Lessons in Radiography Using the X-Ray Simulation Program Student Booklet

Transcription

1 Lessons in Radiography Using the X-Ray Simulation Program Student Booklet Prepared by the North Central Collaboration for Education in NDT. Partial support for this work was provided by the NSF-ATE (Advanced Technological Education) program through grant #DUE Opinions expressed are those of the authors and not necessarily those of the National Science Foundation.

2 Table of Contents Introduction... 1 Contents of this Training Packet... 1 Lesson One: Film Properties... 2 Introduction... 2 Objectives... 2 Log Relative Exposure Calculations... 4 Sensitometric Curve Calculations for AGFA Film... 5 Simulator Exercise... 6 Laboratory Exercise... 9 Lesson Two: Kilovoltage Effect on Contrast Introduction Objectives Calculations Simulator Exercise Writing Assignment Laboratory Exercise Lesson Three: Milliamperage and Time Relationships Introduction Objective Reference Terms Formulas for Time and milliamprage Calculations Simulator Exercise Laboratory Exercise Lesson Four: Creating an Exposure Chart Introduction Objective Reference Terms Calculations Simulator Exercise Laboratory Exercise Lesson Five: Source-to-Film Relationships Introduction Objective Reference Terms Simulator Exercise Calculations for source to film distance change Laboratory Exercise Lesson Six: Radiographic Equivalency Factors Introduction Objective Reference Terms Calculations Simulator Exercise Laboratory Exercise Lesson Seven: Defect Content Introduction Objective... 38

3 Reference Terms Simulator Exercise Laboratory Exercise Lesson Eight: Defect Shape, and Relationship to X-ray Source Introduction Objective Reference Terms Simulator Exercise Laboratory Exercise Lesson Nine: Image Magnification, Beam Divergence and Distortion Introduction Objective Reference Terms Geometric Enlargement (Magnification) Calculations Geometric Unsharpness Calculations Beam Coverage Calculations Simulator Exercise Laboratory Exercise Lesson Ten: Developing a Radiographic Technique Card Introduction Objective Reference Terms Simulator Exercise Laboratory Exercise... 56

4 Introduction Successful radiography depends on numerous variables that affect the outcome and quality of a radiograph. Many of these variables have a substantial effect on the results while others have only a minor effect. One of the difficulties encountered when learning the basics of radiography is that there is a relatively long lag time between when an exposure is made and when the resulting radiograph is ready for viewing. This makes it a time consuming exercise when the lesson call for multiple exposures to be made in order see the effects that certain exposure variables have on image quality. These lessons use an X- ray inspection simulation program that allows beginning radiographers to experiment with set-up and exposure variables. Simulated radiographs are produced in a matter of seconds providing immediate feed on the impact that these variables have on the image quality. The X-ray simulator (XRSIM) program used for these exercises was developed by researchers at the Center for NDE at Iowa State University. The XRSIM program allows physically accurate radiographic inspections to be simulated using a computer aided design (CAD) model of a part. These lesson plans that make use of the XRSIM program were developed with funding from the National Science Foundation (NSF). Contents of this Training Packet Each of the ten lessons in this packet includes: an introduction to the main subject of the lesson statement of the objectives definition of new terms exercises that make use of the X-ray simulation program exercises for hand-on lab activities 1

5 Lesson One: Film Properties Introduction In this lesson, the relationship between film speed and exposure will be evaluated. Calculations will be made using sensitometric curves to determine the best choice of film for a given exposure or visa-versa. The manufacturer of the film makes no difference, as all films will work in the same manner. Review the film manufacturer s literature available at your school before beginning this exercise. Objectives To develop an understanding of industrial radiographic film properties including the characteristic curves for various films. To learn to calculate changes in film speeds while maintaining a constant density of the radiograph. Reference Terms Radiographic sensitivity is the general term used to describe the smallest detail that can be seen using a radiograph. The term also refers to the ease with which images can be seen or details can be detected. Radiographic sensitivity, in other words, is the amount of information that can be found on a radiograph. There are two independent sets of factors that determine the level of radiographic sensitivity, definition and contrast. Definition refers to the sharpness of the outline in the image. This sharpness depends on the type of screens and film used. The sharpness of the outline also depends on the radiation energy used, the geometry of the radiographic specimen and the spot size of the radiation source. Radiographic contrast is the difference between the areas of radiographic density. This difference in the density depends on two factors, subject contrast and film contrast. Subject contrast is the ratio of X-ray intensities transmitted through two or more portions of different thickness in a specimen. The energy of the radiation used and its distribution, and the intensity of the radiation used have to be taken into account when determining the subject contrast. Film contrast refers to the slope of the characteristic curve of the film. This contrast depends on three factors, the type of film being used, the density of the film, and the way the film is processed. The process used to expose the film is also important. Of particularly importance is whether the film has been exposed using lead screens or fluorescent screens. 2

