DEXA Body Composition Analysis & DXA Clinical Applications 1 — Questions and Answers
Question 1: Which three compartments does a standard DXA body composition scan quantify?
- Lean mass, fat mass, and bone mineral content (Correct answer)
- Visceral fat, subcutaneous fat, and muscle mass
- Water, protein, and mineral
- Intracellular fluid, extracellular fluid, and bone
Correct answer: Lean mass, fat mass, and bone mineral content
DXA partitions the body into lean soft tissue mass, fat mass, and bone mineral content (BMC), making it a three-compartment model.
Question 2: In DXA body composition analysis, the term 'android fat distribution' refers to excess fat primarily located in the:
- Hips and thighs
- Arms and lower legs
- Abdominal and trunk region (Correct answer)
- Chest and back
Correct answer: Abdominal and trunk region
Android (apple-shaped) fat distribution is characterized by excess adipose tissue in the abdominal and trunk region, which is associated with higher cardiometabolic risk.
Question 3: What is the Visceral Adipose Tissue (VAT) region automatically estimated by advanced DXA software typically approximated from?
- A biopsy of abdominal fat
- The android region scan data combined with validated algorithms (Correct answer)
- Direct measurement of the peritoneal cavity with ultrasound
- MRI co-registration
Correct answer: The android region scan data combined with validated algorithms
Modern DXA systems estimate VAT using the fat distribution within the android region combined with proprietary validated algorithms, without requiring MRI or CT.
Question 4: Which metric best describes the ratio of appendicular lean mass to body weight squared, commonly used to screen for sarcopenia?
- Fat mass index (FMI)
- Relative skeletal muscle index (RSMI) (Correct answer)
- Body mass index (BMI)
- Visceral fat area (VFA)
Correct answer: Relative skeletal muscle index (RSMI)
The Relative Skeletal Muscle Index (RSMI) is calculated as appendicular lean mass (kg) divided by height squared (m²) and is a primary DXA-based criterion for diagnosing sarcopenia.
Question 5: When performing a whole-body DXA scan for body composition, the patient's arms must be:
- Raised above the head to avoid overlap with the trunk
- Positioned palms down, slightly separated from the body sides (Correct answer)
- Crossed over the chest to reduce scan area
- Held tightly against the thighs
Correct answer: Positioned palms down, slightly separated from the body sides
Arms are placed palms down with a slight gap from the torso so that the software can correctly separate arm from trunk regions during segmentation.
Question 6: A 'total body less head' (TBLH) measurement is frequently reported in pediatric DXA because:
- Children cannot hold their heads still during the scan
- The head contains a disproportionately large amount of bone mass in children, skewing whole-body BMD (Correct answer)
- Head bone density is not affected by growth hormone
- Radiation to the cranium is prohibited in patients under 18
Correct answer: The head contains a disproportionately large amount of bone mass in children, skewing whole-body BMD
In children, the skull contributes disproportionately to total BMD; excluding the head reduces variability and improves the sensitivity of the measurement for assessing skeletal growth and disease.
Question 7: Which of the following clinical conditions is a primary indication for using DXA body composition analysis rather than a standard bone density scan?
- Screening for osteoporosis in a 65-year-old woman
- Monitoring lean mass changes in a patient undergoing cancer cachexia treatment (Correct answer)
- Diagnosing vertebral fracture in a post-menopausal patient
- Measuring bone age in an adolescent athlete
Correct answer: Monitoring lean mass changes in a patient undergoing cancer cachexia treatment
Monitoring lean (muscle) mass changes during cancer cachexia treatment is a body composition application of DXA that goes beyond bone density assessment.
DEXA Body Composition Analysis & DXA Clinical Applications 1
Which three compartments does a standard DXA body composition scan quantify?