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Utilization of Radiographic Grids in Medical Imaging: Everything You Wanted to Know . . . but Were Afraid to Ask

George Tsoukatos, BPS, RT(R)

  *Digital Product Specialist, NY Imaging Service, Inc.
   Address correspondence to: George Tsoukatos, BPS, RT(R), Digital Product Specialist, NY Imaging Service, Inc, 85 Dickson Street, Suite #103, Newburgh, NY 12550. E-mail: georget@nyimagingservice.com.

Disclosures: The author reports having no significant financial or advisory relationships with corporate organizations related to this activity.

ABSTRACT

In radiography, the use of radiographic grids is the best-known, most significant, and effective means of eliminating scatter radiation from an image. Scatter radiation can decrease the contrast of an image and prevent visualization of details, which compromises image quality. Because contrast and image detail are 2 of the most important elements used to create a superior image, using radiographic grids can positively impact the overall image quality. The goal of this article is to review concepts and grid applications, as well as provide an update on the requirements for grid utilization for digital radiologic technologists.

Figure 1Introduction
Scatter radiation is inevitably produced when primary radiation passes through a subject (Figure 1).1,2 A general rule suggests that the higher the kilovolt peak (kVp), the greater the amount of scatter. When a radiographic grid is not used, radiographic fog is produced on the receptor (whether using film or a digital capture device used in computed radiography [CR] or digital radiography [DR]), which degrades the diagnostic quality of the image. 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 passing through the patient is scatter.3

There are 3 factors that determine the amount of scatter radiation produced:

  1. Field size of the radiographic beam
  2. Patient thickness/size of the area to be radiographed
  3. kVp level

A radiographic grid (Figure 2) is a device used to improve the contrast of the radiographic image. It does this by absorbing scatter radiation before it can reach the film or the digital capture device (eg, an imaging phosphor plate used Figure 2in CR or a DR detector plate). The use of radiographic grids in digital imaging is an important tool in the "battle" to keep unwanted scatter radiation from affecting image quality. Regardless of whether you use standard film-screen (analog/conventional) or digital technology, radiographic grids provide the best method for improving image quality and contrast. Grids are highly recommended for use when you are using more than 70 kVp, the anatomy is more than 12 cm thick, and in chest radiographs of patients who measure more than 24 cm around.4

The following article will provide a thorough review of radiographic grids, beginning with the history of their use and examining their construction and other technical parameters. Several different radiographic grids will be outlined to help radiologic technologists determine which grid should be used in various situations, and common errors and portable applications will be discussed.

History
The very first radiographic grid was made in 1913 by the American radiologist Gustav Bucky (1880-1963).5 Dr Bucky's first radiographic grid consisted of wide strips of lead that were spaced 2 cm apart and running in 2 directions-along the length of the film and then across the film. This crude design created an image of the radiographic grid that was superimposed on the patient's image. Despite having to view the anatomy through this checkerboard pattern, the original radiographic grid did remove scatter and improve image contrast.

In 1920 Hollis Potter (1880-1963), a Chicago radiologist, improved Dr Bucky's radiographic grid design.5 Dr Potter realigned the lead strips so that they ran in only 1 direction, made the lead strips thinner and therefore less obvious on the image, and then designed a device (now known as the Potter-Bucky diaphragm), which allowed the radiographic grid to move during the exposure. By moving the radiographic grid, the lead strips became blurred and were no longer visible on the film. All these improvements resulted in a practical grid device for radiographic image applications.

