DEXA scan (Dual-Energy X-Ray Absorptiometry) is the most widely used and clinically validated method for measuring bone mineral density (BMD) and diagnosing osteoporosis. This article provides a comprehensive patient and clinician guide to how DEXA technology works, what T-scores and Z-scores mean, how to prepare for the examination, and how results guide fracture prevention strategies.

Osteoporosis affects approximately 200 million people worldwide, with postmenopausal women accounting for the majority of hip and vertebral compression fractures.^[1] Early detection through DEXA screening allows for timely pharmacologic and lifestyle interventions that can reduce fracture risk by up to 50% over 3 years.^[2] For radiographers, radiologists, and hospital administrators, understanding DEXA protocol standards ensures accurate, reproducible measurements that directly impact patient care pathways.

Clinical background and pathophysiology

Bone is a dynamic, living tissue that undergoes continuous remodeling through the coupled actions of osteoclasts (bone-resorbing cells) and osteoblasts (bone-forming cells). Peak bone mass is typically achieved by age 30, after which gradual bone loss occurs. In osteoporosis, the rate of bone resorption exceeds formation, leading to progressive microarchitectural deterioration and increased fracture susceptibility.

Relevant anatomy and measurement sites

DEXA scanning focuses on two primary anatomical regions because they are the most predictive of future fracture risk:

• Lumbar spine (L1-L4): Primarily trabecular bone, highly metabolically active. Early site of bone loss in postmenopausal women and patients on glucocorticoids.
• Proximal femur (hip): Includes the femoral neck and total hip regions. The femoral neck is the single best predictor of hip fracture risk.
• Forearm (radius/ulna): Measured when hip or spine cannot be evaluated, or in patients with hyperparathyroidism where cortical bone loss predominates.

Epidemiology and risk factors

Osteoporotic fractures occur in 1 in 3 women and 1 in 5 men over age 50 worldwide.^[3] Key risk factors include:

• Advanced age (especially >65 years for women, >70 for men)
• Postmenopausal estrogen deficiency or early menopause (<45 years)
• Family history of hip fracture
• Low body weight (BMI <18.5 kg/m²)
• Chronic glucocorticoid use (>3 months at prednisone-equivalent ≥5 mg/day)
• Smoking, excessive alcohol intake, and sedentary lifestyle
• Secondary causes: hyperthyroidism, celiac disease, rheumatoid arthritis, chronic kidney disease

Imaging protocol and technique

DEXA technology passes two X-ray beams of different energy levels through the body. Soft tissue absorbs one beam preferentially, while bone absorbs the other. By subtracting soft tissue attenuation, the system calculates bone mineral content (BMC) in grams and bone mineral density (BMD) in grams per square centimeter (g/cm²). Modern fan-beam systems complete a total body or regional scan in under 5 minutes.

Patient preparation and positioning

Proper preparation ensures accurate, reproducible results. Follow these standardized steps:

1. Medication and supplement hold: Stop calcium supplements, vitamin D megadoses, and antacids containing calcium at least 24 hours before scanning. Continue all other medications unless directed otherwise.
2. Attire: Wear comfortable clothing without metal zippers, buttons, snaps, or underwire bras. Remove jewelry, wallets, keys, and mobile phones.
3. Recent contrast studies: If the patient had a barium study or CT with oral contrast within the past 7 days, reschedule the DEXA to avoid artifact.
4. Pregnancy screening: Although radiation exposure is negligible (~1-10 µSv), confirm the patient is not pregnant per institutional ALARA policy.
5. Height and weight measurement: Accurate anthropometric data are essential for correct machine calibration and body composition analysis.

Acquisition parameters and quality control

DEXA systems require rigorous daily quality assurance (QA) using a phantom supplied by the manufacturer. Key technical parameters include:

• Radiation dose: Lumbar spine and hip DEXA = 1-10 µSv (less than a cross-country flight)
• Scan modes: Standard (15-30 seconds), fast (5-10 seconds for body composition), and high-resolution research modes
• Precision error: Should be <1.5% at the spine and <2.0% at the hip for longitudinal monitoring
• Least significant change (LSC): Typically 0.025-0.035 g/cm² depending on skeletal site and manufacturer

Image interpretation and diagnostic criteria

DEXA results are reported using two standardized scoring systems: the T-score (compares BMD to a healthy young adult reference population) and the Z-score (compares BMD to an age-, sex-, and ethnicity-matched population). Understanding these values is essential for accurate diagnosis and treatment decisions.

Normal imaging appearance

On DEXA images, bone appears as high-density white regions against a gray soft tissue background. The software automatically identifies the boundaries of the femoral neck, trochanteric region, Ward's triangle, and lumbar vertebral bodies (L1-L4). Normal bone demonstrates homogeneous density without focal lucencies, cortical thinning, or endplate deformities.

T-score classification (WHO criteria)

The T-score is the primary diagnostic criterion for postmenopausal women and men aged 50 and older:

• Normal: T-score ≥ -1.0 (within 1 standard deviation of young adult mean)
• Osteopenia (low bone mass): T-score between -1.0 and -2.5
• Osteoporosis: T-score ≤ -2.5
• Severe osteoporosis: T-score ≤ -2.5 plus one or more fragility fractures

Z-score interpretation

The Z-score is used for premenopausal women, men under 50, and children. A Z-score of -2.0 or lower is considered "below the expected range for age" and warrants evaluation for secondary causes of bone loss, such as celiac disease, hyperparathyroidism, or anorexia nervosa.

