Fluoroscopy: Don’t Miss the Show!
Fluoroscopy produces real-time images that appear as an “x-ray movie,” providing a helpful view for dynamic processes.
Fluoroscopy is an advanced diagnostic tool that helps the veterinary team refine a diagnosis by compiling a series of radiographic images into a movie. The result is similar to a child’s cartoon flipbook in which multiple single images on separate pages, each with a tiny change, become a short animated clip when stapled together and flipped through quickly.
Fluoroscopy is used in research and primary care facilities to visualize dynamic processes, such as gastrointestinal motility, cardiovascular function, blood and urine flow, collapsing trachea, and swallowing, in real time. Fluoroscopic guidance is helpful in contrast procedures and in the placement of medical devices and surgical hardware. This article provides an overview of the history, technology, safety, and use of fluoroscopy in the veterinary hospital.
HISTORY OF FLUOROSCOPY
In 1896, one year after Wilhelm Conrad Roentgen discovered x-rays, Thomas Edison invented the fluoroscope.1 This first fluoroscope was handheld, could not record images for later viewing, and had to be used in the dark, making it impractical for medical use. Fluoroscopy was used for entertainment purposes for many years until the dangers of radiation were recognized.
In 1940, John Coltman’s invention of the electronic image-intensifying tube allowed doctors to use a completely hands-free fluoroscopy machine in regular room lighting, and in 1953, Westinghouse commercialized the image intensifier.1 These developments allowed for improved patient and personnel safety and improved diagnoses. The same basic design is still used in veterinary medicine today, with recording capabilities and digital advancements.
A fluoroscopic unit uses a standard x-ray tube head. In the tube head, excited electrons, emitted by a cathode, strike a tungsten target on an anode, releasing x-ray photons. The angle of the anode directs the x-rays downward, forming the primary beam. The beam exits the tube head through a beryllium window. The primary beam then passes through a collimator made of lead shutters, which helps limit it to just the area of interest.2
In fluoroscopic units, after the x-ray beam passes through the area of interest, it is intercepted on the other side by an input phosphor made of cesium iodide and is converted to light photons by the image intensifier. These photons are converted to photoelectrons that are then seen on a monitor.2 Fluoroscopy units are usually equipped with multiple intensifier sizes, which allow the imager to vary the field of view on the screen, similar to magnifying an image.
The real-time image produced appears to be an “x-ray movie,” displayed on a video monitor. This image appears as an inverted radiograph, flipping the bright (radiopaque) areas with the dark (radiolucent) areas. This inversion is helpful to improve the viewer’s visualization of structures. Some units allow recordings of the study, known as a cine loop. Permanent or still images can also be acquired during fluoroscopy; these are typically referred to as spot films.
There are 2 types of full-sized fluoroscopy units used in veterinary facilities: mobile C-arm units and R&F (radiography and fluoroscopy) rooms. Small handheld units, often used in large animal hospitals to image equine distal extremities, also exist. These units do not produce high-quality images and often overheat with prolonged use; therefore, they are not commonly used today in small animal practice.
C-arm units are more compact than R&F systems, and unlike table units, they are portable, making them useful in both large and small animal practice, as well as research facilities. These units have the fluoroscopy head and the image intensifier on opposite ends of the C-arm, which can be positioned to present anatomy in different angles and directions while the patient lies on a radiolucent surface within the C shape. C-arm units are used during surgery to help evaluate orthopedic fracture repair and reduction (Figure 1), identify catheter placement, and guide placement of stents and medical devices. They can also be used for contrast procedures that do not require the patient to be standing, such as cystography and angiography.
R&F units contain an advanced stationary radiography table with an integrated fluoroscope. The 2 separate tube heads, one for radiography and one for fluoroscopy, allow the machine to be used for both radiography and fluoroscopic imaging, but not simultaneously. The operator must know how to initiate the correct tube for the desired study. The fluoroscopy tube is mounted beneath the table, which can be moved into a vertical orientation for standing views (Figures 2 and 3). During use, the image intensifier is positioned over the patient, and the under-table tube and intensifier can be manually moved simultaneously to cover the area of interest. R&F units can be used for procedures similar to C-arm units, as well as standing procedures such as collapsing trachea and swallow studies.
A contrast agent is often given during a fluoroscopic examination to enhance the area of interest and help highlight its function or delineate it from the surrounding area. Contrast agents can be negative or positive. Negative contrast agents are radiolucent; that is, they have a low density and allow almost all x-rays to pass to the recording surface. They appear white on fluoroscopic images (black on radiographs). Positive contrast agents have a high atomic number and high density and are radiopaque; that is, they absorb most x-rays and allow very few to pass to the recording surface. They appear black on fluoroscopic images (white on radiographs).
Fluoroscopic examinations mostly rely on positive contrast agents to obtain maximum enhancement of the area of interest. The first contrast agent, lead subacetate, was used to view the stomach of a guinea pig in 1896.3 Lead subacetate was chosen because of its high density.
Today’s positive contrast agents fall into 2 categories: barium sulfate and iodinated contrast agents. Barium sulfate is insoluble, meaning the body cannot absorb it. Barium sulfate is very useful for gastrointestinal studies (Box 1); however, it is suggested that iodinated contrast media is used in place of barium when a perforation is suspected.
