Chapter 11 Radiation Concepts Learning Objectives 1. Discuss the basics of radiologic science. 2. Describe how x-rays are produced. 3. Describe the elements of the electromagnetic spectrum. 4. Explain the relationship between energy2 frequency3 and wavelength. 5. Explain the 12 properties of x-rays. 6. Describe the function and location of the components of an x-ray machine. 7. Describe the use of and components of the imaging log book. Key Terms Bucky tray Cathode Collimating device Electromagnetic spectrum Energy Frequency Heel effect Rotating anode Stationary anode Wavelength Basic Concepts of Energy There are many different forms of energy4 including electrical5 chemical6 mechanical7 thermal8 nuclear9 and electromagnetic. Regarding radiographs and x-ray production10 the most important of these forms is electrical. Knowledge of the electromagnetic spectrum is needed to understand x-ray production. The electromagnetic spectrum can be broken down into three components11 Energy 12eV1314 Frequency 15Hz1617 and Wavelength 181920. X-rays fall between the gamma ray and ultraviolet wavelength range21 x-rays have between 0.03223 nanometer wavelengths 23Figure 1.124. These are nonvisible waves. Frequency is the number of waveforms that strike an object during a given time period. The frequency consists of one crest and one trough and is measured in Hertz 25Hz26. This is an important concept to understand when dealing with x-ray production but is more important when looking at the production and use of ultrasound for diagnostic purposes. Wavelength is comprised of both frequency and amplitude. The height of the wave is termed amplitude and the distance between crests is frequency. A long wavelength has a low frequency27 is relatively weak28 and most often associated with microwaves29 VHF30 and radiofrequency. Short wavelengths have a high frequency31 are powerful32 and are most associated with gamma rays such as x-rays33 contact therapy34 and supervoltage therapy 35Figure 1.236. Short wavelengths require less time to be produced and are more powerful with the potential for penetration. This is especially important when radiographing patients that are not sedated or when trying to obtain diagnostic-quality thoracic radiographs. Figure 1.1.jpg Figure 1.137 Electromagnetic spectrum. Figure 1.2.jpg Figure 1.238 Short wavelength vs. long wavelength. X-ray Properties The veterinary technician must have a basic understanding of x-ray production and the steps needed to ensure the safety of both the patient and the veterinary team while taking radiographs. X-rays are nonluminous electromagnetic radiation. The x-rays themselves are similar in nature to visible light but with a much shorter wavelength39 which gives them greater energy. Because the x-ray beam is nonluminous40 great care must be taken to ensure the safety of the patient and staff members. Gowns41 gloves42 thyroid collars43 and dosimetry badges should always be worn during imaging procedures. These protective measures are in place for the safety of all staff members and must be adhered to every time a radiograph is taken. Professor Wilhelm R44ntgen is credited with the discovery of x-rays. Through his research45 R46ntgen was able to describe 12 properties of x-rays. The 12 properties are47 1. Highly penetrating invisible rays that form electromagnetic radiation. 2. Electrically neutral48 not affected by electric or magnetic fields. 3. Produce a wide variety of energies and wavelengths. 4. Release small amounts of heat as they pass through matter. 5. Travel in straight lines. 6. Travel at the speed of light in a vacuum. 7. Ionize matter. 8. Cause fluorescence of specific crystals. 9. Not focused by lens. 10. Affect photographic film. 11. Create chemical and biological changes in matter due to ionization and excitation. 12. Produce secondary and scatter radiation. X-ray Tube Anatomy A basic understanding of the anatomy of the x-ray machine is essential to safely produce diagnostic radiographs and to troubleshoot when there are issues with radiograph quality. All x-rays are the result of a high impact of a large volume of heated negatively charged electrons with a hard surface. When this impact occurs the electrons rupture with the conversion of electrons to x-radiation. The x-ray machine consists of an x-ray tube head49 an x-ray control center50 tabletop51 Bucky tray52 transformer53 and generator. At the top of the machine is the x-ray tube head. Inside the x-ray tube head is where the anode54 cathode55 glass enclosure56 and window are located 57Figure 1.358. Figure 1.3.png Figure 1.359 X-ray machine. Cathode Depending on the x-ray unit60 the cathode usually consists of two filaments similar to an incandescent light bulb 61Figure 1.462. These filaments are made of thoriated tungsten. The benefit of using thoriated tungsten is that it has a very high melting point 63664170 65F66. Pressing the pre-exposure button on the x-ray machine causes