What is a DEXA Scan?
A DEXA (Dual-Energy X-ray Absorptiometry) scan is a specialized two-dimensional X-ray imaging technology that measures bone mineral density (BMD) to diagnose osteoporosis, predict fracture risk, and monitor treatment response. 1
Technical Overview
How DEXA Works:
- DEXA uses two different X-ray energy beams to differentiate between bone and soft tissue, allowing precise calculation of BMD at vulnerable skeletal sites 2
- The scan measures BMD in grams per square centimeter (g/cm²) at the spine, hip, and sometimes forearm 3, 4
- It is a relatively inexpensive, efficient, and low-radiation imaging modality 1
Clinical Applications
Primary Uses:
- Diagnosis of osteoporosis using WHO criteria based on T-scores 1
- Fracture risk prediction as a stand-alone tool or incorporated into risk calculators like FRAX 3, 2
- Treatment monitoring by tracking BMD changes over time 1
- Preoperative assessment for spinal fusion surgery to identify patients at risk for complications 1
Interpretation of Results
T-Score Classification (for postmenopausal women and men ≥50 years): 1
- Normal: T-score ≥ -1.0
- Osteopenia (low bone mass): T-score between -1.0 and -2.5
- Osteoporosis: T-score ≤ -2.5
Z-Score Usage:
- Used for children, adolescents, premenopausal women, and men under 50 years 1
- A Z-score ≤ -2.0 indicates "bone mineral density below the expected range for age" 1
Why DEXA is the Gold Standard
Key Advantages: 1
- Consensus exists for interpreting results using WHO T-score definitions
- Proven ability to predict fracture risk across populations
- Demonstrated effectiveness at targeting antifracture therapies
- Capability to monitor treatment response with acceptable precision
- Widely available worldwide with standardized protocols
Important Limitations
Technical Pitfalls to Recognize: 1, 5
- Degenerative changes (osteophytes, facet arthritis) can falsely elevate lumbar spine BMD measurements
- Scoliosis can lead to inaccurate spine assessments
- Positioning errors and inconsistent patient placement affect precision
- Artifacts from surgical hardware, contrast material, or calcifications can interfere with measurements
When DEXA May Be Less Accurate:
- Patients with significant spinal degeneration may benefit from alternative sites (hip, forearm) or quantitative CT 1
- CT Hounsfield unit (HU) values may be more accurate than DEXA in patients with advanced degenerative disease 1, 6
Quality Control Requirements
Essential Technical Standards: 1
- Daily or thrice-weekly phantom scanning to ensure machine stability
- Each facility must determine its own precision error and calculate the least significant change (LSC)
- Repeat scans should use the same machine, software, positioning, and analysis protocol
- Maximum acceptable LSC is 5.0% for total hip and 5.3% for lumbar spine
Additional Capabilities
Vertebral Fracture Assessment (VFA):
- DEXA machines can perform lateral spine imaging to detect vertebral compression fractures 1
- This adds valuable fracture risk information beyond BMD alone
- Recommended for patients with T-scores < -1.0 and additional risk factors 7
Common Clinical Pitfall: Many clinicians are unaware that DEXA accuracy can be significantly compromised by degenerative spinal changes, leading to underdiagnosis of osteoporosis when relying solely on lumbar spine measurements. Always evaluate hip BMD in patients with spinal degeneration. 1