6 Film grains are the materials that capture the image, which after development becomes a radiograph. Film grains are made-up of extremely fine silver bromide crystals. There can be billions of grains in one square centimeter. The larger the grain size the faster the film. Industry uses a number of film speeds to produce the desired radiographic image depending on the quality of the radiograph sought. Logarithm is the number of the exponent of the power of 10 that a number is raised. Consider a six-inch scale as a linear form of measurement. If we wanted to represent a great distance in the physical distance of six inches, we would use a logarithmic measurement or scale. The logarithmic scale moves by factors of ten, and becomes more compressed the closer to ten the numbers become. Once ten is reached we began counting by one hundred, and then by one thousand. In radiography, and ultrasound the logarithmic scale will be used often. Sensitometric charts are comprised of a group of curves representing the relative sensitivity of various X-ray films to exposure. The curves are a plot of the film density versus the logarithm of the relative exposure they must receive to reach a particular density. Relative exposure means that one of the curves is used as the standard and all other curves are related to this curve by a factor. The charts are used to make comparisons between various films and to make exposure and density calculations when changing from one film to another. 3

7 Log Relative Exposure Calculations Use the sensitometric curves in Figure 1 on page 9 to perform the following calculations: Example. A density of 2.0 was produced using Type I film with an exposure time of 3.0 milliampminutes. A density of 3.5 on the same film is desired. Find the 2.0 density on the left of the chart; move horizontally to film Type I and then move down to the log relative exposure number at the base of the chart (1.30). Repeat these steps to find the log relative exposure number for a density of 3.5 (1.65). Subtract 1.30 from 1.65, which leaves Take the inverse log of this number, which is Now multiply the original time (3.0) by 2.24 to get the new exposure setting of 6.72 ma-minutes (4 ma for 1 minute 41 seconds) to achieve a 3.5 density on the same film. If the density were to be decreased, or if a second faster film were used (further to the left on the chart.), the original exposure time would be divided by the exposure factor. 1. If a density of 2.0 is produced using Type I film and an exposure of 25-mA-minutes, what exposure setting is needed to produce a density of 1.5 using the same film? 2. If an exposure of 8-mAm produces a density of 2.5 when a Type I film is used, what exposure is needed to produce the same density using a type 2 film? 3. Using a Type II film, a density of 2.0 is produced with an exposure of 11-mAm. What exposure is required to increase the density to 3.5 using the same film? 4. If a density of 1.5 is produced using Type II film and an exposure of 8-mAm, what exposure is necessary to produce a density of 2.5 on type 3 film? 5. If a density of 2.25 is produced using Type III film and an exposure of 21-mAm, what exposure in necessary to produce the same density on type II film? 6. If an exposure of 5-mAm produces a density of 2.0 using a Type I film is, what exposure is necessary to produce the same density using a type III film? 7. Using a Type II film, an exposure of 9-mAm produces a density of 2.5. What exposure is necessary to increase the density to 3.0 on the same film? 8. If an exposure of: 7-mAm density of 3.0 on a Type II film, what exposure is required to produce a 2.0 density on a type 3 film? 9. Using a Type I film, if an exposure of 14-mAm produces a density of 3.0, what exposure is needed to reduce the density to 1.5 on the same film? 10. If a density of 2.4 is produced on a type III film with an exposure of 26-mAm, what exposure is required to produce the same density on a type II film? 4

8 Sensitometric Curve Calculations for AGFA Film Using the film chart Figure 2 on page 10, calculate the exposure times needed to accommodate the density or film type changes. Example: An exposure of D-4 film for 1.5 minute produced a radiograph with a density of 1.5. The specification requires a density of 2.5. Use the sensitometric curves to select a new exposure time. First find the 1.5 density point then move horizontally to the D-4 film. Then move down the chart to locate the log number (2.51) for that exposure. Next move to the new density 2.5 and move horizontally to the D-4 film, and then down the chart to locate the log number (2.75). Now subtract the two log values and take the inverse log of this value ( = Take the inverse log of 0.24 to get ). Round this number to the nearest tenth and multiply it by the original exposure time (1.5 X 1.7 = 2.55 minutes) to get the exposure time needed for a 2.5 density on your radiograph. 1. If a density of 1.5 is produced on D8 film with an exposure of 9-mAm, what exposure is required to produce a density of 3.0 on D4 film? 2. What exposure is required to change the density on D5 film to 3.5 when an exposure of 12-mAm produces a density of 2.3? 3. If a density of 3.0 is produced with an exposure of 22-mAm when D3 film is used, what exposure will produce a density of 3.0 when D7 film is used? 4. When D2 film is used, a density of 2.25 is produced with an exposure of 17-mAm. What exposure is required to produce a density of 3.5 on D4 film? 5. Using D4 film, what exposure is required to increase the density to 3.0 when an exposure of 7-mAm produces a density of 1.5? 6. If an exposure of 3-mAm produces a density of 3.5 on D7 film, what exposure is necessary to produce a density of 3.0 on D5 film? 7. If a 24-mAm exposure produces a density of 1.5 on a D4 film, what exposure will produce a density of 2.5 on D7 film? 8. If a density of 2.0 is produced on D8 film with an exposure of 6-mAm, what exposure is needed to produce a density of 2.5 on D5 film? 9. Using D4 film, what exposure is required to raise the density to 3.5 when an exposure of 6-mAm produces a density of 2.5? 10. If a 7-mAm exposure produces a density of 3.5 on D5 film, what exposure is needed to produce a density of 2.0 on D3 film? 5