the filaments to become hot. When this occurs67 the electrons are excited and start to move rapidly back and forth. When the electrons reach their peak excitement68 they are released to the anode. If the unit consists of Loading... two filaments69 one filament is usually shorter than the other. Which filament is heated depends on the current to the machine and the settings chosen. The filaments are located inside a structure known as a focusing cup 70Figure 1.571. This focusing cup allows for the excited electrons to be released in a straight pattern directed toward a focused target on the anode. If there is no focusing cup72 the electrons leave the cathode and move in all directions toward the anode. This tends to result in a radiograph that lacks quality 73Figure 1.674 Figure 1.4.png Figure 1.475 Cathode. Screen Shot 2021-06-17 at 10.48.24 AM.png Figure 1.576 Focusing cup. Figure 1.5.png Figure 1.677 A78 Cathode without a focusing cup. B79 Cathode with a focusing cup. Anodes There are two types of anodes 80stationary and rotating81 that may be present depending on the type of x-ray machine being used. Most standard stationary x-ray machines have a rotating anode 82Figure 1.783. These anodes are positively charged structures that attract the negative electrons released from the cathode. When activated84 rotating anodes will rotate between 38520086387600 rpms. This rotation is the sound you hear when preparing to take a radiograph. Most of the anodes found in x-ray machines are made of a tungsten molybdenum alloy. This combination allows for great heat absorption and distribution88 thus decreasing the wear and tear on the anode. There are benefits and drawbacks to both types of anodes. Figure 1.7.png Figure 1.789 Rotating anode. The benefits to the rotating anode include greater heat resistance. The dissipation of heat due to the rotation increases the useful life of the anode. The rotating anode also serves as an electrical conduction unit to complete the circuit of electricity and as the site for the electron target. The drawback to the rotating unit is that many times these machines are big and bulky and do not allow movement of the machine. All rotating anodes have a set degree of angulation. This angulation results in partial limitation of the x-ray beam on the anode side of the x-ray tube as compared with the cathode side. This is called the anode heel effect and is used to advantage when obtaining images of a patient with great differences in thickness between tissues. The rotating anode is a disc-shaped structure with an angled edge90 which allows the electrons to strike the anode target and direct the x-ray beam down to the patient. Figure 1.891 Figure 1.992 and Figure 1.10 are damaged and cracked rotating anodes. On most portable x-ray units93 a stationary anode is used in place of a rotating anode 94Figure 1.1195. The stationary anode serves the same function as the rotating anode96 except that copper is used in place of tungsten and an angled block used in place of the disc. Considering the stationary unit does not move97 there is greater concern for heat buildup and potential pitting of the anode block. When using a stationary unit98 allow the anode enough cooling time prior to the next exposure. A drawback to the stationary anode is that its lack of movement can result in damage99 such as pitting of the anode100 which would yield non-diagnostic quality images. The stationary anode also does not allow for thick structures to be radiographed due to the limited settings of the machine and capacity of the anode to withstand the heat that is produced while taking radiographs. The conversion rate for the energy from the electrons at the time of contact with this anode results in the production of 99101 heat and 1102 x-rays. Loading... Figure 1.8.png Figure 1.8103 Damaged rotating anode. Figure 1.9.png Figure 1.9104 Cracked rotating anode. Figure 1.10.png Figure 1.10105 Malfunctioning anode. Figure 1.11.png Figure 1.11106 Stationary anode. Glass Enclosure Loading... The positively charged anode and the negatively charged cathode are both located within a glass enclosure that provides a vacuum area. This prevents dust particles from interfering with the electron107s movement from cathode to anode as well as the movement of the x-ray from the anode to the patient. The glass enclosure is a special heat-protected glass. To aid in heat dissipation108 the outer portion of the glass is surrounded by oil that helps reduce the heat of the glass enclosure. On the bottom side of the glass enclosure is a beryllium window. This window allows x-rays to pass from inside the glass enclosure down to the patient with little to no disruption of the flow of x-rays or absorption and filtration of the x-rays that were produced. Aluminum filters can be placed between the window of the glass enclosure and the tube head to help absorb weak x-rays that serve no benefit to radiograph production. The