Ideal Radiography Question And Answers

Ideal Radiography

Write short note on Ideal Radiograph.
or
Describe in detail Ideal Radiograph and factors affecting it.
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

Read And Learn More: Oral Radiology Question And Answers

Characteristics of an Ideal Radiograph are:

• Visual characteristics:
  • Density
  • Contrast.
• Geometric characteristics:
  • Sharpness or resolution
  • Magnification
  • Distortion.
• Anatomical accuracy of radiographic image.
• Adequate coverage of the anatomic region of interest:

Ideal Radiograph Visual characteristics

• Density
• Contrast

Density

Density is the overall blackness or darkness of a dental radiograph.

Various factors affecting the density of a radiograph are:

First-degree Factors:

• Milliamperage (mA)
• Exposure time
• Operating kilovoltage peak (kVp)

Milliamperage (mA)

• 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.

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.

Operating kilo-voltage peak (kVp)

• 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.

Source film distance

• The intensity of an X-ray beam varies inversely as the square of the source–film distance, density also varies inversely as the square of the source–film distance.

Second-degree factors:

• Subject thickness
• Development conditions
• Type of film
• Screens
• Grids
• Amount of filtration used
• Fog.

Subject 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.

Ideal Radiograph of development conditions

Radiograph can be light or dark, this depends on the factor that films are overdeveloped or underdeveloped.

Type of film

• 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.

Screens

Use of screens requires less mAs in order to obtain a density change.

Grids

The use of grids requires more mAs in order to obtain a density change.

Amount of Filtration Used

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

Fog

Film fog may result in an undesirable form of darkening of the film.

Ideal Radiograph of characteristic curve

• Hurter and Driffield (1890) first described the relationship between film density and exposure.
• A graphical relationship between film density and exposure is called a characteristic curve or Hand D Curve.
• This curve is typical of a screen–film combination. It reveals information about film contrast, speed, and latitude.
• Curve denotes that as exposure is increased, density also increases.
• Curve also denotes that the film has greatest diagnostic value, at the relatively straight portion of the graph.

Contrast

• Contrast is the difference in the degree of blackness (densities) between adjacent areas on a dental radiograph.
• A dental radiograph has ‘high contrast’ if it has very dark areas and very light areas, as the dark and the light areas are strikingly different.
• Adental radiograph that does not have very dark and very light areas, but instead has many shades of grey is said to have a ‘low contrast’.
• Radiographic contrast depends on the following factors, i.e.
  • Subject contrast
  • Film contrast
  • Fog and scatter

Subject Contrast

• Subject contrast refers to the characteristic of the subject which influence the radiographic contrast.
• It depends upon:
  • Differences in thickness of subject
  • Differences in density of subject
  • Differences in tissue atomic number or photoelectric absorption.

Film Contrast

• It is the characteristic of the film which influence the radiographic contrast.
• It is an inherent property of the film itself.
• It determines how the film will respond to the different exposures it receives after the X-ray beam has passed through the patient.
• Film contrast depends upon four factors:
  • Characteristic curve of the film
  • Optical density or degree of blackening of the film
  • Type of film, i.e. direct or indirect action.
  • Film processing.

Fog and Scatter

Radiographic contrast decreases as a result of stray radiation which reaches the film either due to background fog or owing to scatter from within the patient. This produces unwanted film density or darkening.

Ideal Radiograph of geometric characteristics

• Sharpness or detail
• Resolution or definition
• Magnification
• Distortion

Sharpness or detail

Sharpness refers to the capability of X-rays to reproduce distinct outlines of an object or to reproduce the small details of an object on dental radiograph.

Resolution or Definition

• Resolution, or resolving power of the film, is a measure of the film’s ability to differentiate between different structures and record separate image of small objects placed very close together and is measured in line pairs per mm.
• A certain degree of unsharpness is present in all dental radiographs.
• The fuzzy, unclear area that surrounds a radiographic image is termed ‘penumbra’.
• Various factors which control sharpness of an image on the X-ray film are:
  • Geometric unsharpness: This type of unsharpness is due to criss-crossing of rays at the edges of the object, resulting in a fuzzy image border. Size of the focal spot and target object distance affect geometric unsharpness.
  • Size of the focal spot: Smaller the focal spot, sharper is the image produced. When a “point source” is used, (the normal focal spot size is 0.6 mm² to 1 mm²) and non-sharpness is produced.
  • Object-film distance: This should be as small as possible to obtain a sharper image.
  • Target-object distance: Should be as large as possible for obtaining a sharp image.
  • Motion unsharpness: It is caused by any movement of either patient, tube or film during the exposure.
  • Absorption unsharpness: It is caused due to variation in object shape. e.g. cervical burn—out at the neck of a tooth.
  • Intensifying screen unsharpness: It is caused by the diffusion as well as spread of the light emitted from intensifying screens.
  • Fog unsharpness: Scattered, stray, leakage or any other radiation not belonging to primary beam is undesirable as it produces film fog.
  • Poor resolution: Resolution is determined mainly by characteristics of the film including: type, direct or indirect action, speed, and silver halide emulsion crystal size.

Magnification

• Magnification refers to a radiographic image that appears larger than the actual size of the object it represents.
• Magnification of a radiographic image results from the divergent paths of the X-rays from the focal spot.
• Factors influencing the image magnification on a dental radiograph are:
  • Target-film distance
  • Object-film distance.
  • Use of intensifying screen

Target-Film Distance

• Distance between the source of X-rays and the film is known as the target-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.

Object-Film Distance

• Distance between the object being radiographed, i.e. the tooth and the dental X-ray film is known as the object—film distance.
• Decrease in object-film distance results in a decrease in magnification while increase in object-film distance results in an increase in image magnification.

Use of Intensifying Screen

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

Distortion

• Dimensional distortion of a radiographic image is a variation in the true size and shape of the object being recorded.
• A distorted image results from the unequal magnification or different parts of the same object.
• The factors influencing dimensional distortion of a radiographic image are:
  • Object-film alignment
  • X-ray beam angulation

Object-Film Alignment

• The object and film must be parallel to each other to minimize dimensional distortion.
• A distorted image may appear too long or too short.

X-ray Beam Angulation

• 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.
• If 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.

Anatomic accuracy of Radiographic images

• Anatomical accuracy occurs when the anatomical structures are reproduced on the film in exact relationship as they normally appear.
• A radiograph with anatomical accuracy will have a minimum of superimposition of images of adjacent tissues.
• A radiograph is said to have anatomical accuracy when:
  • Labial and lingual cementoenamel junctions of anterior teeth are superimposed.
  • Buccal and lingual cusps of posterior teeth are superimposed.
  • Contacts of the teeth are opened in at least one of the projections of given area.
  • Buccal portion of alveolar crest is superimposed over the lingual portion of the alveolar crest.
  • No superimposition of the zygoma over the roots of maxillary molars.

Adequate coverage of the anatomic Region of interest

• It is important that the area of interest is well covered in the radiograph.
• Adequate coverage of the area of interest depends upon various factors:
  • Proper alignment of the film and the radiation beam to the area of interest.
  • Proper selection of the film types as well as the projection techniques.

Factors affecting the Production of an ideal Radiograph

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