Radiographic Imaging Science

RADIOGRAPHIC IMAGING SCIENCE

DUPLITEZED FILM

• The duplitizing of film is a method of gaining the benefits of increased emulsion
• thickness with few of the drawbacks. Duplitized film is designed for use with two
• intensifying screens and is still the most commonly used type of X-ray film in the

X-ray department.

Advantages of duplitized fil

• (1) Increased sensitivity, i.e. adequate image density can be achie\'ed from a smaller
• radiation exposure. This has two important consequences:
• (a) Radiation doses to patients and staff are reduced;

(b) Wear on the X-ray tubes is reduced, thus extending their working life.

Radiographic Imaging Science

Increased image contrast: the contribution made by film emulsion to the eventual contrast of the image is greater because of the greater effective emulsion thickness.

Disadvantage

• (1) Loss of image quality: the use of a duplitized film and twin-screen system may
• introduce a loss of image quality compared with other image recording systems.
• For example, the crossover of light from each intensifying screen to the

emulsion on the 'wrong' side of the film leads to a loss of sharpness in the image.

Radiographic Imaging Science

(2) Economic reasons: duplitized films use a higher coating weight of silver than single-coated films, and silver is an expensive and diminishing world resource.

Photographic characteristics of x ray film

Exposure is proportional to the product of the milliamperes of x-ray tube current and the exposure time. Thus, an exposure of 100 milliamperes for 1 second is expressed as 100 milliampere-seconds, usually written 100 mAs.

Radiographic Imaging Science

Exposure (mAs) of the x-ray film produces film blackening, or density. The quality of the x-ray beam (kVp) has more effect on image contrast.

• mAs controls film density
• kVp controls image contrast

Photographic density

When the x-ray beam passes through body tissues, variable fractions of the beam will be absorbed, depending on the composition and thickness of the tissues and on the quality (kVp) of the beam.

defines an image as "a mental representation of anything not actually present to the senses."

Radiographic Density

Density is the amount of the overall blackness produced on the image after processing A radiograph that is too light has insufficient density to visualize anatomic structures while if its too dark, has excessive density and anatomic parts can not be visualized.

Factors affecting density

X-ray Absorption: Dense structures like bones absorb more X-rays, resulting in brighter areas on the image. In contrast, softer tissues absorb fewer X-rays and appear darker.

Exposure Settings: The amount of radiation exposure (controlled by mAs – milliampere-seconds) directly affects image density. Higher mAs produces darker images, while lower mAs produces lighter images.

Radiographic Imaging Science

the degree of film blackening is directly related to the intensity of radiation reaching the film or intensifying screen. The measurement of film blackness is called "photographic density“

Radiographic Imaging Science

Higher density means a blacker film (less light transmission). In routine x-ray work, a density of 2 (1% of light transmitted) is black when viewed on a standard view box, and a density of 0.25 to 0.3 (50% of light transmitted) is very light.

Radiographic Imaging Science

The important part of them characteristic curve is between the toe and shoulder, and in this region the curve is almost a straight line. In this "straight line“ portion the density is approximately proportional to the log relative exposure.

Contrast

• Contrast is the degree of difference between adjacent densities.

It is the photographic density difference between two adjacent areas on a film/image The ability to distinguish between densities enables differences in anatomical tissues to be visualized.

Contrast can be evaluated best when the radiographic density is adequate to visualize density differences

Radiographic Imaging Science

Radiographic contrast depends on subject contrast and on film contrast. Subject contrast depends on the differential attenuation of the x-ray beam as it passes through the patient.

Subject contrast was seen to be affected by the thickness, density, and atomic differences of the subject, the radiation energy (k V p ), contrast material, and scatter radiation.

Radiographic Imaging Science

The radiographer is required to understand the anatomic structure to be radiographed for him/her to determine the factors required to achieve desired level of radiographic contrast.

• Factors affecting contrast;
• Kilovoltage
• Grids
• Collimation
• Object to Image receptor distance
• Anatomic part
• Contrast media

Processing

Film contrast

Radiographic contrast is the density difference between image areas in the radiograph. There are many definitions of contrast, but we will use the simple definition that contrast is the difference in density existing between various regions on the film.

Film contrast depends on four

• factors:
• characteristic curve of the film
• film density
• screen or direct x-ray exposure
• film processing

Characteristics curve

• Is the relationship between the exposure a film receives and the density produced by the

exposure.

Radiographic Imaging Science

The relationship between exposure and density is plotted as a curve, known as the "characteristic curve" or "H and D curve" (named after F. Hurter and V.C. Driffield, who first published such a curve in England in 1890).

