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ISCD CBDT: Bone Densitometry Technology

ISCD CBDT · Work through your exam, one topic at a time.

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Bone Densitometry Technology

What the exam tests

Bone Densitometry Technology covers the instrument, the acquisition, and the analysis - the parts of a DXA study the technologist owns. Expect questions on dual-energy physics, region-of-interest placement, reference databases, precision assessment, and quality control.

Core rules and concepts

  • DXA fires two X-ray energies (roughly 70 and 140 kVp equivalents, or a K-edge filter) and uses differential attenuation to subtract soft tissue, yielding areal BMD in g/cm².
  • Areal BMD is a 2D projection. Thicker bone and larger body habitus raise BMD independent of true density, so results are only meaningful against a matched reference database.
  • Standard sites: PA lumbar spine L1–L4 (minimum two evaluable vertebrae), total hip, femoral neck, and 33% radius when spine and hip are unusable.
  • T-score compares a patient to the young-adult mean of the same sex and ethnicity: (patient BMD − young-adult mean) / young-adult SD. Z-scores use age-matched means.
  • T-scores ≤ −2.5 define osteoporosis; −1.0 to −2.5 is low bone mass; ≥ −1.0 is normal. A Z-score ≤ −2.0 is "below the expected range for age."

Worked example

A 66-year-old woman's L1–L4 mean is 0.850 g/cm². The young-adult reference mean is 1.047 g/cm² with SD 0.110. T = (0.850 − 1.047) / 0.110 = −1.79. That is low bone mass - not osteoporosis - because −1.79 lies between −1.0 and −2.5.

Traps that cost marks

  • Reporting a T-score from a single vertebra, or from L1–L4 without excluding a vertebra whose T-score differs from an adjacent evaluable vertebra by more than 1.0 unit (fracture, focal sclerosis, vertebroplasty, laminectomy).
  • Using spine phantoms, or the wrong manufacturer's reference database, for the hip.
  • Quoting T-scores in premenopausal women, men under 50, or children, where Z-scores are correct.
  • Comparing serial scans from different machines, or before the least significant change (LSC) is exceeded.

Quality control

Daily: scan the spine phantom, plot the mean BMD on a control chart, investigate drift beyond about 1.5% of the baseline mean. Precision: in-vivo precision from repeat scans in 15 patients (or 30 duplicate scans), RMS-SD, then LSC = 2.77 × precision error.