RADIOGRAPHIC GRIDS

Abstract Grids are devices that are used to improve contrast on a radiographic image. This improvement of contrast is achieved by absorption of scatter radiation produced by the patient as the primary beam interacts with the patient’s tissues. This contrast improvement also comes at a cost. As the grids’ ability to improve contrast increases, so does the patient’s radiation dosage per radiographic exposure. In general when a radiographic exposure is performed using a grid device, the primary photons will either: 1. Pass through the body tissue unaffected. 2. Become absorbed by the tissues within the body or 3. Interact with body tissues and change direction (Compton’s scatter).

Objectives and Outline At the completion of this lecture, the participant will: Be able to describe the rationale for using a grid. Know the history of radiographic grids. Explain grid construction. Explain grid efficiency. Employ grid conversion factors in setting techniques.

Objectives and Outline History 1. Grid invention a. 1913 1. Dr. Gustav Bucky b. 1920 1. Dr. Hollis Potter B. Grid construction 1. Grid material 2. Inter-space material 3. Grid design a. Parallel b. Focused 1. Grid radius c. Cross Hatched C. Grid Efficiency 1. Grid ratio 2. Grid frequency D. Grid conversion factors 1. Factor assignment re. Grid ratio

What is a Grid? A device placed between the patient and film for the purpose of absorbing scatter radiation before it can interact with the imaging receptor.

History - First grid was made by Dr. Gustav Bucky. Consisted of wide strips of lead approx. 2cm apart in a crisscross pattern. Despite the crudeness, it removed enough scatter to improve contrast.

History 1920 – Dr. Hollis Potter improved the grid device. Realigned the lead strips to run in one direction. Made the lead strips thinner. Designed the Potter-Bucky diaphragm which allowed the grid to move during the exposure.

Grid Construction Grid Materials A series of radiopaque lead strips which alternate with radiolucent materials. a. Strips are held firmly together then sliced into flat sheets. b. Lead is the radiopaque material of choice. Interspace materials are radiolucent. a. Aluminum b. Plastic fibers

Grid Construction Aluminum is more common than plastic fiber b/c of ease to manufacture, durability and provides additional absorption of low energy photons. Disadvantage when using low kVp technics. Fiber Interspace grids are preferred when using low kVp technics (pediatric radiography).

Grid Construction 3. Grid design a. parallel grids 1. All lead strips are straight up and down. 2. Less commonly employed than focused grids. 3. Best used with longer SID’s b/c beam is straighter and more perpendicular at longer SID’s. b. focused grids 1. Lead strips are tilted toward the center to correspond with the divergence of the X-ray beam. 2. Convergence lines: A distance in space where if the grid lines were extended above the grid surface they would intersect.

Grid Construction Grid radius: The distance from the grid face to the points of convergence of the lead strips. Each focused grid will identify the focal range within which the tube should be located. Grid patterns Linear All lead strips run in the same direction & are straight up and down. Crisscross Contains two sets of lead strips at 90 degrees from one another. Cross-hatched Equivalent of two linear grids not quite at 90 degrees.

Grid Efficiency The ability of a grid to clean up scatter and improve contrast. Criteria for efficiency measurement: 1. Selectivity 2. Contrast Improvement Ability.

Grid Efficiency Selectivity measures a grid’s ability to absorb a greater percentage of scatter than primary radiation. Measured by: % of primary radiation transmitted % of scatter radiation transmitted. Thus, a grid with high lead content would have a greater selectivity.

Grid Efficiency Contrast Improvement Ability is measured by how well a grid functions to improve contrast in the clinical setting.

Grid Efficiency This is measured by: K= Radiographic contrast with the grid Radiographic contrast w/o the grid. Note that this is dependent upon the kVp used and the volume of tissue irradiated. Most grids have a K of 1.5 to 3.5. Thus the higher the K factor, the greater the contrast improvement.

Grid Ratio Calculated by the height of the lead strips divided by the distance between them. i.e., strips 1.2mm high; 0.1mm apart = 12:1 grid ratio.

