracers bind directly to amyloid plaques, the sticky protein clumps that are a defining feature of the disease. A positive amyloid PET scan, where the tracer lights up across broad regions of the cortex, provides evidence that Alzheimer’s pathology is present, which can be crucial for diagnosis when symptoms alone are ambiguous.⁵ This has become increasingly relevant as new treatments targeting amyloid have entered the market, since confirming the presence of amyloid plaques is typically required before starting those drugs.

For Parkinson’s disease, PET can measure how well the brain’s dopamine system is functioning. Using a tracer called F-DOPA, which tracks dopamine production, scans can reveal reduced uptake in parts of the brain that rely on dopamine, helping distinguish Parkinson’s from conditions that mimic it.⁶ PET research in Parkinson’s has also extended beyond dopamine, exploring glucose metabolism patterns and neuroreceptor activity to better understand how the disease progresses.⁷

What the Heart Tells a PET Scanner

Cardiac PET is a growing application that tackles two main questions: is blood flow to the heart adequate, and is damaged heart muscle still alive?

The second question, called viability assessment, is where FDG PET really shines. After a heart attack, some of the heart muscle may appear to have stopped working on a standard perfusion scan, but that does not always mean it is dead. Some of that tissue may be “hibernating,” alive but conserving energy because blood flow has been reduced. If doctors can identify hibernating muscle, revascularization surgery could wake it up and improve heart function. An FDG PET scan can detect this: areas that show poor blood flow on a perfusion scan but still take up FDG are viable and potentially salvageable.⁸ In one study of patients with ischemic cardiomyopathy, about three-quarters showed this mismatch pattern, meaning a significant majority had viable tissue that might benefit from intervention.⁹

For blood flow assessment, PET can also use dedicated perfusion tracers like rubidium-82, which is produced by a bedside generator rather than requiring a nearby cyclotron. This has made cardiac PET more accessible to hospitals that could not previously offer it.¹⁰

Infection, Inflammation, and Fever of Unknown Origin

Activated immune cells are glucose-hungry, which means infections and inflammatory conditions can light up on FDG PET with an intensity that rivals cancer. This is a double-edged sword: it creates false positives when the scan was ordered to look for tumors, but it also makes PET a powerful tool when infection or inflammation is the actual diagnostic target.

Bacterial infections, fungal infections, tuberculosis, and other mycobacterial diseases can all produce intense FDG uptake in the chest, abdomen, or elsewhere.¹¹ One of the most valuable clinical uses in this space is tracking down the cause of a fever of unknown origin, where standard tests have come up empty. FDG PET/CT can guide clinicians toward the site of hidden infection or inflammation with high sensitivity, reported in one study at over 90%.¹²

Sarcoidosis is another condition where FDG PET has found a clear role. The scan can reveal the extent of the disease, showing FDG-avid lymph nodes, lung tissue, and sometimes unexpected extrapulmonary sites all at once. In one case report, a PET scan ordered to evaluate possible cardiac sarcoidosis incidentally revealed FDG-avid blood vessels, pointing toward systemic vasculitis as an additional diagnosis.¹³ FDG PET is now recommended by international guidelines for evaluating large-vessel vasculitis, including giant cell arteritis and Takayasu arteritis, in addition to cardiac sarcoidosis.¹⁴

Even cardiovascular infections like endocarditis and infected vascular grafts can produce FDG uptake around the affected structures, making PET a useful addition to the diagnostic toolkit when echocardiography or CT alone leave uncertainty.¹⁵

Specialized Tracers Beyond FDG

FDG gets most of the attention, but a growing roster of alternative tracers has opened up PET scanning to diseases that sugar-based imaging handles poorly. Each tracer targets a different biological feature, so different things “light up” depending on which one is used.

For prostate cancer, tracers targeting a protein called PSMA (prostate-specific membrane antigen) have transformed the field. PSMA-based PET scans use radiolabeled ligands that bind to PSMA, which is overexpressed on most prostate cancer cells. These tracers have significantly improved the ability to detect recurrent or metastatic prostate cancer compared to older imaging methods.¹⁶ In the United States, two PSMA tracers are now FDA-approved and widely available.

For neuroendocrine tumors, the story is similar. Well-differentiated neuroendocrine tumors often express somatostatin receptors on their surface, and tracers like gallium-68 DOTATATE are designed to bind to those receptors. This makes DOTATATE PET the preferred imaging method for finding neuroendocrine tumors, selecting patients for targeted radionuclide therapy, and locating primary tumors that other scans have missed.¹⁷ The tracer uptake correlates closely with how densely the tumor expresses those receptors, which also helps predict whether the patient will respond to treatment.¹⁸

The amyloid tracers used in Alzheimer’s diagnosis are yet another example. None of these specialized tracers tracks sugar metabolism at all. Instead, they each bind to a specific molecular target, which means the list of what “lights up” on a PET scan depends entirely on which tracer was given.

