Chapter 7

Grids

 

 

   

The performance of a film/screen imaging system is a function of many variables. The geometry, kVp, patient size, and amount of scatter produced are just a few of the factors that must be considered. In this section on Grids we will discuss the scatter fraction produced and methods of control.

The grid is a flat plate with a series of lead foil strips that is made in various sizes and is used to improve the diagnostic quality of radiographs by absorbing the greater part of the scatter radiation (Fig. 7-1). It is positioned between patient and film (Fig. 7-2).

Scatter radiation is probably the biggest single factor contributing to decreased film quality. It is the result of a redirection of the primary x-ray beam and production of new x-rays following the interaction with the patient. Therefore, scatter radiation is present in each radiographic

 

 GRID USE.

X-ray beam interaction with an object creates scatter radiation that will result in film fog unless a device such as a grid is used to absorb a part of 1~ this new direction x-ray.

graphic examination. The effect of scatter radiation is to produce a generalized photographic fog on the film which reduces the contrast between adjacent areas on the radiograph. The intensity of the scatter radiation is dependent on:

It has been shown that the scatter fraction following passage of the x-ray beam through 5 cm of polystyrene is 50% and following passage through 10 cm of polystyrene is 70%. This means that 50% and 70% of the total beam after passage through the patient is scatter (Motz, et. al., 1978).

Grid transmission is determined by what happens to the incident primary beam which is

Factors in the transmission of the scatter radiation through the grid are:

 

GRID USE.

Relationship between tube, object, grid and film in routine diagnostic radiography.

 

 

Grid Construction  

Drawings and photograph show the structure of the lead strips and radiolucent interspacers.

lead strips,

History The first grid was built in 1913 by Dr. Gustave Bucky of Germany using the principle of lead foil strips standing on edge separated by x-ray transparent inter spacers. Dr. Hollis E. Potter of Chicago introduced in 1920 a method of moving the grid to avoid the presence of shadows of the lead strips appearing on the radiograph. Improvements have been made in the fabrication of better grids from the standpoint of uniformity of pattern and fineness (number of lines per cm (inch) ). The grid has thus been called the Potter-Bucky diaphragm.  

 

GRID TYPES

Linear grids have lead strips only in one direction. Crossed grids are usually constructed by placing linear grids one over the other with the lead strips at right angles to each other.

7.1 Construction

Lead strips approximately 0.5 mm in thickness and numbering between 500 and 1500 are set on edge. The total number of lead strips, thickness, height, and number per cm (inch) can vary. Sizes of the grids are usually 2 cm larger than the radiographic film sizes. The lead strips run parallel to the long dimension on most grids.

Transparent interspacers are placed between the lead foil strips to provide support. The interspacers are of fiber, plastic or some organic material of low density. Some grids use aluminum as the interspacers because of ease in construction. It has been shown that transmission of scatter is almost the same for fiber and aluminum inter spaces. However, transmission of the primary beam is an average of 15% greater if fiber interspaces are used (Jen sen and Schmeid,&endash;). Since, the aluminum absorbs a greater percentage of the primary beam, the exposure required to produce a satisfactory radiograph would be higher.

A binder is used to hold the many strips of lead and interspacer together. A covering material, such as aluminum, provides strength and prevents damage to the interspacers and lead strips (Fig. 7-3).

It is possible to construct a cassette with a grid permanently positioned in the front face of the cassette. This facilitates use of the grid since you need handle only one object and are not required to position a grid plus cassette. The disadvantage is the high cost of the grid-cassette and the fact that the film must be removed from the cassette and a new film loaded before another exposure can be made.

7.2 Grid Characteristics (Fig. 7-3a)

Linear grid Orientation of the lead strips is such that they are parallel to each other and have their length in the same direction. The grid is usually placed so the strips are parallel with the length of the table. This type of grid has the advantage that the primary x-ray beam may be angled along the length of the grid without loss of primary radiation because of excessive absorption of x-ray by the lead strips.

Crossed grids Generally two linear grids are con structed one on top of the other; with the lead strips of one at right angles to the lead strips of the second. This has the advantage of absorbing more of the secondary   

 

 GRID TYPES.

Drawing of a focused and parallel grid that shows the different alignment of the lead strips shown as dark lines within the grid. 

