Chapter 18 physics The Grid

Anthony Torres
Flashcards by Anthony Torres, updated more than 1 year ago
Anthony Torres
Created by Anthony Torres over 5 years ago


The Grid

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Question Answer
Grid -A grid is a device used to improve the contrast of the radiographic image.) It does this by absorbing scatter radiation before it can reach the image receptor. -When an x-ray beam passes through the body, one of three things will occur with the primary photons that originated at the target.
Grid Continued -When an x-ray beam passes through the body, one of three things will occur with the primary photons that originated at the target. -They will (1) pass through the body unaffected, (2) be absorbed by the body, or (3) interact and change direction
Grid Continued -The photons that pass through the body unaffected will interact with the image receptor to create the image. -These are the photons responsible for creating the contrast (differences in the image receptor exposures or densities) on the image. -These differences exist because some photons pass through the body while others are absorbed.
Grid Continued -Scattered photons add an overall exposure to the image receptor and, as a result of this overall graying of the image, contrast is lowered. -An important point to remember is that the percentage of Compton interactions increases with increased kVp. -(Therefore, scatter increases and contrast is further impaired as kVp increases.)
Grid Continued -The greater the atomic number of the tissue, the less will be the quantity of scatter created. -For example, less scatter is produced in bone than in soft tissue, because bone absorbs more photons photoelectrically. -This is the result of changes in the number and types of atoms that are present for interaction. -(In summary, the amount of scatter radiation increases with (1) increases in patient thickness, (2) larger field sizes, and (3) decreases in atomic number of the tissue.)
Grid Continued -Because a grid is designed to absorb the unwanted scatter radiation, it is necessary to use a grid with thicker, larger body parts and with procedures that require higher-kVp techniques. -As a general rule, a grid is employed when: 1. body part thickness exceeds 10 cm 2. kVp is above 60
What is a Grid made of? -A grid is a thin, flat, rectangular device made by placing a series of radiopaque lead strips side by side and separating the strips by an interspace material that is radiolucent. -The lead strips are of very thin foil and the interspace material is thicker and usually made of aluminum. -These strips are then encased in a plastic or aluminum cover to protect them from damage.
Who made the very first grid? -The very first grid was made in 1913 by the American radiologist Gustav Bucky (1880-1963). -Dr. Bucky's first grid consisted of wide strips of lead spaced 2 cm apart and running in two directions, along the length of the image and across the image.
Who is Hollis Potter? and what did he design? -In 1920 Hollis Potter (1880-1963), a Chicago radiologist, improved Dr. Bucky's grid design. -Dr. Potter realigned the lead strips so they would run in only one direction, made the lead strips thinner and therefore less obvious on the image, and then designed a device (now known as the PotterBucky diaphragm) that allowed the grid to move during the exposure.
How did Hollis Potter design and upgrade the bucky? -By moving the grid, the lead strips became blurred and were no longer visible on the image. -All these improvements resulted in a practical grid device, which significantly improved contrast without impairing the view of the patient's anatomy.
What kind of materials is the grid made of? -A grid is a series of radiopaque strips that alternate with radiolucent interspace strips. -These strips are bonded firmly together and then sliced into flat sheets. -The radiopaque strips are needed to absorb the scatter radiation and must therefore be made of a dense material with a high atomic number. -Lead is the material of choice because it is relatively inexpensive and is easy to shape into very thin foil.
Why is Aluminum used for a Grid? -Several organic and inorganic materials have been tried but only two are commercially available—aluminum and plastic fiber. -Ideally, this material should not absorb any radiation. -However, in reality, it does absorb a small amount. -Aluminum is more commonly used than plastic fiber because it is easier to use in manufacturing and is more durable.
what are the disadvantages and advantages in using Aluminum for a Grid? -Also, because it has a higher atomic number than fiber, it can provide additional absorption of low-energy scatter. -With its higher atomic number, aluminum also increases the absorption of primary photons. -This can be a disadvantage, especially with low-kVp techniques where this absorption would be greater. -Fiber interspace grids are preferred when using low-kVp techniques where their application can contribute to lower patient dose, such as in mammography
Why are Fiber interspace grids preferred when using low kVp? -Fiber interspace grids are preferred when using low-kVp techniques where their application can contribute to lower patient dose, such as in mammography
Grid ratio -(Grid ratio has a major influence on the ability of the grid to improve contrast. -It is defined as the ratio of the height of the lead strips to the distance between the strips. -This is expressed in the formula: Gridratio= h/w where: h= lead strip D= interspace width
What is the relationship between distance and height of grid strips? - An inverse relationship exists between the distance between the lead strips and grid ratio when the height of the grid strips remains the same.