Scattered Radiation in the Imaging Process
Scatter radiation's origin is the Compton scatter, which is when the incoming high-energy photon uses a part of its energy to eject an outer electron. By doing so, the photon changes its direction but retains much of its initial energy.2

Scattered radiation is present in all radiographic studies and is an important consideration, especially in the majority of diagnostic examinations that use broad area beams, instead of narrow (fan or pencil) beam geometry. The image formation process in diagnostic radiology essentially captures a radiographic "shadow" created by the body of X-rays from a source point. The accuracy of this "shadow" depends on the photons being highly directional. However, scattered radiation is not emitted by a single source point. Rather, it strikes the imaging receptor (either film or CR/DR plate) from random directions and carries little useful information, unlike the directional primary photons that arise from the source.6 A useful way to describe the amount of scatter in a radiographic signal is the scatter fraction F defined as follows6:
Formula
As X-ray energy increases, the relative number of X-rays that undergo Compton scattering increases.2 In theory, when scattered X-rays come in contact with a radiographic grid they are absorbed and do not reach the image receptor. Scatter radiation affects image contrast, which is one of the most important characteristics of film quality because it allows for the differentiation of soft tissue versus bone. If a radiograph was taken with only scatter radiation, the image would have low contrast and a dull gray appearance.

Radiographic Grid Design, Construction, and Composition
An antiscatter radiographic grid is composed of high X-ray transmitting material, as well as high X-ray absorbing material, each aligned alternately and regularly (Figure 3).7 Aluminium is an example of one type of transmitting material (aluminium strips). An example of an absorbing material is lead strips. The (grid bars) are usually made of lead; openings (interspaces) between the bars can be made of carbon fiber, wood (older material), or aluminum.4 When designing an antiscatter radiation grid, it is important that it be designed so that 80% to 96% of the scatter radiation is removed prior to the image receptor. Most antiscatter radiographic grids allow transmission of 60% of the primary beam to the image receptor (Figure 4).7 The goal of any radiographic grid is to minimize the amount of scatter radiation that reaches the image receptor, while allowing the primary radiation to pass through to it. The term "grid clean-up" pertains to the amount of absorption of scatter radiation by the grid.1

Figure 3Figure 4

 

 

 


Figure 5Grid Patterns
Parallel Linear Grid
The lead strips run parallel to each other in this type of grid pattern. Strips are never aligned with the primary beam because they are all vertical (except for strips directly under the central ray). The parallel linear grid allows for tube angulation, minimizing the risk of grid cutoff. There are 2 linear arrangements, as seen in Figure 52:

  • Short dimension: Lead strips parallel to short dimension, also known as a "decubitus" grid
  • Long dimension: Lead strips parallel to long dimension, which is the standard configuration7

The grid lines in a short dimension radiographic grid are arranged along the short axis of the grid; this allows for crosswise placement and gives the technologist the ability to angle cephalic or caudal without having grid lines on the final image. Short dimension grids are ideal for performing portable radiographic images on patients with large body habitus, where the radiographic grid needs to be placed crosswise.8 Short dimension grids are also often called decubitus grids; they are great for performing chest, abdominal, and barium enema decubitus views.

Figure 6Focused Linear Grid
The focused linear grid is the most effective for reducing scattered radiation. In focused linear grids the lead strips are tilted progressively as they move away from center, whereas in parallel linear grids, the lead strips are aligned parallel to each other (Figure 6).2 Correct and careful use of focused linear grids is of the outmost importance if their full effectiveness is to be realized. The following are helpful tips:

  1. The focused linear grid should be used with the focal film distance (FFD)/source interceptor distance it was designed for. For example, common configurations are 34- to 44-in FFD or 40- to 72-in FFDs. A slight deviation downwards and a somewhat larger deviation upwards are possible.
  2. It is of critical importance that the grid is carefully centered during the exposure.
  3. The grid must be perpendicular in relation to the center beam during exposure.

Figure 7Grid radius applies to focused linear grids; it is the distance from the surface of the grid to the point above the grid where all of the strips would meet.7

Criss-Cross or Cross-Hatch Grid
A criss-cross grid, also referred to as a cross-hatch grid (Figure 7), is a composite of 2 grids with the lead strips at right angles to each other.9 This design generally increases contrast improvement and is used in certain "special procedure" radiographic applications. When criss-cross grids are used, no tube tilt is permitted because any angulation would result in grid cutoff because lead strips are running in both directions. A criss-cross grid is mainly suitable where a grid with a very high ratio is required (eg, Mammography and special procedure imaging, such as cross-table angiography).

