film optical density spectral matching & sensitometry

Film Optical Density

Spectral Matching &

sensitometry

Film screen

• Radiographic Density

• Overall blackening of the radiographic

• image as a result of the energy deposited

• inthe film emulsion.

Film screen

Radiographic Contrast / Grayscale

• The difference between two adjacent

• radiographic densities in the image.

• The overall number of density shades

• present in the image.

Densitometer

Density

• .25 TO -2.5

• The straight line of the H&D curve

To see changes in optical density

• In order to see changes in optical density on a radiograph you must increase you mAs by at least 20 – 30%

• To double the density on a radiograph you must double your density

Optical Density• A numerical value indicating the degree of blackening on

the film. (average OD seen on a radiograph = 1.2 Range is 0.21 –

2.5)

# of photons coming through film = OD # # of photons hitting film

OD= 1 2 31 = 0 100 1 = 1 101 1 = 2 102 1 = 3 103

1 10 100 1000

• Carltons – CH 21 (pg 307) – increments of optical density • • Density log = incident light intensity • transmitted light intensity• • OD # % of light photons transmitted• 0 100 %• 1 10 %• 2 1 %• 3 0.1 %• If the quantity of black metallic silver on a radiograph• is such that it allow• 1 % of the light to pass through – what is the OD?• 10%• 100%

• An unexposed and processed film will have a density of about?

1. zero

2. 0.1

3. 1.0

4. 2.5

If the quantity of black on the film allows 1% of light to pass through, that film has a density of

1. 0.01

2. 0.1

3. 1.0

4. 2.0

If the quantity of black on the film allows 10% of light to pass through, that film has a density of

1. 0.01

2. 0.1

3. 1.0

4. 2.0

• The relationship between the intensity of light striking a film and the intensity of

light transmitted through the film is an expression of which of the following?

1. Radiographic contrast

2. Radiographic density

3. Recorded detail

4. Radiographic filtration

Image-forming x-rays

• Two kinds of photons are responsible for the OD and contrast on an image: Photons that pass through without interacting and those that are scattered though Compton.

• X-rays that exit the patient are remnant and those that interact with the IR are image-forming.

Optical Density

• Controlling factor: mAs • mAs determines the quantity of x-

rays

• What is the formula to determine quantity of x-rays?

How is OD measured on a monitor

Optical Denisty H & D curveSpectral Matching

RT 244

Week 13/14

Rev 11/09

Log denisty H & D curve

• If the exposure of the film results in optical densities (OD) that lie in the toe or shoulder regions, where the slope of the curve is less, contrast is reduced.

• Slope of the straight-line portion of the characteristic curve is greater for image receptor A than for image receptor B. Image receptor A has higher contrast.

• How does the slope affect

• Latitude?• Contrast?

Spectral Matching (F/S)

• What does it mean?

• Name the two types of screen phosphors

• What light spectrum do they emit?

Spectral Matching (F/S systems)

Spectral Matching (F/S systems)

Red safe light

Safe lights

• What wattage bulb?

• Distance from counter top?

Sensitometry

• What is the purpose?• What does it measure?• Why is it important?

More on this in RT 255 QA/QC

Sensitometric curve

• Developer temp• Can effect• Slope of curve• Less exposure is

needed for higher• Dev temps

• What is the ave temp? for a 90 sec processor?

100º90º

Log of relative exposure

DE

NS

ITY