thickness of these filters109 and the amount of weak x-rays they absorb110 may need to be taken into consideration when developing the technique chart for the radiograph machine. A collimating device is also present. This is used to restrict the size of the x-ray beam and thus also reduces the amount of scatter radiation. Tabletop The next structure to look at is the tabletop. The function of the tabletop is pretty straightforward in that the tabletop is a place to put the patient for radiographic exposure. Below the table is a structure known as a Bucky tray. The purpose of the Bucky tray is to hold the x-ray film cassette under the tabletop while taking the radiograph 111Figure 1.12112 Most Bucky trays have a grid that is located between the tabletop and the tray itself. The purpose of the Bucky tray will be discussed in Chapter 2. Figure 1.12.png Figure 1.12113 Bucky tray. Radiograph units have some form of a control panel. This control panel usually consists of the following114 On115 Off switch116 kVp selector117 mAs selector118 time selector119 x-ray prep120 and x-ray exposure button 121Figure 1.13122. These components will be described in greater detail in Chapter 2. These control panels are usually either on the machine itself123 on the tube head124 or under the table as a pull-out door. Some of these panels will be completely detached from the x-ray unit and either be in a different room or behind a protective brick wall or lead-lined glass. Some exposure buttons are located on the floor as a foot pedal for use by the operator while restraining the patient. This foot pedal has both pre-exposure and x-ray exposure capabilities. Figure 1.13.png Figure 1.13125 Control panel. Transformer and Rectifiers The last two items of the anatomy of the x-ray unit are the transformer and the rectifiers. The transformer is located under the x-ray table. Inside the transformer is a step-up unit as well as a step-down unit. Most radiograph units run on 220 volts. To allow the radiograph unit to heat the filament enough to adequately excite the electrons126 the step-up transformer raises the voltage from 220 to approximately 125127000 volts. Once the step-up transformer has increased the volts in the machine128 the step-down or filament transformer reduces the number of volts to a pre-set number selected by the technician when the milliamperage 129mA130 was selected. The filament transformer regulates the voltage that enters the filament131 thus determining the number of x-rays that can be produced. In the U.S.132 electrical current is transmitted as an alternating current. The alternating current moves at 60 cycles per second. Just as with a waveform133 there is a positive movement and a negative movement. One cycle consists of a positive and a negative movement. The cathode only receives positive movements134 so without the rectifiers135 the x-ray machine only receives half of the current that reaches the machine. The rectifier works on three different levels. A half-wave rectification allows for electrons to move from the cathode to the anode on the positive movement of the incoming current. This results in a short exposure time. These are usually seen in portable units commonly used for equine radiography. Full-wave rectifiers convert the negative movement into a positive movement so both positive and negative movements can be used during x-ray production. Faster times can be used with full-wave rectifiers and more x-rays are produced compared to the half-wave unit. The third method is the three-phase approach. This results in a more continuous movement136 which allows for higher energy137 more electrons138 and more constant energy in the x-ray beam. Record Keeping Radiographs are part of the patient medical record and must be properly filed for easy retrieval. Completed film radiographs are usually placed in folders or envelopes that are made of materials that cannot react with the film or film processing chemicals. In general139 all views of one radiographic study are placed in a single folder and labeled with the patient and client name and the date and views taken 140Figure 1.14141. A radiographic study usually refers to all the views taken of the same body part on the same patient at the same time. The radiographic technique may also be included in case the radiographs need to be repeated in the future. Film filing folders are usually color-coded with either alphabetical or numerical folder labels142 depending on the type of filing system used at the clinic. Figure 1.14.png Figure 1.14143 Film folders. Radiology logs are used for tracking and cross-referencing radiographic studies 144Figure 1.15145 The log generally includes the following information146 Loading... Client147s name Patient148s name Date the image was taken Technique Figure 1.15.png Figure 1.15149 Radiology logs. Copies of radiographs may be provided to owners on occasion for the purpose of seeking a second opinion