3.1 - The Controlling Factors

mAs

- Milliampere-seconds

- Controls the quantity of x-ray photons.

kVp

- Kilovoltage potential

- Controls the quality and penetrating power of x-ray photons.

What is meant by the quantity of the beam?

Number of photons

What is meant by the quality of the beam?

- Strength of photons

- Determined by wavelength and frequency

Steps of photon production

1. Electrical input generates electrons

2. Electrons move in tube

3. Electrons generate photons

4. Photons leave tube

What is the effect of higher current (mA) in radiography?

More electrons are produced.

What happens when the time current is increased in radiography?

More electrons are produced.

What is the relationship between the number of electrons and the number of photons in radiography?

More electrons result in more photons.

What is the effect of increasing the number of photons in radiography?

Higher optical density is achieved.

What happens to density if you double mAs?

Density doubles.

What happens to density if you half mAs?

Density is halved.

What type of relationship exists between mAs and density?

A direct linear relationship.

Adjusting for image density

Adjust mAs first, as it only affects density and not contrast.

How would mAs change with thickness?

Increase mAs by half for a small vs bigger patient

Effect of increasing kVp

Higher kVp = higher energy electrons = more photons = more optical density

kVp values for hand/finger

60 kVp.

kVp values for cervical spine

70 kVp.

kVp values for AP Lumbar

80 kVp.

kVp values for Lateral Lumbar

90 kVp.

What happens to density when kVp is increased by 15%?

Density doubles.

What happens to density when kVp is decreased by 15%?

Density halves.

What type of relationship does kVp have with density?

Exponential relationship.

What is the relationship between kVp and image contrast?

Higher kVp = lower contrast

What happens to image contrast when kVp is lowered?

Lower kVp = higher contrast

What type of relationship exists between kVp and image contrast?

This is an inverse relationship.

Best contrast for soft tissue imaging

Low contrast:

- Higher kVp

- Many shades of gray

Best contrast for bone imaging

High contrast:

- Low kVp

- Black and white (few shades of gray)

What does it mean if the image looks 'too white' (wonder bread)

- Image is underexposed (low density)

- mAs is too low

- double mAs to fix

What does it mean if the image looks 'very dark' (burned toast)

- Image is overexposed (high density)

- mAs is too high

- halve mAs

What does it mean if there are very dark blacks and bright whites (few shades of gray)

- High contrast

- kVp is too low

- increase kVp by 15%

- Decrease mAs by half

What does it mean if the image is "gross gray" (too many shades of gray)

- Low contrast

- kVp is too high

- decrease kVp by 15%

- increase mAs by 2x

Evaluating film quality steps

1. Check orthogonal views

2. Ensure anatomy is present

3. Evaluate image quality.