 

Table 7-1

EXAMPLES OF GRIDS AVAILABLE COMPARING GRID RAT10, LINES/CM (INCH), AND FOCUSING RANGE:

Grid Ratio Lines/cm

(inch)

Focusing Range

5:1

31(80)

71 cm to 183 cm(28" to 72")

5:1

33(83)

122 cm to inf (48" to Inf)

71 cm to 122 cm(28" to 48")

124 cm to 183 cm(49" to 72")

6:1

24(60)

122 cm to inf (48" to inf)

66 cm to 81 cm (26" to 32")

86 cm to 112 cm (34" to 44")

6:1

33(85)

122 cm to inf (48" to inf)

66 cm to 81 cm (26" to 32")

86 cm to 112 cm (34" to 44")

6:1

41(103)

122 cm to inf (48" to inf)

71 cm to 122 cm (28" to 48")

124 cm to 183 cm(49" to 72")

8:1

31(80)

66 cm to 81 cm (26" to 32")

86 cm to 112 cm(34" to 44")

122 cm to 183 cm(48"to 72")

8:1

33(85)

66 cm to 81 cm (26" to 32")

86 cm to 112 cm (34" to 44")

122 cm to 183 cm (48" to 72")

10:1

41(103)

75 cm to 140 cm (30" to 56")

91 cm to 112 cm (36" to 40") 152 cm to 183 cm(60" to 72")

91 cm to 112 cm (36" to 40")

152 cm to 183 cm (60" to 72")

13:1

-

86 cm to 120 cm (34" to 48")

16:1

-

95 cm to 105 cm (38" to 42")

 

  GRID RATI0.

The distance between lead strips may remain constant so the thickness of the grid must increase as grid ratios increase. It is possible to appreciate the smaller angle of deflection of the x-ray photon that will pass through the 16:1 ratio grid. Thus, high ratio grids usually "clean-up the beam," removing scatter radiation more effectively than low ratio grids.

  radiation than a linear grid. The disadvantages are:

Focused grid The lead strips are angled at a progressively greater angle in such a way that planes drawn through each lead strip and continued beyond the grid will intersect at a line a specified distance from the surface of the grid (Fig. 7-4). The distance from the surface of the grid to the point of intersection of these planes is called the focal distance of the grid. Most grids in use are focused grids. Both linear and crossed grids can be focused. Within the focusing range are the focal-film distances (FFD) that can be used satisfactorily with a specific grid and not have excessive grid cutoff. The range is rather great at a low grid ratio but much less with high ratio grids. Focal ranges are generally divided in short range (65-80 cm), medium range (81-114 cm), and long range (115-180 cm).

Examples of grids available are listed comparing grid ratio, lines/cm (inch), and focusing range (Table 7-1). 

Parallel Grid The leads strips are perpendicular to the face of the grid. Since they focus at infinity, they have no convergent line or focal distance (Fig. 7-4). Their 

 

 GRID RATI0.

The figure illustrates the influence of grid ratio on the angle of acceptance of the x-ray beam. The angle can change from 7° down to 3° with increasing grid ratio thus causing greater absorption of the beam.

greatest value is with a small field of exposure or at great target-film distances ( 180 cm). They are used in spot-film devices in association with fluoroscopic examinations. 

Grid Ratio Grid ratio is the relationship of the height of the lead strips to the distance between them, i.e., if lead strips are five times as high as the space between them, the ratio is 5:1 (Fig. 7-5) . The grid ratio expresses the ability of a grid to satisfactorily absorb scatter radiation. The higher ratio grid will absorb scatter radiation to a greater degree but has the disadvantages of:

The ratio of crossed grids can be assumed to equal the sum of the two linear grids used in its construction. The grid ratio, how ever, does not tell us anything about height and thickness of the lead strips, even though these factors also have a great influence on the absorption of "stray" radiation. Thickness of the lead strips tends to decrease with increasing grid ratios.

Lines Per Cm (inch) Grids can be produced with varying number of lead strips per centimeter. This is an important consideration for several 

 

  GRID COMPARISON.

Comparison of lines per centimeter and lead content of three grids that have the same grid ratio to demonstrate the reason for different behavior from grids of the same ratio.

reasons. If the grid is used as a stationary grid, the more lines per cm probably means that the lines are more narrow and, therefore, less objectionable as they appear on the radiograph. The thickness of the lead strips is O .3 mm in grids with 28 lines/ cm 0.25 mm in grids with 33 lines/cm, and 0.2 mm in grids with 40 lines/cm. However, as the number of lines per cm increases, the width of the lead strip decreases and the grid is less efficient in the absorption of higher energy scatter radiation. A trend toward stationary, fine-line grids with 53 lines per cm has not been successful because of the lower lead content. The inability to identify the grid lines when used as a stationary grid was commendable, but the cleanup of the scatter radiation was not as good. The number of lines per centimeter is not as important if the grid is used as a moving grid since the lines are not visualized in the radiograph. 

