DEXA DEXA Quality Assurance & Equipment Calibration 2 — Questions and Answers
Question 1: Which measurement is used to evaluate in-vitro precision of a DEXA scanner?
- Root mean square coefficient of variation (RMS-CV%) (Correct answer)
- Pearson correlation coefficient
- T-score standard deviation
- Hounsfield unit range
Correct answer: Root mean square coefficient of variation (RMS-CV%)
RMS-CV% quantifies the percentage variability of repeated phantom or patient measurements and reflects scanner precision.
Question 2: What is the recommended minimum number of repeat scans used to calculate an in-vivo precision assessment for a DEXA technologist?
- 30 patients scanned twice or 15 patients scanned three times (Correct answer)
- 10 patients scanned once
- 5 patients scanned five times each
- 50 patients scanned once and averaged
Correct answer: 30 patients scanned twice or 15 patients scanned three times
ISCD guidelines recommend 30×2 or 15×3 repeat scans on real patients to calculate a clinically valid precision error.
Question 3: A DEXA technologist calculates an in-vivo spine precision of 1.8% RMS-CV. What is the least significant change (LSC) at 95% confidence?
- ~5.0% (Correct answer)
- ~1.8%
- ~3.6%
- ~9.0%
Correct answer: ~5.0%
LSC at 95% confidence = 2.77 × RMS-CV; 2.77 × 1.8% ≈ 5.0%, meaning a change must exceed ~5% to be considered real.
Question 4: Which factor most directly degrades technologist repositioning precision in spine DEXA?
- Inconsistent patient positioning and ROI placement between visits (Correct answer)
- Scanner brand differences
- Time of day the scan is performed
- Patient age
Correct answer: Inconsistent patient positioning and ROI placement between visits
Repositioning errors from inconsistent patient setup and ROI placement are the leading cause of poor in-vivo precision.
Question 5: A DEXA facility is switching from one manufacturer's system to another. What must be done before comparing serial BMD results?
- Perform cross-calibration using ISCD-recommended methods and ideally rescan patients on both systems (Correct answer)
- Simply apply a manufacturer conversion factor from a chart
- Use only T-scores as they are universally standardized
- No adjustment is needed if both machines are FDA-cleared
Correct answer: Perform cross-calibration using ISCD-recommended methods and ideally rescan patients on both systems
Different DEXA systems are not interchangeable; cross-calibration studies are required to establish BMD conversion equations.
Question 6: What is the typical acceptable long-term coefficient of variation (CV) for a well-maintained DEXA phantom?
- Less than 0.5–1.0% (Correct answer)
- Less than 5%
- Less than 10%
- Less than 2.5%
Correct answer: Less than 0.5–1.0%
A well-maintained DEXA system should achieve long-term phantom CV of less than 0.5–1.0%, indicating stable and reproducible performance.
DEXA DEXA Quality Assurance & Equipment Calibration 2
Which measurement is used to evaluate in-vitro precision of a DEXA scanner?