A DEXA scan measures bone mineral density, or BMD, by using two low-dose X-ray energies to estimate how much mineral is present in a specific area of bone. The scan usually examines the hip and lower spine, where bone loss and fractures are clinically important.
But how does that actually work?
The basic idea is surprisingly simple. Different tissues absorb X-rays differently. A DEXA machine measures how much of each of two X-ray energies passes through the body, separates the contribution from soft tissue, and uses the remaining signal to estimate bone mineral content and bone mineral density.
The result is reported as a BMD value, usually in grams per square centimeter, along with a T-score and/or Z-score, depending on the person's age and clinical situation.
 |
| DEXA Scan: Understanding Bone Density |
What Is a DEXA Scan?
DEXA, also written DXA, stands for dual-energy X-ray absorptiometry.
It is a low-dose X-ray imaging test used to measure bone mineral density and help evaluate osteoporosis and fracture risk. It is commonly performed on the lumbar spine and hip.
Unlike a conventional X-ray, which primarily creates an image of bones and other structures, a DEXA examination is designed to make quantitative measurements of bone mineral density.
The procedure is generally quick, painless and noninvasive.
What Does a DEXA Scan Actually Measure?
A DEXA scan measures bone mineral density (BMD).
Bones contain minerals, particularly calcium and phosphorus. In general, bone containing more mineral is denser and tends to be stronger.
The machine doesn't simply ask:
"How thick does this bone look?"
Instead, it estimates the amount of mineral in a defined region and calculates an areal BMD value.
DEXA reports commonly express BMD in g/cm² (grams per square centimeter).
This is an important detail because DEXA's BMD is an areal measurement, not a direct measurement of true three-dimensional mineral density inside the bone.
How Does DEXA Use X-Rays to Measure Bone Density?
This is the key part of the technology.
A DEXA machine sends two X-ray beams with different energy levels through the area being examined.
Different materials absorb those X-rays differently.
For example, the body contains:
- soft tissue
- muscle
- fat
- bone
The two X-ray energies allow the system to estimate how much attenuation is coming from soft tissue and how much is attributable to bone mineral.
The machine then uses software to calculate the bone measurement.
A simple example
Imagine the X-ray beam traveling through your hip.
It encounters:
Skin → fat → muscle → bone
The machine records how the two X-ray energies are reduced as they pass through these tissues.
The software estimates the soft-tissue contribution.
What remains provides information about the mineral content of the bone in that region.
That information is then used to calculate BMD.
Why Does DEXA Use Two X-Ray Energies?
Using two different energies helps the machine distinguish bone mineral from soft tissue.
A single X-ray energy cannot separate those components as effectively.
With dual-energy measurements, the system can estimate the amount of attenuation attributable to soft tissue and bone separately.
That's where the name comes from:
Dual-energy X-ray absorptiometry = DEXA/DXA
What Happens During a DEXA Scan?
For a central DEXA scan, you generally lie on a padded table.
An X-ray source is positioned below you, while a detector moves above the area being examined.
For a spine measurement, your legs may be supported in a particular position to help align the lower spine.
For a hip measurement, the foot may be positioned in a brace to rotate the hip appropriately for scanning.
The technologist asks you to remain still while the scanner collects the measurements.
The process does not require anesthesia and is generally painless.
Why Are the Hip and Spine Usually Scanned?
The hip and lumbar spine are standard sites for central DXA measurement.
The International Society for Clinical Densitometry (ISCD) recommends measuring BMD at the PA spine and hip for adult patients undergoing appropriate testing.
These locations are especially useful because osteoporosis commonly affects bones that are important for mobility and fracture risk.
A DEXA examination may also include the forearm in certain circumstances, such as when the hip or spine cannot be measured or interpreted, or in specific clinical situations.
What Is BMD on a DEXA Report?
BMD stands for bone mineral density.
It is the measured amount of bone mineral in the scanned area, reported as an areal density value.
For example, a report might show a value such as:
BMD: 0.850 g/cm²
The number by itself doesn't tell you whether a person's result is considered normal, low or in the osteoporosis range.
That's because interpretation depends on the reference population and the score derived from the BMD value.
This is where the T-score and Z-score come in.
What Is a T-Score?
A T-score compares a person's BMD with the average BMD of a healthy young-adult reference population.
The result is expressed in standard deviations from that reference value.
For diagnostic classification, T-scores are generally used for postmenopausal women and men age 50 and older, with specific clinical guidance applying to different populations.
A simplified interpretation is:
These WHO-based thresholds are used for appropriate adult diagnostic contexts; the complete interpretation should consider the scanned site and clinical circumstances.
Simple example
Suppose two people have:
Person A: T-score −0.4
Person B: T-score −2.1
Person B has a BMD substantially below the young-adult reference level and falls into the low-bone-density range by the standard classification.
A third person with:
T-score −2.7
would meet the BMD threshold used for osteoporosis classification in the appropriate adult population.
What Is a Z-Score?
A Z-score compares BMD with people of similar age and other relevant characteristics, rather than comparing it primarily with a young-adult reference group.
