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• radiographic image quality - refers to the fidelity with which the anatomical structure that is being examined is rendered on the radiograph. A radiograph that faithfully reproduces structure and tissues is identified as a high-quality radiograph.
• most important characteristics of radiographic image quality are

  • spatial resolution, contrast resolution, noise, and artifacts
• Resolution - ability to image two separate objects and visually distinguish one from the other
• Spatial resolution - refers to the ability to image small objects that have high subject contrast, such as a bone–soft tissue interface, a breast microcalcification, or a calcified lung nodule.
• Screen-film radiography / conventional radiograohy has excellent spatial resolution.
• Contrast resolution - is the ability to distinguish anatomical structures of similar subject contrast such as liver–spleen and gray matter–white matter.
• The actual size of objects that can be imaged is always smaller under conditions of high subject contrast than under conditions of low subject contrast.
• Visibility of detail - refers to the ability to visualize recorded detail when image contrast and optical density (OD) are optimized.
• Noise - is an undesirable fluctuation in

  optical density of the image
• true or false. Lower noise results in a better radiographic image because it improves contrast resolution?

  true
• Radiographic noise has four components:

  • film graininess
  • structure mottle
  • quantum mottle
  • scatter radiation
• The principal source of radiographic noise — scatter radiation
• Noise has Two major types:

  Film graininess (uncontrolled by RT)

  Quantum mottle (controlled by RT)
• Film graininess

  - refers to the distribution in size and space of silver halide grains in the emulsion.

  -similar to Structure mottles but refers to the phosphor of the radiographic intensifying screen.

  - ____ and structure mottle are inherent in the screen-film image receptor.

  - not under the control of the radiologic technologist, and they contribute very little to radiographic noise, with the exception of mammography.
• Quantum mottle - is a principal contributor to radiographic noise in many radiographic imaging procedures.
• If an image is produced with just a few x-rays, the quantum mottle will be higher than if the image is formed from a large number of x-rays. The use of very fast intensifying screens results in increased quantum mottle.
• Resolution and noise are

  intimately connected with

  speed
• Fast image receptors = high noise, low spatial resolution, and low contrast resolution.
• low noise and slow image receptors = High spatial resolution and high contrast resolution
• The study of the relationship between the intensity of exposure of the film and the blackness after processing is called sensitometry
• The two principal measurements involved in sensitometry are the exposure to the film and the percentage of light transmitted through the processed film.
• The two principal measurements involved in sensitometry are the exposure to the film and the percentage of light transmitted through the processed film. Such measurements are used to describe the relationship between OD and radiation exposure. This relationship is called a characteristic curve, or sometimes the H & D curve after Hurter and Driffield, who first described this relationship.
• Parts of Characteristic Curve

  Shoulder (High exposure level)

  Toe (Low exposure level)

  Straight-line portion

  (Intermediate region in which a

  properly exposed radiograph

  appears.)

  (Area where very small changes in

  exposure results in large changes in

  density)
• sensitometer and a densitometer (optical step wedge) - a device that measures OD
• aluminum step wedge or penetrometer - an alternative to the sensitometer.
• Optical Density - a measure of the degree of

  film darkening
• Formula: OD

  OD = log10 I o / I t

  I o = level of light incident on a

  processed film

  I t = level of light transmitted

  through the film
• Radiographic film contains ODs that range from near 0 to 4. These ODs correspond to clear and black, respectively.

  An OD of 4 actually means that only one in 10,000 light photons (10 >4) is capable of penetrating the x-ray film
• Useful range in general radiography is from 0.25-2.5 OD
• Most radiographs, however show image patterns in

  the range of 0.5 - 1.25 OD
• The ODs of unexposed film (0.1 - 0.3) are attributable to base density and fog density
• Base density - is the OD that is inherent in the base of the film (approx 0.1)

  - It is attributable to the composition of the base and the tint added to the base to make the radiograph more pleasing to the eye.
• Fog density - is the development of silver grains that contain no useful information. Fog density results from inadvertent exposure of film during storage, undesirable chemical contamination, improper processing, and a number of other influences.

  - should not exceed 0.1
• Base plus Fog Density Range is from 0.1-0.3

  • Should be never above 0.3
• Radiographic contrast is the product of two separate factors:

  • Image receptor contrast - is inherent in the screen-film combination and is influenced somewhat by processing of the film.
  • Subject contrast - is determined by the size, shape, and x-ray–attenuating characteristics of the anatomy that is being examined and the energy (kVp) of the x-ray beam
• Controlling factors of Optical density

  • milliamperage
  • exposure time

  influencing factors

  • kilovoltage
  • distance
  • grids
  • film screen speed
  • collimation
  • anatomic part
  • anode heel effect
  • reciprocity law
  • generator output
  • filtration
  • film processing
• controlling factor of Density -

  Milliampere second or mAs

  
  

  mA and exposure time have an inverse relationship when

  maintaining the same mAs.
• 15% increase in kvp = doubling the mAs

  15% decrease in kvp = dividing the mAs
• To increase density:

  Multiply the kVp by

  1.15
• To decrease density:

  Multiply the kVp by

  0.85