Normal Things That Light Up and Confuse the Picture

Because FDG is a sugar analog, plenty of normal physiological activity generates uptake that can be mistaken for disease if you do not know what to look for.

Brown fat is one of the most common culprits. Unlike regular body fat, brown fat burns calories to generate heat, and when it is activated, it can take up so much FDG that it mimics lymph node involvement in the neck, shoulders, or along the spine. Brown-fat uptake shows up in roughly 2.5 to 4% of scans and is more common in women, younger patients, people with low body mass, and during cold weather.¹⁹ It can appear in atypical locations beyond the classic neck-and-shoulder distribution, which adds to the potential for misinterpretation.²⁰ Keeping the patient warm before and during the uptake period helps suppress brown-fat activity.

Muscle activity is another frequent issue. Muscles that were recently active gobble up FDG, and the patterns can be surprisingly specific. One study found that about 12.5% of patients had excessive muscle uptake traceable to activities during or shortly before the scan. The patterns matched the activity: neck muscles lit up in patients who had been straining on a stretcher, the jaw muscles in someone who had been chewing gum, vocal cords in patients who had been talking, and forearm muscles in someone who had been holding up reading material.²¹ This is why patients are typically told to avoid strenuous exercise for one to two days before the scan and to rest quietly during the uptake period after injection.

The bowel can also show patchy FDG uptake that is entirely normal, caused by smooth muscle contractions or the metabolic activity of gut bacteria. But intense focal uptake in the bowel deserves attention. When researchers followed up on 25 foci of intense bowel uptake found incidentally on PET/CT, 84% turned out to be genuine abnormalities on endoscopy or surgery, including premalignant polyps and outright cancers.²² So while diffuse bowel uptake is usually dismissed, a bright focal spot in the colon often triggers a referral for colonoscopy.

How Blood Sugar and Timing Affect Results

Since FDG competes with regular glucose for entry into cells, blood sugar levels at the time of injection directly affect scan quality. When blood sugar is high, the body’s own glucose crowds out FDG, reducing how much tracer gets into tumors and other tissues of interest. The relationship is not straightforward, either. In the brain, the effect of rising blood sugar on FDG uptake follows a nonlinear, exponential pattern rather than a simple one-for-one trade-off.²³ This is why most imaging centers require patients to fast for at least four to six hours before the scan, and why diabetic patients may need special scheduling or insulin protocols to bring glucose levels into an acceptable range.

Timing also matters in the context of treatment. After radiation therapy, the treated area can remain inflamed and FDG-avid for weeks. Current consensus recommends waiting at least 12 weeks after completing radiation before scanning to evaluate treatment response, because earlier scans carry a high risk of false-positive findings from inflammation rather than residual tumor.²⁴ Post-surgical changes can similarly produce uptake that mimics disease, making the clinical context and scan timing critical for accurate interpretation.

How PET Scan Technology Has Changed

The scanners themselves have undergone dramatic improvement since PET imaging began in the 1970s. Spatial resolution has improved by a factor of ten and sensitivity by a factor of forty from those early designs to today’s high-performance machines. A whole-body scan that once took an hour or more can now be completed in under ten minutes on a modern PET/CT system.²⁵ That speed means less time lying still for patients and sharper images with fewer motion artifacts.

The combination of PET with CT (and increasingly with MRI) also changed what lights up in a practical sense. Early PET scanners produced metabolic images without detailed anatomy, so a bright spot could be hard to place precisely. Fusing the PET image with a CT scan lets radiologists pin that bright spot to a specific lymph node, bone, or organ, dramatically reducing ambiguity. This is why modern reports almost always refer to PET/CT rather than PET alone, and why the fusion approach has helped distinguish true disease from benign hot spots like brown fat or active muscle more reliably than either scan could alone.

More recent developments include total-body PET scanners with detector rings long enough to image the entire body at once, rather than stitching together multiple bed positions. These systems capture more of the emitted radiation, enabling even shorter scan times or the use of lower tracer doses, which reduces radiation exposure. They also open the door to studying how tracers move through the body over time, tracking the dynamic process of uptake rather than capturing a single snapshot. For patients who need repeated scans over months or years of treatment monitoring, the reduced radiation per scan is a meaningful practical benefit.