Grid Specifications and Technical Parameters
Orientation of the Grid Lines
Depending on the type of radiographic grid, the lines can be vertical, horizontal, or crossed (criss-cross/cross-hatch). The different orientation of the grid lines affects the absorption of the scatter radiation.

Grid Ratio
Figure 8The grid ratio is the ratio of the height of the lead strip to the distance between the strips by the interspace material (Figure 8).2 Typical grid ratios are 6:1, 8:1, 10:1, and 12:1. Higher grid ratios require more precise centering of the X-ray beam and remove the greatest amount of scatter from the primary X-ray beam. Also, the higher the grid ratio, the greater the tendency to improve contrast. An 8:1 grid ratio is recommended when imaging below 90 kVp and a 10:1 or 12:1 grid ratio is recommended for examinations requiring kVp ranges that are greater than 90 kVp.2

Grid Frequency
Grid frequency refers to the number of lead strips per inch or centimeter. Typical grid frequencies are 103 line pairs/in (lpi), 178 lpi or 200 lpi.

Contrast Improvement Factor
The contrast improvement factor is a ratio of the contrast of a finished radiograph made with a radiographic grid compared to the contrast of a radiograph made without the antiscatter radiographic grid.2 The contrast improvement factor in a radiographic grid is represented as the "K" factor. The "K" factor compares the radiographic contrast of an image with a grid to the radiographic contrast of an image without a grid. Typical K factors range between 1.5 to 3.5.2

Grid Cutoff
Grid cutoff refers to the uneven density or loss of density on the resultant image due to undesirable absorption of the primary X-ray beam by the radiographic grid. Grid cutoff most commonly occurs when the primary beam is angled into the lead; the lead will absorb an undesirable amount of primary radiation, resulting in an underexposed image or underexposed edges of an image.

Density of the Lead Septa
The density of the lead septa is the actual thickness of the lead strips. There is better attenuation of scatter/secondary radiation striking the lead septa, particularly at higher kVp settings. The downside of thicker lead septa is that there is greater absorption of the primary beam, causing the need for increased radiation when imaging.Other benefits of using lead is it is relatively inexpensive and easy to share.

Composition of the Interspace Material
Interspace material influences dose efficiency. Usually, the interspace material in radiographic grids is plastic fiber, carbon fiber, or aluminum.The most common material used is aluminum, which has high atomic number, helps absorb scatter, and has less visible grid lines

Bucky Factor
Bucky factor is the ratio of incident radiation intensity reaching the radiographic grid to the transmitted radiation intensity passing through the radiographic grid. This is the factor by which milliampere seconds and thus, patient dose, must be increased when using the radiographic grid.2

Grid Selectivity
Grid selectivity is the ratio of primary to scatter radiation. Grid selectivity is influenced by the grid ratio.

Focal Length
Focal length determines the amount of slant of the slits in the radiographic grid. Adjusting this will alter the amount of scatter radiation that the grid absorbs.

Canting
Canting is the process used to tilt the lead strips when forming a focused linear grid.2  They are angled to match the beam divergence, which helps reduce scatter radiation.

Radiographic Grid Positioning and Mounting
A radiographic grid is used either in a stationary position or mounted in a Potter-Bucky diaphragm to move it during exposure. Most radiology departments have a supply of stationary grids in various sizes that can be mounted to the front of a cassette. These are generally made approximately 1 inch larger than the film or digital detector size that they are intended to cover. Stationary grids are used primarily in portable procedures or for upright or horizontal beam views. Stationary grids are typically used for portable and cross-table images. Reciprocating, or oscillating grids (also known as Buckys), are usually found in tables and chest stands, and they move during the actual exposure.10 By moving, reciprocating grids blur the grid lines and it is typically unnecessary to change the grid from that which was originally used for film imaging.

