DEXA Scan Operation & Technique 5 — Questions and Answers
Question 1: Which patient condition requires the technologist to exclude a vertebra from lumbar spine BMD analysis?
- Mild disc space narrowing at L3–L4
- A vertebra with BMD more than 1 SD different from adjacent vertebrae due to artifact or deformity (Correct answer)
- A patient age over 70 years
- A BMD result in the osteopenic range at any level
Correct answer: A vertebra with BMD more than 1 SD different from adjacent vertebrae due to artifact or deformity
A vertebra that differs by more than 1 SD from adjacent vertebrae due to artifact, fracture, or degenerative disease should be excluded from the mean BMD calculation.
Question 2: During a forearm DXA scan, the non-dominant arm is typically scanned. When should the dominant arm be scanned instead?
- When the patient is left-handed
- When the non-dominant arm has a fracture, cast, or cannot be positioned correctly (Correct answer)
- Whenever the patient requests it
- When the non-dominant forearm BMD is within normal limits
Correct answer: When the non-dominant arm has a fracture, cast, or cannot be positioned correctly
The dominant forearm is substituted when the non-dominant arm is unavailable due to injury, cast, surgical hardware, or positioning limitations.
Question 3: How is the 'least significant change' (LSC) used in serial DXA monitoring?
- It defines the T-score threshold for starting medication
- It is the minimum BMD change that exceeds measurement precision error and can be considered real (Correct answer)
- It is the average BMD decline expected per year in osteoporosis
- It represents the maximum allowable difference between left and right hip BMD
Correct answer: It is the minimum BMD change that exceeds measurement precision error and can be considered real
The LSC, calculated from precision data, defines the threshold above which a BMD change is statistically significant and not attributable to measurement error.
Question 4: A patient receiving long-term glucocorticoid therapy is referred for a DXA scan. Which additional scan site should be considered beyond the standard spine and hip?
- Calcaneus
- Distal forearm (radius) (Correct answer)
- Skull
- Tibial shaft
Correct answer: Distal forearm (radius)
ISCD guidelines recommend adding a forearm scan for patients on long-term glucocorticoids because steroid-induced bone loss may be more pronounced at cortical sites like the radius.
Question 5: What is the correct method for measuring the ulnar length during a forearm DXA scan setup?
- From the elbow to the wrist with the arm extended
- From the olecranon to the ulnar styloid with the elbow bent at 90° (Correct answer)
- From the radial head to the radial styloid
- From the medial epicondyle to the ulnar styloid
Correct answer: From the olecranon to the ulnar styloid with the elbow bent at 90°
Ulnar length is measured from the olecranon process to the ulnar styloid with the elbow flexed to 90°, and is used by DXA software to calculate the 33% radius site location.
Question 6: Which action should a DXA technologist take if the daily QC phantom scan result falls outside the control limits (e.g., ±1.5% of the baseline mean)?
- Continue scanning patients and flag the phantom result as an outlier
- Rescan the phantom; if still out of range, do not scan patients and contact the manufacturer or medical physicist (Correct answer)
- Adjust the baseline mean to incorporate the new value
- Scan patients but add a note that QC failed
Correct answer: Rescan the phantom; if still out of range, do not scan patients and contact the manufacturer or medical physicist
A phantom scan outside control limits indicates potential machine malfunction; the phantom should be rescanned and, if still out of range, clinical scanning must stop until the issue is resolved.
Question 7: When analyzing a total hip DXA scan, which sub-regions are combined to calculate the 'total hip' BMD value?
- Femoral neck and greater trochanter only
- Femoral neck, trochanter, and intertrochanteric region (Correct answer)
- Femoral neck and Ward's triangle
- Trochanter and shaft only
Correct answer: Femoral neck, trochanter, and intertrochanteric region
The total hip BMD is calculated from the combined area and BMC of the femoral neck, trochanteric, and intertrochanteric sub-regions.
DEXA Scan Operation & Technique 5
Which patient condition requires the technologist to exclude a vertebra from lumbar spine BMD analysis?