Z-scores are particularly important when evaluating:
- premenopausal women
- men younger than 50
- children and adolescents
For these groups, ISCD guidance emphasizes Z-score interpretation rather than using the adult T-score diagnostic framework.
A Z-score of −2.0 or lower is described by ISCD as "below the expected range for age" in the relevant younger populations.
What Is the Difference Between BMD, T-Score and Z-Score?
These three terms are related, but they aren't interchangeable.
BMD
The actual measured bone mineral density value.
T-score
Compares your BMD with a young-adult reference population.
Z-score
Compares your BMD with an age-appropriate reference population.
A simple way to remember it is:
BMD = What was measured
T-score = How does it compare with young-adult peak bone mass?
Z-score = How does it compare with people of similar age?
Does a DEXA Scan Measure Bone Strength?
Not directly.
DEXA measures bone mineral density, which is an important part of assessing osteoporosis and fracture risk, but bone strength depends on more than mineral density alone.
Fracture risk can also be influenced by factors such as:
- age
- previous fractures
- medications
- medical conditions
- falls
- bone structure and quality
- other clinical risk factors
NIAMS notes that BMD testing can help assess fracture risk, but clinicians use the results alongside the broader clinical picture.
So a DEXA scan should not be interpreted as a complete measurement of every aspect of bone quality.
Can DEXA Show How Much Bone You Have Lost?
It can help monitor changes in BMD over time.
For example, a person might have one scan today and another later to see whether BMD has changed.
But there's an important catch:
Small differences between two scans do not automatically mean real bone loss or gain.
Measurement precision, positioning and the characteristics of the DXA system can affect the result.
The ISCD recommends that DXA facilities determine their own measurement precision and least significant change (LSC) so that clinicians can distinguish a meaningful change from normal measurement variability.
Why Should Follow-Up DEXA Scans Ideally Use the Same Machine?
Suppose your first scan is performed on one DXA machine and your next scan is performed on a different system.
Even if both machines are designed to measure BMD, their numerical results may not be directly interchangeable.
The ISCD recommends appropriate cross-calibration when systems change and advises against applying an LSC or reporting BMD change between instruments that have not been appropriately cross-calibrated.
That is one reason follow-up scans are often most useful when performed on the same DXA device, where appropriate.
How Accurate Is a DEXA Scan?
DEXA is the established standard technique for measuring BMD in clinical practice.
It is quick, noninvasive and uses a very small amount of ionizing radiation.
However, "accurate" doesn't mean that every measurement is perfectly identical every time.
Differences can arise from:
- patient positioning
- movement
- artifacts
- equipment
- analysis
- measurement precision
That's why clinical DXA interpretation includes quality control and precision assessment.
How Much Radiation Does a DEXA Scan Use?
DEXA uses a very small dose of ionizing radiation.
RadiologyInfo notes that the radiation exposure is less than one-tenth of the dose from a standard chest X-ray.
Even so, patients should tell the healthcare team if there is a possibility of pregnancy or if they recently had certain contrast or barium examinations, because these circumstances can affect when the examination should be performed.
Does a DEXA Scan Hurt?
No.
The scan is generally quick and painless, and anesthesia isn't required.
You simply need to lie still while the scanner collects the measurements.
Some positioning may feel slightly awkward, but the examination itself is noninvasive.
How Long Does a DEXA Scan Take?
A DEXA examination is generally a relatively quick procedure.
The exact time depends on the equipment, the number of regions being measured and the facility.
Because the scan requires you to remain still during image acquisition, following the technologist's positioning instructions is important for obtaining reliable measurements.
What Can Affect a DEXA Result?
Several factors can complicate interpretation.
Examples include:
- incorrect positioning
- movement
- degenerative changes in the spine
- previous surgery
- metal or other artifacts
- structural abnormalities
- differences between machines
The ISCD provides specific criteria for excluding vertebrae affected by local structural changes or artifacts when analyzing the lumbar spine.
This is why the scan should be interpreted by an appropriately trained clinician rather than by looking at a single number in isolation.
Can Arthritis Affect a DEXA Scan?
Degenerative changes in the spine can affect the measured BMD and therefore complicate interpretation.
The ISCD has specific rules for excluding vertebrae that are affected by local structural changes or artifacts.
This is one reason the hip and spine are analyzed using standardized regions of interest rather than simply taking one overall visual impression of the bone.
Why Can Two DEXA Scans Show Slightly Different Numbers?
Imagine that your first scan reports:
BMD = 0.900 g/cm²
and a later scan reports:
BMD = 0.892 g/cm²
It might be tempting to conclude that your bones have become less dense.
But the difference could be within the expected measurement variability.
The ISCD recommends using the facility's least significant change to determine whether a change between scans is statistically meaningful.
That's why comparing two raw BMD numbers without knowing the measurement precision can be misleading.
What Does a Typical DEXA Report Contain?
An appropriately prepared DXA report can include:
- the BMD value
- the scanned skeletal site
- the T-score and/or Z-score
- the region of interest
- the DXA machine information
- diagnostic classification where appropriate
- comparison with previous scans when valid
- information about significant change
These reporting elements are outlined in the ISCD's official adult positions.
