2.2 - Imaging Terminology and Basic Interpretation

What does optical density refer to in film?

It refers to the blackness of the film.

How does the number of photons affect the optical density of film?

More photons result in more blackness and higher density.

Darker film

It indicates a higher optical density (more blackness), meaning more X-ray photons hit the film.

Lighter film

It indicates a lower optical density (less blackness), meaning fewer X-ray photons hit the film.

Lucent (Lucency)

Area of blackness where it should be white.

Opaque (Opacity)

Area of whiteness where it should be black.

Sclerotic (Sclerosis)

Area with more whiteness than normal (usually bone).

mAs (Milliampere-seconds)

Mainly controls density.

kVp (Kilovolt potential)

Mainly controls contrast.

Radiographic contrast

Contrast is the amount of difference between visible shades of gray on an image.

High contrast

High contrast = big differences between black and white ( fewer shades of grey).

Low contrast

Low contrast = many shades of gray

What is the ideal contrast for soft tissue films?

Low contrast:

- High kVp

- Shades of grey

- Long scale contrast

- Fewer blacks and whites

What is ideal contrast for bone films?

High contrast:

- Low kVp

- Few shades of gray

- Short scale contrast

- Intense black and whites

High kVp effect on contrast

- Increases Compton scatter

- Produces lower contrast (more gray)

Low kVp effect on contrast

- Reduces scatter

- Produces higher contrast (more black & white)

Primary radiographic densities

Air, Fat, Water, Bone, Metal.

What is the relationship between higher object density and optical density?

Higher object density (e.g., metal) results in lower optical density (appears white).

What is the relationship between lower object density and optical density?

Lower object density (e.g., air) results in higher optical density (appears black).