Lead Content of Grids The lead content of grids is expressed in gm/cm2. This is easy to understand if you consider a square centimeter marked on the surface of the grid. The weight of the grid contained within that square centimeter is expressed in gm/cm and is independent of the thickness of the grid. The lead content of the grid is a good indicator of the quality of the grid, however, the construction of the grid is important also. A lead sheet would have the highest lead content and yet it would not possess the ability to improve contrast effectively. A consideration of 

Table 7-2

CONTRAST IMPROVEMENT FACTOR*

Grid ratio

Lead content (mg/cm2)

Contrast improvement factor (K)

34

170

1.95

2 * 3.1

310

1.95

11

340

2.1

7

390

2.1

9

460

2.35

15

460

2.6

2 * 7

680

2.95

15

900

2.95

*(Boldingh, 1964) 

lead content in connection with lines per cm and grid ratio is important (Fig. 7-7). If the grid ratio remains constant and the number of lines per cm is increased, the lead content must decrease. The only way to keep the same grid ratio and increase the lines per cm is to decrease the thickness of the lead strips or the interspace. If the lead strips are made thinner, the grid ratio remains the same and the number of lines per cm increases. However, the lead content drops . If the width of the innerspaces is decreased the thickness of the lead strips must be made less in order to keep the grid ratio constant. This also results in a low lead content. This places a limitation on the number of lines per cm that can be placed in a grid since the lead content decreases with the addition of lines.

It is the practice to decrease both the width and height of the lead strips when increasing the number of lines per cm. These grids are then thinner (called thin line) and the lead content is low. For this reason, a 33 line per cm, 8:1 ratio grid improves film contrast much more than a 53 line per cm 12:1 ratio grid. 

Contrast Improvement Factor It has been considered necessary in evaluating a grid to establish a quality factor that is independent of: l) the number of lead strips per cm, 2) the thickness and heights of these strips, and 3) the ratio of the interspacing material. Therefore, a term "contrast improvement factor" has been introduced. Contrast improvement is the relationship of the contrast noted on the radiograph made without use of a grid when compared with the contrast noted on the radiograph utilizing the grid. It is the grid's ability to improve contrast that is its primary function (Table 7-2).

Three factors determine the amount of scatter radiation produced:

The higher the grid ratio, the greater is the tendency to improve contrast. This is generally true but as mentioned before is probably due to the lead content of the grids rather than the ratio. The relationship between the grid ratio and lead content of the grid is not constant but generally lead content increases with increasing grid ratio. 

Table 7-3

BUCKY FACTOR TO COMPENSATE FOR EXPOSURE REDUCTION DUE TO GRID USE*

Ratio

at 70 kVp

at 95 kVp

at 120 kVp

no grid

1

1

1

5:1

3

3

3

8:1

3.5

3.75

4

12:1

4

4.25

5

16:1

4.5

5

6

5:1 coss

4.5

5

5.5

8:1 cross

5

6

7

 *(Characteristics and Applications of X-Ray Grids. Leibel Flarsheim Co., 1968).

To permit comparison of different grids, the contrast improvement factor is usually determined at 100 kVp with a large field, a focal-film distance of 100 cm and a phantom of water 20 cm thick. While the contrast improvement factor is determined at 100 kVp, the relative quality of two grids appears to remain constant, that is, a grid with a good contrast improvement factor at 100 kVp will also have a good factor at 60 kVp and at 120 kVp.

 Bucky Factor The bucky factor refers to the alteration in exposure factors (mA and time) required due to the attenuation of the primary beam and absorption of 50 to 75% of the scatter radiation. It is the multiplication factor needed for the mAs value when the grid is used under repeated kVp settings (Table 7-3). The higher the bucky factor, the greater must be the exposure to compensate for the expected loss of radiation through use of the grid. This means that the higher the bucky factor, the greater is the radiation exposure to the patient. It is also possible that the increased exposure required due to use of the grid would be accomplished by increasing the exposure time and, therefore, there is a greater possibility of object motion on the radiograph. In reading Table 7-3, it is perhaps easiest to consider the bucky factor as the mAs of the exposure. You can then more easily appreciate the increase in exposure required with the use of the various ratios. Remember, the bucky factor depends on:

 

Table 7-4

kVp CHANGE TO COMPENSATE FOR GRID USE

Grid ratio

kVp change

no grid

-

5:1 grid

add 8 kVp

6:1 grid

add 12 kVp

8:1 grid

add 20 kVp

12:1 grid

add 23 kVp

16:1 grid

add 25 kVp

 

The figures in Table 7-3 must be considered as guides only.

