Chapter 4 Radiographic and Ultrasonographic Techniques
The purpose of the radiograph is to provide a lasting record of maximum information. The sequence of the major operations involved in transforming the altered morphology and tissue density within a diseased animal into a two-dimensional, black-and-white radiograph and then reaching a diagnosis is complex and includes the following steps: (1) making a properly exposed and positioned radiograph; (2) recording the x-ray picture with the assistance of accessory equipment; (3) reviewing radiographs in proper conditions and in a systematic and detailed manner; (4) recognizing lesions—therefore [requiring] a knowledge of normal radiographic anatomy and its variation by age, species, and breed and the ability to recognize and understand artifacts; and (5) evaluating radiographic abnormalities with respect to clinical and laboratory findings.
X-RAY MACHINE
Milliampere-Second and Kilovolt Peak
• mA (milliampere) × seconds (time) = mAs (milliampere-second), which affects the degree of blackness (density) of the radiograph with no effect on contrast. A direct relationship exists between mA and radiographic density. Time and mA both influence the number of x-rays produced but have no effect on the penetrating ability of the beam. To quickly check the adequacy of the mAs on a film, hold the film up to room light and place a white sheet of paper about 1 inch behind the film. Place a finger between the film and the paper. If the finger can be readily seen through the film in the black area (the most exposed), increase the mAs.
• kVp (kilovolt peak) is the only machine factor that influences radiographic contrast and has some control over the amount of radiographic density (blackness). The contrast can be expressed as being low, which means that there are many shades of gray (long scale) between the extremes of black and white. The term high contrast means that there are few shades of gray (short scale). Lowering the kVp increases the contrast, and raising the kVp reduces the contrast.
Recommendations
• The 300-mA machine may have adjustable mA stations of 25, 50, 100, 200, and 300 and two (1 and 2 mm or smaller) focal spots.
• A timing device is necessary to control the duration of an x-ray exposure. Modern x-ray machines have electronic timers with ranges that can control motion in the patient and prevent blurring. With a timed exposure of 1/120 second, all significant motion is stopped.
• The tube stand moves along the full length of the table and has an adjustable height from 0 to 60 inches (152 cm). The tube is able to rotate 90 degrees around the vertical axis and 180 degrees around the horizontal axis.
• The collimator decreases scatter radiation and human and patient exposure as it increases film quality. Always leave a clear margin of collimation on every film. The collimator is lighted and has a centering mark. A high-quality, dial-adjustable, multileaf lead shutter collimator is highly recommended.
• Filters have the primary function of reducing patient radiation dose by removing scatter radiation and increasing (mean beam energy) quality. Most x-ray equipment has inherent filtration equal to 1.5- to 2.0-mm aluminum. Increased film quality can be obtained by adding an additional 2.0-mm aluminum filtration. Total filtration must be at least 2.5-mm aluminum.
• Grids consist of a flat plate with a series of lead foil strips separated by transparent spacers. They are made in various sizes and improve the diagnostic quality of radiographs by absorbing the greater part of the scatter radiation. Position grids between the patient and the cassette, usually under the table. Use only for body parts thicker than 10 cm. The reciprocating grid (Potter-Bucky diaphragm) includes a mechanism that moves the grid during exposure to eliminate grid lines from the radiograph. This grid is optional equipment but recommended for the best-quality radiographs.
• Grids can be classified in three ways: the lead content (g/cm2), the number of lines per inch, and the ratio of lead strip height to the space between the lead strips. In general, high-ratio grids absorb scatter better but are more expensive. More lines per inch give better quality, because the lead strips are narrower and therefore lines become less prominent; however, they cost more. The most common grid is an 8:1 with 103 lines per inch.
• Exposure switches include the two-position exposure switch on the console and the two-position exposure foot switch, with a cord of sufficient length.
• High-frequency x-ray machines have a few advantages over single-phase x-ray machines. A 150-mA high-frequency generator can produce a quantity of x-rays equal to that of a 300-mA, single-phase x-ray machine. Many of the units use 110 volts. They are reliable, with less downtime than single-phase machines. Their cost, at present, is greater than that of single-phase machines but most likely will decrease in the future.