9 Simulator Exercise Use the XRSIM program to evaluate the relationship between film speed and exposure time by completing the following exercise. 1. Review the AGFA sensitometric curves in Figure 2. This chart shows the speeds and characteristics of various films. Review Table 1, which shows ratios of film speeds. Notice in Table 1 that all films are referenced to D-7 film. 2. In the XRSIM program, open the stepwedge11cm CAD file and select 2024Al (aluminum alloy 2024) as the material. Below your material select the display tab, and check translucent to allow the defect to be viewed within the part. Using the defect tab select the sphere and choose air as the material. Locate the defect below the surface on step five. Select the detector tab and change the film to D-7. Use the top view and click and drag the film to expand it so the film extends beyond the part (The stepwedge may need to be moved slightly to reveal the film.) Use the HOMX160 generator with an initial setting of 60 kv, 1 ma, and 7 seconds to produce an image with a density reasonably close to 2.5 on step five of the stepwedge. Use the view/slice option to evaluate the density of the stepwedge. NOTE: Remember to change the name of the XRSIM density image (xyz file) each time you produce a simulated radiograph or the system will save one file over the other. To open multiple images for viewing, go to open, change file type to density image and select the desired file. Open the slice option by left clicking on the Imageslice icon. When the slice window opens, select horizontal and move your cursor over step six on the stepwedge to measure the density. 3. Using the film characteristic curves, calculate a change of density from 2.5 to 3.5 on D-7 film. Produce an image using XRSIM to confirm the calculation. 4. Change the detector to D-5 (which has a smaller grain and is a slower film) and use the appropriate calculations to determine the setting needed to generate a density of 3.5 on step five. Produce an image to confirm the calculation. 5. Manufacturer s film speed ratio charts give an overview of the characteristic curve relationships. On Chart 3, note the speed ratio differences for the films produced by this company. Notice that the manufacturer has chosen D-7 as the standard or 0 film. D-7 will be the standard against witch other films will be compared. For example, if you exposed a D-7 film for 33-milliamp seconds, the chart indicates that a D-5 film would need to be exposed 1.8 times as long. Note the differences between the other films. Also note the change of exposure ratios as kilovoltage is increased. Why is this? 6

10 6. Using the film characteristic curves for AGFA films (Figure 1), compare two films, D- 4 and D-7. If a density of 2.0 were achieved when D-4 film is exposed for 43 mas at 100 kv, what would be the correct exposure if D-7 film were used? 7. After calculating the time difference look on the "relative exposure factor" chart and find the ratio between the two films. 8. Was the time change 3 to 1 as predicted by the exposure factor chart? Density Figure 1. Film sensitometric (characteristic) curves for three typical film types. 7

11 Figure 2. Film sensitometric curves for Agfa Structurix industrial X-ray films. Film Type Radiation 100 KV 200 KV ir 192 D D D D D D Figure 3. Approximate Relative Exposure Factors for Agfa Structurix Industrial X-ray films. 8

12 Laboratory Exercise This exercise requires the use of an X-ray generator. Prior to the operation of this equipment, the required safety training must have been completed and instructions on the safe operation of the X-ray equipment must have been received. 1. Produce a film radiograph of a 0.1 to 0.5-inch steel stepwedge using the X-ray system of your choosing in the laboratory. Select an exposure setting (kv, ma and time) for any one of the steps from an existing exposure chart for the system. Place a lead letter on the edge of the step and make the exposure. Did your exposure chart correctly predict the density you achieved? If not, what was the difference in density? 2. Using the manufacture exposure chart for the film used above, and the density produced on your first exposure, calculate the correct exposure time for a change of film (faster or slower). Show your calculations, and new time below. 3. Produce a second radiograph using the calculated change in time to compensate for the change of film. Allowing for some deviation (+/- 5 percent), did your calculations produce the density predicted? If not, identify some of the possible reasons that for the discrepancy. 9

13 Introduction LESSON TWO: KILOVOLTAGE EFFECT ON CONTRAST In this lesson, the relationship between energy of the X-rays and the image contrast will be evaluated. The energy of the X-rays is controlled by the electrical potential of the X-ray tube. This potential is generally in the range of thousands of volts. X-ray generators will have a control that allows the voltage to be adjusted and this is generally labeled in kilovolts. Therefore, it is common to speak of the energy of X-rays in terms of the voltage used to produce the X-rays and this value is often referred to as the kilovoltage. The kilovoltage has an effect on the radiographic image because the energy used to produce the X-rays is related to the penetrating power of the X-rays. The first requirement in developing a radiographic procedure is to determine what level of radiation is necessary to penetrate the object being inspected and produce an image in a practical exposure time. If too high of kilovoltage is used, image quality deteriorates. A number of methods have been developed for determining the kilovoltage needed to radiograph an object of certain thickness and material. The two methods that will be covered here are the Half-Value Layer Method (HVL) and the Fixed Exposure Method. Objectives To evaluate how X-ray energy (kilovoltage) effects radiograph quality. To investigate how different radiographic films affect image contrast. To lean to apply several multiple methods of determining the kilovoltage required for an exposure. Reference Terms Kilovoltage is a measure of the electrical potential between the anode and the cathode in the X-ray tube. The higher the kilovoltage, the shorter the wavelength and the greater the penetrating power of the X-rays produced. The energy level used for an exposure will be greatly influenced by the material being radiographed because the X- rays must have enough energy to penetrate through the material being inspected. The kilovoltage should be chosen carefully because: 1. scatter increases as the energy increases scatter increases 2. contrast sensitivity decreases as energy increases Latitude refers to the amount of thickness variation that a particular film will be able to image in one exposure. As latitude increases contrast will diminish, and vice versa. Half Value Layer is the thickness of a given material that will reduce the radiation passing through it by one half. The energy of the ray, and the atomic make-up of the material determine half value layers. As kilovoltage increases the half value layer will decrease. 10

