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

Sizin üçün oyun:

Google Play'də əldə edin


Yüklə 460 b.
tarix25.01.2019
ölçüsü460 b.


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.




Dostları ilə paylaş:
Orklarla döyüş:

Google Play'də əldə edin


Verilənlər bazası müəlliflik hüququ ilə müdafiə olunur ©muhaz.org 2017
rəhbərliyinə müraciət

    Ana səhifə