There are 2 movement mechanisms used today. The movements are described as reciprocating and oscillating.11 With the reciprocating grid, a motor drives the radiographic grid back and forth during the exposure for a total distance of no more than 2 to 3 cm.11 With the oscillating grid, an electromagnet pulls the grid to one side and then releases it during exposure.11 The grid oscillates in a circular motion within the radiographic grid frame.

TableExposure and Correction Factors for Grid Utilization
The Table lists the recommended corrections for various grid ratios. Corrections with changes in grid ratio can be accomplished by using kilovoltage. However, it must be remembered that kilovoltage affects the overall scale of contrast in conventional imaging, whereas in digital imaging this factor is controlled by the various "look-up tables" and pre-programmed algorithms, as well as post-processing selections. In most instances, using kilovoltage is not the preferred method because the kilovoltage adjustments may be as high as 20 to 25 kVp.1

Grid Selection
The selection of the appropriate grid ratio for a given examination requires the consideration of several factors. Although radiographic grids with higher ratios eliminate more scattered radiation, they tend to increase patient exposure and X-ray tube loading, as well as require more precise positioning.

Several factors must be considered when selecting a radiographic grid for a specific application. In most cases, a radiographic grid is selected that provides a reasonable compromise between contrast improvement and patient exposure, machine loading, and positioning.

The advantages of a 6:1-ratio radiographic grid are that it is easy to use and does not require critical positioning. Its use must be restricted, however, to situations in which the amount of scattered radiation is relatively small (ie, thin body section, low kVp) or in which maximum image contrast is not necessary.2 On the other hand, a 16:1-ratio grid produces high contrast recovery but significantly increases patient exposure. With a high ratio grid of this type, there also is very little latitude in positioning. Many applications are best served by grid ratio values between these 2 extremes. Such grids generally represent compromises between image quality and the other factors discussed.

In stationary grid applications in which lines in the image are undesirable, grids with a high spacing density (lines/cm) can be used. An increase in the spacing density generally requires a higher ratio grid to produce the same contrast improvement.

Some manufacturers are recommending high-fre­quency stationary grids, which are more expensive and difficult to use clinically. At higher frequency, higher ratio grids are required to produce equivalent scatter cleanup compared to lower fre­quency grids, which also have a higher Bucky factor (dose penalty).2 However, in certain situa­tions, grid cutoff is more significant a problem than aliasing. For instance, in applications such as bedside radiography, grid cutoff (absorption of primary radiation) caused by tilted grids can be problematic, particularly for cross-wise imaging of the chest, and can result in very poor image quality. In addition, histogram analysis can yield different results with and without grids.10 Specific menu selections (or processing algorithms) must be considered for grid and non-grid examinations to produce optimal results when using digital technologies in portable radiography.10

Grid Errors
The biggest problem with radiographic grids is misalignment. Grid misalignment can result in an underexposed image or the edges of the image can be underexposed or "hazy."

The presence of grid cutoff on any finished radiograph detracts from the image. Grid cutoff can be caused by 1 or more of these factors (Figure 9)1,6,12,13:

  • The central ray is not perpendicular to the grid surface (off-level).
  • The central ray is not aimed at the center of the grid (off-center).
  • The distance from the X-ray tube head to the grid surface is beyond a given tolerance (off-focus).
  • The grid is upside down.

Figure 9

Disadvantages of Using Radiographic Grids
The major disadvantages of using a radiographic grid besides the potential errors listed above include the following:

  1. It requires greater exposure to the patient because of the increased absorption of the primary X-ray beam.
  2. The additional weight added to the image receptor with the grid and encasement could increase the potential of error in certain radiographic views if not properly secured in a cassette holder.
  3. It adds cost because it is one more item added to the overall budget of an imaging department as well as payment for proper care and routine quality assurance testing of the device.