It is also possible to vary the kVp setting when use of a grid is contemplated. The grid conversion chart in Table 7-4 can be used when the kVp range is between 60 and 80 kVp. 

7.3 Grid Use

A grid should be used in the radiography of any anatomical structure which is solid and is more than l l cm thick. Radiography of the thorax presents some specific problems in deciding the thickness at which to use the grid. This is because the normal aerating lung causes much less attenuation of the primary x-ray beam and much less secondary radiation is formed. Because of this, a grid is probably not needed in thoracic radiography until thoracic size exceeds 18 cm. However, a grid should be used in the radiography of the thorax when there is:

If your machine has capabilities in excess of 300 mA, it is possible to recommend use of a grid technique regardless of the tissue thickness. This is possible because the adjustment in technique can be made and still permit exposures with short time intervals. Thus, motion is not a problem. It still must be appreciated that the exposures with the grid are higher than they would be without the grid. Consequently, patient exposure and scatter radiation to a technician is increased.

After purchase of a ray machine, the most important accessory is the grid. Without a good grid the quality of the radiographs drops precipitously.

There are two ways that a grid may be used. In one technique the grid is stationary during.the exposure while in the other the grid moves during the exposure. This motion is usually lateral or reciprocating but may be rotational in direction. Exposure factors and contrast ranges are essentially the same for exposures made, regardless of whether the grid is stationary or moving. 

 

 EXAMPLE OF ELABORATE GRID HOLDER. Large animal radiography of the thorax and upper extremities requires use of a grid. By having the x-ray tube united with the grid and cassette it is possible to always have the central beam (line) centered and perpendicular to the grid.

 

 

 

COMPARISON OF STATIC~NARY AND MOVING GRIDS. Grid lines are visible if a stationary grid (A) is utilized and may compromise the quality of the radiograph as compared to the radiograph made with a moving grid (B),

 

7.4 Stationary Grids

Grids that remain stationary during the exposure are used in the following situations. They are:

In these situations the grid does not move during the exposure and grid lines are easily identified on the radio graph. Visibility of the grid lines varies with the number of lines per cm (Fig. 7-9). If the lead strips are not too wide, the lines on the resulting radiograph are not disturbing and are easily ignored by the viewer. Use of stationary grids has the advantage of having less limitations placed on where and how it can be used. Regardless of situation, the grid can be placed over the film holder and the exposure made. However, the central beam must be perpendicular to the surface of the grid, which may be of greater difficulty if both tube and film holder are held or suspended independently. If the grid is focused, the central beam must be directed along the midline of the grid. This is easily determined by noting this line marked on the surface of the grid. If the grid is nonfocused, the central beam may be directed anywhere on the surface of the grid, but it must be perpendicular to the face of the grid.

Until recently, most radiographic tables have had a bucky tray with reciprocating grid. Many grids did not move with sufficient speed to be used successfully with short exposure times of V30 and V60 sec. With the development of grids with a high number of lines, the use of a less expensive grid cabinet beneath the table top has become more popular. In this mode the grid is stationary and there is no limitation on use due to an ultra short exposure time. Grid lines are not objectionable because they are thin; however, they are present on every radiograph made with the grid. One disadvantage of this method results from the lower lead content found in the grids with a large number of lines per cm. They are not as efficient in removal of secondary or scatter radiation as are grids with a higher lead content. Still, the lower cost, absence of movable parts and ability to be used, regardless of exposure time, make them desirable for the average veterinary radiographic facility.

Stationary grids used with horses' feet or used with a horizontal x-ray beam are more easily damaged and special care is needed. This care can be provided through use of specially constructed cassette and grid holders. It is also possible to purchase cassettes with grids built into the face of the cassette . This, of course, provides good protection for the grid, but the high cost of the cassette plus grid prohibits purchase of many cassettes and limits the number of exposures before having to reload the cassette(s)

Grids are available now in multiple sizes that are attached to metallic channels that permit the cassette to slide into the holder and maintain the cassette and grid in a fixed position. 

