Screen / Film Imaging Roland Wong, Sc. M., D. A. B. M. P, D. A. B. R

Screen / Film Imaging

• Roland Wong, Sc.M., D.A.B.M.P, D.A.B.R.

Outline

Projection Radiography

Projection Radiography

• In the ideal world, the focal spot is a geometric point.

• Initial imaging was direct film exposure.

• Very high spatial resolution.

• Very high dose.

• Dental radiography is still direct film exposure.

Inverse Square Law

Geometric Principles

Geometric Principles

The Film/Screen Cassette

Intensifying Screens

Intensifying Screens

Screen Composition

Screen Composition

Screen Composition

Screen Composition

Screen Function & Geometry

Screen Function & Geometry

Screen Function & Geometry

Screen Thickness Effect

Screen Function & Geometry

Spatial Resolution Depends Upon Screen Thickness

Reflective Layer of Screens

Crossover of Light Through One Emulsion To The Emulsion on the Other Side

Film Cassette

Parallax

Summary of Screen Effects on Spatial Resolution

• Thin screens have better spatial resolution.

• Thin screens have less absorption efficiency – More patient radiation dose.

• Reflective layer reduces patient radiation dose – But worsens the spatial resolution.

• Dyes can be added to screens to decrease light spread & improve spatial resolution – more patient dose.

Effect of Dyes in the Screen

Conversion Efficiency (CE)

Conversion Efficiency (CE)

Conversion Efficiency

CaWO4 Low Conversion Efficiency Screens

Rare Earth High Conversion Efficiency Screens

Absorption Efficiency (AE)

Absorption Efficiency (AE)

Absorption Efficiency

Thicker Screens Have Higher Absorption Efficiency

Overall Efficiency

Overall Efficiency

Matching Screen Light to Film Response

• If film sensitivity is not matched to screen light => Patient radiation dose increases.

• Some of the light from the screen can be lost if the film is not sensitive to the total spectrum of emission.

• CaWO4 screens can emit continuous blue light.

• Rare earth phosphors emit discrete hues in the green, yellow or UV.

Intensifying Screen Materials

K-Edge of Phosphor Material Makes Screens are kVp Dependent

SPEED vs. X-RAY kVp

Noise Effects of CE vs. AE

Radiographic Mottle (Image Noise)

• Three Main Components of Mottle

• Quantum Mottle
• Screen Mottle
• Grain Mottle

Radiographic Mottle (Image Noise)

• Quantum mottle is the variation in the # photons/ mm2 used to form the image.

• Screen Mottle is the variation in phosphor thickness and density.

• Grain mottle is the variation in # silver grains in film / mm2.

Quantum Mottle

Different Screen-Film Combinations have Mottle that have Different Appearances

Mottle in a Clinical Image

Advantages of Rare Earth Screens

• For the same thickness as CaWO4, rare earth decreases the patient dose, with the same resolution and more quantum mottle.

• For the same patient dose, Rare earth screens are thinner, thus have better resolution and more quantum mottle.

RADIOGRAPHIC MOTTLE (IMAGE NOICE)

• THREE MAIN COMPONENTS OF MOTTLE

• QUANTUM MOTTLE
• SCREEN MOTTLE
• GRAIN MOTTLE

• QUANTUM MOTTLE IS VARIATION IN # PHOTONS / mm2 USED TO FORM IMAGE

• SCREEN MOTTLE IS VARIATION IN PHOSPHOR THICKNESS & DENSITY

• GRAIN MOTTLE IS VARIATION IN # SILVER GRAINS IN FILM / mm2

Basic Principles

• Film determines the image contrast.

• Only the screen determines spatial resolution.

• The combination of film and Screen determines the speed (dose) & quantum mottle.

• Film processing affects everything except resolution.

Conclusions about Quantum Mottle (Q.M.)

• For a give type of phosphor, the thickness of the intensifying screen does not increase QM – Only speed (dose)

• Screens with low spatial resolution decrase QM – noise is blurred out.

• Changing to a faster film does increase QM.

Conclusions about Q.M.

• High contrast film increases visibility of QM

• Changing film processing can affect QM.

• Changing from CaWO4 to various rare earth screens increases Q.M.

• Q.M. is only important when trying to visualize low contrast objects.

Film Composition & Function

Optical Density

Optical Density

The Hurter and Driffield (H&D) Curve

The Hurter and Driffield (H&D) Curve

Contrast of Film (Average Gradient)

Contrast of Film (Average Gradient)

Sensitivity or Speed

Sensitivity or Speed

Sensitivity or Speed

Latitude

Latitude

The Screen-Film System

The Screen-Film System

Contrast and Dose

Contrast and Dose

Scattered Radiation

Scattered Radiation

Scattered Radiation

The Antiscatter Grid

Grid Construction

The Antiscatter Grid

The Antiscatter Grid

Parallel Grid

Parallel Grid

Focused Grids

• Focused grids have a range of focal distances.

• Distance de-centering, Lateral de-centering & a combination of both cause cut-off of primary radiation.

• Upside down focused grids show only a narrow area in the center of the image receptor.

Crossed Grids

Crossed Grids

• The effective grid ratio of two crossed grids is the sum of the individual ratios.

• Crossed grids clean up (remove) the scattered radiation in two orthogonal directions.

• Crossed grids are more sensitive to improper alignment.

Focused Grid at the Focal Point

Focused Grid – Upside Down

Focused Grid – Distance De-centered

Focused Grid – High and to Left

Focused Grid – Low and to the Right

Contrast Improvement Factor (K)

• The removal of scattered x-rays by the grid improves the contrast.

• Ratio of contrast with scatter plus grid devided by contrast with scatter without grid is “K”

Contrast Improvement Factor of Grids

Bucky Factor (B)

• Because the grid removes scattered x-rays that would have exposed the film,

• Fewer x-rays reach the image receptor.
• Radiation dose must be increased to maintain the film’s OD.

• B = (DOSEWITH GRID) / (DOSENO GRID)

Extra Radiation Needed With Use of a Grid

Extra Radiation Needed With Use of a Grid

Dependence of “B” & “K”

• “B” and “K” Depend on grid ratio

• Both increase with higher g

• “B” and “K” Depend upon ( S / P )

• Both increase with more scatter.

• “B” & “K” are nearly the same numerically.

• “B” > “K”

• A typical value of “B” IS ~ 3 – 5

Grid Artifacts and Air Gap

Grid Artifacts and Air Gap

Alternative to Grids

• Air Gaps of 20-30 cm can be effective in reducing scattered radiation to the image receptor.