Questions and Answers
What is the inverse square law?
The intensity of the x-ray beam varies inversely with the square of the source to film distance.
What will be influenced if the source-to-film distance (SFD) is changed?
The density of the radiographs.
What are the two rules of thumb to maintain the original density when a change in SFD occurs?
- If SFD is doubled, the exposure time should be multiplied by 4. 2. If SFD is halved, exposure is divided by 4.
If an 8 inch PID with 15 mA, 80 kVp, and 0.01 impulses is changed to a 16 inch PID with the same mA and kVp, what would the impulses be?
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If a 16 inch PID with 15 mA, 90 kVp, and 20 impulses is changed to an 8 inch PID, what would the impulses be?
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What does mA control?
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What is the rule of thumb for changes in mA to maintain density?
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If 5 mA at 0.30 seconds is increased to 10 mA, what is the new time?
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If 10 mA at 0.20 seconds is decreased to 5 mA, what is the new time?
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What does kVp control?
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A change in kVp will alter what?
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What are the rules of thumb for kVp?
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If 60 kVp at 0.50 seconds is increased to 75 kVp, what is the new time?
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If 85 kVp at 0.50 seconds is decreased to 70 kVp, what is the new time?
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A good radiograph was made using a source-to-film distance of 16 inches and an exposure of 20 seconds. If the source-to-film distance is decreased to 8 inches, what would the correct exposure time be?
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A radiograph is exposed using 85 kVp at 0.15 seconds. The kVp is decreased to 70 kVp. What will the new time be?
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If a satisfactory radiograph is produced using a source-to-film distance of 8 inches and an exposure time of 1 second, what would the correct exposure time for a source-to-film distance of 16 inches be?
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A diagnostic film produced using 90 kVp and 0.25 seconds. What exposure time is needed to produce the same film at 75 kVp?
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If a diagnostic radiograph is produced using 5 mA at 0.30 seconds, what exposure time is needed to produce the same film using 15 mA?
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Study Notes
Inverse Square Law
- The intensity of the x-ray beam decreases as the distance from the source increases, specifically varying inversely with the square of the source-to-film distance (SFD).
Effects of Changing SFD
- A change in SFD will directly influence the density of the radiographs produced.
Rules of Thumb for Changing SFD
- Doubling the SFD requires an increase in exposure time by a factor of four.
- Halving the SFD allows for a decrease in exposure time to one-fourth of the original duration.
Impulses Adjustment with SFD Change
- When changing from an 8 inch PID to a 16 inch PID while maintaining the same mA and kVp, the impulses needed will double (e.g., 0.01 becomes 0.04).
- Conversely, changing from a 16 inch PID to an 8 inch PID will require the impulses to be reduced to a quarter (e.g., 20 impulses becomes 5).
Milliamperes (mA) Control
- mA controls the quantity or number of x-rays produced during exposure.
mA Adjustment Rules
- An increase of 5 mA necessitates halving the exposure time to maintain density.
- A decrease of 5 mA requires doubling the exposure time to preserve image quality.
Time Adjustment with mA Changes
- If mA is increased from 5 to 10, the time is halved from 0.30 seconds to 0.15 seconds.
- If mA decreases from 10 to 5, the time doubles from 0.20 seconds to 0.40 seconds.
Kilovoltage Peak (kVp) Control
- kVp influences the quality and penetrating power of x-rays, playing a minor role in the quantity produced.
Density Adjustment with kVp Changes
- A modification in kVp alters both the density and the quantity of x-rays generated.
kVp Adjustment Rules
- Increasing kVp by 15 requires halving the exposure time to maintain density.
- Decreasing kVp by 15 necessitates doubling the exposure time to achieve the same effect.
Time Adjustment with kVp Changes
- If kVp increases from 60 to 75, exposure time is shortened from 0.50 seconds to 0.25 seconds.
- Reducing kVp from 85 to 70 requires time to be adjusted from 0.15 seconds to 0.30 seconds.
SFD and Exposure Time Relationship
- A satisfactory radiograph exposed at 16 inches for 20 seconds will only need 5 seconds when the SFD is reduced to 8 inches.
- For an SFD of 16 inches, if starting from an exposure of 1 second at 8 inches, the time required becomes 4 seconds.
Final Time Adjustment Scenarios
- Reducing kVp from 85 to 70 while maintaining exposure time requires multiplying the initial time (0.15 seconds) by 2, resulting in 0.30 seconds for the new exposure duration.
- Producing a diagnostic radiograph with increased mA to 15 from 5 mA at 0.30 seconds requires dividing the time by 4, yielding 0.075 seconds for proper density maintenance.
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Description
Study the fundamentals of the inverse square law in radiology with these flashcards. Explore key concepts such as the relationship between x-ray intensity and source-to-film distance, as well as practical rules of thumb for maintaining radiographic density. Perfect for radiology students and professionals looking to reinforce their knowledge.