FRAX 10-year fracture probability

The WHO FRAX tool integrates DEXA-derived femoral neck BMD with clinical risk factors (age, sex, BMI, smoking, alcohol, family history, glucocorticoid use, rheumatoid arthritis, secondary osteoporosis) to calculate 10-year probability of major osteoporotic fracture and hip fracture. Treatment thresholds vary by country; in the USA, the National Osteoporosis Foundation recommends pharmacologic therapy when 10-year major fracture probability is ≥ 20% or hip fracture probability is ≥ 3%.^[4]

Common pitfalls and artefacts

Accurate DEXA interpretation requires awareness of technical and physiological factors that can falsely elevate or depress BMD measurements. These pitfalls are a frequent source of diagnostic error and inappropriate treatment decisions.

Technical artefacts

Common technical issues and their mitigation strategies:

• Undissolved calcium tablets: Appear as focal hyperdense regions over the lumbar spine, artificially elevating BMD by 0.03-0.10 g/cm². Mitigation: 24-hour calcium hold and patient interview.
• Metal implants: Hip replacements, spinal fusion hardware, and surgical clips create beam-hardening artifacts. Use contralateral hip or forearm as alternative site.
• Body positioning errors: Hip internal rotation <15 degrees or spine lateral tilt increases measurement variability. Use positioning aids and laser alignment.
• Soft tissue interference: Severe obesity or very low body weight can affect beam attenuation calculations. Ensure accurate weight entry and use appropriate scan mode.

Interpretation traps

Physiological conditions that mimic or mask true bone loss:

• Degenerative disc disease and osteophytes: Common in patients over 60, these can falsely elevate lumbar spine BMD by 0.05-0.15 g/cm². Rely on hip T-score or exclude affected vertebrae.
• Aortic calcification: Overlies the L1-L4 region and artifactually increases spine BMD. Review the bone density image, not just the numerical report.
• Vertebral compression fractures: Collapsed vertebrae appear denser due to reduced height. Exclude fractured vertebrae from analysis per ISCD guidelines.
• Ankylosing spondylitis and diffuse idiopathic skeletal hyperostosis (DISH): Syndesmophytes and flowing ossification falsely elevate BMD. Use hip or forearm for diagnosis.

Management implications

DEXA results directly guide clinical management pathways, from lifestyle counseling to pharmacologic intervention. Radiologists and referring clinicians must communicate findings clearly, including T-scores, Z-scores, fracture risk assessment, and specific recommendations.

Interventional planning and treatment thresholds

Management decisions integrate DEXA BMD with FRAX probability and clinical context:

• Normal BMD (T-score ≥ -1.0): Reassurance, lifestyle counseling (calcium 1,000-1,200 mg/day, vitamin D 800-1,000 IU/day, weight-bearing exercise), and rescreening in 10-15 years for low-risk patients.
• Osteopenia (T-score -1.0 to -2.5): Lifestyle optimization plus FRAX assessment. Treat if 10-year major fracture risk ≥ 20% or hip fracture risk ≥ 3%. Consider bisphosphonates, denosumab, or selective estrogen receptor modulators (SERMs).
• Osteoporosis (T-score ≤ -2.5): Initiate pharmacologic therapy. First-line options include oral bisphosphonates (alendronate, risedronate) or IV zoledronic acid. Monitor with repeat DEXA in 1-2 years.
• Glucocorticoid-induced osteoporosis: Initiate therapy at T-score ≤ -1.0 if prednisone-equivalent ≥ 7.5 mg/day for ≥ 3 months, regardless of baseline BMD.

Follow-up and surveillance intervals

Repeat DEXA timing depends on baseline status, treatment initiation, and clinical risk factors:

• On treatment: Repeat DEXA every 1-2 years until stability is confirmed, then every 2 years.
• Untreated osteopenia: Repeat every 2-5 years depending on FRAX risk.
• Significant interval change: A decrease ≥ the Least Significant Change (LSC) suggests treatment failure, nonadherence, or secondary cause progression.
• Post-fracture: Obtain baseline DEXA after a fragility fracture to guide treatment, even if prior scans were normal.

Conclusion

The DEXA scan remains the gold standard for bone mineral density assessment, providing rapid, low-radiation, highly reproducible measurements that form the foundation of osteoporosis diagnosis and fracture risk stratification. By understanding T-score and Z-score interpretation, recognizing common artifacts, and following standardized acquisition protocols, radiology departments can deliver consistent, clinically actionable results that directly improve patient outcomes.

For hospital administrators, investing in modern fan-beam DEXA systems with VFA capability expands service offerings while maintaining cost efficiency. For radiographers, meticulous attention to patient positioning and quality assurance ensures the precision required for longitudinal monitoring. As populations age globally, the demand for bone health screening will continue to grow—making DEXA expertise an essential component of any comprehensive imaging department.