ACCESSORY RADIOGRAPHIC EQUIPMENT
Intensifying Screens
• Screen speed depends on the thickness of the phosphor layer, the size of phosphor crystals, and the efficiency of phosphor crystals at absorbing x-rays and converting them to light.
• Screen classification varies because each company has a slightly different system for labeling screen speed. Resolution ability of the screen is inversely related to speed. Increased speed gives decreased resolution. Have a technique chart available for each screen speed. Par speed is the starting point for comparison of screens. High speed has a speed 2 times that of par speed. Ultraspeed is 4 times par speed. High detail speed is half that of par speed.
• Types of Screens
• Calcium tungstate screens reduce the amount of radiation necessary to expose film by 10 times compared with film exposed without a screen. They are also less expensive than rare-earth screens. Calcium tungstate emits a broad spectrum of light in the ultraviolet and blue range.
• Rare-earth screens are more expensive than calcium tungstate screens. The light emitted is in the ultraviolet, green, or blue range. The major advantage of rare-earth over calcium tungstate screens is that rare-earth screens are fast, because of a more efficient production of light. Therefore, they decrease the production of scatter, the exposure time, the radiation dose, the chance of motion and subsequent blurring, and the wear and tear on the x-ray tube.
• System speed is the speed of film and screen in combination. It is not an additive system. A very low system speed results when screens and films with different color spectrum sensitivities are put together. Ask the film dealer what system speed you have with your particular screens and film. The higher the system speed number, the more sensitive the system and the less radiation necessary to make an exposure. System speeds can vary from 50 to 3200 and higher. The higher the system speed number, the less radiation to the patient and to personnel within the room; however, the lower the system speed number, the better the resolution qualities. When you know the system speed, changing the technique chart is simpler. If the system speed doubles, change the mAs on the technique chart by half. If the system speed decreases by 50%, double the mAs on the technique chart.
• Exposure time
• For thoracic radiographs, exposure times of 1/60 second or shorter stop the effects of respiratory and heart motion.
• For abdominal radiographs, employ exposure times of 1/40 second or shorter to eliminate gastrointestinal motion.
• Rare-earth screens have a system speed that allows the shortest exposure times possible to eliminate motion problems and to lower radiation exposure to patient and personnel. In general, when using a 300-mA machine and a 400 to 800 speed system, with an 8:1 grid, good-quality radiographs are attainable even for large-breed dogs. In a feline practice, a slower system, such as a 100 to 250 speed system, provides excellent quality. As the crystal size gets larger, the system gets faster but resolution is reduced.
Film
• Film is made of a light-sensitive emulsion, composed of gelatin and silver halide with other ingredients attached to a plastic (polyester) base. The silver halides are sensitive to light and change when exposed to light to produce a latent image. The process of developing changes silver ions into black silver, thus producing the radiographic image. Fixer removes all unexposed silver from the film. The emulsion may be attached to one or both sides of the base. X-ray film may be most sensitive to direct x-ray exposure (nonscreen film) or to blue, green, or ultraviolet light.
• Films vary in their contrast. Some films appear more black and white after exposure and development. Use the film that is recommended by your screen manufacturer to match the spectrum of light produced by your screens. Choose a film that results in the contrast range most pleasing to you. Use a film that gives you a system speed that results in quality radiographs. Make sure to match film, screens, and processing chemicals.
• Film must be sensitive to the type of light emitted by the screens in use. Film speed determines the amount of light required to produce an image on the radiograph. Fast film has large crystals (silver halide), requires less exposure, and produces a grainy image. Slow film has small crystals, requires greater exposure, and produces a sharper image.
• Screen film is manufactured with crystals that are sensitive to fluorescent light. Nonscreen film is a direct exposure–type film and is manufactured to be sensitive to x-rays. Nonscreen film requires 10 to 25 times more radiation than screen film.
• Film is available in both metric and nonmetric measurements. The most common sizes used in small animal practice are 8 × 10 inches, 10 × 12 inches, and 14 × 17 inches.
