University of Guyana BMI 2202 – Science and Instrumentation II Lecturer – Ms. Schimze Sagon Presented by – Patrick Fung 1013992 Explain what is a radiographic grid, including materials used, grid ratio and grid frequency Describe and differentiate between grid type and grid pattern Explain the relationship of grid selection to patient dose and image receptor exposure Describe methods for evaluating grid performance. Grid cut off and the effects of these errors on the radiographic image Discuss the physical characteristics and uses of immobilization devices and how they help to reduce patient dose and/or improve image quality What is a radiographic grid? • Radiographic grids or grids, invented by Dr. Gustave Bucky in 1913, are “sheets” made of parallel strips of high attenuating material, lead is the radiopaque material of choice, separated by spaces filled with low attenuating material, e.g. Aluminum or carbon fiber. • They are placed between the patient and the x-ray film to improve image contrast by reducing scattered radiation, mainly via Compton Effect. • It is most useful when imaging thicker patients where there is a more probability for scattered radiation. Grid Ratio • The grid ratio is the working ability of the grid. • It is calculated as the height of the lead strips divided by the distance between them .i.e., strips 1.9mm high; 0.2mm apart = 19:2 grid ratio. Grid Frequency • Grid frequency is the amount of grid lines or radiopaque strips per inch or centimeter. • In general as the radiopaque content increases, the ability of the grid to remove scatter and improve contrast increases. Grid Pattern • This is the orientation of the lead strips in their longitudinal axis as viewed from the top • Some of the main patterns are; 1. Crossed 2. Linear or Parallel 3. Focused FOCUSSED GRIDS A grid in which the absorbing strips are slightly angled towards the focal spot. The grid can therefore be used only at a specified focal distance. Otherwise the grid will absorb the primary radiation and parts of the film are barely exposed. Focused grids may be linear or crossed LINEAR OR PARALLEL GRIDS A grid where the lead strips are parallel to each other in their longitudinal axis. Most linear grids are focused, i.e. their strips are slightly tilted, converging at a line in space (the convergent line). A non-focused linear grid also has strips that are parallel when viewed in cross-section; this is called a parallel grid. Many X-ray tables are equipped with linear, focused grids, and the strips in these grids are parallel with the long axis of the table allowing the X-ray tube to be tilted in this direction. CROSSED GRIDS A grid consisting of two superimposed parallel grids having the same focusing distance. Such grids are very efficient in removing scattered radiation but must, however, be arranged at exactly 90 degrees to the beam. This makes the use of crossed grids limited. There are two types of grids used for radiographic purposes 1. Moving Grids The moving grid was invented by Hollis E. Potter in 1920. A grid which is set into motion (one way or oscillating) just before an X-ray exposure starts. The intention is to remove or reduce the shadows produced by the lead strips in the grid. This method increases patient dose. 2. Stationary Grids In this grid, the X-ray opaque strips are so thin and so close together that the grid can remain stationary without the shadows of the strips being sufficiently visible to interfere with the image detail of the film Grids absorb scatter radiation, scatter adds density to the radiographic image and decreases the overall contrast. Thus, the higher the efficiency of 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 technique using below 90kVp 8:1 grid recommended. Techniques using above 90kVp 12:1 grid recommended. There are three main method of grid performance; 1. Primary Transmission This is the measurement of the percentage of primary radiation transmitted through a grid . 1. Made with the grid in place to determine the intensity of the radiation through the grid . 2. 2. Made after removal of the grid to determine the intensity of the radiation directed at the grid . Primary Transmission (Tp) = Intensity with grid/intensity without grid X 100 (%) • The ideal Primary transmission is 100%, however typical values are ~55-75% • The theoretical calculation accommodates for the fraction of the grid that is interspace. Theoretic Cal. Tp (%)= 100 X W / (W+w) where W = Interspace Thickness w = Lead strip thickness 2. Bucky Factor Bucky Factor is the ratio of the incident radiation on the grid to the transmitted radiation passing through the grid . • Indicates the actual increase in exposure dur to the grid’s presence. • Measure of the Grid’s ability to absorb scatter radiation • Unlike primary transmission , Bucky factor indicates the absorption of both primary and secondary radiation. • Higher Grid Ratio = Higher Bucky Factor B= INCIDENT RADIATION / TRANSMITTED RADIATION 3. Contrast Improvement Factor This is the ultimate test of a grid’s performance. It is the ratio of the contrast with a grid to the contrast without a grid, and is a measure of the grid’s ability to improve contrast, which is its primary function. • It depends on : 1. kVp 2. Field size 3. Phantom thickness Higher Grid Ratio = Higher Contrast Improvement Factor • Grid cut off is the loss of primary radiation that occurs when the images of the lead strips are projected wider than they would be with ordinary magnification. • Cut off is complete and no primary radiation reaches the film when the projected images of the Lead strips are thicker than the width of the interspaces. • Amount of cut off is always greatest with high ratio grids and short grid focus distances. Situations that produce grid cut off are; 1. Focused grids used upside down 2. Lateral Decentering or grid angulation 3. Distance decentering 4. Combined decentering When a focused grid is used upside down, there is severe peripheral cut off with a dark band of exposure in the center of the film with no exposure at the periphery. The higher the grid ratio, narrower the exposed area . When the X-ray tube is positioned lateral to the convergent line but at the correct focal distance. There is uniform loss of radiation over the entire surface of the grid, producing a uniformly light radiograph. When a linear grid is tilted , there is uniform loss of primary radiation across the entire surface of the grid . Effect on the film being same as that of lateral decentering This occurs when the target of the X-ray tube is correctly centered to the grid , but it is positioned above or below the convergent line . The central portion of the film isn't affected but the periphery is light. The loss of primary radiation is directly proportional to the grid ratio and the distance from the center line. This causes an uneven exposure resulting in a film that is light on one side and dark on the other side. The effect is directly proportional to the grid ratio and decentering distance and inversely proportional to the focal distance of the grid . • One of the many factors that affect diagnostic quality is motion. For instance the wiggly hand of a 3 year old or the shaking hand of a badly injured patient. The resultant image for these instances would not be of appropriate diagnostic quality • Forms of immobilization or restraint can be simple or intricate. • The simplest techniques involve the use of a radiolucent positioning sponge to support the anatomic area of interest or gently laying a sandbag across a patient’s forearm to minimize shaking caused by patient anxiety. • More complex techniques might involve completely wrapping an infant or small child in a sheet (often referred to as a mummy wrap) or securing an accident victim to a backboard Curry TS, Dowdey JE, Murry RE. Christensen ́s Physics of Diagnostic Radiology 4 Ed. Lippincott Williams & Wilkins. (1990) ISBN:0812113101. 2. JPI Healthcare Korea, X-Ray Grids 3. Bushberg et al, The Essentials of Physics and Medical Imaging, Williams & Wilkins Publisher 4. Upstate Medical University, https://www.upstate.edu/radiology/education/rsna/radiogr aphy/scattergrid.php 5. https://radiologykey.com/immobilization-techniques/ 1.