DEXA Example: How the Numbers Fit Together
Imagine a hypothetical report for an older adult:
Lumbar spine BMD: 0.820 g/cm²
T-score: −1.8
The BMD number is the measurement.
The T-score tells you where that measurement falls relative to the relevant young-adult reference population.
Because −1.8 lies between −1.0 and −2.5, it falls within the low-bone-density category under the standard adult classification framework.
Now imagine another person has:
Hip BMD: 0.650 g/cm²
T-score: −2.7
In the appropriate adult population, a T-score at or below −2.5 meets the BMD threshold used for osteoporosis classification.
These examples are simplified. A real report should be interpreted using the correct skeletal site, reference database and clinical context.
Can DEXA Measure Bone Density Anywhere in the Body?
Central DXA normally focuses on the hip and lumbar spine.
Peripheral DXA devices can measure areas such as the wrist, heel or finger, but central DXA remains the standard approach for diagnosing osteoporosis.
The ISCD recommends the hip and PA spine as the principal sites for adult BMD measurement, with the 33% radius used for diagnosis in certain circumstances.
DEXA vs a Normal X-Ray: What's the Difference?
A conventional X-ray primarily produces an anatomical image.
A DEXA scan is specifically designed to quantify bone mineral density using dual-energy X-ray measurements.
A normal X-ray may show a fracture or other structural abnormality, but it isn't the standard test for quantitatively measuring BMD.
That's why someone can have an ordinary X-ray without it providing the same information as a DEXA scan.
DEXA vs Quantitative Ultrasound
Quantitative ultrasound, or QUS, can be used to evaluate bone, particularly at peripheral sites such as the heel.
But QUS does not directly measure BMD in the same way that DXA does.
NIAMS notes that if QUS suggests bone loss, a DXA test may still be needed to diagnose osteoporosis.
What Is the Main Purpose of a DEXA Scan?
DEXA can be used to:
- measure BMD
- detect low bone density
- help diagnose osteoporosis
- estimate fracture risk
- monitor BMD over time
- assess response to treatment
The precise reason for ordering the scan depends on the person's risk factors and clinical situation.
Read Here: What Do T-Scores Mean for Osteoporosis Risk?
Frequently Asked Questions
How does a DEXA scan measure bone density?
A DEXA machine sends two low-dose X-ray energies through the area being examined. Because bone and soft tissue absorb the two energies differently, the scanner can estimate the amount of bone mineral and calculate bone mineral density.
What does DEXA actually measure?
DEXA primarily measures bone mineral density (BMD), usually at the hip and lumbar spine.
What does a DEXA T-score mean?
A T-score compares BMD with a healthy young-adult reference population. In appropriate adult populations, −1.0 or higher is generally considered normal, between −1.0 and −2.5 indicates low bone density, and −2.5 or below meets the BMD threshold for osteoporosis classification.
What does a DEXA Z-score mean?
A Z-score compares BMD with an age-appropriate reference population. It is particularly important for premenopausal women, men younger than 50 and children.
Is DEXA the same as DXA?
Yes. DEXA and DXA refer to the same dual-energy X-ray absorptiometry technique.
Does a DEXA scan measure bone strength?
Not directly. It measures BMD, which is an important component of bone health and fracture-risk assessment, but fracture risk also depends on other clinical and skeletal factors.
Is a DEXA scan painful?
No. It is generally a quick, painless and noninvasive test.
Does a DEXA scan use radiation?
Yes, but the radiation dose is very small. RadiologyInfo reports that it is less than one-tenth the dose of a standard chest X-ray.
Why are the hip and spine scanned?
They are standard skeletal sites for central DXA because they are important locations for assessing bone mineral density and fracture risk. ISCD recommends measuring the PA spine and hip for adult BMD testing.
Can DEXA show whether bone density has improved?
Yes, serial DXA testing can be used to monitor changes in BMD, but a small numerical difference isn't necessarily a real biological change. Clinicians use the facility's precision and least significant change when interpreting follow-up results.
Why is the same DEXA machine useful for follow-up scans?
Different DXA systems may produce results that aren't directly comparable unless appropriate cross-calibration has been performed. ISCD therefore recommends using appropriately comparable measurements when assessing change over time.
Final Thoughts
A DEXA scan measures bone mineral density by using two low-dose X-ray energies to distinguish bone mineral from surrounding soft tissue.
The machine converts those measurements into a BMD value and, where appropriate, T-scores or Z-scores.
The important takeaway is that a DEXA result is more than one number. The measured BMD, the relevant reference score, the skeletal site, scan quality and a person's clinical risk factors all matter when interpreting the result.
If you receive a DEXA report, discussing the numbers with your healthcare professional is the safest way to understand what they mean for your individual situation.
Editorial note: This article is intended for health education, not individual diagnosis. DEXA interpretation depends on the person's age, sex, menopausal status, clinical history, skeletal site and the quality/comparability of the scan.