or further care at a referral practice. The original radiograph is always owned by the clinic and it is up to the clinic to determine what will be given to clients. The radiology log can also contain a column to document removal of the radiograph from the premises. In some cases150 the clinic may choose to maintain the original radiograph and provide a copy to the owner. Loading... Radiographs taken with digital systems can usually be easily incorporated into the patient151s electronic medical record and should contain all the identifying information already attached to the file. Digital imaging systems provide images in a format known as DICOM. DICOM is an acronym that refers to digital imaging and communications in medicine152 the universally accepted format for the dispersion and storing of medical information. Digital images can be copied onto a CD or flash drive if they need to be provided to a client or another veterinary facility. Key Points X-rays fall between the gamma ray and ultraviolet wavelength range153 x-rays have between 0.031543 nano-meter wavelengths. The x-ray machine consists of an x-ray tube head155 an x-ray control center156 tabletop157 Bucky tray158 trans-former159 and generator. The anode160 cathode161 glass enclosure162 and window are located in the x-ray tube head. Anodes may be either a rotating or stationary type. Completed film radiographs are usually placed in folders or envelopes that are made of materials that cannot react with the film or film processing chemicals. Radiology logs are used for tracking and cross-referencing radiographic studies. Radiographs taken with digital systems are incorporated into the patient163s electronic medical record as DICOM format images. Loading... Review Questions Loading... Loading... Question Question Radiographs are legal property of the _________. A Clinic B Client C Patient Show Answer Check Answer Question Question Which of the following is used to restrict the size of the x-ray beam and reduce the amount of scatter radiation164 A Anode B Grid C Collimator D Cathode Show Answer Check Answer Question Question What is the significance of the wavelength range of x-rays between 0.031653 nanometers in relation to other electromagnetic waves166 A X-rays have a longer wavelength than gamma rays167 resulting in higher energy levels. B X-rays have a shorter wavelength than ultraviolet rays168 making them safe for repeated medical imaging. C X-rays fall between gamma rays and ultraviolet rays in wavelength169 allowing them to penetrate materials for imaging. D The wavelength of x-rays is longer than ultraviolet rays but shorter than visible light170 offering a unique application. Show Answer Check Answer Question Question What holds the x-ray cassette in place within the tray under the x-ray table171 A Grid B Collimator C Screen D Bucky Show Answer Check AnswerChapter 2172 Radiograph Equipment Learning Objectives 1. Describe the purpose of grids and how their use affects radiograph quality. 2. List the parts and function of intensifying screens. 3. Compare types of intensifying screens and identify the pros and cons of each type. 4. Explain how milliamperage and kilovoltage affect radiographic contrast and density. 5. Describe the differences between standard radiography and fluoroscopy. Key Terms Milliamperage Kilovoltage Time Cassette Intensifying screens Focused grid Grid cutoff Grid ratio Caliper Contrast Density Parallel grid Screen speed Radiograph Units There are two main types of x-ray units available173 portable and non-portable. In Chapter 1174 we discussed the anodes of each type of machine and the benefits and drawbacks associated with each. It is important to remember that portable units contain a non-rotating anode whereas non-portable 175stationary176 units contain a rotating anode. Having an anode that can rotate allows for the use of a higher mA and kVp setting without undue concern for damage to the anode. When selecting the type of x-ray machine to purchase or use177 the technician must determine the caseload of the clinic as well as the availability of resources such as electrical access for portable units 178Figure 2.1179. Figure 2.1.png Figure 2.1180 Portable x-ray unit. Evaluating caseload requires calculation of an average number of radiographs that will be taken each week. A low-volume clinic may only use the x-ray machine for large animal extremities and only a couple of times per week. In this case181 a portable unit would be best suited for the clinic. A medium-volume clinic may have a combination of radiographs that are needed each week182 including large animals and companion animals. These clinics may be in a situation where both styles of x-ray machines are needed. If the clinic is a high-volume clinic with limited large animal work183 a stationary unit is most likely the best purchase for that clinic. Milliamperage 184mA185 A number of values can be adjusted on the x-ray machine to