Figure 10Grid Encasements
Due to the high cost of both conventional and custom CR/DR grids, it is highly recommended that a protective encasement be purchased with the grid. Encasements come in a variety of options including a grid cap ("drop on"), an encasement with channels ("slip on"), or a flat encasement (Figure 10). Radiographic grid encasements usually are made of aluminum or lightweight polypropylene.

Gridded Cassette or Embedded Grid
Some imaging departments also may purchase a special cassette with a radiographic grid built into it. This design is called a gridded cassette, also known as an embedded grid.

Embedding an antiscatter radiographic grid into the CR or conventional cassette offers the radiologic technologist the full flexibility of having a grid within a digital capture device, in a ready-to-go mode. Examples of imaging procedures that would benefit from this new radiographic grid option with 10- x 12-in size cassettes could include orthopedic examinations, such as knee and hip imaging. Utilizing a 14- x 17-in gridded cassette for lateral lumbar imaging in an operating room is another example of a potentially beneficial use of this new format.14

Gridded cassettes are designed to work with department imaging processes and therefore, they eliminate the time-intensive tasks associated with adding and removing exterior grids. Using this new technology can help improve department productivity significantly because the radiologic technologist will no longer need to remove exterior radiographic grids prior to scanning, or replace radiographic grids in preparation for the next image acquisition. Most companies color code the CR gridded cassette so it stands out from the other grids and is not used for applications where a stationary grid would not be applicable. Another advantage is that the embedded grid will not slip or move when placed under a patient, and this is critical for portable radiographic examinations or trauma imaging. The main negative of a gridded cassette is the weight. Because the grid and encasement are built into the CR cassette, the overall weight is greater.

Radiographic Grids for Digital Imaging Capture Devices (CR and DR)
In digital radiology, the image contrast can be altered by digital manipulation. The relevant image quality parameter is then the signal-to-noise ratio.15

Whenever possible with CR, radiologic technologists should use a reciprocating grid or Potter-Bucky device, which causes the grid to move prior to the radiographic exposure. It is this motion that "blurs" out the grid lines and does not allow them to appear on the final image. It is also important to see how the CR cassettes are scanned by the image reader (ie, the laser will most often scan the photostimulable phosphor plate perpendicular to the direction of travel). If the CR system scans along the short axis of the phosphor screen (the same axis as the short dimension grid), the technologist should make sure that the grid frequency (not grid ratio) is slightly higher than the scan frequency. This will prevent an aliasing/Moiré effect.15 When using CR and grids in a mobile or stationary environment, very specific grid frequency (line spacing) on the grid is required. For example, it is one manufacturer's recommendation that the grid lines be spaced at 178 lpi, or 70 lines/cm, in order to avoid the Moiré effect.16 The Moiré effect is a summation artifact caused by the scanning laser beam overlapping with the grid line structure. Once you have this Moiré effect on the image, the image has to be repeated.

Radiographic Grid Suppression Software
Radiographic grid suppression software removes the Moiré lines from an image. When software is in place, a standard 103-lpi grid is sufficient and customary. In the absence of this software, a 152-lpi grid or higher is required. Grid pattern removal processing intelligently suppresses display of Moiré patterns associated with the use of nonoptimal stationary grids, such as those with a low number of grid lines per inch (80-100 lpi) and radiographic grids with horizontal grid lines. Specific menu selections (or processing algorithms) must be considered for radiographic grid and non-grid examinations to produce optimal results.10

Specific Applications for Radiographic Grids
Portable Applications
There are many factors that work against producing a quality image in a mobile environment. Unfortunately, many radiologic technologists find it cumbersome to perform portables examinations and use a grid with an encasement and a cassette. The new gridded cassette will hopefully eliminate this issue and improve image quality, especially during portable examinations.

