Chapter 17 Radiographic Quality

 

 

Radiographic quality is that feature of diagnostic radiology that describes how faithfully you have been able to reproduce shadows on the radiograph that clearly depict the anatomical organs under investigation.

17.1 Density, Contrast, and Detail

Radiographic quality is dependent on radiographic density, radiographic contrast, and geometric factors affecting radiographic detail(Table 17-1).

Radiographic density is determined by the number of photons that reach the film. This is influenced by beam quality as well as the type and thickness of tissue. Radiographic density is determined by the number of electrons present in a given thickness of tissue which influences the photon stopping power that the tissue possesses. The electrons may be placed so that the mass per unit volume is great or the number may be high because of the thickness of tissue. If the density or thickness of the tissue doubles, the number of photons reaching the film is halved and

Table 17-1

RADIOGRAPHIC QUALITY

*(important factors)

    

RADIOGRAPHIC DENSITY.

Radiographic density is influenced by tissue density. Bone is almost twice as dense as muscle.

radiographic density is halved (Fig. 17-1). Therefore, radiographic density is inversely proportional to tissue density (Table 17-2).

Since radiographic density is a result of the number of photons that reach the film it is also influenced by:

• l) kVp,
• 2) mAs and
• 3) forms of beam attenuation such as filters or grids.

Contrast is divided into radiographic contrast and subject contrast. Subject contrast is the most important factor in determination of radiographic contrast and it is helpful to introduce both terms at this time.

Radiographic contrast refers to the observed differences in radiographic density between adjacent areas in the radiograph. These differences in film exposure permit identification of shadows on the radiograph. Radiographic contrast depends on

• l) subject contrast,
• 2) inherent film contrast,
• 3) kVp level,
• 4) scatter radiation and
• 5) film fogging.

Subject contrast is created by the difference in density and mass of two adjacent anatomic structures that permits transmission of x-ray photons through the two organs to a different degree. Subject contrast depends on the

• l) thickness of the part,
• 2) density of the part and
• 3) atomic number difference.

The intensity of the transmitted beam is inversely proportional to the thickness of the part. If

the amount of tissue is doubled the intensity of the transmitted beam is cut to one half. The

Table 17-2

TISSUE DENSITY

   RADIOGRAPHIC CONTRAST.

Contrast is dependent on kVp range. Low kVp produces high radio graphic contrast while high kVp produces low radiographic contrast.

greater the density (mass per unit volume) the greater is the ability of the object to attenuate the beam. Photoelectric absorption is one of three methods of attenuation of the x-ray beam, and this form of absorption is increased in tissue with high atomic numbers. Bone will attenuate many more x-rays than muscle or fat assuming equal thickness.

The use of contrast media obviously affects subject contrast markedly. Positive contrast media contain iodine or barium, while negative contrast media are air, COz, or NOz The amount of contrast media plus their concentration determine the effect on tissue density. Use of a large quantity of undiluted barium sulfate in the stomach creates a much more dense shadow than use of a small amount of diluted iodinated contrast media in the shoulder joint of a dog. In all cases the purpose in using contrast media is to artificially alter subject density to influence subject contrast.

The type of x-ray film used affects the radiographic contrast produced by its inherent film contrast. Some types of film have a steep H and D curve (characteristic curve) and produce radiographs of high contrast. Other film has a more sloping curve to the H and D curve and the film produces radiographs of lower contrast.

The ability of an x-ray photon to penetrate tissue is also dependent on its energy. Low kVp settings produce high radiographic contrast provided the energy of the incident beam is great enough to penetrate the part to be examined. High kVp settings produce lower radiographic contrast referred to as long-scale contrast because there is a long scale of shades of gray with little difference between each separate shade (Fig. 17-2).

Any fogging or scatter radiation affects radiographic contrast because it decreases the density difference between two adjacent shadows, thus decreasing film contrast. Thus, film processing influences radiographic contrast greatly. The use of high temperature solutions, short developer times, and fast processing chemistries accentuates the affect on radiographic contrast.

