Infection/Inflammation Imaging
- Infection and inflammation light up on functional imaging because they recruit a swarm of metabolically hungry white blood cells.
- The toolkit: FDG-PET/CT (the sensitive generalist), labeled-WBC scintigraphy (the specific specialist), gallium-67 (the old reliable), and bone scintigraphy (great for "is the bone involved?").
- FDG is gloriously sensitive but not picky — tumors, healing wounds, and infection all glow, so context is everything.
- Labeled-WBC imaging follows your own white cells to where they're partying, which makes it the more specific answer when "is this actually infected?" is the question.
- Classic jobs: fever of unknown origin, prosthetic joint and vascular graft infection, and complicated bone infection.
Anatomy imaging asks "what does it look like?" Functional imaging asks the nosier question: "what is it doing?" And it turns out that infected and inflamed tissue is doing a lot — burning sugar, summoning white blood cells, and generally throwing a metabolic house party. Our scanners are just the neighbors who can see the lights on from outside.
That's the whole trick here. We don't image the bug. We image the body's frantic, glucose-guzzling response to the bug.
Why infection glows: the hungry-cell story
When tissue gets infected or inflamed, it floods the area with white blood cells and they start burning glucose like it's going out of style. FDG — fluorodeoxyglucose — is a radioactive glucose look-alike. Cells gobble it up thinking it's sugar, then can't fully metabolize it, so it gets trapped inside, glowing for the camera.
The catch: cancer does the exact same thing. So does a healing surgical scar. FDG is the metabolic equivalent of a smoke detector that goes off for toast, candles, and actual fires alike — wonderfully sensitive, frustratingly indiscriminate.
FDG-PET/CT is extremely sensitive for infection and inflammation but not very specific. A bright spot tells you "something energetic is happening here," not "this is pus." You bring the clinical context; the scan brings the flashlight.
The toolkit, and which tool for which job
There's no single "infection scan." You pick based on the question. Here's the cheat sheet I wish someone had handed me on day one.
| Agent / study | What it tracks | Best at |
|---|---|---|
| FDG-PET/CT | Glucose-hungry cells (any cause) | Fever of unknown origin, whole-body survey, vascular graft infection, vasculitis |
| Labeled-WBC scintigraphy | Your own tagged white cells | "Is this actually infected?" — prosthetic joints, abscess vs. tumor |
| Gallium-67 | Binds iron-handling proteins; taken up by inflammation/tumor | Spine infection (discitis-osteomyelitis), some chronic infections |
| Bone scintigraphy | Bone turnover | Screening the whole skeleton for sites of bone involvement |
For labeled-WBC studies, we literally draw the patient's blood, tag their white cells with a radiotracer, and re-inject them — then watch where they migrate. Because only white cells are labeled, they pile up at genuine infection rather than at tumor. That's why it's the more specific answer to "infected or not?"
Fever of unknown origin: the great treasure hunt
This is FDG-PET's signature gig. Someone has been febrile for weeks and nobody can find the source. Because FDG surveys the entire body in one go, it can spotlight an occult abscess, an inflamed vessel, or an unexpected tumor hiding in a place nobody thought to scan.
In fever of unknown origin, FDG-PET earns its keep precisely because it isn't specific — you want a sensitive, whole-body net that catches infection, inflammation, and malignancy, since any of the three could be the culprit.
Hardware infection: the painful prosthesis
A patient has a painful hip replacement. Loosening or infection? Plain films and even CT struggle because the metal scatters everything. Functional imaging steps in. The trouble is that normal bone remodeling around an implant is also metabolically busy, so any single scan can mislead — which is why labeled-WBC imaging, often paired with a marrow study, is frequently the tie-breaker for "is there real infection here?"
Don't read post-surgical FDG uptake as infection. A healing incision, recent surgery, and reactive bone all glow for weeks to months. Always ask when the operation was — early postoperative uptake is expected, not damning.
Bone and spine: where it's burning
For the skeleton, a bone scintigraphy study finds sites of increased bone turnover anywhere in the body, and a three-phase technique adds timing information that helps separate active bone infection from old changes. When the question is complicated bone infection — say, a diabetic foot — this is where nuclear medicine and the dedicated osteomyelitis workup overlap heavily.
Match the tracer to the question: FDG to find something anywhere, labeled-WBC to confirm it's infection, bone scintigraphy to ask whether bone is involved.
The one thing to carry out
Infection and inflammation imaging isn't about seeing germs — it's about reading the body's energetic, glucose-fueled, white-cell-summoning response. FDG is your sensitive floodlight; labeled-WBC is your specific spotlight. Pick the one that answers the question being asked, and never forget that a fresh surgical scar glows just as eagerly as the thing you're hunting.