14 Calculations Half-Value Layer Method (HVL) The HVL Method is a simplification of a very complex calculation and only takes a few variables into consideration. For example, it is assumed that the energy is a single wavelength and scatter is not a concern. The HVL Method uses values that have been previously calculated and can be found in Attenuation Coefficient Tables contained in various references such as the Nondestructive Testing Handbook, Volume Three on Radiography and Radiation Testing. Using this method: ideally the energy level is selected so that the part thickness to be penetrated is 5 Half-Value Layers. satisfactory results can be obtained if the deviation from this rule is no greater than a factor of 2 half-value layers. Therefore, the target range is 3 to 7 half-value layers, but at a value of three image contrast is questionable and at a value of 7 exposure time will be excessive. the attenuation coefficients for the material must be located from reference materials and converted to Half-Value Layers using the following formula. HVL AttenuationCoefficient Below are linear attenuation factors to be used with aluminum and steel at a number of kilovoltage settings. Note from the table below that kilovoltage selection has a great affect on the HVL. Kilovoltage Material Attenuation coefficient 50 Al Al Al Al St St St St St St 1.15 Example: A steel part that is 1.27 cm thick must be inspected. Choose a kilovoltage that should produce a radiograph in a reasonable amount of time. From experience, 150 kv is know to provide good penetrating power and should produce a radiograph in a relatively short period. From the table, 1.54 is the value that corresponds to 150 kev for steel. Using the formula, divided by 1.54 equals 0.45 cm cm is the half-value layer for 150 kv. Divided the part thickness by the half-value layer to get the number of half value layers for the chosen kilovoltage setting divided by.45 equals 2.8 or rounded up 3.0 halfvalue layers. This value is at the lower limit of the range and could result in a radiograph with questionable contrast sensitivity. The calculation should be repeated using a lower kilovoltage. 11

15 Work the calculation at 100kv and determine the number of half value layers. Half value layers at 100 kv. Which setting is closer to five half value layers 100 or 150 kv? Fixed Exposure Method In this method, the kilovoltage is adjusted to maintain a constant film density for radiographs of objects with different material thicknesses and properties. Time, milliamperage, and source-to-film distance are held constant. This method is not as widely used as the HVL Method because it requires considerable work experience to develop the knowledge to produce acceptable radiographs. Without these skills this method requires much trial and error. As you can imagine, this method would require numerous exposures and added time without the aid of the simulator. 12

16 Simulator Exercise Simulator Stepwedge Thickness. Step Step Step Step Step Step Step Step Step Step cm 1.0 cm 1.5 cm 2.0 cm 2.5 cm 3.0 cm 3.5 cm 4.0 cm 4.5 cm 5.0 cm 1. Using the simulator, open the stepwedge11cm CAD file and select 2024Al (aluminum alloy 2024) as the material. Select step nine on the stepwedge and place a void (air) defect 0.55 cm by 0.45 cm by 0.65 cm. in the center of the step. 2. Using the HVL Method, calculate five half-value layers to get the optimum kilovoltage for step nine on the stepwedge. Create an image with a density of 2.5 on the step using D-7 film. Save the image in a file (xy1) for comparison with images you will produce later. Show your calculations below. Optimum Kilovoltage for step nine. 3. Create two more images using D-7 film and maintaining a 2.5 density. Create the first image with the kilovoltage reduced by 50 percent and the second image with an increase in kilovoltage 3 times the optimum value calculated above. Which exposure produced the most easily recognizable defect? 4. Using the slice option to evaluate density, record the contrast ratio between the defect and the stepwedge for each exposure. Exposure 1. Exposure 2. Exposure 3. Was the exposure with the greatest contrast sensitivity the one where the defect was easiest to see? 5. Using the slice option to evaluate density of the steps. Which exposure produced the greatest number of steps within a density range of 1.0 to 4.0? Exposure 1. Exposure 2. Exposure 3. Did this exposure have the highest or lowest contrast sensitivity? 13

17 9. Note 6. Recalculate and repeat your three exposures using D-2 film. Show your calculations below. Which exposure produced the most easily recognizable defect? 7. Using the slice option to evaluate density, record the contrast ratio between the defect and the stepwedge for each exposure. Exposure 1. Exposure 2. Exposure 3. Was the exposure with the greatest contrast sensitivity the one where the defect was easiest to see? 8. Using the slice option to evaluate density of the steps. Which exposure produced the greatest number of steps within a density range of 1.0 to 4.0? Exposure 1. Exposure 2. Exposure 3. Did this exposure have the highest or lowest contrast sensitivity? any differences seen when changing films. Describe the contrast and latitude differences between the two films. 14

18 Writing Assignment Describe your observations of this laboratory activity in a two-paragraph summary, and be prepared to discuss those observations during the next class. In your summary, discuss which method of determining kilovoltage feel is best, and discuss why. Summarize your observations on why latitude increases and contrast decreases as kilovoltage increases. 15