obtain the optimal image. These adjustment areas include186 milliamperage 187mA188189 kilovoltage 190kV191192 and time 193s194. Milliamperage is the setting used to control the quantity of electrons that will be produced and released from the filament during x-ray production. Most stationary small animal units can use a setting between 50 and 300 mAs195 whereas a large animal portable unit may only produce 10-20 mAs 196Figure 2.2197. Figure 2.2.png Figure 2.2198 kVp199 mA200 time controller. Loading... When an adjustment is made to the mA201 the density of the finished radiograph is affected. When the mA is increased202 the radiograph density or degree of blackness is increased. When the mA is decreased203 the overall radiograph density is lightened204 and the film is less black. Many times205 the mA adjustment is compared to that of a toaster. If the setting is increased206 the toast will become dark black. If the toaster is set at a lighter setting207 the toast will not be as dark. Kilovoltage 208kVp209 Kilovoltage refers to the energy of the electrons when they reach the anode and is also referred to as kilovolt age potential 210kVp211 or peak kilovoltage. This setting is used to control the quality of the x-ray beam. A higher kV allows the electrons to move with greater speed and results in greater penetrating power. The higher the kV the more penetrating power the x-ray beam will have. The thicker the tissue the greater the kV needed to penetrate the tissue and reach the cassette and film. The kV can affect the density of the radiograph due to the number of x-rays that pass through the patient. Most veterinary x-ray units will be able to produce between 40 and 150 kV. The kV is usually associated with the contrast of the image. Contrast refers to the shades of gray that are produced on the finished radiograph 212Figure 2.3213. The greater the number of shades of gray present214 the higher the quality of the image. Higher contrast allows for small differences in tissues to be better visualized due to the subtle differences in gray scale. The selection of a particular kV varies depending on the characteristics of the tissue being radiographed and the type of contrast and density that needs to be produced. In general215 a higher kV and lower mA setting is used for soft tissue such as abdominal and thoracic imaging. Lower kV and higher mA setting are used for extremities where sharper differences are needed with fewer shades of gray. Loading... Figure 2.3.png Figure 2.3216 a217 Short scale contrast218 b219 Long scale contrast. Loading... Time 220s221 Time is the duration in seconds that the anode has a positive charge and is receiving electrons. Time and milliamperage are closely related in that the longer the time the anode is positively charged222 the more electrons will be produced from the cathode. To reduce the risk of blur223 due to patient movement during x-ray exposure224 use a high mA and the lowest time possible. This will be extremely beneficial when taking thoracic radiographs of the patient where respirations can result in a blurred image due to patient motion. Many x-ray machines will have a setting that is the mathematical combination of both milliamperage 225mA226 x time 227s228 and will be indicated on the x-ray machine as mAs. For example229 an mA of 100 and a time setting of 123010 second will produce 10 mAs. Similarly231 if the machine were set at 200 mA and 123220 of a second233 this also results in 10 mAs234 but this mAs would have less of a chance of blur due to patient motion because of the shorter exposure time. Loading... The x-ray machine has a pre-exposure switch that allows the cathode to charge the filament and start to excite the electrons. This is occurring as the anode starts to rotate if it is a stationary unit. Once the machine has enough electrons excited and the anode is at the correct speed235 the x-ray light will illuminate notifying the technician that the x-rays can be released. As the exposure button is depressed236 the electrons are released from the cathode237 striking the anode238 and are directed down toward the patient and cassette. This is also the time that the exposure light will be illuminated. Some machines rely on the control panel to allow for prepping and exposing the image239 while others have the same option as a foot control 240Figure 2.4241. The foot control allows for the technician to help restrain the patient if needed while simultaneously taking the image. Figure 2.4.png Figure 2.4242 Two-stage foot pedal. In addition to the portable and stationary units found in most veterinary medical hospitals243 fluoroscopy is another type of radiographic machine that can be used in veterinary medicine. It is important to understand that fluoroscopy should not be used in place of standard radiographs244 but as an adjunct to help identify the underlying medical issues of the patient. Fluoroscopy is different from standard radiography