Chp. 18

Grid history

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 *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 the very first grid was made in *1913* by American radiologist *Gustav Bucky* his grid consisted of wide strips of lead spaced 2cm apart and running in two directions "checkerboard" along the length and across. the grid was superimposed on the patients image It helped remove scatter and improve contrast *1920 Hollis Potter* a Chicago radiologist, improved Bucky's grid design by realigning the lead strips so they would run in only one direction, made the lead strips thinner, and less obvious on the image. He designed the device *Potter-Bucky Diaphragm* that allowed the grid to move during the exposure so the strips became blurred and no longer visible on the image. this significantly improved contrast without impairing the anatomy

the moire effect

Grid error that occurs with digital image receptor systems when 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 exams such as portables or translateral hip images. if the grid lines run in the same direction as the movement of the laser beam that is scanning the imaging plate, the moire effect happens most imaging plates are scanned across the short axis of the plate, and most often grid lines run parallel to the long axis of the imaging plate. in these instances, the moire effect will not demonstrate. to prevent this error, it is recommended that high frequency grids of 103 lines per inch or greater be used for digital image receptor systems when a stationary grid is needed.

grid selection/conversions

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. compensations must be made to increase this exposure. this is 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. the amount of mAs needed can be calculated using the following formula: *grid conversion factor (GCF) = mAs with the grid/ mAs without the grid* this formula is referred to as the Bucky factor as well as the grid conversion factor. *grid conversion factors (GCFs) increase with higher grid ratios and increasing kVp* when converting from one grid ratio to another the following formula is used: *mAs1/mAs2 = GCF1/GCF2* mAs1 = original mAs mAs2 = new mAs GCF1 = original grid conversion factor GCF2 = new grid conversion factor

grid types

Grids are either parallel or focused grids *parallel grids* - 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*- designed so that central grid strips are parallel and as the strips move away from the central axis they become more and more inclined. The focused design results in a grid with lead strips designed to math the divergence of the x-ray beam. if these lead strips were extended, the strips would intersect along a line in space known as the *convergence line* the distance from the face of the grid to the points of the convergence of the lead strips is called the *grid radius* *for the grid to be properly focused, the x-ray tube must be located along the convergence line* Each focused grid will identify the focal range within which the tube should be located. -short, medium, and long focal ranges depending on the distance for which they are designed short focal range grids (14-18 in or 36-46cm) are made for use in mammography long focal range grids (60-72 in or 152-183cm) used for chest radiography 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 are less commonly employed than focused grids. 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. parallel grid is best employed at long SIDs because the beam will be a straighter, more perpendicular one

Grid

a device used to improve the contrast of the radiographic image. it does this by absorbing the scatter radiation before it can reach the image receptor. when the x-ray beam passes through the body, one of three things occur with the primary photons that originated at the target. *1) pass through the body unaffected* *2) be absorbed by the body* *3) interact and change direction*

grid material

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 inexpensive and is easy to shape into very thin foil *Interspace material must be radiolucent*, it allows radiation to pass easily through it -aluminum or plastic fiber is used it should not absorb any radiation, but 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. 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 increased the absorption of primary photons. this can be a disadvantage, especially with low kVp technique 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 mammography

grid frequency

defined as the number of grid lines per inch or centimeter 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) *grids with higher frequencies have thinner lead strips* very-high frequency grids of 178-200 lines/inch are recommended for stationary grids used with digital image receptor systems to minimize the possibility of seeing the grid lines on the image the grids content that is most important in determining the grids efficiency at cleaning up scatter. lead content is measured in mass per unit area, or grams per square centimeter 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 performance evaluation

efficiency of a grid in cleaning up or removing scatter can be measured. the International Commission on Radiologic Units and Measurements (ICRU) handbook 89 defines two criteria for measuring a grids performance: *-selectivity* *-contrast improvement ability*

upside down

focused grid has an identified tube side based on the way the grid strips are angled. 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.

grid patterns

grid strips can be made to run in one or two directions *linear grids*: running in only one direction *criss-cross* or *cross hatched*: placing two linear grids on top of one another so the grid lines are running at right angles "Dr. Bucky's original grid"

selectivity

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. defined by the Greek sigma selectivity = % primary radiation transmitted/ % scatter radiation transmitted 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*

grid ratio

major influence on the ability of the grid to improve contrast. defined as the ratio of the height of the lead strips to the distance between the strips Grid ratio = h/D h (lead strip height) D (interspace width) if the height of the grid is constant, decreasing the distance between the lead strips would result in an increase in the grid ratio. *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 allow less scatter radiation to pass through their interspace material to reach the image receptor. the higher the grid ratio, the straighter the scattered photon has to be in order to pass through the interspace material. higher grid ratios are more effective at removing scatter, 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 expressed in terms of grams per square centimeter (g/cm^2) or occasionally in the US as grams per square inch (g/in^2)

linear and criss-cross grids

more commonly used because it allows 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 typical x-ray table, the grid strips run along the long axis of the table which allows angulation of the tube towards the head or feet of the patient. if the primary beam is angled into the lead, the lead will absorb an undesirable amount of the primary radiation resulting in a problem known as *grid cutoff* when criss cross grids are used, no tube tilt is permitted, as any angulation would result in grid cutoff because the lead strips are running in both directions. as a result, criss cross grids have limited applications in radiography grids are also sometimes classified according to the direction of grid lines run within the grid. most grid lines run along the long axis of the grid, but it is possible to obtain short-axis grids, which have grid lines running across the short axis of the grid. short axis grids are useful for portable chest procedures to decrease the change of grid cutoff when the cassette is placed crosswise