19 Laboratory Exercise Using the Half-Value Layer method, calculate 3, 5 and 7 half-value layers and note the corresponding kilovoltage for a steel or aluminum stepwedge. Using an exposure chart developed for the system, compare the kilovoltage range for your materials. Are the kilovoltages reasonably close? Write a paragraph that discusses the correlation of the kilovoltage values produced by the half-value method and those on the exposure chart. Note any similarities and differences. 16

20 LESSON THREE: MILLIAMPERAGE AND TIME RELATIONSHIPS Introduction In this lesson, the relationship between milliamps and the time of an exposure will be examined. An understanding will be developed of the interaction between the amperage and time controls, and their relationship to the density of a radiograph. Calculations to adjust for time or milliamperage to obtain a certain density on the radiograph will be introduced. Objective To develop an understanding of the effects of milliamperage and time changes on the density of a radiograph. To learn to calculate the approximate density change on a radiograph when the milliamperage or time setting of the exposure are changed. Reference Terms Milliamperage is a term used to describe the amount of radiation produced by the X-ray system. The milliamperage is actually the units of the electrical current that is used to produce the radiation. Milliamp-minutes (mam) are the product of the amount of milliamps and the amount of time used to make the exposure. Since the exposure time is directly tied to the amount of current used (i.e. the more current used, the less time needed to make an exposure), it is convenient to note the product of these two value rather that the two individual values. Formulas for Time and milliamprage Calculations There is roughly a one-to-one direct relationship between a milliamperage or time change and the resulting change in image density. If time is held constant and only the milliamperage is varied, the following equation can be used. M M 1 2 D D Where: M 1 = Milliamp original M 2 = Milliamp new D 1 = Film density original D 2 = Film density new

21 Example calculation: With a ma setting of 3.0, a density of 1.0 is produced. What ma setting would be needed to increase the density to 2.0 with time and Kv remaining constant? 3.0 / x = 1.0 / 2.0. Crossmultiply and divide. 3.0 times 2.0 = 6. Divide 6 by 1.0 = 6.0. Your new ma setting will be 6.0 ma. Use the previous equation to calculate the missing variable. 1. M 1 = 5.0 M 2 =? D 1 = 3.0 D 2 = 2.4" 2. M 1 =? M 2 = 3.0 D 1 = 2.1 D 2 = M 1 = 6.0 M 2 = 4.0 D 1 = 3.8 D 2 =? 4. M 1 = 5.0 M 2 = 3.0 D 1 =? D 2 = M 1 = 2.0 M 2 = 4.0 D 1 =? D 2 = 3.3 Time will now be the variable, and milliamperage will remain constant. The calculation will be the same as above except that time replaces milliamperage as one of the variables. T1 T 2 D D 1 2 Where: T 1 = Time (in minutes or seconds) original T 2 = Time (in minutes or seconds) new D 1 = Film density original D 2 = Film density new 18

22 Use the equation above to determine the missing variable. 1. T 1 = 2 T 2 = 8.0 D 1 = 1.1 D 2 =? 2. T 1 = 15.0 T 2 =? D 1 = 2.1 D 2 = T 1 =? T 2 = 9.0 D 1 = 2.4 D 2 = T 1 =? T 2 = 12.0 D 1 = 2.6 D 2 = 2.1 Now put the two calculations together to address all three variables at the same time. In this calculation, remember that the milliamps and time are a product and need to be multiplied before working the calculation. M 1 T1 D1 M T D 1. M 1 = 3.0, T 1 = 12.0 M 2 = 4.0, T 2 = 5.0 D 1 = 3.6 D 2 =? 2. M 1 = 1.0, T 1 = 63.0 M 2 = 4.0, T 2 = 20.0 D 1 =? D 2 = M 1 = 4.0, T 1 = 40.0 M 2 = 4.0, T 2 = 27.0 D 1 = 3.9 D 2 =? 4. M 1 = 3.0, T 1 = 15.0 M 2 = 5.0, T 2 =? D 1 = 2.9 D 2 =

23 Simulator Exercise After participating in class discussion and viewing demonstrations with the simulator, complete the calculations for change of density, milliamperage, and time. Next, complete the following activity using the X-ray simulation program. 1. Use the following settings as a starting point to produce three images: Part CAD File Part Material Source to film distance Detector Energy Current Time Stepwedge11cm Titanium (Ti) 100 cm. D-5 film 150 kv 1 ma 7 seconds These settings should generate an image with a density close to Change the milliamperage by increasing the setting to 2.0. What is the density on step eight? 3. Reduce your milliamps to 1.0 and increase the time to 14 seconds. What is the density? 4. Was the resulting density the same for an increase in milliamps as it was for and increase in time? Explain the relationship between time and milliamperage. 5. An industry rule-of-thumb is that if the exposure time is doubled, the density will approximately double. Use the slice option to verify the density results. 6. Continue with the settings used above (1ma and 7 seconds). a. Calculate the change in time needed to produce a density of 2.5 b. Calculate the change in time needed to produce a density of Write a statement noting the results and conclusions drawn from this lesson. 20