in that fluoroscopy is a live time image of the area being assessed. With standard radiographs245 a one-time exposure is taken and evaluated. With fluoroscopy246 the x-rays are continuously being produced during the entire procedure. The image is then captured on a screen where it is converted into a visible image and viewed live or stored in video format for later evaluation. Cassettes Properly functioning cassettes247 in which the film is placed to protect it from light exposure248 are vital to producing diagnostic-quality radiographs 249Figure 2.5250. Figure 2.5.png Figure 2.5251 Properly functioning cassettes. Cassettes can be made of plastic252 light metal253 or graphite. Cassettes are expensive and should be treated with the utmost care to ensure the functional quality as well as the longevity of the cassette. Most x-ray cassettes that are produced today meet a specific set of criteria. X-ray film cassettes must254 Be sturdy under the patient255s weight to prevent cracking Be weather resistant 256cold and heat257 Be inflexible 258no warping or bending259 Have secure260 properly functioning latches Have a radiolucent front cover Contain a lead foil backing to absorb and decrease scatter radiation Contain a washable outer covering Ensure good intensifying screen contact Intensifying Screens Inside the cassette are the intensifying screens. These screens are white with a smooth shiny surface. The screens are made from tiny crystals that fluoresce 261emit light262 once they are exposed to x-rays. Some cassettes do not have intensifying screens. This type is commonly used when obtaining dental radiographs. This will be discussed in greater detail in the dental radiography chapter. 1. X-ray film is more sensitive to visible light than radiation263 therefore264 the use of intensifying screens will help decrease the amount of radiation needed to produce a diagnostic image. The intensifying screen also helps increase the contrast of the image resulting in an improved radiographic detail. Intensifying screens possess many structural components that allow them to function properly 265Figure 2.6266. Each screen is made of a cardboard or Mylar backing. The reflecting layer of the screen is made of titanium dioxide267 which helps reflect the light from the active layer of the screen back to the film. There is also plastic coating on the screens that helps reduce static electricity and allows the screens to be cleaned. Screens should be cleaned monthly or more often if they become dirty or artifacts are present on the finished radiographs. These screens contain sensitive material and should only be cleaned with an approved cleaning agent. Intensifying screens do not last forever with most screens failing after about 10-15 years of regular use with appropriate care 268Figure 2.7269. Figure 2.6.png Figure 2.6270 Intensifying screen with lead blocker. Figure 2.7.png Figure 2.7271 Intensifying screen structural components. Screen Speed Intensifying screens are described based on the speed of the screen. Screen speed is involved with allowing the crystals in the screen to convert x-rays into visible light to be used by the film. The speed is associated with the amount of radiation needed to produce an image on the film. In veterinary medicine272 the following screen types are used273 fast274 regular275 medium276 par277 and detailed. Fast speed screens require less radiation than medium or par screens to produce the same degree of blackness. While the benefit of fast screens is the decreased amount of radiation used278 the drawback of fast screens is that the image detail is poorer than that of the medium or par screens. Fast screens have large crystals279 which allows for the decrease in radiation but results in decreased image quality. Slow or detailed screens on the other hand280 have small crystals that while they require more radiation281 produce a more detailed image and better resolution. Detailed screens are usually used in bird and exotic animal radiography and medium or par screens are used for small animal imaging. Intensifying screens have the potential to produce either a blue or green color when contacted by x-rays 282Figure 2.8283. Older intensifying screens284 that used calcium tungstate285 produced a light that was in the blue color spectrum. Rare earth phosphors286 such as lanthanum oxybromide and gadolinium oxysulfide287 produce light that is found in the green color spectrum. With rare earth phosphors288 less radiation is needed than that of the older phosphors. Rare earth phosphors absorb more x-rays per crystal and produce more light per absorbed photon. In addition to the benefits of being more efficient289 rare earth screens also have the following benefits290 Decreased exposure time Decreased motion artifacts Improved contrast Increased tube life due to decreased tube current Reduced patient radiation dose Loading... Figure 2.8.png Figure 2.8291 Intensifying screen