Radiographic sharpness or detail is the feature of the radiograph that describes the definition of the edge of an anatomic structure. There are several types of unsharpness or loss of detail. Geometric unsharpness is due to

• l) focal spot size,
• 2) focal-film distance,
• 3) object-film distance, and
• 4) film - screen contact.

Motion unsharpness is due to motion of the tube, object or film during an exposure. Screen unsharpness or inherent unsharpness is due to the use of film and intensifying screens and is dependent on the speed and type of screen and film selected (Table 17-3). Absorption unsharpness is due to the shape of the structure radiographed. Round or oval objects have the greatest degree of unsharpness. Parallel unsharpness is due to the film emulsion being placed on both sides of the film base. This has a negligible effect on image unsharpness.

Another aspect of radiographic unsharpness is referred to as distortion. Distortion is the phenomenon in which there is enlargement of the various parts of the radiographic image by differing amounts. This will occur if the focal spot is not vertically above the object. The recording plane (film) may not be parallel to the plane of the object which causes different object-film distances and results in a distorted image.

If the object is separated from the plane of the film but the focal spot is directly overhead we appreciate equal distortion of the image and refer to this as magnification and may use it as an aid to diagnosis.

In the radiography of bones it is often possible to make the central ray nearly perpendicular to the anatomical structure and also have the bone parallel to the film. This results in near anatomical reproduction of the image on the radiograph. Compare that description to one of the position of the small bowel loops in the abdomen in which case the central ray may not be perpendicular to the bowel loop and the loop may be positioned at a large angle with the film. For this reason the radio graphic reproduction of bowel loops is often bizarre and causes great problems in interpretation.

Screen mottle. A method of measuring sharpness of image detail is to determine the line spread function. Light from an intensifying screen spreads causing blurring of an image edge. The line spread function is a method of measuring this effect.

A 10µ thick slit in metal placed in close contact with the film causes such marked collimation of the x-ray beam that it can be considered a line source x-ray beam.

RESOLUTION RESULTS OF SCREEN/FILM

COMBINATIONS.*

  *(Koblik, et al. 1980)

A perfect imaging system would produce an image on the developed film of a line 10µ thick. However, this does not happen because of light diffusion by the screens and the width of the exposed line on the film is wider extending several hundred microns to each side of the line. Measurement of the width of this line provides a method of expressing the faithfulness of the screens in production of a sharp image edge. The speed of the screens obviously affects the width of the exposed line with high speed screens causing a much wider line than slow speed screens.

Line spread function can also be used to measure the influence on the image of other factors such as:

• (l) x-ray film,
• (2) fluoroscopic screens,
• (3) tube focal spot size,
• (4) image intensifiers, and
• (5) grids.

Modulation transfer function is a concept formulated to provide an objective measurement of the combined effects of sharpness and resolution. The MTF represents a ratio between the information recorded and the total amount of information available. The MTF can never be greater than 1 since the information recorded can never exceed the available information. The information taken from determination of the line spread function is treated mathematically and presented on a graph when the percentage of information transferred to the film is determined for progressively larger number of line pairs per mm. The transfer of information at a low number of line pairs is better than that transferred at a high number of line pairs.

Radiographic mottle is defined to mean the impression gained by an observer when looking at the radiograph that there is marked graininess present which interferes with the interpretation of the radiograph. This is another cause of loss of radiographic detail. There are several components that create the course density fluctuations observed on screen films. The first is the density fluctuation that results from the x-ray photons that are absorbed in the intensifying screens. If the number of x-ray photons absorbed per unit area by the intensifying screen becomes low enough, adequate film density may be obtained but the statistical distribution of the x-ray photons prevents development of a sharp image on the film. This is called quantum mottle. Second, structural inhomogenities in the phosphor coating of the screens can cause a density fluctuation called structure mottle. The combination of these two phenomena is referred to as screen mottle. The third component is film graininess which is due to the built-in random distribution of exposed grains in the emulsion. Of these three, quantum mottle is the most significant (Rossman, 1963) (Table 17-4).