24 Laboratory Exercise Name: 1. Produce two radiographs on any X-ray system in the laboratory using the steel or aluminum stepwedge. First use an available exposure chart to expose a step near the center of the stepwedge. 2. Choose any three steps on the processed radiograph and calculate a density change to raise or lower the density to 2.0 for the steps chosen. Show your work and results below. 3. Once your calculations are complete expose and process the stepwedge and film. Note the density on the appropriate step. Did you produce a density change as calculated? Attach your film and explain your findings below. 21

25 Lesson Four: Creating an Exposure Chart Introduction This lesson will explain how to create a radiographic exposure chart. As material thickness changes exposure times must be adjusted to produce an acceptable density. Exposure charts provide the proper milliamperage and time settings for a constant kilovoltage over a range of material thickness. Exposure charts are developed with the following held constant: kilovoltage, density, film type, film processing, source to film distance, use of lead screens, and material type. The materials thickness is the variable and the chart provides a fast and reliable way of determining exposure time that produces a certain density. Objective To learn to interpret radiographic exposure charts to select exposure settings. To learn to make density conversion calculations. To create radiographic exposure charts that will be used as guides for establishing exposure settings for a particular X-ray system, film and film processing parameters. Reference Terms Densitometer is an instrument for measuring photographic densities. A number of different types are available commercially. An important property is reliability; that is, the densitometer should reproduce readings from day to day. X-Ray Exposure Chart is a chart that shows the relationship between material thickness, kilovoltage, and exposure time. Film Development is the process used to turn a latent image into a viewable radiograph. Processing variables such as manual or automatic processing, time and temperature will affect the final radiograph and the outcome of the exposure chart. Film Speed is a system used to classify the relative exposure times needed to expose various films. The film speed is dependant on the grain size of the film and larger the film grain size, the less exposure time required to produce a certain density of the processed film. 22

26 Calculations Density Conversion Calculations Use the following equation to solve for the missing information. M 1 T1 D1 M T D Example: If 20.0 mam produces a density of 2.0, what exposure (amperage times time) would be needed to produce a density of 4.0? 20.0mAm? mam 2 min 4 min (20.0 mam times 4.0 min divided by 2 min = X) The new amperage would be 40.0 mam. 1. If 20.0 mam produces a density of 2.0, what exposure would be necessary to produce a density of 4.0? 2. If 15.0 mam produces a density of 2.3, what exposure would be necessary to produced a density of 3.6? 3. If 24.0 mam produces a density of 3.0, what exposure would be necessary to produced a density of 2.0? 4. If 10.0 mam produces a density of 1.5, what exposure would be necessary to produced a density of 3.1? 5. If 15.0 mam produces a density of 1.3, what exposure would be necessary to produced a density of 2.6? 6. If 22.0 mam produces a density of 1.8, what exposure would be necessary to produced a density of 2.6? 7. If 8.0 mam produces a density of 2.1, what exposure would be necessary to produced a density of 2.0? 8. If 18.0 mam produces a density of 1.3, what exposure would be necessary to produced a density of 2.6? 9. If 21.0 mam produces a density of 1.7, what exposure would be necessary to produced a density of If 14.0 mam produces a density of 2.6, what exposure would be necessary to produced a density of 3.5? 23

27 Simulator Exercise 1. Create two exposure charts using the simulator. Review the exposure chart on the following page before making your exposure chart. Use step eight on the simulator stepwedge. The step thicknesses are: Step cm Step cm Step cm Step cm Step cm Step cm Step cm Step cm Step cm Step cm 1. Place a 0.8 by 0.8 by 0.8 cm defect on the center of step eight for ease of location. To locate the defect, left click on the defect tab and then open. Choose the sphere, and it will appear on your stepwedge. Under the Flaw Operation menu, choose scale enter the dimensions for the flaw. Left click and drag the defect to the position you choose. To aid in location of the defect use the translucent option. Left click on the sample tab. At the lower right of the window there is a box marked translucent, left click. This will let you see the defect as you move it around in the stepwedge. 2. Using the knowledge gained in Lessons 1 and 2, calculate and choose the appropriate kilovoltage for aluminum and steel material. Choose an appropriate film. Film speed and type must always be considered when making an exposure chart in order to keep the exposure time reasonable. Denser materials, such as steel usually require a faster film than do less dense materials such as aluminum. Industry production demands and sensitivity requirements will dictate which film should be used. It often is not practical to x-ray steel with a very fine grain film that takes an extended time to expose, when a faster film will locate the needed defects. 3. Calculate the kilovoltage for steps one, eight and ten to determine the kilovoltage range for the span of thicknesses of the stepwedge. Record the kilovoltage. Steel Thickness step one Kilovoltage Thickness step eight Kilovoltage Thickness step ten Kilovoltage Aluminum Thickness step one Kilovoltage Thickness step eight Kilovoltage Thickness step ten Kilovoltage 5. Select the calculated kilovoltage for step number eight of the steel stepwedge and set the milliamperage and time to produce a density as close to 2.0 as practical on the step (not on the defect). 24

