X- RAY FILM, INTENSIFYING SCREENS AND GRIDS

According to use:

  • Intra oral films.
  • Extra oral films.

According to coating of emulsion:

  • Single coated films: Produce sharper image but patient exposure is more.
  • Double coated films: Have emulsion on both side and patient exposure is less.

According to speed:

  • Slow speed films: A,B,C.
  • Fast speed films: D,E,F.
  • Hyper speed films: G.

According to packaging:

  • Single film packet.
  • Double film packet.

According to use with intensifying screen:

  • Direct exposure films.
  • Indirect exposure films.
  • Film intended to be exposed by x-ray is called direct exposure film.
  • All intraoral films are direct exposure films.
  • The emulsion is thicker than screen film.
  • Renders excellent detail.
  • In indirect exposure films the x ray beam is converted into the light by intensifying screens and this light expose the photographic type films.
  • A wide range of different films are available both the blue- sensitive and green – sensitive.

Used in:

  • Panoramic
  • Cephalometric
  • Skull
  • Tomographic x- ray techniques

Because they decrease the x-ray dose to patient and still result in a properly exposed film.

Two principle components:

  1. Emulsion
  • Sensitive to x rays and visible light.
  • Records radiographic image
  1. Base
  • Plastic supporting material and emulsion coated over it.

Two principal components

  1. Silver halide grains
  • Photosensitive.
  • Composed of silver bromide crystals.
  • In inside films these are flat tabular crystals with diameter about 1.8micro meter.
  • Ultra-Speed film is composed of globular shaped crystals about 1 micro meter.
  1. Gelatin matrix
  • Supports silver halide grains.
  • Made up from cattle bone.
  • Gelatin is clear so that it will transmit light.
  • It absorbs the processing solution and facilitates the reaction between chemicals and silver halide crystals. 
  • It supports the emulsion.
  • Made up of polyester polyethylene terephthalate.
  • Thickness 0.18mm.

Iodide, it disrupts the regularity of AgBr crystals and increases its sensitivity to x ray radiation.

  • By trace amount of Sulphur containing compound.
  • Trace amounts of gold.

Outer package

  • Made up of soft vinyl or paper.
  • Protects film from moisture saliva and light.
  • Have two sides: tube side and label side.
  • Tube side is plane white in color with identification dot.

Black paper

  • Film encased in black paper wrapper.
  • Black paper protective sheet covers the film on either side and shield it from light exposure.

Lead foil

  • It shields the film from backscatter (secondary) radiation, which fogs the film and reduces subject contrast (image quality).
  • Positioned in packet behind the film away from tube side
  • It absorbs some of residual x ray beam and decreases patient exposure

In this case most of the radiation absorbed by the lead foil and resulting radiograph is light and source embossed pattern in the lead foil that is known as tyre track or dotted pattern.

  • It indicates the exposure surface.
  • Placed towards the occlusal or incisal surface:
  1. 5 mm upward to posteriors.
  2. 2 mm upwards to anterior.
  • Intraoral x ray films are double emulsion film that is emulsion coated on each side of the base.
  • 0 used for small children (22X35mm).
  • 1 narrow, used for anterior (24X40mm).
  • 2 standard film size, used for adults (31X41mm).
  • 0 for small children.
  • 1 in children.
  • 2 for adults.
  • 3 long size also available.
  • Bitewing films have proper tab projecting from middle of film on which patient bites to support the film
  • 57 X76 mm or 5.7X7.6cm.

Characteristic

Photographic film

Radiographic film

Mode of image capture

By reflected light

By penetrated/ transmitted light

Emulsion layer

On one side

Either on one side or both

Speed

Slow

Fast

Grain size

Small

Large

Resolving power

1000 lines/mm

6-10 lines/mm

Irradiation

by Light only

Light as well as x-ray

Non curl back layer

Always present

Absent in double coated

Characteristic

Single coated

Double coated

Emulsion layer

One side

Both side

Patient Radiation dose

More

Less

Noncurl back layer

Present

Absent

Radiographic detail

More

Less

Average gradient (G)

Very less

More

Parallax effect

No

Yes

Contrast

Less

More

  • Used for extra oral radiography.
  • It is placed between two intensifying screen.
  • Intensifying screens absorbs the x rays and emits visible light which exposes the screen films.
  • Silver halide crystals are inherently sensitive to ultraviolet (UV) and blue light (300 to 500 nm) and thus are sensitive to screens that emit UV and blue light.
  • It creates an image receptor system that is 10 to 60 times more sensitive to x rays than film alone.
  • Reduce the patient’s exposure.
  • The resolving power of screen related to speed (slower speed greater resolving power and vice versa).

Because it reduces the resolution of resulting image below that necessary for diagnosis of much dental disease.

Intensifying screens are used in pairs, one on each side of the film, and they are positioned inside a cassette.

  • Base supporting material.
  • Phosphor layer.
  • Protective polymer coat.
  • Polyester plastic (0.25 mm thick).
  • Provide mechanical support to other layer.
  • In some screens base is also reflective.
  • In some intensifying screen base is non reflective and coating of titanium dioxide applied to base material that serve as reflective layer.
  • Composed of phosphorescent crystals suspended in polymeric coat.
  • These crystals absorb x-ray and fluoresce.
  • Phosphor crystals contain rare earth elements, like lanthanum and gadolinium.
  • Their fluorescence can be increased by the addition of small amounts of elements such as thulium, niobium or terbium.
  • Previously crystalline calcium tungstate (CaWO4) was used that fluoresces in blue portion of spectrum.
  • Now a days rare earth intensifying screens using terbium activated gadolinium oxy sulfide (Gd2O2S: Tb) and thulium activated lanthanum oxybromide (LaOBr: Tm) are available which fluoresces in green portion of spectrum.
  • A protective polymer coat (up to 15µm thick) is placed over the phosphor layer to protect the phosphor and provide a surface that can be cleaned.
  • The intensifying screens should be kept clean because any debris, spots, or scratches may cause light spots on the resultant radiograph.