Factors that affect radiographic contrast usually are not so severe that they affect detail visibility. Graying of the radiograph from fog certainly affects the density of the parts of the film that would otherwise have been normally unexposed. Problems in geometry are the usual cause of loss of radiographic detail. These films do not have diminished contrast or density. However; if the production of scatter is so severe, it is possible that loss of detail may result as well as a diminished contrast. The following chart lists what are felt to be the factors most likely to cause change in density, contrast, or detail, recognizing that all three factors may be commonly affected by some technical errors.

17.2 Technical Errors in Film Handling and Film Processing

Technical errors due to exposure and film handling are found with both manual (wet tank) and automatic film processing. These various technical errors are encountered frequently in radiography and often destroy the diagnostic qualities of the radiograph. The failures arise from not appreciating correct radiographic procedures and radiographic processing and, therefore, incorrectly handling and processing a radiographic film. The following chart lists most common technical errors, their appearance on the radiograph, their cause, and method of correction .

Examination of the film with transmitted light and with reflected light often assists in determining the cause of the technical error. Any problem of too great or too little exposure of the film by visible light or radiation will result in an alteration in the number of silver crystals present on the film. This is detected by transmitted light. If the film is examined with reflected light, the film emulsion is noted to be intact and without damage or altered appearance. Errors that result from too rough handling of the film of ten damage the film emulsion. These may appear with transmitted light to be due to a lack or excess of silver crystals. However, reflected light may more clearly detect the damaged emulsion.

The method and frequency of film quality control obviously will be determined by the level of success you are having with radiographic techniques. The more problems, the more rigid must be the method of film quality control until you solve the difficulties. Remember that you have need for this control with both wet tank processing and with automatic processing (Tables 17-5,17-6).

One of the most common technical errors concerns the tendency for radiographs to turn yellow or yellow brown with age. The archival quality of the radiograph, or its ability to avoid this color change, is dependent on the residual ammonium thiosulfate that remains after the final wash. If the residual thiosulfate is greater than 25 ugm/in2, the film will turn yellow, and if the residual thiosulfate is greater than 100 ugm/in2, the film will turn brown within one year.

Table 17-5

TECHNICAL ERRORS (OTHER THAN PROCESSING).

Table 17-6

TECHNICAL ERRORS THAT RESULT FROM IMPROPER MANUAL (WET TANK) FILM PROCESSING

Most of these problems deal with rough handling of the film to be processed, failure to maintain correct solution temperature and proper strength of developer and fixer solution, and failure to move the film through the processing cycle at the prescribed time intervals.

Table 17-7

TECHNICAL ERRORS THAT RESULT FROM IMPROPER AUTOMATIC FILM PROCESSING

The automatic processor removes the chance of many of the errors created by the technician but creates some new ones. Errors in manual processing are often random in their distribution and do not follow specific patterns. Technical errors in automatic processors tend to recur until corrected since they are "machine generated. "Most of the problems relate to incorrect replenisher rates, incorrect temperatures, and problems in film transport.

FILM QUALITY COMPROMISED BY STATIC ELECTRICITY

References

• Bushong, S. C.: Radiologic Science for Technologists. 3rd ed., C.V. Mosby Cp., St. Louis, 1984
• Koblik, P., Hornoff, W.J. and O’Brien, T.F.: Rare Earth Intensification Screen for Veterinary Radiography: An evaluation of two systems. Vet. Rad. 21:224-32, 1980
• Reynolds, J.A.: Factors Affecting Radiographic Quality. Vet. Med. 50:187, 1955
• Rossman, K.: Spatial Fluctuations of X-Ray Quanta and the Recording of Radiographic Mottle. Amer. J. Roentgenol. 90:863, 1963.
• Sweeney, R. J: Radiographic Artifacts: Their cause and control. J.B. Lippincott Co., New York, 1983