The concept of the characteristic curve of an x-ray film exposed by light from x-ray intensifying screens is illustrated in Figure 11-4.

Film density is plotted on the vertical axis and

film exposure on the horizontal axis. The shape and location of this curve on the graph are important film exposure we refer to the product of the intensity of the exposure (milliamperes of x-ray tube current) and time of exposure (expressed in seconds).

Radiographic Imaging Science

Exposure is expressed in terms of milliampere-seconds, usually abbreviated mAs. One way to produce a characteristic curve is to expose different areas of a film with constant kilovoltage and milliamperage while varying the time of exposure

Radiographic Imaging Science

Analysis of the characteristic curve of a particular x-ray film provides information about the contrast, speed (sensitivity), and latitude of the film.

Radiographic Imaging Science

Even at 0 exposure the film density is not 0 but will usually be 0.2, or less. This density is made up of fog (development of unexposed grains of silver halide in the emulsion) and base densities (opacity of the film base),

Radiographic Imaging Science

Therefore, total density on an exposed and developed film will include base and fog densities . Second, note that at low density (toe) and high density (shoulder), the film shows little change in density despite a relatively large change in log relative exposure

Film latitude

Latitude refers to the range of log relative exposure (mAs) that will produce density within the accepted range for diagnostic radiology (usually considered to be density 0.25 to 2 .0)

Cross over exposure

Crossover exposure, also called "print through exposure," occurs when a double emulsion x-ray film is exposed in a cassette containing two intensifying screens. Ideally, each film emulsion would receive light only from the screen in contact with the emulsion.

Radiographic Imaging Science

Crossover is the exposure of a film emulsion to light emitted by the screen opposite the emulsion. The main cause of this crossover is incomplete absorption of light by the adjacent emulsion.

This unabsorbed light passes through the film base to reach the opposite emulsion.

Radiographic Imaging Science

The crossover light is spread because of diffusion, scattering, and reflection caused by the film base and interfaces between the emulsions and film base. Crossover exposure is a significant contributing factor to unsharpness in film-screen systems.

Radiographic Imaging Science

The ideal way to reduce print-through is to increase light absorption in the silver halide grains of the film emulsion.

This would improve image quality without reducing system speed.

The original film designed to reduce print-through had a light absorbing dye coated on both sides of the film base.

This anti crossover dye absorbed light attempting to diffuse through the base into the opposite emulsion,

Spatial resolution

Resolution is the ability to image two separate objects and visually distinguish one from the other.

Spatial resolution is the ability to image small structures that have high subject contrast such as bone-soft tissue interface.

When all of the factors are correct conventional radiography has excellent spatial resolution

Noise

Noise is random variation in image brightness that can obscure details, often appearing as graininess:

Quantum Noise: Caused by the limited number of X-ray photons reaching the detector, it is more noticeable in low-dose imaging.

Electronic Noise: Arises from the electronic components of the imaging system, particularly in digital detectors.

Scatter: Scattered radiation (from Compton scattering) contributes to noise, which can be managed using techniques like grids that absorb scattered photons before they reach the detector.

Reducing noise while maintaining diagnostic quality often involves balancing the dose and exposure settings.

Density is expressed as a number that is actually a logarithm, using the common base 10.

• Photographic density is defined by
• D = loglo
• I,
• D = density
• 10 = light incident on a film

I, = light transmitted

Refer to Figure 11-1. If ten arrows (photons)

• of light strike the back of the film,
• and only one photon passes through the
• film, then I0 = 10 and I, = 1:

D . 10

• ens1ty = log 1 = I og 1 0 = 1
• Note that !c! measures the opacity of the
• I,
• film (the ability of film to stop light). The
• reciprocal of density, ..!:, measures the fracIo

tion of light transmitted by the film, and is called transmittance.

Higher density means a blacker film (less

• light transmission). In routine x-ray work,
• a density of 2 (1% of light transmitted) is
• black when viewed on a standard viewbox,
• and a density of 0.25 to 0.3 (50% of light

transmitted) is very light.

Typical characteristic curve of a

• screen-type x-ray film, exposed with x-ray intensifying

screens

Cross over exponsure

Crossover exposure, also called "printthrough exposure," occurs when a doubleemulsion x-ray film is exposed in a cassette containing two intensifying screens. Ideally, each film emulsion would receive light only from the screen in contact with the emulsion.

The ideal way to reduce print-through

• is to increase light absorption in the silver
• halide grains of the film emulsion. This
• would improve image quality without reducing
• system speed. The original film designed
• to reduce print-through had a lightabsorbing
• dye coated on both sides of the

film base.

This anticrossover dye absorbed light attempting to diffuse through the base into the opposite emulsion,