sensitivity. Grids The grid functions to decrease scatter radiation while increasing the contrast of the radiograph. Using a grid requires an increase in time since the grid does absorb some of the primary x-ray beam. As the thickness of the area being radiographed increases292 the kVp needs to be increased as well. When the thickness of an area is greater than 10 cm293 a grid should be used to help reduce scatter radiation 294Figure 2.9295. Figure 2.9.png Figure 2.9296 Using a grid to absorb x-ray beams. The grid is made of thin linear strips of alternating radiodense and radiolucent material. The radiodense strips are made of lead whereas the radiolucent spacers are made of plastic297 aluminum298 or fiber. Grid function and quality is determined by a ratio. The ratio is determined from the height of the strip in relationship to the width of the radiolucent spaces. The ratios can vary from 52991 up to 163001 and 60 lines to 120 lines per 2.5 cm. The greater the ratio and greater number of lines per 2.5 cm the more absorption occurs. For example301 if the height of the strip is 10 times greater than the thickness of the space of radiolucent material302 it will have a 103031 ratio. Grids with a higher ratio will absorb more scatter radiation304 so an increase in mAs will be needed to compensate for the increased grid ratio. For most applications in veterinary medicine305 an ideal grid ratio is 83061 with 103 lines per 2.5 cm 307Figure 2.10308. FIgure 2.10.png Figure 2.10309 Correct vs. off-center grid positioning. Parallel and Focused Grids There are two types of grids available310 parallel and focused. Parallel grids have lead strips that are set perpendicular to the surface. This allows the x-rays to penetrate through the grid while absorbing x-rays where the strips are located. A disadvantage to parallel grids is that as the x-ray beam is released from the tube head there is a divergence at an increasing angle. This results in a decrease in the amount of x-ray reaching the outer edge of the grid and film. This is referred to as grid cutoff. Focused grids have lead strips that are placed in increasing angles to match that of the divergence of the x-ray beam. The advantage of focused grids is that an unobstructed number of x-rays penetrate through the center and edges as long as the grid is parallel to the axis of the strips. These grids can only be used at specific focalfilm distances set by the manufacturer. If the distance is greater or less than that specified by the manufacturer311 grid cutoff will occur. The grid focal distance in veterinary medicine is between 34 to 44 in 31286-112 cm313 in length. Other options that are available include linear grids that allow the x-ray beam to be angled along the length of the grid without absorption by the lead strips. A crossed grid has two linear grids placed on top of one another. This type of grid does remove more scatter radiation314 but the tube cannot be tilted without causing grid cutoff. A Potter-Bucky diaphragm is a movable grid. This grid is moved in time with the exposure of the radiograph. If a movable grid is used315 the time and kilovoltage must be increased. The air gap technique can be used by increasing the distance of the patient to the cassette. Scatter radiation does not decrease in this method316 but less scatter does reach the film. This technique also decreases the sharpness of the image due to greater subject-film distance. Caliper Another piece of vital equipment used in veterinary radiology is the caliper 317Figure 2.11318. The caliper is used to measure the area of the body that is to be radiographed. It is vital that when measuring the animal319 the caliper is square and true as possible to prevent the possible over or under measurement of the animal320 resulting in a non-diagnostic image. Measurement of the animal should be taken over the widest portion that is to be radiographed while the animal is in the position that it will be in when radiographed. If the measurement is taken with the animal sitting or standing321 a falsely increased measured value may occur322 resulting in an image that is too dark323 therefore resulting in a non-diagnostic image and extra exposure to the patient and the technician when the image needs to be retaken. Proper measurement locations will be discussed in greater detail in Chapters 11-15. Loading... Figure 2.11.png Figure 2.11324 Caliper. Film Identification Proper film identification and labeling is necessary to maintain the image as a legal document. Films should be labeled with the following information as a minimum325 name of veterinary practice326 date of exposure327 patient name328 and owner first and last name. Some clinics also record the image taken and staff present during exposure. There are several methods available to label the films. One method is the use of lead numbers and letters placed on the cassette during exposure of the film 329Figure 2.12330. These are