DEXA Body Composition Analysis & DXA Clinical Applications 2 — Questions and Answers
Question 1: What is the primary clinical significance of a high fat mass index (FMI) combined with a low lean mass index (LMI) on DXA body composition analysis?
- It indicates primary hyperparathyroidism
- It suggests sarcopenic obesity, a condition associated with increased metabolic and functional risk (Correct answer)
- It confirms a diagnosis of anorexia nervosa
- It is a normal finding in elite endurance athletes
Correct answer: It suggests sarcopenic obesity, a condition associated with increased metabolic and functional risk
Sarcopenic obesity is defined by simultaneous excess fat mass and deficient lean mass, and DXA is a key tool for identifying this high-risk phenotype.
Question 2: On a DXA whole-body scan printout, the 'gynoid' region of interest (ROI) encompasses primarily the:
- Lumbar spine and pelvis
- Hips, buttocks, and upper thighs (Correct answer)
- Upper arms and shoulders
- Lower legs and ankles
Correct answer: Hips, buttocks, and upper thighs
The gynoid ROI covers the hips, buttocks, and upper thighs, capturing fat distribution characteristic of the pear-shaped body pattern.
Question 3: Which DXA-derived measurement is most useful for monitoring response to resistance training in a clinical exercise program?
- Total bone mineral density (BMD)
- Appendicular lean mass (ALM) (Correct answer)
- Visceral adipose tissue (VAT) volume
- Fat mass index (FMI)
Correct answer: Appendicular lean mass (ALM)
Appendicular lean mass (arms + legs) is the most responsive DXA metric to resistance training adaptations because skeletal muscle in the limbs increases measurably with progressive overload.
Question 4: When comparing DXA body composition results across serial scans, the technologist should ensure:
- The patient fasts for 24 hours before each scan to standardize hydration
- The same DXA system and software version are used for each scan (Correct answer)
- The patient wears the same clothing for all follow-up scans
- Scans are always performed in the morning to control circadian variation
Correct answer: The same DXA system and software version are used for each scan
Using the same DXA machine and software version is critical for longitudinal comparability because cross-calibration between different systems introduces systematic errors.
Question 5: The precision error (CV%) for DXA whole-body fat mass measurement in a well-controlled clinical setting is typically:
- Less than 1%
- 1–3% (Correct answer)
- 5–10%
- Greater than 15%
Correct answer: 1–3%
Under standardized conditions, DXA whole-body fat mass has a coefficient of variation (CV%) of approximately 1–3%, making it suitable for longitudinal monitoring.
Question 6: A patient with severe obesity (BMI > 40) presents for a DXA body composition scan. The main technical limitation to address is:
- The scan will overestimate bone mineral density due to beam hardening
- The patient may exceed the table weight limit and scan field width, causing tissue truncation (Correct answer)
- The radiation dose will be too high for the patient to receive the scan
- The DXA software cannot generate a body composition report above a BMI of 35
Correct answer: The patient may exceed the table weight limit and scan field width, causing tissue truncation
Obese patients may exceed the weight limit of the DXA table (commonly 350–450 lbs) and may be wider than the scan field, leading to truncation artifacts that invalidate body composition results.
Question 7: In DXA body composition analysis, the 'R-value' (or tissue ratio) is used to:
- Calculate the patient's 10-year fracture risk
- Differentiate lean soft tissue from fat based on differential X-ray attenuation at two energy levels (Correct answer)
- Measure the ratio of android to gynoid fat
- Estimate visceral fat from subcutaneous fat thickness
Correct answer: Differentiate lean soft tissue from fat based on differential X-ray attenuation at two energy levels
The R-value is the ratio of high-energy to low-energy attenuation for soft tissue pixels, and it distinguishes lean from fat tissue because these tissues have different attenuation ratios.
DEXA Body Composition Analysis & DXA Clinical Applications 2
What is the primary clinical significance of a high fat mass index (FMI) combined with a low lean mass index (LMI) on DXA body composition analysis?