7.5 Moving Grids

Whenever a grid is used in the making of a radio graph, the lead strips cast a shadow. If the grid is stationary during the exposure, the shadows will be easily detected on the films. If the number of lines per cm is rather high, the shadows on the film are less objectionable. If the grid can be made to move during the exposure, the shadows cast by the lead strips are blurred and cannot be identified on the radiograph (Fig. 7-9).

Dr. Hollis E. Potter introduced a method in 1920 of moving the grid in a direction at right angles to the lead strips during the exposure. This resulted in concealment of the grid shadows. The original term used to describe this apparatus was a Potter-sucky diaphragm. As of late, this has been shortened to that of a "Bucky diaphragm," "Bucky tray," or simple "sucky."

Early grids were spring-loaded and traveled across the film once. These were called catapult grids. It was necessary to manually pull a lever "cocking" the grid. Of ten a timer could be set that determined the length of time required for the grid to travel across the film once. More recent units have reciprocating grids that move continuously back and forth during the exposure. Usually they start moving at the time the tube anode starts to rotate. Two precautions must be taken in the use of a moving grid. The grid must move fast enough or grid lines can be identified, at least in part. This may mean that the shortest exposure time possible on the machine may not be used with the reciprocating grid. Grids are designated to be "par speed" or "high speed," indicating something of the shortest exposure time that may be used with the grid.

It is also important that the grid not move at such a speed that it is in synchrony with the pulses of the x-ray generator. If this were to happen, pulses of x-ray would be generated at a time when the lead strips were in the same position relative to the film and the radiographs would be compromised by your being able to identify grid lines on the film. You have no controls or adjustments that can be made to correct this problem, and service personnel are required to correct the situation.

The advantage of a moving grid is removal of objectionable grid lines from radiographs. Disadvantages relate to cost of the unit and the fact that it, like any other mechanical device, may be subject to malfunction. A grid may be noisy during operation, and this is objectionable when working with animals. If grid motion is slow, its use places a limit on how short exposure times can be. A moving grid requires slightly more exposure because of an increased grid cutoff (20% ) due to lateral decentralization as the grid reciprocates. 

Rotating grids Rotating grids are simple disk-shaped focused linear grids rotated about their central axis. They are used almost exclusively for angiography since it has been shown that conventional grid lines impair visualization of fine vascular detail. The rotary motion used with rotating grids at an angular velocity of 600 revolutions per minute effectively eliminates grid lines even with short exposures normally used in angiography.The cutoff characteristic of rotating grids are different from those of linear grids since the orientation of the grid strips changes as they rotate and as a result they effectively focus to a point in space, the convergent point. Lateral decentering, focus-grid distance decentering or combined lateral and focus-grid distance decentering can result. For any of these decentering errors the loss of primary radiation is approximately one-third less than that experienced by a comparable stationary grid. It should be recognized that oblique radiography can never be accomplished with a rotating grid (Bull, et al., 1975). 

7.6 Care of Grids

Grids installed in a Potter-Bucky diaphragm are well protected under the table top and generally require little care. It is possible for liquid contrast agents to flow onto the surface of a grid and present a repetitive pattern or artifact on each radiograph. This is corrected by removing the grid from the tray and wiping its surface.

Stationary grids attached to a cassette are obviously more prone to injury as a result of being dropped or bent while in use. Construction of the grid is such that little force is needed to bend the grid and separate the lead strips. This type of damage creates a permanent artifact on all films exposed using this grid.

Care must be exercised in cleaning grids since excess water will damage the filler material between the lead strips in some grids. 

7.7 Incorrect Use of Grids

Grid Cutoff From Incorrect Use One of the primary disadvantages associated with use of grids is the absorption of a part of the primary x-ray beam by the lead strips. Another problem associated with their use is the additional absorption of the primary beam if the beam is not directed in such a way that it passes through the grid with the least absorption possible. This problem is primarily associated with focused grids but does exist with parallel grids to a lesser degree (Fig. 7-10). Grid cutoff or primary cutoff is that additional absorption of the primary beam resulting from improper use of the grid. It may be uniform attenuation affecting the entire film equally but usually is expressed more on one part of the film than another. The radiograph or part of the radiograph appears to be exposed more or less than was assumed. There are four situations that will produce grid cutoff with varying degrees of significance:

Combinations of the above errors are also possible. 

Focused Grid Used Inverted (Fig. 711) Focused grids are usually identified on one surface with the words "tube side" or "tube side up". If the linear focused grid is used upside down, there is severe cutoff on the two edges with a more normal exposure along the center of the grid. The higher the ratio, the narrower will be the strip of "near normally" exposed film. If a focused crossed grid were used inverted, only a small square in the center would be properly exposed. Use of a focused grid inverted is obviously a serious error and requires a repeat study. 

