DEXA & Body Composition
- DEXA (dual-energy X-ray absorptiometry) is a low-dose X-ray scan that measures bone mineral density — basically, how solid your skeleton is.
- It works by firing X-rays at two different energies and seeing how much each gets eaten, which lets it separate bone from soft tissue.
- The headline number is the T-score: how your bones compare to a healthy young adult. The Z-score compares you to your own age group.
- The radiation dose is tiny — far less than a chest X-ray, and a rounding error next to a CT.
- The same trick that weighs bone can also split soft tissue into fat and lean mass — that's the "body composition" half.
Imagine you could weigh someone's skeleton without the inconvenience of removing it first. That's the whole pitch for DEXA: a quick, cheap, low-radiation scan that tells you whether someone's bones are dense and sturdy or quietly thinning toward a fracture. It's the workhorse test for diagnosing osteoporosis, and it does its job lying flat on a padded table while the patient checks their phone.
The two-energy trick
Here's the clever bit. A normal X-ray fires one beam through you and measures what survives. Bone eats a lot of the beam, soft tissue eats some, air eats almost none — and that's enough to make a picture, but not enough to put a number on the bone.
DEXA cheats by using two X-ray energies — that's the "dual-energy" in the name. Bone and soft tissue don't dim the two energies by the same proportion. Think of shining a flashlight and a red laser through a stack of pancakes versus a stack of poker chips: each material has its own fingerprint for how it dims different beams. By comparing how much of each energy gets through, the machine can do algebra at every pixel and solve for two unknowns — how much is bone, and how much is everything else.
If the physics of "how much beam gets eaten" feels shaky, it's worth a detour through DEXA physics, where the absorptiometry math lives.
DEXA measures areal density — grams of mineral per square centimeter of the projected image (g/cm²), not true grams per cubic centimeter. It's a 2D shadow of a 3D bone, so a bigger bone can look "denser" simply because there's more of it stacked in the beam's path. It's an approximation, but a well-validated, reproducible one.
Where it scans and what it reports
The two sites that matter are the lumbar spine and the hip (specifically the femoral neck and total hip). These are weight-bearing, fracture-prone, and rich in the spongy trabecular bone that thins first. The forearm is a backup when the spine or hip can't be measured.
The scan spits out a T-score and a Z-score, and confusing the two is a rite of passage.
| Score | Compared to | What it's for |
|---|---|---|
| T-score | A healthy young adult at peak bone mass | Diagnosing osteoporosis in postmenopausal women and older men |
| Z-score | People of the same age, sex, and size | Flagging when bone loss is worse than expected for age — a nudge to hunt for a secondary cause |
The diagnostic categories hang off the T-score using the WHO definitions: a T-score down to about −1 is normal, between roughly −1 and −2.5 is osteopenia (the "your bones are a little thin, let's keep an eye on it" zone), and −2.5 or lower is osteoporosis. The minus signs trip everyone up — a more negative number is worse, like a sad bone credit score.
Use the T-score for diagnosis in older patients, but reach for the Z-score in younger adults, premenopausal women, and children. A T-score in a 30-year-old is comparing them to a peak they haven't reached or left yet — it'll mislead you. A low Z-score at any age says "this is more bone loss than age explains," which should prompt a search for a secondary cause.
The fine print that wrecks a scan
DEXA is gloriously simple to run and gloriously easy to fool. Anything dense sitting in the beam's path gets misread as bone and falsely raises the score — which is dangerous, because it can hide real osteoporosis.
Degenerative changes are the classic spoiler at the spine. Osteophytes, facet arthritis, aortic calcification, vertebral compression fractures, even a retained surgical clip or a swallowed calcium pill — all of it can inflate the lumbar number. When a vertebra reads suspiciously high compared to its neighbors, it usually gets excluded from the average. This is exactly why the hip is often the more trustworthy site in an older spine.
The radiation is basically nothing
People hear "X-ray" and brace for a dose. DEXA's effective dose is genuinely tiny — a small fraction of a single chest radiograph, and a microscopic sliver of what a PET/CT delivers. It's one of the lowest-dose X-ray exams in the building. The narrow, low-energy beams that make the dose small are the same beams that make the image low-resolution — DEXA is for numbers, not for spotting a hairline fracture.
The other half: body composition
Remember the two-unknowns algebra? In areas without bone, the machine can split soft tissue into fat mass and lean (mostly muscle) mass, and map where each lives. A whole-body DEXA becomes a detailed accounting of fat versus muscle — useful for tracking sarcopenia, obesity research, athletic monitoring, and disease that wastes muscle. Same scanner, same physics, just pointed at the soft stuff instead of the skeleton.
One machine, one quick low-dose scan, two answers: how strong the skeleton is (bone density, via the T-score) and what the body is made of (fat versus lean mass). The two-energy trick is the engine under both.
So that's DEXA — the test that weighs your skeleton, ranks your bones against a 25-year-old, and somehow also knows your fat-to-muscle ratio, all while you lie still for a few minutes. If you remember nothing else: dual energy separates bone from tissue, the T-score drives the diagnosis, more negative is worse, and dense junk in the path lies to you by reading high.