Radiology Inverse Square Law Flashcards

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?

Double the impulses; 0.01 x 4 = 0.04 impulses. Signup and view all the answers

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?

5 impulses; 20 / 4 = 5. Signup and view all the answers

What does mA control?

The quantity of number of x-rays produced. Signup and view all the answers

What is the rule of thumb for changes in mA to maintain density?

<ol> <li>An increase of 5 mA will increase the number of x-rays produced, divide the time by 2. 2. A decrease in 5 mA will decrease the number of x-rays, multiply the time by 2.</li> </ol> Signup and view all the answers

If 5 mA at 0.30 seconds is increased to 10 mA, what is the new time?

0.15 seconds; 0.30 / 2 = 0.15. Signup and view all the answers

If 10 mA at 0.20 seconds is decreased to 5 mA, what is the new time?

0.40 seconds; 0.20 x 2 = 0.40. Signup and view all the answers

What does kVp control?

Quality or penetrating power of x-rays; it is a minor factor in controlling quantity of x-rays. Signup and view all the answers

A change in kVp will alter what?

Density or quantity of x-rays produced. Signup and view all the answers

What are the rules of thumb for kVp?

<ol> <li>If kVp is increased by 15 kVp, it will increase the number of x-rays; to maintain density, divide time by 2. 2. If kVp is decreased by 15, it will decrease the number of x-rays; multiply time by 2.</li> </ol> Signup and view all the answers

If 60 kVp at 0.50 seconds is increased to 75 kVp, what is the new time?

0.25 seconds; 0.50 / 2 = 0.25. Signup and view all the answers

If 85 kVp at 0.50 seconds is decreased to 70 kVp, what is the new time?

0.30 seconds; 0.15 x 2 = 0.30. Signup and view all the answers

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?

5 seconds; 20 / 4 = 5 seconds. Signup and view all the answers

A radiograph is exposed using 85 kVp at 0.15 seconds. The kVp is decreased to 70 kVp. What will the new time be?

0.30 seconds; 0.15 x 2 = 0.30 seconds. Signup and view all the answers

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?

4 seconds; 1 x 4 = 4 seconds. Signup and view all the answers

A diagnostic film produced using 90 kVp and 0.25 seconds. What exposure time is needed to produce the same film at 75 kVp?

0.50 seconds; 0.25 x 2 = 0.50. Signup and view all the answers

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?

0.075 seconds; for each increase of 5 mA, the time must be divided by 2, so 0.30 / 4 = 0.075. Signup and view all the answers

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.