While using a portable DR in a clinical setting, the medical physicist and the radiologic technologist have to work together to determine the need for radiographic grid and other technical factors. This process should save exposure to the patient and prevent dose creep, as well as assist in eliminating repeat imaging in this environment, which is costly in time, dose, and obtaining critical diagnostic information for the clinician.

Chest Radiography
There are a variety of densities that must be visualized on a chest X-ray: air, fat, soft tissue, and bone. This is what makes taking the perfect chest X-ray so challenging. To demonstrate this wide range of densities, a long scale of contrast is needed, generally requiring the use of 100 to 130 kilovolts, in conjunction with fine line grids. Generally, the higher the kVp, the greater the amount of scatter radiation. Using the proper radiographic grid with high kVp when performing a chest radiograph will allow for proper "clean up" of the scatter with a reduction in radiation dose to the patient. 17

Bariatric Imaging
When imaging an obese patient, improvements in image quality may be gained by taking measures that control scatter radiation with a radiographic grid. The amount of scatter produced is a function of exposure area and thickness, so anything that can be done to limit these factors will improve the final image quality.18

Figure 11Standing Spinal Radiography
Many departments have protocols for doing erect spinal imaging for pathologies such as scoliosis. This protocol requires the use of a stationary cassette holder, wall Bucky, or a mobile cassette holder (Figure 11).19 Conventional imaging uses a gradient cassette and screen in large sizes, such as 14 x 36 in. CR uses multiple cassettes in a specialized holder with "stitching" software. Due to the size of the body part, either technology requires a grid. Grids are available in larger sizes such as 15 x 37 inch or 15 x 51 inch for these types of applications. Radiologic technologist need to carefully select the correct grid frequency as recommended by the manufacturer to avoid artifacts in these types of specialty examinations.

Cervicothoracic Junction
Various techniques have been used to adequately visualize the cervicothoracic junction, which is the disk between the seventh cervical vertebra and first thoracic vertebra. Performing high-quality imaging of this area can be challenging, and alternative positioning techniques of the cervical spine, such as a swimmer's or supine oblique views, may be necessary to adequately visualize the cervical spine on X-ray. The swimmer's view, which requires the patient's arms to be placed above his or her head, may not be possible when imaging patients who have upper extremity injuries because the upper extremity trauma may prohibit the positioning of the patient's arms for this view. In addition, this technique can have high dose and high scatter.

Using an antiscatter radiographic grid and filter can improve the visualization of the cervicothoracic junction on the initial radiograph as well as improves the signal-to-noise ratio and thus the image quality and scatter on the resulting image. However, their use is often at the discretion of the radiologic technologist, who should evaluate the risks of using a grid (grid cutoff, etc) in this scenario compared to these benefits. The cervicothoracic junction view is most often required in an emergency department or trauma setting. The clinical outcome of obtaining a proper view is a critical aide in the clinician's decision-making process on how to stabilize and treat the patient.20

Grid Artifacts Associated with Digital Imaging: Moiré Patterns
The Moiré effect is an image artifact that can appear when using either parallel or focused grids. Generally, it appears if a set of parameters is fulfilled and disappears if one of the parameters is altered. A Moiré pattern looks like an image of parallel lines superimposed on the real image. The frequency of the lines and their orientation can differ.

The Moiré pattern is really an interference pattern. It is generated by the coincidence of the grid lines with the scanning laser beam. Thus, if the scanning laser beam is traveling in the same direction as the grid lines, then the Moiré pattern is introduced. It is the summation of both of those linear structures. If the grid is rotated through 90°, then the Moiré pattern is eliminated because now the scanning direction is going one way, while the grid lines are going a different way, at 90º.10 If the imaging plate cannot be rotated, another option is to get a radiographic grid with different linear spacing, so that it no longer marries up to the line scanning frequency of the CR system.10

Radiographic Grid Care
Grids are fragile and expensive. Radiologic technologists need to take good care of them. If the lead strips become bent or the radiographic grid gets warped, a number of grid artifacts will be produced that will degrade image quality. Radiographic grids should be stored lying flat and handled in a gentle manner. They should only be cleaned with solutions as recommended by the manufacturer.