Term rare earth is used because these elements are difficult to separate from the earth and each other, not because the elements are rare.

Rare earth elements used in intensifying screens:

EMISSION

PHOSPHOR

Green

Gadolinium oxysulfide, terbium activated

Blue and UV

Yttrium tantalate, nobium activated

  • Phosphor type and phosphor conversion efficiency.
  • Thickness of phosphor layer and coating weight (amount of phosphor/unit volume).
  • Presence of reflective layer.
  • Presence of light-absorbing dye in phosphor binder or protective coating
  • Phosphor grain size.

For dental use, following screen film sizes are available:

  • 5X7 inches.
  • 5X12 inches (Panoramic).
  • 6X12 inches (Panoramic).
  • 8X10 inches.
  • 10X12 inches.
  • Invented by Dr. Gustav Bucky in 1913.
  • Grids are placed between the patient and the x-ray film to reduce the scattered radiation (produced mainly by the Compton effect) and thus improve image contrast.

Hollis E Potter.

Grid is composed of alternate strips of radiopaque material usually lead and strip of radiolucent material usually plastic.

High atomic number, absorbs scatter, dense.

  1. Where the anatomy is >10 cm like:
  2. Abdomen
  3. Skull
  4. Spine (except lateral cervical)
  5. Contrast studies:
  • IVU
  • RGU
  • MCU
  • barium studies (including lateral cervical)
  1. Breast (mammography): uses 4:1 grid ratio.
  2. Soft tissue structures to increase contrast.
  3. Structures affected by pathological condition that would increase scatter production.
  4. With high Kvp.
  • The ratio of the thickness of grid to the width of radiolucent spacer is known as grid ratio.
  • The higher the grid ratio, the better the image contrast but at a cost of increased patient dose.
  • Grid ratio of 8:1 is generally used for 70-90 kVp technique and 12:1 is used for >90 kVp technique.

Focused grids (most grids):

  • In a focused grid the strips of radiopaque material are all directed toward

     a common point, the focal spot of the x-ray tube, some distance away.

  • Because the lead strips are angled toward the focal spot, their direction coincides with the paths of diverging photons in the primary x-ray beam.

Parallel grid:  

  • Made with the lead and interspace strips running parallel to one another, never intersecting.
  • Used with very small x-ray field or long target-grid distance
  • Parallel grids are less commonly used.

Moving grids (also known as Potter-Bucky grids):

  • A bucky is typically used for table or wall mounted x-ray systems and holds the x-ray cassette and grid.
  • A bucky, is a device found underneath the exam table, a drawer like device that the cassette and grid is slid into before shooting x-ray.
  • A reciprocating bucky is a device that moves the grid while the x ray is being taken.
  • The motion keeps the lead strips from being seen on the image.
  • The finer the lead strips, the less movement is needed.

Three methods:

  • Primary transmission (Tp)
  • Bucky factor (B)
  • Contrast improvement factor (K).
  • It is fraction of a scatter-free beam passed by grid.
  • Ideally 100% (never achieved).
  • Typical values: 55 – 75%
  • Theoretic calculation:

Tp (%) = 100 X W / (W+w)

Where W = Interspace thickness, w = lead strip thickness

The Bucky factor (B) is the ratio of x-rays arriving at the grid (incident radiation), and those being transmitted through the grid.

The Bucky factor describes how much the output of the x-ray tube must be increased in order to compensate for the removal of incident radiation by a grid.

B = incident radiation / transmitted radiation.

  • It is test of grid’s performance.
  • I.F= Contrast with a grid/Contrast without a grid.
  • Higher the grid ratio, higher the C.I. factor.

The grid frequency is a measure of the number of grid lines per unit distance (inches or centimeters).

Range:

  • 40-50 lines/cm (100-120 lines/inch) for low frequency grids.
  • 50-60 lines/cm (120-150 lines/inch) for medium frequency grids.
  • 60-70+ lines/cm (150-170+ lines/inch) for high frequency grids.

Grid cutoff is an unwanted absorption of x-rays via an x-ray grid, observed when a grid is employed incorrectly, most often seen with parallel grids.

The term cutoff stems from the phenomenon in which the primary x-ray beam is ‘cut off’ by grid lines, leading to an overall decrease in optical density or a decrease in radiographic exposure (more opaque).

  • Focused grids used upside down.
  • Lateral decentering (grid angulation).
  • Focus grid distance decentering
  • Combined lateral and focus-grid distance decentering.
  1. Goaz PW, White SC X-ray film and its properties, intensifying screens and grids Book of oral radiology, principles and interpretation 2nd edition 1990;97-112.
  2. White SC, Pharaoh MJ X-ray film, intensifying screen and grid. Book of oral radiology principles and interpretations 6th edition 2009;70-85.
  3. Langlang OE, Langlais RP Special radiographic techniques. Book of principles of dental imaging 1st edition 1997;275-280.
  4. Hapugoda S, Goel A et al. Grids. Available from: https://radiopaedia.org/articles/grids.
  5. Glick Y, Murphy A. Grid cutoff. Available from: https://radiopaedia.org/articles/grid-cutoff.
  6. Scatter Removal Grids. Available from: http://www.upstate.edu /radiology /education/rsna/radiography/scattergrid.php
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