Higher Grid ratios -This means that higher grid ratios are more effective at removing scatter. - For the same reason, higher-ratio grids require greater accuracy in their positioning and are more prone to grid errors. -Grids are sometimes rated according to their weight instead of ratio.
GRID FREQUENCY -Grid frequency is defined as the number of grid lines per inch or centimeter. -Grids are made with a range in frequency from 60 to 200 lines/inch (25–80 lines/ cm). -Most commonly used grids have a frequency of 85–103  lines/inch (33–41  lines/cm). -In general, grids with higher grid frequencies have thinner lead strips
what is the most important thing in determining the grid's efficiency when cleaning scatter? -By combining information about grid ratio and frequency, one can determine the total quantity of lead in the grid. -It is the grid’s lead content that is most important in determining the grid’s efficiency at cleaning up scatter.
What does the content of lead do for a grid? -In general, the lead content is greater in a grid that has a higher grid ratio and lower grid frequency. -As the lead content of a grid increases, the ability of the grid to remove scatter and improve contrast increases
GRID PATTERNS/Linear Grids -Grid strips can be made to run in one or two directions. -(Grids with lead strips running in only one direction are called linear grids)
Criss-cross or cross hatched grids - Grids may also be made by placing two linear grids on top of one another so the grid lines are running at right angles. -These grids are termed criss-cross or cross-hatched (Figure 18-6). -Dr. Bucky’s original grid was made using this pattern.
Linear Grids -Linear grids are more commonly used in clinical practice because they can be used when performing procedures that require tube angulation. -Linear grids allow the radiographer to angle the tube only along the direction that the lines are running. -For most grids, this is along the long axis. - In a grid located in a typical x-ray table, the grid strips run along the long axis of the table, which allows for angling the tube toward the head or feet of the patient.
Grid-cutoff - If the primary beam is angled into the lead, the lead will absorb an undesirable amount of primary radiation, resulting in a problem known as grid cutoff.
Criss-cross grids - When criss-cross grids are used, no tube tilt is permitted, as any angulation would result in grid cutoff because lead strips are running in both directions. - As a result, criss-cross grids have limited applications in radiography.
Short-axis grids -Short-axis grids are useful for portable chest procedures to decrease the change of grid cutoff when the cassette is placed crosswise.
GRID TYPES/Parallel grids -Parallel grids are made with the lead and interspace strips running parallel to one another. -This means that if the grid lines were extended into space they would never intersect
Focused grids -Focused grids are designed so that the central grid strips are parallel and as the strips move away from the central axis they become more and more inclined
grid radius -The distance from the face of the grid to the points of convergence of the lead strips is called the grid radius
What does it take for the grid to be properly focused? -For the grid to be properly focused, the x-ray tube must be located along the convergence line.
Short/long focal range grids -Short-focal-range grids (14–18 inches or 36–46 cm) are made for use in mammography; -long-focal range grids (60–72 inches or 152–183 cm) are used for chest radiography.
Focused grids -Focused grids with lower grid ratios allow for greater latitude in the alignment of the tube with the grid. -With higher grid ratios, proper alignment of the grid with the tube is more critical.
Parallel grids -Parallel grids are less commonly employed than focused grids. -Because the strips do not try to coincide with the divergence of the x-ray beam, some grid cutoff will occur along the lateral edges, especially when the grid is employed at short SIDs. -The parallel grid is best employed at long SIDs because the beam will be a straighter, more perpendicular one
GRID USES/Stationary grids -These are generally made approximately one inch larger than the image receptor size they are intended to cover. -Stationary grids are used primarily in portable procedures or for upright or horizontal beam views. -Some departments may also purchase a special cassette with a grid built into it. -This design, called a grid cassette, requires reloading between exposures using the grid.
Moving grids -The most common use of the grid is for procedures using the Potter-Bucky diaphragm (usually called the Bucky). -This device is mounted below the tabletop of radiographic and radiographic/ fluoroscopic tables and holds the cassette in place below the grid. -movinIt can move the grid during the exposure so that grid lines will be blurred and
moving grids cont -The direction in which the grid moves is important if it is to accomplish the job of blurring the grid lines. -The lead strips of the grid run along the long axis of the table. -To blur the lead lines, the grid must move at a right angle to the direction of the lines. -This means that it will be moving back and forth across the table and not from top to bottom.
Two movements mechanisms/Reciprocating grid - With the reciprocating grid, a motor drives the grid back and forth during the exposure for a total distance of no more than 2–3 cm.
Two movements mechanisms/Oscillating grid - With the oscillating grid, an electromagnet pulls the grid to one side and then releases it during exposure. -The grid oscillates in a circular motion within the grid frame
Grids and scatter -Grids absorb scatter and scatter adds exposure to the image receptor. -The more efficient a grid is at absorbing scatter, the less exposure will be received by the image receptor. -Therefore, compensations must be made to increase this exposure.