• X-ray film is sensitive not only to light and x-ray photons but also to humidity, chemicals, and physical stress. Film is stored on end to reduce pressure on the face of the film. High humidity causes film fogging, and low humidity causes film static; therefore, between 30% and 40% humidity is appropriate for film storage. Storage temperature should not exceed 50°F to 70°F (10°C to 21.1°C). Store the film away from developing chemicals and ionizing radiation. Do not put pressure on the film when loading or unloading film.
Cassettes
• Rigid cassettes protect both screens and film from physical damage and film from exposure to light. The cassette provides snug contact between the film and the screens. The front of the cassette is usually a rigid plastic, aluminum, or other substance that absorbs relatively few x-ray photons. Usually, a small rectangle in the corner of the cassette shields the film from x-rays to allow an unexposed area for film identification. The back of the cassette is lined with lead to absorb backscatter radiation. The back of the cassette is also equipped with latches to provide a lightproof seal.
• Cassettes for a nonscreen film only protect the film from light exposure. They are usually made from cardboard.
Miscellaneous Accessories
• Use safety devices (lead aprons, lead gloves, glasses, and lead thyroid shield) when personnel are required in the x-ray room for patient positioning.
• Film markers consist of right and left lead film markers, Mitchell markers (for horizontal radiographs), and time markers (for upper gastrointestinal and intravenous urography radiographs).
• Positioning devices include sponge wedges, sandbags, Plexiglas or foam trough, rope, tape, and agents for chemical restraint to avoid human exposure associated with hand-holding whenever possible.
• Monitoring devices (film badges) for employees and room monitoring, a film viewer (at least one double bank), and a hot light (high-intensity light) are needed.
Table 4-6 RADIOGRAPHIC CONTRAST AGENTS
| Type of Contrast Agent | Brand Names | Manufacturers |
|---|---|---|
| Barium products | Intropaste (barium paste) | Lafayette Pharmacal (Lafayette, IN) |
| Barosperse (barium suspension) | Mallinckrodt (St. Louis, MO) | |
| Novopaque (barium suspension) | Lafayette Pharmacal (Lafayette, IN) | |
| E-Z-ML- 196 (barium suspension) | E-Z-EM (Westbury, NY) | |
| Iodinated gastrointestinal contrast agent | Gastrografin | Bracco Diagnostics (New Brunswick, NJ) |
| Omnipaque (iohexol) | Amersham Health (Piscataway, NJ) | |
| Iodinated contrast agent | Renografin 60 or 76 | Bracco Diagnostics (New Brunswick, NJ) |
| Hypaque-76 | Amersham Health (Piscataway, NJ) | |
| Conray 30 or 400 | Mallinckrodt (St. Louis, MO) | |
| Myelographic contrast agent | Omnipaque (iohexol) | Amersham Health (Piscataway, NJ) |
| Isovue (iopamidol) | Bracco Diagnostics (New Brunswick, NJ) |
CHECKING X-RAY MACHINE ACCURACY
Milliampere Station Check
• Use an aluminum step wedge to check mA station (setting) accuracy. Use the step wedge to determine if your mA stations are linear. It cannot determine if all your mA stations are off by the same amount. A step wedge (see Table 4-3 for sources) gives a general idea about the mA stations. A step wedge is inexpensive ($100) and easy to use. Place the step wedge on a loaded cassette; make many separate exposures of the step wedge, changing the mA but always having the same mAs and kVp. If you cannot keep the same mAs throughout all mA stations, do as many as possible at one mAs setting then go back and check the rest at another mAs.
Compare the densities of all exposures. They should be the same for all mA stations taken at the same mAs. An exposure that varies from the average shows that there is a problem with that mA station. This test does not tell you if all mA stations are accurate, but it does tell you if a particular station has a problem.
Kilovolt Peak Check
The only accurate way to check the kVp is the Wisconsin test cassette ($1300; see Table 4-3 for sources).
• The Wisconsin test cassette has many kVp settings listed on the front of the cassette. Under each of these settings is a certain amount of material to attenuate the beam. After exposure and processing, this film provides information to determine if the kVp settings are accurate.
