X-Ray Certification Exposure Factors 3 — Questions and Answers
Question 1: A radiographer uses 200 mA at 0.05 s. What is the resulting mAs?
- 4 mAs
- 10 mAs (Correct answer)
- 40 mAs
- 100 mAs
Correct answer: 10 mAs
mAs = mA × time = 200 × 0.05 = 10 mAs.
Question 2: Which combination represents equivalent mAs to 300 mA at 1/10 s?
- 100 mA at 1/10 s
- 150 mA at 1/5 s
- 600 mA at 1/20 s (Correct answer)
- 200 mA at 1/5 s
Correct answer: 600 mA at 1/20 s
300 × 0.1 = 30 mAs; 600 × 0.05 = 30 mAs, confirming reciprocity.
Question 3: Reducing focal spot size from large to small primarily improves which image quality factor?
- Radiographic density
- Recorded detail (spatial resolution) (Correct answer)
- Radiographic contrast
- Dynamic range
Correct answer: Recorded detail (spatial resolution)
A smaller focal spot reduces geometric penumbra, improving spatial resolution and recorded detail.
Question 4: A chest radiograph taken at 120 kVp / 4 mAs is to be repeated at 80 kVp. Using the 15% rule, approximately what mAs is needed?
- 2 mAs
- 8 mAs
- 16 mAs (Correct answer)
- 32 mAs
Correct answer: 16 mAs
Each 15% kVp reduction halves receptor exposure, requiring mAs to double per step; two steps down from 120 to 80 kVp requires 4× mAs = 16 mAs.
Question 5: At a fixed mAs, what is the primary trade-off when selecting a very high mA with a very short time?
- Reduced patient dose
- Increased focal spot blooming at high mA (Correct answer)
- Improved contrast scale
- Reduced penumbra from shorter time
Correct answer: Increased focal spot blooming at high mA
Very high mA causes space charge and thermal effects that cause focal spot blooming, enlarging the effective focal spot and reducing detail.
Question 6: Which statement about automatic exposure control (AEC) is correct?
- AEC adjusts kVp to terminate exposure
- AEC terminates exposure when the detector reaches a preset charge level (Correct answer)
- AEC increases mA automatically when motion is detected
- AEC selects the appropriate grid ratio for each exam
Correct answer: AEC terminates exposure when the detector reaches a preset charge level
AEC uses ionization chambers or solid-state detectors that terminate the exposure when the receptor reaches a preset exposure level.
Question 7: A lateral lumbar spine requires significantly more mAs than an AP lumbar spine primarily because of:
- Increased SID for lateral projections
- Greater part thickness in the lateral plane (Correct answer)
- Lower grid ratio used laterally
- Less anode heel effect laterally
Correct answer: Greater part thickness in the lateral plane
The lateral lumbar spine passes through a much thicker cross-section of tissue, requiring more exposure to penetrate adequately.
X-Ray Certification Exposure Factors 3
A radiographer uses 200 mA at 0.05 s.
What is the resulting mAs?