 

  GRID CUT-OFF WITH THE USE OF A PARALLEL GRID.

Lead strips directly under the tube anode cast a shadow the same width on the film. All other lead strips cast shadows of varying width dependent on the distance from the center of the grid. This primary beam cut off results in a progressively decreasing film density as you approach the edge of the film.

primary radiation is approximately one-third less than that experienced by a comparable stationary grid. It should be recognized that oblique radiography can never be accomplished with a rotating grid (Bull, et. al., 1975).

 7.6 Care of Grids

Grids installed in a Potter-Bucky diaphragm are well protected under the table top and generally require little care. It is possible for liquid contrast agents to flow onto the surface of a grid and present a repetitive pattern or artifact on each radiograph. This is corrected by removing the grid from the tray and wiping its surface 

 

 GRID CUT-OFF WITH USE OF AN INVERTED GRID.

Demonstration of the abnormal x-ray beam cut-off that is created by using a focused grid inverted. Film density decreases rapidly as you move from the grid center. 

 

 GRID CUT-OFF WITH USE OF A LATERALLY CENTERED TUBE.

Demonstration of the abnormal x-ray beam cut-off that is created by using a focused grid with the x-ray tube off-center. Film density is decreased more on the side toward the tube shift.

 

Stationary grids attached to a cassette are obviously more prone to injury as a result of being dropped or bent while in use. Construction of the grid is such that little force is needed to bend the grid and separate the lead strips. This type of damage creates a permanent artifact on all films exposed using this grid.

Care must be exercised in cleaning grids since excess water will damage the filler material between the lead strips in some grids.

7.7 Incorrect Use of Grids

Grid Cutoff From Incorrect Use One of the primary' disadvantages associated with use of grids is the absorption of a part of the primary x-ray beam by the lead strips. Another problem associated with their use is the additional absorption of the primary beam if the beam is not directed in such a way that it passes through the grid with the least absorption possible. This problem is primarily associated with focused grids but does exist with parallel grids to a lesser degree (Fig. 7-10) . Grid cutoff or primary cutoff is that additional absorption of the primary beam resulting from improper use of the grid. It may be a uniform attenuation affecting the entire film equally but usually is expressed more on one part of the film than another. The radiograph or part of the radiograph appears to be exposed more or less than was assumed. There are four situations that will produce grid cutoff with varying degrees of significance:

Combinations of the above errors are also possible.

Focused Grid Used Inverted (Fig. 7-11) Focused grids are usually identified on one surface with the words "tube side" or "tube side up." If the linear focused grid is used upside down, there is severe cutoff on the two edges with a more normal exposure along the center of the grid. The higher the ratio, the narrower will be the strip of "near normally" exposed film. If a focused crossed grid were used inverted, only a small square in the center would be properly exposed. Use of a focused grid inverted is obviously a serious error and requires a repeat study.

Focused Grid Used with the Central Bealn Perpendicular but Lateral to the Center Line (Fig. 7-12) The x-ray tube is positioned lateral to the center line on the grid but at a correct focus-grid distance and with the x-ray beam perpendicular to the surface of the grid. The lead strips absorb the primary radiation unequally on one side of the central line than the other. Greater absorption takes place on the side toward which the decentralization of the primary beam has occurred. The opposite side experiences some cutoff but not as much. Three factors affect the degree of cutoff:

The amount of cutoff increases as:

Using grids with a ratio of 5:1 or 8:1 at a 100 cm focal-grid distance, the amount of decentralization of the primary beam can reach 2 .5 cm on either side of the midline and probably not require the study be repeated because of only a slight a decrease in film density.

The effect of the central beam not being centered on the grid is much greater when using a crossed grid since any movement away from the central point results in grid cut-off. One of the advantages of using a linear parallel grid(non-focused) is that the cutoff of the primary beam is constant regardless of where the beam is centered as long as it remains perpendicular to the surface of the grid.

This error in grid use is the most common one identified in practice. It is usually due to shifting in position of the grid in the bucky tray that cannot be noted visually. Another cause is the undetected lateral shifting of the x-ray tube as it hangs over the table top. A malpositioned collimation

 

GRID CUT-OFF WITH USE OF TILTED GRID.

Demonstration of the abnormal x-ray beam cut-off that is created by using a focused grid when the grid surface is not perpendicular to the central x-ray beam (off level). Film density decreases rapidly as you move to the right of the drawing and actually increases as you move to the left.