• Another less accurate way to obtain a general check of kVp accuracy is to vary mAs and kVp. Go through all kVp settings, employing density-changing factors to keep the densities the same throughout all these exposures.
| 100 mAs at 50 kVp | 3.1 mAs at 81 kVp |
| 50 mAs at 55 kVp | 1.5 mAs at 90 kVp |
| 25 mAs at 61 kVp | 0.8 mAs at 99 kVp |
| 12.5 mAs at 67 kVp | 0.4 mAs at 108 kVp |
| 6.2 mAs at 74 kVp | 0.2 mAs at 124 kVp |
Exposure Timer Check (Single-Phase Machine)
Use a spinning-top test tool. It can be purchased from the same company as the machine (see Table 4-1) or from an accessory company (see Table 4-3). This tool is inexpensive ($50) and easy to use. It is a flat metal spinning top with a hole in one side. The top is set on a loaded cassette and spun. Take an exposure at 1/120 second. Move the top to another corner of the film, spin again, and expose at 1/60 second. This maneuver is repeated at 1/40 and 1/30 second. At 1/120 second, only one dot should be seen; at 1/60, two dots; at 1/40, three dots; and at 1/30, four dots. If more or less dots are on the film than expected, the timer is not accurate.
Line Voltage
Line voltage is the amount of current coming into the machine. The amount of current may vary, depending on electrical wiring, consistency of voltage in the area, and usage of current on the same circuit in the particular practice. Almost all machines used for small animals have a line voltage-check device. Some have a meter and a dial to adjust line voltage and do not permit exposure until the voltage is manually adjusted. Other machines have an automatic line voltage adjustment. Almost all equipment in small animal practice uses 120 volts (i.e., right out of the socket).
FILM PROCESSING
Manual Processing
Film developing is a chemical process and therefore depends on both time and temperature.
Radiograph Labeling
• A radiograph is a legal document and must therefore have permanent labeling. On the label, include the hospital or veterinarian’s name, the date the radiograph was taken, and the owner’s name or the animal’s file number. Label the radiograph with right or left, dorsoventral or ventrodorsal (DV/VD), a time marker on contrast studies, and a Mitchell marker when a horizontal beam is used. Basic types of permanent marking systems are available:
• Lead letters and numbers in a holder with the hospital or veterinarian’s name, placed on the cassette during exposure.
• Radiopaque marking tape (lead-impregnated tape). Information may be written on the tape, and the tape is placed on the cassette before exposure.
• A darkroom printer transfers data from a card to the corner of the x-ray film that was shielded from radiation during exposure. This system requires cassettes with special windows. The film is removed from the cassette, and the corner of the film where it was blocked from light exposure is imprinted by exposing the patient information on a 5 × 7-inch card onto the film.
Procedure for Manual Film Processing
• Place the hanger with the film into the developing tank. Rap hard against the tank wall to dislodge air bubbles. Agitate by pulling film out of the developer, and let the developer drain to one lower corner. Return film to the developer. Repeat agitation every minute. Manufacturers recommend a specific temperature for the developed solution that they produce, usually 68°F (20°C). Adjust for change in temperature (increased temperature, decreased time, and vice versa). The chemical manufacturer can provide a time-temperature development chart.
• At the end of the developing time, remove the hanger from the developer and drain over the rinse tank. Agitate film in rinse water for 30 seconds.
Automatic Processing
• Processors are a good investment for most veterinary practices. New, small, tabletop models are priced from $3500. Most of these processors develop an excellent quality film in 90 to 210 seconds. They can use cold water for processing; therefore, no special needs exist for plumbing. They are also easy to maintain.
• Equipment includes processor, safelight, water for processing, chemicals, sponges to clean rollers of processor, and processor cleaning solution.
• Tabletop film processors are easy to install (no special plumbing or wiring). Different models process films as fast as 90 seconds or as long as 31/2 minutes. Most are made for easy care and cleaning. They are relatively inexpensive (from $3000 to $7000).
• Large, hard-wired, 90-second film processors need special plumbing and electrical wiring, but they are able to process many films. They need special cleaning and repairs and are expensive ($15,000–$25,000).