28 6. Create the image and use the slice option to record the densities from the steps beginning with the lightest. 7. Use the density conversion calculations to determine the exposure needed to produce a density of 2.0 on each of the steps. 8. Plot the exposure versus thickness on log graph paper. On the vertical edge of the log graph paper, plot the exposure in milliamp-minutes and on the horizontal line of the graph plot the material thickness. Place a mark at the intersection of these values and continue until all points for that kilovoltage setting are marked. Now connect the points to produce a kilovoltage reference line for a range of material thickness and time settings. The exposure chart on the next page can serve as an example. 9. Next, select the optimum kilovoltage calculated for step one and create another line on the chart. Repeat the process for step ten of the stepwedge. When complete the chart should show three kilovoltage lines. Remember to record all information including film, development method used, screens used, and source-to-film distance (SFD) in an information box. 10. Repeat the process for the aluminum material and produce a second chart. 11. Once the charts are complete select one thickness (step) for each kilovoltage listed on the exposure charts and produce an image using the simulator. Using the slice option record the density of the steps. Theoretically, the densities should all be 2.0 but some variation is to be expected. Exposure chart 1 Material Step thickness: Density Step thickness: Density Step thickness: Density Exposure chart 2 Material Step thickness: Density Step thickness: Density Step thickness: Density 25

29 Example X-ray Exposure Chart 100 milliamp seconds YX - 5 Film / No screens Auto Process 85 degrees Density: 2.0 FFD: 39 inches Source: X-ray / Model 160 Serial No. RA Material: Aluminum 1 0 ½ 1 1 ½ 2 2 ½ 3 3 ½ Thickness (inch) 26

30 Laboratory Exercise Name: Produce an exposure charts for steel or aluminum on one of the laboratory X-ray systems using a process similar to that used in the simulator exercise. Use a densitometer to measure the film density that corresponds to each step of the stepwedge on the radiograph. Generate lines for at least four kilovoltage settings on the chart. Kilovoltage settings should be in increments of 10 kilovolts or more. Remember to list any variable in the information box on the chart. When finished, turn in the following documents: Calculations used to develop the exposure chart. One exposure chart, for aluminum or for steel. Charts will show four kilovoltage settings over a given thickness range. A two paragraph description of your experience developing an exposure chart 27

31 LESSON FIVE: SOURCE-TO-FILM RELATIONSHIPS Introduction In this lesson, Newton s inverse square law will be used to calculate the film density when the source-to-film distance (SFD) has changed. Newton s inverse square law states that the intensity of the electromagnetic energy is inversely proportional to the square of the distance from the source. Therefore, reducing the SFD by one-half will increase the intensity of the radiation by a factor of four. Since the exposure time is directly related the radiation intensity, reducing the SFD by one-half would also reduce the exposure time by a factor of four. Often it is not practical to make exposures at the distance listed on an exposure chart. Reducing the SFD distance is sometimes necessary to speed the radiography of extremely thick parts, and increasing the SFD may be necessary when the physical shape of the part does not permit using distances listed on the exposure chart. Objective To develop an understanding of the effects of distance change on the density of a radiograph. To calculate and maintain a density on a radiograph when the source-to-film distance has been altered. Reference Terms Milliamperage-Distance Relationship is the dependence between the milliamperage required for a given exposure and the source-to-film distance. This relationship has been standardized to meet with manufactures ratings on the various x-ray tubes. Time-Distance Relationship is the dependence between the time required for a specific exposure and the source-to-film distance. The exposure time is indirectly proportional to the square of source-to-film distance. Newton s Inverse Square Law describes the radiation intensity as a function of distance from the source. The intensity reaching the specimen is inversely proportional to the square of the distance. Anytime the distance doubles the intensity of the radiation reaching the film is changed by a factor of four. Milliamprage, Time, Distance is the relationship between the milliamprage, and time required for a given exposure and source- to- film distance. This relationship can be easily calculated for any source- to- film distance change. Source-To-Film Distance is the measured distance the x-ray tube is from the film to be exposed. This distance will change for various reasons. Often there is a need to speed up production, hence the distance will be reduced thereby shorting the exposure time at a given density. 28

32 Simulator Exercise 1. After reading and classroom discussion, the following calculations should be completed and evaluated in class. 2. The following problem should be calculated and the answers confirmed on the number eight step of the aluminum stepwedge and D-3 film. Place a "void" flaw in the step for ease of location. Choose a kilovoltage setting appropriate for the thickness you are exposing (See lesson on kilovoltage selection). Establish a density of 2.5 at the default setting of 100 cm. This image will be saved in the density file as xyz.xbm. Verify the density using the slice option. Now change the source to film distance to (35cm), recalculate the new time setting for the distance change. Using the full and top view note the change in the radiation cone coverage of your film. Produce an image on the simulator using your new calculations. Name your image file xy1.xbm. Remember you must name the file before you produce the simulated image or the image will write over the xyz.xbm file. Check your density. Did the density remain at 2.5? if not what was the density? was it within 10 percent of your calculated change? 3. Next select the stepwedge choose Fe as your material. Establish a 2.5 density using AA 400 film on step seven. Place a void flaw in the step for ease of location. Calculate a change of distance to 125 cm while maintaining a 2.5 density. Save this image in a density file xy2.xbm. Using the full and top views note the change in the radiation cone. Retrieve the three radiographic image files from the density file and write a short description of the results. 4. Using the attached chart for change of distance produce another image with AA 400 film and a source to film distance of 25 inches (remember to convert cm to inches). The density on step seven should be near 2.5. Using the chart attached recalculate a change of distance to 40 inches. Did you maintain the 2.5 density? Write a short description of the results comparing the calculation method to use of the chart to determine Milliamp and time settings for a distance change. 29