Grid Ratio Grid ratio plays a major role in the grid’s ability to improve contrast Thus if the height of the grid is constant, the distance b/w the lead strips was decreased, this would result in an increase in the grid ratio.

Grid Frequency The number of grid lines per inch or centimeter. Grid frequency ranges from 60 to 196 lines/inch (25-78 lines/cm) Most commonly used grids have a frequency of 85-103 lines/inch (33-41 lines/cm). In general as the lead content increases, the ability of the grid to remove scatter and improve contrast increases.

Grid Selection/Conversion Note: Grids absorb scatter radiation, scatter adds density to the radiographic image and decreases the overall contrast. Thus, the more efficient is the grid the less density is produced on the image receptor. As a general rule, any anatomical part measuring 10cm or greater should be imaged with a radiographic grid. Any radiographic technic using more than 70 kVp should employ the use of a grid device.

Grid Selection/Conversion Grid conversion factor is best used when it is necessary to change grids while maintaining a similar density on a subsequent radiographic image.

Grid conversion Factor GCF = mAs with a grid mAs w/o a grid

Grid conversion Factor Example: A CXR is produced using 5 mAs at 85kVp w/o a grid. A second film is to be produced using a 12:1 grid. What mAs is needed to produce a satisfactory radiographic image with a similar density? X 5 =----- 5 Answer: 25 mAs

Grid conversion Factor When converting from one radiographic grid to another, the following formula should be used. mAs1 GCF1 ------ = ------ mAs2 GCF2 mAs1 = original mAs mAs2 = new mAs GCF1 = Original grid conversion factor GCF2 = new grid conversion factor

Grid conversion Factor Example: A satisfactory radiographic image of the abdomen is produced using an 8:1 Grid, 35mAs and 86kVp. A second film is requested using a 12:1 grid. What mAs is needed to produce a second satisfactory image? 35 4 ------- = ------- X 5 4X = 175 X = 43.75

Grid Conversion Factors Grid Ratio Conversion Factor no grid 1 5:1 2 6:1 3 8:1 4 10:1 5 12:1 5 16:1 6

Grid Errors Off Level Grid Error This occurs when the CR is angled across the long axis of the grid strips/across the radiographic table. This most commonly occurs during bedside radiography. E.g. pt’s body weight is not evenly distributed on the grid device.

Grid Errors When this occurs, there is an undesirable absorption of the primary beam which results in a radiograph with a decreased density across the entire image.

Grid Errors Off Center Grid Error This occurs when the CR is not properly centered to the grid device. It results in a decrease in density across the entire film.

Grid Errors Off Focus Grid Error This error occurs when a grid is used at a distance other than that is specified as the focal range. This results in grid cut-off along the peripheral edges of the film. This is especially common when using higher grid ratio grids.

Grid Errors Upside-Down grid error This type of grid error occurs when the radiographic grid is used upside-down. Severe peripheral grid cut-off occurs. The radiation will pass through the grid along the central axis where the grid strips are most perpendicular.

Air Gap Technique An alternative to the use of a grid. Its primary usage is magnification radiography. Results in an increased OID, thus creating an air gap b/w the patient and the film. The end result is that less scatter radiation reaches the image receptor. Contrast is is improved. Primary disadvantage is a loss in detail and sharpness of the image. It has been demonstrated by Gould and Hale that a 25cm air gap is equivalent to a 15:1 grid for a 10cm body part.

References   Bushberg et al, The Essentials of Physics and Medical Imaging, Williams & Wilkins Publisher. Bushong, S., Radiologic Science for Technologists, Physics, Biology and Protection, 8th Edition, C.V. Mosby Company. Cullinan, A., Producing Quality Radiographs, Lippincott, New York, 1987. Carlton et al, Principles of Radiographic Imaging, An Art and Science, Delmar Publishing. Selman, J., The Fundamentals of X-Ray and Radium Physics, 8th Edition, Charles C. Thomas Publisher.