Alternatives to Radiographic Grids for Scatter Control
Figure 12Air Gap Technique
One alternative technique to the use of a radiographic grid is known by using an "air gap" being introduced between the object and the receptor. This technique can have a similar effect to that of the grid but with many limitations.

In an air gap technique the image receptor is positioned 10 to 15 cm from patient in order to reduce scatter radiation. When using an air gap technique patient dose is higher than when using other non-grid techniques, but less than when using a radiographic grid. 

First, in order to create an air gap, the object to interceptor distance (OID) must be increased. Thus, instead of using the lead strips to absorb the scatter radiation, it provides extra space for the scatter radiation to pass the film, therefore reducing the scatter on the resulting image. Increasing the OID also results in magnification and a loss of detail.21

Another downfall to using the air gap technique instead of a radiographic grid is that it is only equal to approximately an 8:1 grid. As discussed previously, radiographic grids can go as high 16:1, thus removing more scatter from the resulting image.

In addition, the air gap technique is not effective when using a high kVp. In these instances, it is best to use a radiographic grid to reduce scatter.

Finally, the distance must be increased to overcome magnification distortion, resulting in increased radiation dose to the patient (Figure 12).21

Beam Restricting Devices
Collimators
A collimator is a device that helps focus the radiographic X-ray beam. A collimator is the most commonly used form of beam restriction. The size of the field or area being irradiated has a significant impact on scatter radiation. When the field size is reduced, the resulting reduction in scatter will reduce the density on the image as well as improve contrast resolution, resulting in improved image quality.11 Because a collimator is attached to the radiographic tube and is adjusted prior to positioning a patient, it is the first-line of defense in controlling scatter radiation. When imaging the lateral lumbar spine, collimation is critical due to the large size of the anatomy which will emanate scatter radiation. Collimation of the X-ray beam within the patient's skin line posteriorly will result in a significant reduction in scatter radiation and a significant improvement in image quality. This will not always be possible but should be considered an objective when performing lateral lumbar spine radiography. The use of lead shielding following the soft tissue line of the lumbar spine should be considered an adjunct to close collimation rather than a substitute for close collimation.22

A collimator focuses the X-ray beam using a set of adjustable lead shutters. A light and mirror then shows the area of beam and collimation. When using a collimator, beam restricting devices such as aperture diaphragms and cones or cylinders are sometimes attached to the collimator to further focus the beam (Figure 13).11,13 Automatic collimator (positive-beam limiting device) are electronic sensing device that are attached to the cassette tray; the sensors are activated to automatically adjust the shutters to the size of the film.

Figure 13

Aperture diaphragm. An aperture diaphragm is a flat piece of lead with a hole that slides into the bottom of the collimator. The size of the hole varies, depending on how much further the beam needs to be narrowed. This is the simplest form of collimation and has a very low cost. This technique can lead to some penumbra, or shadowing, on the image.11

Cones and Cylinders. Using a cone or cylinder allows the radiologic technologist to modify the aperture. They are similar to the aperture diaphragm with an extension cone or cylinder. They also slide into a slot at the bottom of collimator. With a cone or cylinder, radiologic technologists can improve geometric sharpness in an image.11

Types of cones include the flared circular cone, the rectangular cone, and the cylindrical cone. Round cones can be usefully employed when the anatomy of interest is round in shape (eg, skull, L5-S1 "coned down" view), when the corners of an image provide no useful information, and/or when a department wants to standardize the coning for a particular radiographic view.