Grids and scatter cont This is generally accomplished by increasing mAs, which, in turn, results in greater patient dose. - The better the grid cleans up scatter, the greater will be the dose given to the patient to achieve an adequate exposure.
GRID CONVERSION FACTOR (GCFS)/ look at worksheets and study chart - Grid- conversion factors (GCFs) increase with higher grid ratios and increasing kVp. -Because grids vary in respect to their ratio, frequency, and lead content, it would be useful to check the grid conversion factor for each of the common grids used in a department.
GRID PERFORMANCE EVALUATION/SELECTIVITY, CONTRAST -The International Commission on Radiologic Units and Measurements (ICRU) Handbook 89 defines two criteria for measuring a grid’s performance: selectivity and contrast improvement ability.
Selectivity -Although grids are designed to absorb scatter, they also absorb some primary radiation. -Grids that absorb a greater percentage of scatter than primary radiation are described as having a greater degree of selectivity.
Selectivity cont -The better a grid is at removing scatter, the greater will be the selectivity of the grid. -This means that a grid with a higher lead content would have a greater selectivity.
Contrast - The contrast improvement factor is dependent on the amount of scatter produced, which is controlled by the kVp and volume of irradiated tissue. -As the amount of scatter radiation increases, the lower will be the  contrast and the lower the contrast improvement factor.
Contrast cont -If K 5 1, then no improvement in contrast has occurred. -Most grids have contrast improvement factors between 1.5 and 3.5. -This means that contrast is 1.5–3.5 times better when using the grid. -The higher the K factor, the greater the contrast improvement.
GRID ERRORS -Errors in the use of the grid occur mainly with grids that have a focused design. -This is because focused grids are made to coincide with the divergence of the x-ray beam. -Proper tube/grid alignment is essential to prevent the undesirable absorption of primary radiation known as grid cutoff.
off-level error -An off-level grid error occurs when the tube is angled across the long axis of the grid strips. -This can be the result of improper tube or grid positioning (Figure 18-9). -Improper tube positioning results if the central ray is directed across the long axis of the radiographic table.
off-level error cont -it is only possible to angle along the long axis of the table with a linear grid and it is not possible to angle at all with a criss-cross grid. -Improper grid positioning most commonly occurs with stationary grids being used for mobile procedures or decubitus views.
Off- center error -The x-ray tube must be centered along the central axis of a focused grid to prevent an off-center (off axis or lateral decentering) grid error . -If the central ray is off-center, the most perpendicular portion of the x-ray beam will not correspond to the most perpendicular portion of the grid. -The result is a decrease in exposure across the entire image (Figure 18-12). The greater the degree of lateral decentering, the greater the grid cutoff.
off-focus error -A focused grid is made to be used at very specific distances identified as the focal range labeled on the front of the grid. -When a grid is used at a distance other than that specified as the focal range, an off-focus error results. -For example, if a grid has a focal range of 36–44  inches (91–112  cm) and it is used at 72 inches, severe grid cutoff will occur.
off-focus error cont -Off-focus errors result in grid cutoff along the peripheral edges of the image (Figure 18-14). -Higher grid ratios require greater positioning accuracy to prevent grid cutoff.
upside down error - If the grid is used upside-down, severe peripheral grid cutoff will occur. -Radiation will pass through the grid along the central axis where the grid strips are most perpendicular and radiation will be increasingly absorbed away from the center. -It is important that the technologist check the tube side prior to using a focused grid.
The Moire Effect error -The Moire effect is a grid error that occurs with digital image receptor systems when the grid lines are captured and scanned parallel to the scan lines in the imaging plate readers. -This error occurs with grids used in a stationary fashion for examinations such as portable radiography or translateral hip images.
The Moire Effect error cont -In order for the Moire pattern to demonstrate on the image, the grid lines must be running in the same direction as the movement of the laser beam that is scanning the imaging plate
AN ALTERNATE SCATTER REDUCTION METHOD— THE AIR-GAP TECHNIQUE -The air-gap technique is an alternative to the use of a grid. - It has primary applications in magnification radiography and, to a lesser extent, in chest radiography. -The technique involves placing the patient at a greater object image receptor distance (OID), thus creating an air gap between the patient and the image receptor.
AN ALTERNATE SCATTER REDUCTION METHOD— THE AIR-GAP TECHNIQUE cont -By moving the patient away from the image receptor, the amount of scatter reaching the image receptor will be reduced. -Although the same amount of scatter will be created during the exposure, less of the scatter will reach the image receptor if the patient is moved farther away. -The result is improved contrast without the use of the grid. -The primary disadvantage of the air-gap technique is the loss of sharpness that results from increased OID.
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