 

GRID CUT-OFF WITH TUBE OUTSIDE RANGE OF FOCUSED DISTANCE.

Demonstration of the abnormal x-ray beam cut-off that is created by using a focused grid at a FFD less than that for which the grid was constructed. Film density decreased rapidly as you move from the grid center.

 

light can cause the central beam to be unknowingly shifted lateral to the mid line.

Grid Tilted (Fig. 7-13) In working with a horizontal beam, it is not uncommon to find the central beam directed on the center line of the grid but not perpendicular to the surface of the grid. This may be as a result of the tube being tilted or the cassette and grid not being held so they are perpendicular to the central beam. Either situation creates the same type of cutoff. The cutoff is not uniform throughout the film but more severe on one side than the other, depending on the direction of angle of the primary beam.

In radiography of small animals, the most common cause for this type of grid misuse is a tube housing that is loose permitting the x-ray tube to shift in position. Use of a centering light from the collimator directs the central beam onto the center of the grid but at an angle.

The lines in a focused grid commonly are parallel with the long axis of the table. In this situation it is possible to tilt the angle of the central beam within this plane. In the discussion of certain radiographic views, it is recommended that the x-ray tube be angled to achieve a certain obliquity with the table top. This can be done successfully only if the tube is angled within the plane of the midline of the long axis of the table.

Tube Outside the Range of the Focal-Grid Distance (Fig. 7-14) Since a parallel grid is non-focused and has no focal-grid distance it will always demonstrate some cutoff on the edges, since the grid is focused at infinity and the tube will, therefore, always be closer than it should be. The problem as it occurs with use of a focused grid is discussed below.

If the distance between the grid and the tube is out side the prescribed range stated on the grid, then excessive cutoff occurs. This can result from the tube being either too close to or at too great a distance from the grid. The amount of cutoff is dependent on the grid ratio. In the lower ratio grids, the amount of cutoff is not great. The central portion of the

 

GRID AND CASSETTE HOLDER.

Drawing of a holder for grid (A) and cassette (B) that provides protection such as would be needed if a horse's foot were to be positioned for a study of P3.

 

film is not affected with the degree of cutoff being progressively greater toward the two edges. If the tube-grid distance is accidentally altered, there will be an alteration in the exposure of the film due to failure to compensate for a new focal-film distance in addition to the non-uniform grid cutoff.

Combined Errors Any combination of errors in use of a grid may occur. One of the most severe combinations of errors is the decentralization of the primary beam with the grid tilted. The amount of cutoff is not uniform across the film and is dependent on the combination of errors. On one side almost no radiation will reach the film, while on the other the film may remain of diagnostic density.

7.8 Selection of a Grid

The selection of a grid is a compromise usually based on the maximum kVp and mA capability of the machine, the type of animals radiographed, and the possibility of having a moving grid. Cost may also influence your choice. It is recommended that a grid ratio of 8:1 or 10:1 be used if most of the exposures are to be under 90 kVp. If exposures of over 90 kVp are common, a grid with higher lead content which probably will have a higher grid ratio should be considered. The increase in exposure factors required must be realized. The grid should be focused at 100 cm and should have 25 to 32 lines per cm (60 to 85 lines per inch).

Consideration of purchase of a fine line type grid to be used as a stationary grid under a table top can be made with the realization that lead content is low and clean up of scatter-radiation is not as good. A grid of this type is probably satisfactory for most small animal examinations.

Purchase of a stationary grid in addition to the grid within the Bucky tray will allow you to make cross-table exposures and also allow you to use the grid in making studies of horses' legs above the carpus and hock.

Sometimes it is advantageous to have a holder for the grid and the cassette that provides protection from damage due to trauma. This is especially important in radiography of the third phalanx of the horse (Fig. 7-15). It can also be helpful in assuring a fixed relationship between

 

GRID AND CASSETTE HOLDER.

Cassette holder (A) with radiolucent front that provides protection for the cassette. The cassette slides into the holder (B). The holder may have a grid front making grid techniques easier since grid and cassette are held together firmly.

cassette and grid if the x-ray table doesn't have a grid holder tray and you are forced to do table-top technique. It also ensures close apposition of grid and cassette so that they are exactly aligned in the event that both grid and cassette must be held when using a horizontal beam technique (Fig. 7-16).

 

AIR GAP.

Drawing illustrates the tube, object, grid and film relationship in conventional radiography and the use of an air gap to decrease the effect of scatter radiation. Note that an increase in the FFD tends to decrease the magnification of the image on the film.

