12.1 Recognition, Impact, and Management of DXA Artifacts

12.1 Recognition, Impact, and Management of DXA Artifacts

Quick Answer: Dual-energy X-ray absorptiometry (DXA) artifacts are categorized as internal (pathological/structural) or external (foreign objects/ingested substances). Internal artifacts—such as degenerative osteophytes, facet sclerosis, abdominal aortic calcifications, and healing compression fractures—falsely elevate measured areal BMD, masking osteoporosis. External artifacts—such as metal clothing fasteners, oral barium sulfate, undissolved calcium pills, and radiopharmaceuticals—severely distort photon attenuation. Technologists must visually inspect raw scan images, apply ISCD vertebral exclusion rules (excluding levels with artifacts or >1.0 T-score discrepancy, requiring at least two evaluable vertebrae, contiguous or not), and transition to the proximal femur or 33% radius when artifacts invalidate central sites.

Densitometric Principles of Artifact Formation

Dual-energy X-ray absorptiometry quantifies areal bone mineral density (g/cm²) by measuring the differential attenuation of high- and low-energy X-ray photons through bone mineral (hydroxyapatite) and soft tissue:

$\text{BMD} = \frac{\text{Bone Mineral Content (BMC, g)}}{\text{Projected Bone Area (Area, cm}^2)}$

Because central DXA generates a 2D planar projection of 3D structures, any attenuating substance situated within the projection beam path—whether anterior, posterior, or internal to bone—is integrated into the total attenuation measurement. Consequently, non-skeletal calcifications, structural bone spurs, or dense foreign substances produce significant artifacts that distort calculated bone mineral density.

Internal Artifacts: Degenerative, Vascular, and Structural Pathology

Internal artifacts originate within patient anatomy. Most internal artifacts add radiopaque density to the beam path, artificially elevating measured BMD and generating falsely elevated T-scores that mask underlying osteoporosis:

Degenerative Spine Disease & Osteophytes

• Pathological Changes: Marginal osteophytes, facet joint osteoarthritis (hypertrophy and sclerosis of posterior articular processes), intervertebral disc space narrowing, and subchondral endplate sclerosis.
• Impact on BMD: PA lumbar spine acquisitions project posterior facet joints and marginal osteophytes directly over the vertebral bodies. This superimposed calcium falsely inflates bone mineral content (BMC) and areal BMD, often elevating measured density by 10% to 30%. Advancing osteoarthritis causes lumbar BMD to paradoxically increase with age, obscuring progressive trabecular bone loss.
• Systemic Arthropathies: Diffuse idiopathic skeletal hyperostosis (DISH, flowing anterolateral ossification across ≥ 4 contiguous vertebrae) and ankylosing spondylitis (bilateral syndesmophytes forming a rigid "bamboo spine") produce marked artificial BMD elevations.

1. Atherosclerotic Vascular Calcifications:
   • Abdominal Aorta Calcification: Atherosclerotic plaques deposit calcium along the abdominal aorta directly anterior to L1–L4. On a 2D projection, calcified aortic walls overlap the lumbar vertebrae, adding false mineral content to the spine ROI.
2. Surgical Hardware and Bone Cement:
   • Spinal Instrumentation: Metallic pedicle screws, rods, and fusion cages exhibit extreme photon attenuation. The scanner software registers metal as hyper-dense bone, generating non-physiological BMD elevations.
   • Vertebroplasty and Kyphoplasty: Injection of polymethylmethacrylate (PMMA) bone cement creates an intensely radiopaque focal mass. The treated vertebral level must be excluded from diagnostic analysis.
   • Hip Arthroplasty: Prosthetic stems, cups, and cerclage wires invalidate proximal femur densitometry on the operated side.
3. Vertebral Compression Fractures:
   • A fractured vertebra collapses in height (>20% loss), compacting trabeculae into a reduced projected area. Healing fracture callus adds further mineralization, causing the fractured level to display a paradoxically higher BMD and T-score than adjacent uninjured vertebrae.
4. Non-Skeletal Calcified Entities:
   • Gallstones (cholelithiasis), renal stones (nephrolithiasis), calcified mesenteric lymph nodes, and calcified uterine fibroids project over the spine or pelvis, causing focal false density spikes.

External Artifacts: Clothing, Ingested Agents, and Contrast Media

External artifacts originate outside biological tissues or within the gastrointestinal tract and can be prevented through rigorous screening:

1. Clothing Fasteners and Personal Items:
   • Zippers, underwires, snaps, metal buttons, belt buckles, coins, jewelry, and piercings directly attenuate X-rays. Over bone, they falsely elevate BMD; over soft tissue baselines, they distort soft-tissue reference values. Patients should change into a metal-free gown.
2. Ingested Radiopaque Contrast Substances:
   • Oral Barium Sulfate: High-density barium suspensions linger in bowel loops. Superimposed over the spine or pelvis, barium produces extreme X-ray absorption, resulting in vast artifactual BMD spikes.
     • Clearance Interval: Central DXA must be postponed 10 to 14 days (1 to 2 weeks) following barium administration to permit complete intestinal clearance. Technologists must verify bowel clearance on raw scout images.
   • Water-Soluble Iodinated CT/IV Contrast: Excreted renally within 24 to 48 hours. Central DXA should be delayed 48 to 72 hours after contrast examinations.
   • Nuclear Medicine Radiopharmaceuticals: Diagnostic radionuclides (e.g., Tc-99m MDP, F-18 FDG) emit gamma or positron radiation that floods DXA detectors, causing count overflow. Central DXA must be postponed 48 to 72 hours to allow radioactive decay and excretion.
   • Undissolved Calcium Supplements: Ingesting calcium tablets shortly before scanning leaves dense, radiopaque pills in bowel loops overlying the lumbar spine. Patients must withhold oral calcium supplements for 24 hours prior to central DXA.

Troubleshooting Protocols and Vertebral Exclusion Criteria

Quality densitometry requires systematic quality control by the technologist:

• Raw Image Scrutiny: Technologists must inspect raw grayscale images to verify accurate bone edge detection, correct L1–L4 intervertebral disc labeling, neutral patient positioning, and absence of foreign materials or soft-tissue artifacts.
• ISCD Vertebral Exclusion Rules:
  • Vertebrae affected by local structural artifacts (severe osteophytes, fracture, aortic calcification, hardware, or PMMA cement) must be systematically excluded from the diagnostic L1–L4 mean.
  • T-Score Discrepancy Rule: If an individual vertebra exhibits a T-score differing by >1.0 standard deviation from adjacent levels, and visual inspection identifies focal artifact, that level must be excluded.
  • Minimum of Two Evaluable Vertebrae: A valid diagnostic lumbar spine evaluation requires at least two evaluable vertebrae; diagnostic classification must never rest on a single vertebra. Contiguous levels are preferable, but ISCD does not impose contiguity as a requirement, so non-adjacent combinations such as L1 and L3 remain usable when the intervening level is excluded. If fewer than two evaluable levels remain, the spine is non-diagnostic.
• Alternative Site Strategy:
  • If the lumbar spine cannot be interpreted due to multi-level artifacts, the technologist evaluates the proximal femur.
  • If bilateral hip replacements or severe bilateral deformities preclude hip densitometry, the technologist must immediately initiate a 33% (one-third) radius forearm scan.