• The more a processor is used, the fewer problems it will have. It is made to be used on a 24-hour basis. Chemical buildup on rollers can cause film artifacts. Rollers age and crack if oxidized chemicals are left on them.
• Clean processors often.
• All processors need cleaning daily when not in use. Take out the rollers and wash them down with a sponge and water. Dry and replace the rollers. Clean the chemical tanks. Once a month, clean with processor cleaning solution. Cleaning a processor takes about 15 minutes a day but saves in wasted film and time.
• Consider purchasing a processor with a standby mechanism. This function helps conserve water, energy, and chemicals.
Darkroom
Recommendations
• Darkroom location: Locate the darkroom close to water and drains for plumbing purposes, and close to radiographic area to reduce unnecessary walking and to increase efficiency.
• Darkroom layout (Figs. 4-1 and 4-2): Darkrooms do not have to take up much room but are at least 6 × 8 feet. Many darkrooms are located where a bathroom might have been. This eliminates the process of bringing in new plumbing and electrical outlets. Try to include a sink for cleanup and processor maintenance. Keep wet and dry areas separate to eliminate contamination of screens and unexposed film by chemicals. Always have good ventilation to keep heat, humidity, and chemical fumes from destroying film and to reduce exposure of personnel to chemical fumes. Paint all walls white to reflect light from the safelights and for a brighter working environment. Use an adequate number and type of safelights. Make sure all safelights are 40 inches from the working area to prevent film fogging.
TECHNIQUE CHARTS
• Have a working knowledge of equipment (tube rating charts and anode cooling curves). A constant focal film distance is recommended. The same film, screens, and darkroom technique are used, as described previously.
• The technique chart is set up to take radiographs of normal animals. Different animals may have the same lateral thoracic measurement but different body types (obese, emaciated, etc.). The emaciated animal may be overexposed with the technique from the chart; therefore, you will need to decrease the exposure (kVp). The obese animal may be underexposed following the chart technique; therefore, you will have to increase the exposure (kVp).
• The technique may need to be increased (2–30%) for numerous reasons (e.g., obesity, pregnancy, ascites, pleural effusion, or disease processes that increase lung opacity, such as pneumonia and atelectasis) and for positive-contrast studies.
Technique Chart (with Grid) (Fig. 4-3)
Thorax with Grid
4. Underneath this lateral measurement, set your kVp at 95—a value in the middle of the ideal kVp parameters for a thorax.
6. Once the kVp values have been assigned, take three chest films at different mAs values. You may begin with 0.8, 1.6, and 3.2 mAs for a rare-earth system and 5, 10, and 20 mAs for calcium tungstate, for example. Select the mAs value at 95 kVp that provides you with the best technique.
Technique Chart (Tabletop without Grid) (Fig. 4-4)
Thorax, Abdomen, and Spine
2. Decrease the kVp as the thickness decreases. For example, if 74 kVp is at 11 cm, use 72 kVp at 10 cm, 70 kVp at 9 cm, and so forth.
3. Adjust the mAs by reducing to half (possibly more depending on the type of grid) the mAs value for the base techniques for the thorax, abdomen, and spine. If films are still overexposed, reduce the mAs by half again.
Extremity
2. Underneath that measurement, set your kVp at 60 (a value in the middle of your ideal kVp parameters for extremities).
4. Take films at three different mAs values (0.8, 1.6, and 3.2 with rare earth; 5, 10, and 20 with calcium tungstate). Continue selecting the mAs values until an appropriate exposure has been made. Your tabletop extremity chart is now complete. Employ this chart for all extremities distal to and including the elbow and stifle.
POSITIONING AND TECHNIQUE
Measuring for Radiographic Studies
• Measurements for the thorax, abdomen, and thoracolumbar spine are all taken at the same location on the animal. Observe the animal from above, taking the lateral measurement from the widest point across the ribs. The DV/VD measurement is made at the same point. The widest point is usually at the thoracolumbar junction.
• Measure the pelvis across the wings of the ilium. This is a very accessible area to measure, giving highly repeatable results. The same measurement is used for both the lateral and the VD.
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