Factors Affecting Quality Of Radiograph

Factors Affecting Quality Of Radiograph

Define Ideal image. Describe in detail various factors affecting Radiographic image.
Answer.

“An ideal radiograph is one which has desired density and overall blackness which shows the part completely without distortion with maximum details and has the right amount of contrast to make the details fully apparent”. HM Worth

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Factors affecting Radiographic image

• Factors related to the radiation beam
  • Exposure time
  • Milliamperage
  • Kilovoltage peak
  • Tube film distance
  • Focal spot size
  • Collimation
  • Filtration
  • Equipment efficiency.

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Radiographic image quality

• Factors related to the absorbing media or object:
  • Object thickness
  • Object density.
• Factors related to the technique:
  • Position of patient’s head
  • Placement and position of the film
  • Angulation of the X-ray beam.
• Factors related to recording of the roentgen image of the object:
  • Reduction in secondary radiation
  • Films and film storage
  • Intensifying screens
  • Film processing.

Factors Affecting Radiograph Quality: Exposure, Positioning, and Technique

Factors Related to the Radiation Beam

Exposure Time

• An increase in the exposure time increases the film density.
• If exposure time is increased, then film density is increased, and if exposure time is decreased then film density is decreased.
• Exposure time and milliamperage are interchangeable and are thus considered as a single factor.

Milliamperage

• An increase in milliamperage produces more X-rays that expose the film and result in increased film density.
• If mA increases, then film density increases.
• If mA decreases, then film density decreases.
• Thus density varies directly and proportionately to the milliamperage or the tube current.

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Kilo Voltage Peak

• An increased kVp increases the penetrating power of X-rays and so increasing the film density.
• If kVp increases, then film density increases. lf kVp decreases, then film density decreases. Thus density varies directly and in proportion to the square of the relative kVp.

Factors Affecting Radiograph Quality: Exposure, Positioning, and Technique

Radiographic contrast and sharpness

Tube Film distance

• Distance between the source of X-rays and the film is known as the tube-film distance or source-to-film distance.
• Longer is the position indicating device (PID) used, more parallel rays from the middle of the X-ray beam strike the object rather than the diverging X-rays from the periphery of the beam. So therefore there is less magnification. Shorter PID and target—film distance result in more image magnification.

Focal spot size

Smaller is the focal spot, sharper is the image produced, If a “point source”, i.e. normal focal spot size 0.6mm2 to 1 mm2 is used no unsharpness is produced

Collimation

• It is the method by which one can control the size and shape of X-ray beam.
• Collimation decreases the size of X-ray beam and decreases amount of scattered radiation.
• Collimation decreases the volume of irradiated tissues of patient and reduces radiation exposure.
• Collimation decreases the film fog and enhances the image quality.

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Filtration

Reduction in the amount of added filtration used will increase the number of photons hitting the film which increases the density.

Equipment Efficiency

Detective quantum efficiency is a basic measure of the efficiency of an imaging system. It encompasses image contrast, blur, speed and noise.

Density and contrast in radiography

Factors Related to the Absorbing Media or Object

Object Thickness

• In a patient having an increased amount of soft tissue or thick dense bones, fewer X-rays will reach the film and the radiograph will appear light and have less density.
• If subject thickness increases, then density decreases. If subject thickness decreases, then density increases.
• Adjustments in the operating mA, kVp or exposure time can be made to compensate for variations in size of the patient and subject thickness. The next lower kVp and/or mA should be used, if patient is thin and has a narrow facial bone structure.

Object Density

Object’s uneven density causes uneven absorptionand scattering of an X-ray beam.

Factors Related to the technique

Position of Patient’s Head

As per the film projection technique, the head of the patient should be adjusted which will allow adequate coverage of anatomical area of interest.

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Placement and Position of the Film

• Film and the object should be placed parallel to each other, because if they are not parallel, and if an angular relationship results, it will produce variation of distances between tooth and the film resulting in distortion. A distorted image can appear too long or too short.
• Tooth and X-ray film should be placed as much close as possible to each other. Closer is the tooth to film, less image enlargement is there on the film. A decrease in object-film distance leads to decrease in magnification and vice-versa.

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Angulation of the X-ray Beam

• To minimize dimensional distortion, the X-ray beam must be directed perpendicular to the tooth and the film, so as to record the tooth and adjacent structures in their true spatial relationship.
• If the vertical angulation is increased, there will be shortening of the image, and if vertical angulation is decreased, there will be elongation of the image.
• lf the horizontal angulation is increased mesially or distally, there will be overlapping of structures.
• Geometric accuracy of an image depends on the position of the X-ray beam, object, and image receptor satisfying certain basic geometrical requirements:
  • Object and the film should be in contact or as close together as possible.
  • Object and the film should be parallel to one another.
  • X-ray tube head should be positioned so that the beam falls at right angles on the object and the film.

Factors Related to Recording of the Roentgen image of the Object.

Reduction in secondary Radiation

• Secondary radiation is undesirable as it produces film fog.
• For intra: oral films, filtration, collimation and film packets with lead back sheets should be used to reduce the secondary radiation.
• For extra-oral films, grids are used to reduce secondary radiation.

Films and Film storage

• Film speed: High speed films require less mAs in order to obtain a density change
• Film latitude: It is measured as a range of exposures that can be recorded as distinguishable densities on a film.
• Radiographic noise: This is the appearance of uneven density of a uniformly exposed radiographic film. It is seen on a small area of film as localized variations in density.

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Digital x-ray radiation safety

Intensifying screens

Usage of these screens increases the film to object distance, which produces certain amount of magnification.

Film Processing

• Film processing is under the control of dental radiographer.
• Increase in developing time produces increased contrast.
• Increase in developer solution temperature will give increased contrast
• Only one in developing solution will give indistinct contrast.
• Only hydroquinone is the developing solution will produce a harsh black and white image.
• Film fog smoothers the whole film with a dull grey shadow, giving a very poor contrast.