held in place either with tape or special holding devices. This style of labeling is not very user friendly in that the markers are easily lost and take time to put the information together prior to exposures. A second style of labeling is graphite impregnated tape. The tape can be written on or typed on and placed on the cassette prior to exposure. Some tape-type labels come with filters that can be placed to help prevent over or underexposure 331Figure 2.13332. Figure 2.12.png Figure 2.12333 Patient information lead letters. Figure 2.13.png Figure 2.13334 Patient information graphite impregnated tape. The most common style of film identification is the light flasher system. Information is written or typed onto a card that is placed into an imprinter. In one of the upper corners of the cassette335 a lead blocker is added to prevent exposure of the film in that area. After the exposure336 this corner is placed in the imprinter under the card and a light is flashed through the card. The information on the card is transferred to the film and will be visible following processing 337Figure 2.14338. Figure 2.14.png Figure 2.14339 Patient information imprinter. Image Markers A marker to identify the right or left side of the patient is also needed when taking radiographs 340Figure 2.15341. This is especially true when comparing extremities. In addition to right and left markers342 more advanced marking systems such as timing markers for contrast studies are available 343Figure 2.16344. Figure 2.15.png Figure 2.15345 Directional clip marker. Figure 2.16.png Figure 2.16346 Left347 Directional marker. Right348 Timing marker. Mitchell markers contain lead balls that move with gravity to help identify the location of fluid levels in equine head images and in some standing radiographs of small animals as well as helping to determine proximal and distal when looking at the long bones of a large animal 349Figure 2.17350. Computerized and digital imaging have made marking and identifying films much easier as most of the information is imprinted on the image when viewed at the viewing station351 but are not best practice and can lead to an increased chance of human error. Figure 2.17.png Figure 2.17352 Mitchell marker. Key Points Milliamperage 353mA354355 kilovoltage 356kV357358 and time 359s360 can be adjusted to obtain the optimal x-ray image. X-ray cassettes can be made of plastic361 light metal362 or graphite and contain intensifying screens. Intensifying screens are made from tiny crystals that fluoresce when exposed to x-rays. The grid functions to decrease scatter radiation while increasing the contrast of the radiograph. Grids are described as either parallel or focused grids. The caliper is used to assist in measuring the area of the body that is to be radiographed. Films should be labeled with the name of the veterinary practice363 date of exposure364 patient name and owner365s first and last name366 and directional markers. Film can be labeled with lead numbers and letters367 graphite tape368 or an imprinter. Review Questions Loading... Review Question 2.3 Mark as369 None 370object Object371 Which setting determines the acceleration of the electrons and their striking energy372 Select an answer and submit. For keyboard navigation373 use the up374down arrow keys to select an answer. a mA b mAs c kVp d Time Show submitted answer This is your recorded answer from classroom or homework Show correct answer Check My Answer Please enter an answer to submit. Review Question 2.5 Mark as375 None 376object Object377 On what structure is the tungsten plate located in the x-ray machine378 Select an answer and submit. For keyboard navigation379 use the up380down arrow keys to select an answer. a Anode b Cathode c Bucky tray 381grid tray382 d Collimator Show submitted answer This is your recorded answer from classroom or homework Show correct answer Check My Answer Please enter an answer to submit. Review Question 2.7 Mark as383 None 384object Object385 Which of the following information is not required on the film identification386 Select an answer and submit. For keyboard navigation387 use the up388down arrow keys to select an answer. a Patient389owner name b Clinic name390phone number c Date of radiograph d kVp391 mA392 and time settings Show submitted answer This is your recorded answer from classroom or homework Show correct answer Check My Answer Please enter an answer to submit. Review Question 2.8 Mark as393 None 394object Object395 What is the function of the grid396 Select an answer and submit. For keyboard navigation397 use the up398down arrow keys to select an answer. a Decrease scatter radiation while increasing contrast b Increase density while decreasing scatter radiation c Decrease contrast and scatter radiation d Increase contrast and decrease density Show submitted answer This is your recorded answer from classroom or homework

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