How Does a DEXA Scanner Work? Dual-Energy X-Rays Explained

How does a DEXA scanner work?

A DEXA scanner passes two X-ray beams of different energies through your body, measures how much of each is absorbed, and uses the difference between them to calculate how dense your bones are. DEXA stands for dual-energy X-ray absorptiometry — "dual-energy" because it uses two beams, and "absorptiometry" because it measures absorption. As the NHS describes it, the scan uses low-dose X-rays where "some radiation is absorbed by the bone and soft tissue, and some travels through your body." NHS, Bone density scan (DEXA scan), 2022

Everything the scanner does hangs on one idea: dense material blocks X-rays more than soft material does. Bone is dense, so it casts a stronger "shadow" than the muscle, fat and organs around it. Measuring the strength of that shadow — precisely, and after removing everything that isn't bone — is the whole job.

Why does a DEXA scan need two X-ray energies, not one?

Because a single X-ray beam can't separate bone from the soft tissue sitting in front of and behind it — but two beams of different energy can. Your spine and hip aren't floating in mid-air; the X-rays have to travel through skin, fat and muscle before and after they reach bone. With only one energy, the machine can't tell how much of the absorption came from bone and how much from that surrounding tissue.

Two energies solve the problem because the two tissue types respond differently to them. A 2020 review notes that "the higher the photon energy, the lowest the attenuation," and that "high-density tissues (such as bone) attenuate the X-ray beam more than low-density tissues." Quantitative Imaging in Medicine and Surgery, Body composition with dual energy X-ray absorptiometry, 2020 By comparing the ratio of how strongly the low-energy and high-energy beams are absorbed at each point, the scanner can mathematically strip the soft tissue out of the picture and leave bone on its own.

How does the scanner turn absorption into a bone density number?

Once soft tissue has been subtracted, the scanner is left with the X-ray signal from bone alone, which it converts point by point into your bone mineral density (BMD). The DEXA system's X-ray source "generates alternating high- and low-energy waves in a thin beam," and as that beam scans across the region it builds up a map of bone density. Journal of Mid-life Health, Dual-Energy X-Ray Absorptiometry Scanning in Practice, 2022

BMD is reported as an areal density — the amount of bone mineral spread over the area that was scanned — which is why two different sites, usually the lower spine and the hip, are measured separately. This raw density figure is precise, but on its own it means little to most people. That's why the scanner takes one more step.

What is a T-score, and how does the scanner calculate it?

A T-score is your bone density expressed as the number of standard deviations above or below the average for a healthy young adult. The 2022 technical review defines it precisely as "the number of standard deviations that the patient's BMD is above or below the young adult reference mean." Journal of Mid-life Health, Dual-Energy X-Ray Absorptiometry Scanning in Practice, 2022 The NHS describes the same comparison: your measurement is set against "a young healthy adult or an adult of your own age, gender and ethnicity." NHS, Bone density scan (DEXA scan), 2022

In other words, the machine converts a raw density into a standardised score, so a number that would otherwise be meaningless becomes a like-for-like comparison against a healthy reference. For what the different scores mean in practice, see our guide to DEXA scan results and your T-score explained. For the wider picture of what a scan can and can't reveal, see what a DEXA scan shows.

What does a DEXA machine look like — and why does its shape matter?

A DEXA scanner is an open, flat table with a scanning arm that moves above you — there is no tunnel to enter. The NHS describes lying "on your back on a flat, open X-ray table" while "a large scanning arm will be passed over your body" delivering "a narrow beam of low-dose X-rays." NHS, DEXA scan: What happens, 2022

That moving arm isn't just for show — it is the mechanism. Because DEXA measures density point by point, the arm slowly sweeps the dual-energy beam line by line across the spine or hip to build the density map. You're asked to keep still for the same reason a long-exposure photograph needs a steady subject: movement blurs the image. You can usually "remain fully clothed," apart from removing anything with metal fasteners, and the scan "usually takes 10 to 20 minutes." NHS, DEXA scan: What happens, 2022 The open design is also why DEXA is comfortable for people who find enclosed scanners claustrophobic.

How much radiation does a DEXA scan actually use?

Very little — a DEXA scan is among the lowest-dose X-ray examinations in medicine. The 2022 technical review reports that the effective dose from a DEXA scan is "less than 10 microsieverts," which it describes as "among the lowest of all the radiological investigations." Journal of Mid-life Health, Dual-Energy X-Ray Absorptiometry Scanning in Practice, 2022 The dose is low enough that the radiographer can stay in the room during the scan. NHS, DEXA scan: What happens, 2022

For a fuller look at the safety side, see is a DEXA scan safe?

Is there a way to check bone density without any X-rays?

Yes. DEXA is the long-established reference standard for bone density, but it isn't the only way to screen your bones. Because DEXA relies on ionising radiation, it isn't ideal when you want to check bone density regularly — for example, while tracking change through menopause or during treatment.

REMS (Radiofrequency Echographic Multi Spectrometry) takes a different route entirely: it is an ultrasound-based, radiation-free bone-density technology, so it can be used for screening without any X-ray exposure at all. That makes it well suited to assessments you may want to repeat over time.

At Screen My Bones we offer radiation-free REMS bone-density screening at assessment days across the UK. It's a screening designed to help you understand where your bone health stands — it doesn't replace advice from your GP or specialist, but it's an easy, X-ray-free first step. You can book a bone health assessment or read more about our bone health assessment.

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This article is part of our screening technology series. To weigh the two approaches side by side, see our comparison of REMS versus DEXA.