7.9 Air Gap

The grid is the most important method of reducing scatter radiation, but there is another technique that may be of value in producing comparable results with decreased patient exposure. Two possible problems may be created by use of the grid:

Use of an air gap between patient and film decreases scatter radiation that reaches the film without the need to use higher kVp or mAs settings. Realize that the same number of scatter photons are produced as the primary beam strikes the patient, but they miss the film because of the increase in object-film distance. The term "air filtration" has been used to describe this technique, but this is a misnomer because there is little attenuation of scatter and no hardening of the x-ray beam (Fig. 7-17).

New problems are created by the use of an air gap . An increased unsharpness of the image occurs because of an increased object-film distance. It might be possible to diminish this problem by increasing the focal-film distance, but this requires an increase in machine settings and consideration of the new problems associated with this change in technique. The air gap is also less effective at higher kVp settings, since a larger percentage of the scatter is in a forward direction.

Air gap techniques are used in two specific situations, magnification radiography and thoracic radiography. Both of these techniques can be used in Veterinary Radiography. The only correction in exposure factors that is required result from the increase in focal film distance. The thicker the part to be radiographed, the greater the decrease in number of scatter photons that reach the film as the gap is increased. The first few cms of gap improves radiographic contrast more than any subsequent gap. Exposure factors must be changed with in corporation of an air gap only if the focal-film distance is changed. However, if the focal-film distance is left unchanged, the exposure can be decreased because of the increase of approximately 35% of the primary photons that would otherwise have been absorbed by the grid.

7.10 Moving-slot Radiography

A new method of reducing the level of scatter radiation that reaches the film is the use of a single moving slot or set of moving slots, one above and one below the patient that permits a long, thin beam(s) of primary radiation that sweeps across the patient. Use of this thin slot removes much of the scatter radiation and film contrast is improved. The primary beam is not attenuated. The duration of the exposure is long ( l - 3 seconds) which will make the technique unacceptable for veterinary radiology at this time (Moore, et al., 1977). This principle has been more recently used with a pair of rotating disks with multiple radial slots without the visible images of the individual slots noted on the radiograph. Substantial reduction in scattered radiation was achieved (1/2 to 1/4) (Sorensen, et al., 1980; Sorenson and Nelson, 1976; Jaffe and Webster, 1975 ) .

References

 

  • Barnes, G. T., Cleare, H. M. and Brezovich, 1. A.: Reduction of Scatter in Diagnostic Radiology by Means of a Scanning Multiple Slit Assembly. Radiol. 120:691-4,1976.
  • Boldingh, W. H.: Grids to Reduce Scattered X-ray in Medical Radiography. Phillips Research Reports Suppl. 1. Eindhoven, 1964.
  • Boldingh, W. H.: Quality and Choice of Potter Bucky Grids Part Vl. Acta Radiol. 56:202-8, 1961.
  • Bonellkamp, J. G. and Boldingh, H.: Quality and Choice of Potter Bucky Girds. 111. The choice of a Bucky grid. Acta Radiol. 52:241, 1959 .
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  • Dick, C. E. and Motz, J. W.: New Methods for the Experimental Evaluation of X-ray Grids. Med. Phys. 5:133-40, 1978.
  • Ilford, X-ray Focus. 2:12, 1958.
  • Jaffe, C. and Webster, E. W.: Radiographic Contrast Improvement by Means of Slit Radiography. Radiol. 116:631, 1975.
  • Jensen, G. and Schmeid, C.: The Performance of Radiographic Grids. General Electric Medical Systems.
  • Mattsson, O.: Practical Photographic Problems in Radiography. Acta Radiol. Suppl. 120,1955.
  • Moore, R., Korbully, D. and Amplatz, K.: Removal of Scattered Radiation by Moving-slot Radiography. Applied Radiology, p. 85, November/December 1977.
  • ott,T.: The X-ray Technician. pp. 335-343, May 1963.
  • Radiation Protection in Veterinary Medicine, NCRP 36, Bethesda, Maryland, 1970.
  • Sorenson, J. A. and Nelson, J. A.: Investigations of Moving-slit Radiog raphy. Radiol. 120:705, 1976.
  • Sorenson, J. A., Nelson, J. A., Niklason, T. and Jacobsen, S. C.: Rotating Disk Device for Slit Radiography of the Chest. Radiol. 134:227 31, 1980.
  • Trout E. D.: The Life History of an X-ray Beam. Rad. Tech. 35:161-70, 1963.