33 Calculations for source to film distance change According to Newton s inverse square law, the intensity of the radiation intensity is inversely proportional to the square of the distance from the source. This can be represented by the following equation. Intensity 1 D 2 Since the exposure of a radiograph is inversely related to the radiation intensity (i.e. as intensity increases, the exposure must decreases to produce a constant density), it can be written that the exposure is directly proportional to the square of the source-to-film distance as represented by this equation. 2 Exposure SFD This equation can then be used to make calculations for changes in the source-to-film distance by setting two equations equal to each other. This equation is Exposure1 2 SFD1 Exposure SFD Example: an exposure for 16mAm with a source to film distance of 20 inches produces a density of 2.5. It is necessary to move the part closer to the source to reduce the exposure time but the density must be kept at 2.5. What would the new time be if the distance were moved to 10 inches from the source and all other factors remained the same? (As before cross multiply and divide. Four milliamp-minutes would be the new time for the distance change.) Use the above equation to solve for the missing variable in order to keep the density constant. 1. If 5.5 mam at a source-to-film distance of 12 inches produces a density of 3.0, what exposure is needed to produce the same density if the source is moved to 24 inches? 2. If 12.0 mam at a source-to-film distance of 21 inches produces a density of 2.50, what exposure is needed to produce the same density if the source is moved to 14 inches? 3. If 16.5 mam at a source-to-film distance of 18 inches produces a density of 3.3, what exposure is needed to produce the same density if the source is moved to 12 inches? 4. If 8.1 mam at a source-to-film distance of 16 inches produces a density of 3.6, what exposure is needed to produce the same density if the source is moved to 9 inches? 5. If 2.0 mam at a source-to-film distance of 6 inches produces a density of 2.0, what exposure is needed to produce the same density if the source is moved to 11 inches? 6. If 15.5 mam at a source-to-film distance of 21 inches produces a density of 3.0, what exposure is needed to produce the same density if the source is moved to 12 inches? 30

34 MILLIAMPERAGE-TIME AND DISTANCE RELATIONS When distances are changed from a given value to another given value on the chart the original time can be multiplied or divided by the factor given in the chart. New Dist. Old Dist. 25" 30" 35" 40" 45" 50" 25" " " " " "

35 Laboratory Exercise Name: 1. Produce a radiograph of a stepwedge using the X-ray system, film, and source-to-film distance of your choice. 2. Change the source-to-film distance and calculate a new exposure setting to produce a radiograph with densities similar to the first radiograph. 3. Compare the two exposures and write a two paragraph on effects of the source-to-film distance on exposure density. 32

36 Introduction LESSON SIX: RADIOGRAPHIC EQUIVALENCY FACTORS This lesson addresses the physical properties of materials and how these properties affect the ability of X-rays to pass through a given amount of material. The material properties of primary interest in radiography are the density and the atomic number. The higher the density of a material, the more difficult it is for radiation to penetrate the material. This concept was studied in Radiation Safety when calculating shielding. Technique charts and other materials used in industry to control radiographic procedures usually address only one material. When presented with a new material of a different density, new exposure times must be determined. This lesson will explain how radiographic equivalency factors can be used to compensate for changes in materials properties when exposure parameters for other materials are known. Objective To investigate the effect that material properties have on radiographic density. To learn to compensate for material changes when making exposures by using the exposure settings at a given kilovoltage for one material to estimate the settings for a second material. Reference Terms X-ray Absorption is a term used to describe the ability of a material to stop or absorb x- rays. X-ray absorption depends on three factors: the thickness of the material, the density of the material and the atomic number of the material. The atomic nature of the material influences the X-ray absorption more than the other two factors combined. Exposure Factor is a term used to describe the combined effect of milliamprage, time and source-to-film distance. A change in any of these factors will affect the density of the radiograph. Image quality and resolution is affected by the contrast, sensitivity, and definition of the image. Equivalence Factor is the value by which the thickness of a material is multiplied to give the thickness of a "standard" material. Equivalence factors provide a means of relating material of different densities so that exposure estimates can be made. 33

37 APPROXIMATE RADIOGRAPHIC EQUIVALENCE FACTORS Material 50 kv 100 kv 150 kv 220 kv 400 kv Magnesium Aluminum (pure) Aluminum Titanium Steel Steel Copper Zinc Brass 1.4* 1.3* 1.3* Inconel X Zirconium 2.3 Lead 14.0 Uranium Aluminum is taken as the standard metal at 50kV and 100kV. Steel becomes the standard at the higher voltages and at gamma ray energy levels. The thickness of the material of interest is multiplied by the corresponding factor to obtain the approximate equivalent thickness of the standard metal. The exposure settings that would normally be used to produce a radiograph of the standard metal at this thickness is used to produce the radiograph of the material of interest. *Tin or lead alloyed in the brass will increase these factors. Example: To radiograph 0.5 inch of copper at 220 kv, multiply 0.5 inch by 1.4 to obtain an equivalency thickness of 0.7 inch for steel. The exposure settings that would be used to produce a radiograph of 0.7 inch of steel are; therefore, use to produce the radiograph of 0.5 inch of copper. 34