an alternate scatter reduction method: the air-gap technique

most important way to improve image quality is to decrease the amount of scatter initially being created. this is best done by restricting the primary beam. collimating to the size of the area being examined is critical to image quality. the body part may also be compressed to decrease the amount of scatter created. the air-gap technique is an alternative to the use of a grid. 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. by moving the patient away from the image receptor, the amount of scatter reaching the image receptor will be reduced. the patient is the source of the majority of scatter. 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 father away the result is improved contrast with the use of the grid. the primary disadvantage of the air-gap technique is the loss of sharpness that results from increased OID. Gould and Hale evaluated the use of a grid and the air-gap technique, showing that a 10 inch air gap has the same degree of clean up of scatter as a 15:1 grid for a 10cm body part.

off-level

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 improper tube positioning results if the central ray is directed across the long axis of the radiographic table 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. Ex. if the patient is lying on the grid and the weight of the patient is unevenly distributed, the grid may not be properly aligned to the tube off-level grid error can occur with a focused grid and it is the only positioning error possible with a parallel grid. when this error occurs, there is an undesirable absorption of primary radiation, which results in a radiograph with a decrease in exposure across the entire image.

grids cont.

photons that pass through the body unaffected will interact with the image receptor to create the image. photons are responsible for creating the contrast (differences in the image receptor exposures or densities) on the image absorption of photons occurs as the result of *photoelectric interaction*. this interaction results in the complete absorption of the primary photon and the production of a secondary photon. the secondary radiation created by this interaction is very weak and is quickly absorbed in the surrounding tissues primary radiation that interacts and, as a result of this interaction, changes direction is known as *scatter radiation* interaction that produces scatter is known as *Compton interaction* these interactions can result in photons that are strong enough to be emitted by the patient and interact with the image receptor because these photons have changed direction, they are no longer able to record exposures on the image receptor that relate to the patients anatomy. no diagnostic value. scattered photons add an overall exposure to the image receptor and as a result of this overall graying of the image contrast is lowered. Compton interactions increases with increased kVp* *scatter radiation increases and contrast is further impaired as kVp increases*

grid errors

poor images can result from improper use of the grid. error 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. the tube must be centered to the focused grid and aligned at the correct distance. a focused grid has a tube side and a receptor side based on the angulation of the grid strips. proper tube/grid alignment is essential to prevent the undesirable absorption of primary radiation known as *grid cutoff*

grids cont.

scatter increases with increases in volume of the tissue irradiated and decreases with increased atomic number of the tissue. the volume of tissue that is irradiated is controlled by the thickness of the patient and the exposure field size. field size can be kept to a minimum by collimation. the atomic number of the tissue will also affect the quantity of scatter. the greater the atomic number of tissue, the less will be the quantity of scatter created. Ex. 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. *the amount of scatter radiation increases with* *1) increases in patient thickness* *2) larger field sizes* *3) decreases in atomic number of the tissue* It is necessary to use a grid for with thicker, larger body parts and with procedures that require higher kVp technique*** *1) body part thickness exceeds 10cm* *2) kVp is above 60*

contrast improvement ability

the best measure of how well a grid functions is its ability to improve contrast in the clinical setting. 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. the contrast improvement factor (K) K= radiographic contrast with the grid/ radiographic contrast without the grid If K= 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*

off-focus

used at 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 if a grid has a focal range of 36-44inches and it is used at 72 inches, severe grid cutoff will occur. off-focus errors result in grid cutoff along the peripheral edges of the image. higher grid ratios require greater positioning accuracy to prevent grid cutoff.

off-center

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. the center grid strips are perpendicular and become more and more inclined away from the center. this design coincides with the divergence of the x-ray beam from the tube. if the central ray is off-center, most of the perpendicular portion of the x-ray beam will not correspond to the most perpendicular portion of the grid - decrease in exposure across the entire image

grid construction

-selection of material -grid ratio -grid frequency

grid uses

-stationary position -mounted in a Potter Bucky diaphragm to move it during exposure *Stationary*: made approximately one inch larger than the image receptor size they are intended to cover. they are used primarily in portable procedures or for upright or horizontal beam views. some departments may purchase a special cassette with a grid built into it. this design is called a *grid cassette* that requires reloading between exposures using the grid this may be an inconvenience in situations where multiple images are needed. Grid lines on the image will usually be noticed on close inspection. this is true with low frequency grids. high frequency grids, like those used for digital imaging have a minimal visual effect *Potter-Bucky diaphragm:* 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. it can move the grid during the exposure so that grid lines will be blurred and therefore not evident on the image. approximately 17" X 19", large enough to cover a 14" X 17" cassette lengthwise or crosswise. 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. it moves back and forth across the table and not from top to bottom. two mechanisms: -reciprocating -oscillating *reciprocating*- a motor drives the grid back and forth during the exposure for a total distance of no more than 2-3cm. *oscillating*- an electromagnet pulls the grid to one side and then releases it during exposure. it oscillates in a circular motion within the grid frame.