Bone Densitometry (DEXA): Measuring Bone Strength
Bone densitometry using dual-energy X-ray absorptiometry (DEXA) is the established clinical standard for quantifying bone mineral density (BMD) and diagnosing conditions such as osteoporosis and osteopenia. The examination uses a low-dose X-ray technique to produce precise measurements that guide fracture risk assessment and treatment decisions across millions of patients annually in the United States. Understanding how DEXA works, when it is ordered, and how its results are interpreted is essential for contextualizing the broader landscape of medical imaging that supports bone health management.
Definition and Scope
Dual-energy X-ray absorptiometry measures the absorption of two different X-ray energy levels as they pass through bone and soft tissue. The differential attenuation between the two beams isolates bone mineral content from surrounding tissue, yielding a BMD value expressed in grams per square centimeter (g/cm²).
The National Osteoporosis Foundation (NOF) and the International Society for Clinical Densitometry (ISCD) both recognize DEXA as the reference standard for BMD measurement. The ISCD publishes official positions on DEXA acquisition and interpretation that clinical facilities are expected to follow, including site-specific protocols for the lumbar spine, proximal femur (hip), and distal forearm.
DEXA falls under ionizing radiation regulation in the United States. The Food and Drug Administration (FDA) classifies DEXA systems as Class II medical devices under 21 CFR Part 892, and facilities operating DEXA units must comply with applicable state radiation control regulations as well as the Mammography Quality Standards Act framework where state programs extend oversight to general X-ray equipment. The regulatory context for radiology establishes the broader federal and state framework within which DEXA facilities operate.
How It Works
A DEXA scanner directs two X-ray beams — one at approximately 70 kVp and one at approximately 140 kVp — through the region of interest. Detectors measure the transmitted energy. Because bone and soft tissue absorb the two energy levels at predictably different ratios, software algorithms calculate BMD by subtracting the soft tissue contribution.
The acquisition process for a standard lumbar spine and hip DEXA involves the following discrete phases:
- Patient positioning — The patient lies supine on the scan table; the lumbar spine is typically imaged with legs elevated on a padded block to flatten the lumbar lordosis, while the hip is imaged with the foot internally rotated approximately 15–25 degrees.
- Scout scan acquisition — A rapid low-dose scout image confirms patient position and identifies the region of interest.
- Density scan acquisition — The detector arm traverses the region, collecting attenuation data across the field.
- BMD calculation — Software generates areal BMD values for predefined sub-regions (e.g., L1–L4 vertebrae individually and combined; femoral neck, total hip, trochanter).
- T-score and Z-score computation — The system compares the patient's BMD to reference databases. The T-score compares BMD to a young adult sex-matched reference mean; the Z-score compares to an age- and sex-matched reference mean.
Effective radiation dose from a standard DEXA examination is extremely low — typically 1–10 microsieverts (µSv) per scan, compared to approximately 3,000 µSv from a standard chest CT — as reported by the ISCD Official Positions.
Common Scenarios
DEXA is ordered across a defined range of clinical circumstances. The U.S. Preventive Services Task Force (USPSTF) recommends BMD screening for women age 65 and older, and for younger postmenopausal women whose 10-year fracture risk equals or exceeds that of a 65-year-old white woman of average risk. The USPSTF notes insufficient evidence to recommend routine screening in men.
Beyond population screening, clinical indications include:
- Patients initiating or on long-term glucocorticoid therapy — The American College of Rheumatology (ACR) recommends DEXA for patients expected to take ≥7.5 mg/day of prednisone (or equivalent) for ≥3 months.
- Monitoring of osteoporosis treatment response — Serial DEXA at minimum 1–2 year intervals tracks therapeutic effect of bisphosphonates, denosumab, or anabolic agents.
- Secondary osteoporosis evaluation — Conditions including hyperparathyroidism, malabsorption syndromes, and hypogonadism warrant BMD assessment regardless of age.
- Vertebral fracture assessment (VFA) — Many modern DEXA units can perform lateral spine imaging to identify vertebral deformities, extending the diagnostic scope beyond BMD alone.
- Body composition analysis — DEXA provides fat mass, lean mass, and regional body composition data used in research and select clinical contexts, such as sarcopenia evaluation.
Decision Boundaries
DEXA results are interpreted using WHO diagnostic criteria (WHO Technical Report 843, 1994), which define thresholds based on T-scores:
| T-score | WHO Classification |
|---|---|
| ≥ −1.0 | Normal bone density |
| −1.0 to −2.5 | Osteopenia (low bone mass) |
| ≤ −2.5 | Osteoporosis |
| ≤ −2.5 with fragility fracture | Severe (established) osteoporosis |
The T-score alone does not determine treatment. The FRAX tool, developed at the University of Sheffield and endorsed by the NOF and WHO Collaborating Centre for Metabolic Bone Diseases, integrates BMD with clinical risk factors to calculate 10-year probabilities of major osteoporotic fracture and hip fracture specifically.
DEXA is distinct from quantitative computed tomography (QCT), which measures true volumetric BMD (mg/cm³) rather than areal BMD, and captures trabecular and cortical compartments separately. QCT carries a substantially higher radiation dose — typically 200–400 µSv per lumbar spine examination — and is used in specific research or clinical scenarios where compartmental analysis is required. Peripheral DEXA (pDEXA) devices measure the wrist, heel, or finger and are used for screening but are not interchangeable with central DEXA for diagnostic classification per ISCD guidance.
The ISCD specifies that BMD results should only be compared across time when obtained on the same scanner model, and that a least significant change (LSC) threshold — calculated from the precision of the specific scanner and technologist — must be exceeded before a change in BMD is considered statistically meaningful at the 95% confidence level.
For patients and clinicians navigating questions about imaging for joint pain and musculoskeletal conditions, bone densitometry represents a distinct, non-structural modality focused on quantitative tissue composition rather than anatomical visualization.
References
- International Society for Clinical Densitometry (ISCD) — Official Positions
- National Osteoporosis Foundation (NOF)
- U.S. Preventive Services Task Force — Osteoporosis Screening Recommendation
- FDA — 21 CFR Part 892 (Radiology Devices)
- WHO Technical Report Series 843 — Assessment of Fracture Risk and Its Application to Screening for Postmenopausal Osteoporosis (1994)
- FRAX — Fracture Risk Assessment Tool, University of Sheffield / WHO Collaborating Centre
- American College of Rheumatology (ACR) — Glucocorticoid-Induced Osteoporosis Guidelines
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