Figure 14Snake-articulated lead shield
The lead snake was designed to reduce scatter radiation associated with the horizontal ray lateral sternum projection (Figure 14). It was clear that when imaging thicker anatomical regions and when there was primary X-ray beam making direct contact with the film/receptor, radiation scatter degradation of the image was considerable. This degradation was inevitable with the lateral sternum projection because it was otherwise impossible to collimate the primary beam within the skinline. The snake is constructed with 1-mm lead sheet that is laminated between 2 pieces of gray polyvinyl chloride or PVC sheet. Unlike lead rubber, the snake is made from solid lead sheet and is very effective at absorbing the primary X-ray beam, and therefore, decreasing the associated resultant scatter radiation. Other applications for the lead snake include the barium enema horizontal ray lateral rectum view.23

Conclusions
Two of the most important concepts in diagnostic image are keeping the patient dose at a minimum while producing the highest quality radiographic image. Keeping scatter radiation to a minimum is important for both objectives. Proper use of a radiographic grid, as well as understanding the concepts behind radiographic grids, will allow radiologic technologists to make the proper choice and selection based upon their imaging needs and applications. Radiographic grid design and technology will continue to evolve to address the ever-changing digital imaging evolution, making a thorough understanding of these changes critical to proper grid selection in the future.

References
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2. Curry TS, Dowdey JE, Murry RC. Christensen's Physics of Diagnostic Radiology. 4th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 1990. Chapter 8.

3. Dick CE, Soares CG, Motz JW. X-ray scatter data for diagnostic radiology. Phys Med Biol. 1978;23:1076-1085.

4. Fuchs AW. Radiographic Exposures and Processing. Springfield, IL: Charles C. Thomas Publisher; 1958:189-195.

5. Compiled and written in part by Pizzutiello J, Cullinan JE, for Eastman Kodak Company. Introduction to Medical Radiographic Imaging. Rochester, NY: Eastman Kodak Company; 1993: Kodak Publication No. MI-18.

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7. Online Digital Imaging Academy (ODIA). Module 1: The X-ray beam, antiscatter grids, controlling the impact of scatter. Available: https://www.asrt.org/Applications/ODIA/ODIALogin.aspx. Accessed February 10, 2011.

8. Sauer P, Fetterly K, Schueler B, et al. Improving adult portable chest radiography performed with computed radiography. Presented at: RSNA 2007; November 25-30, 2007; Chicago, IL. Poster #LL-PH5191

9. A guide to the proper use of Lysholm scatter radiation grids. Available at: http://www.gridline.se/assets/doc/guidelysholmsgrids.pdf. Accessed April 6, 2011.

10. Seibert JA, Bogucki TM, Cinoa T, et al. Acceptance testing and quality control of photostimulable phosphor imaging systems. A report of the American Association of Physicists in Medicine (AAPM) Task Group 10. Available at: http://www.aapm.org/pubs/reports/RPT_93.pdf.Accessed February 5, 2011

11. Stockley S. A Manual of Radiographic Equipment. 1st ed. New York, NY: Churchill Livingstone; 1986.

12. Bontrager KL, Lampignano JP. Textbook of Radiographic Positioning and Related Anatomy. 7th ed. St Louis, MO: Mosby Elsevier; 2010:41-42.

13. Carlton RR, Adler AM. Principles of Radiographic Imaging: An Art and a Science. 4th ed. Clifton Park, NY: Thompson Delmar Learning; 2006:263-267.

14. Carestream Health. CR Gridded Cassette. Available at: http://www.carestreamhealth.com. Accessed February 28, 2011.

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Utilization of Radiographic Grids in Medical Imaging: Everything You Wanted to Know . . . but Were Afraid to Ask

 
» Comment From: Jenny » Posted on: 06/17/2011 16:04 PM
Good article! Thanks.
 
» Comment From: sreeves » Posted on: 08/13/2011 12:18 PM
As always, very good
 
» Comment From: BC Jones » Posted on: 09/05/2011 16:55 PM
Excellent! I wish they were all this straight froward.More like this please!!!!
 
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