Water-soluble iodinated contrast agents may be ionic or nonionic. Ionic iodinated contrast media are hypertonic solutions, meaning they have high concentration of solutes that will cause surrounding cells to dehydrate. Nonionic contrast agents have a low osmolality and do not separate in solutions, making them up to 6 times safer than ionic contrast media (Box 2); however, they are very expensive and do not have a long shelf life.4 Nonionic contrast agents are useful for vascular and urinary studies. These agents are absorbed into the bloodstream and excreted by the kidneys, and they can be administered intravenously, orally, or intrathecally.
Unlike in the early days of fluoroscopy, the diagnostic use of x-rays must now be justified and the radiation dose kept “as low as reasonably achievable” (ALARA). Fluoroscopic procedures can expose both patient and personnel to a high dose of radiation; therefore, fluoroscopy should never replace standard radiography. The veterinary team must remember that the radiation exposure during a fluoroscopic examination is higher than that associated with a single radiograph.
During a fluoroscopic examination, the greatest occupational hazard to the veterinary team performing the examination is scatter radiation (i.e., secondary radiation), owing to the long exposure time and increased kilovoltage peak (kVp). Because the configuration of a fluoroscopy unit differs from standard radiography, the veterinary team needs to understand where the radiation is coming from and where potential scatter radiation is highest. Table units, with the tube head below the table, project most scatter upward.
Personal Protective Equipment
All staff who remain in the room during the examination must wear proper personal protective equipment (PPE). The suggested PPE to wear during fluoroscopy is a lead apron and a thyroid shield with a 0.5-mm lead equivalent, and, optimally, lead glasses or goggles.5,6 Any personnel handling the patient or whose hands are positioned close to the primary beam should also wear front- and back-sided, full-coverage, 0.5-mm lead gloves. As with standard radiography, anyone who remains in the room must wear a dosimetry badge to measure the dose of scatter radiation.
While most lead aprons are suitable for frontal coverage, it is suggested that two-sided lead aprons covering both front and back, often referred to as 360-degree coverage, be worn during fluoroscopic procedures. This is because of the amount of scatter radiation generated while the system is collecting images; personnel in the room should never turn their back to the table or unit. Table units have a lead curtain to further protect the personnel performing the examination and manipulating the image intensifier position from scatter radiation.
Radiation Dose Reduction
During fluoroscopy, x-rays are continuously being released and converted to produce the images being displayed on the video monitor. Depending on the specific system, frame rates of 1 to 30 fps (frames per second) may be available. To help keep the radiation dose ALARA, the lowest frame rate that provides the best diagnostic quality is the best choice. The veterinary nurse or clinician should always adjust the collimation to the area of interest to reduce radiation exposure to the patient and veterinary team.
Different technical factors can also help reduce radiation exposure. It is important for the veterinary nurse operating the unit to be familiar with the machine’s capabilities. Using pulsed fluoroscopy with a lower pulse rate can reduce the patient’s radiation exposure by almost half.1 The pulse rate is directly proportional to the applied dose. If the image is fairly static, with not much movement, a lower pulse rate of 4 to 8 p/s (pulses per second) will lower the patient and operator dose significantly. If the situation warrants viewing a moving object, such as during a motility study, at least 8 to 12.5 p/s might be required.
Increasing or decreasing the magnification over the area of interest can also increase or decrease the amount of radiation the patient receives. Exposure, kVp, and milliamperage (mA) can also be adjusted to ensure the lowest diagnostic dose. Fluoroscopy units tend to use a low mA and higher kVp. Most units have an automatic and/or manual exposure rate or a set kVp and mA. Automatic exposure rates are preset tube voltage and current levels based on the object being imaged.
Contrast Agent Dose and Administration
Different contrast agents are used for specific studies, and not all agents are administered the same way. Contrast agents often vary in concentration based on the brand. Care should be taken in selecting and administering contrast agents, as many are nephrotoxic when given repeatedly or in large quantities. Ionic contrast agents can cause severe dehydration, bradycardia, congestive heart failure, nausea and vomiting, peripheral vasodilation, immune-mediated responses, and allergic reactions.3 Contrast studies should be at least 24 hours apart. Caution should be used when administering barium because peritonitis can occur when barium leaks out of the gastrointestinal tract or is accidentally aspirated. Peritonitis can cause permanent adhesions and granulomas.
All contrast agents and doses must be prescribed by the veterinarian. It is the veterinary nurse’s role to ensure the specific details for each procedure and to make sure the correct contrast dose is administered via the route intended.
Veterinary Team Education
Each practice should develop specific protocols for fluoroscopic examinations. These should include examination-specific details and principles to be followed by the veterinary team to help eliminate errors. Fluoroscopy units should be operated under the direction of educated personnel who are familiar with the technical and safety requirements and operational guidelines of the specific system. All personnel involved in an examination should be aware of the appropriate radiation factors to ensure that radiation exposure is kept to a minimum.
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2. Ayers S. Radiographic system. In: Small Animal Radiographic Techniques and Positioning. Ames, IA: John Wiley & Sons; 2012:24-25.
3. Wallack ST. Introduction to barium. In: The Handbook of Veterinary Contrast Radiography. San Diego, CA: San Diego Veterinary Imaging, Inc.; 2003:1-4.
4. Wallack ST. Introduction to iodinated contrast agents. In: The Handbook of Veterinary Contrast Radiography. San Diego, CA: San Diego Veterinary Imaging, Inc.; 2003:5-7.
5. Han CM, Hurd CD. X-ray equipment. In: Practical Diagnostic Imaging for the Veterinary Technician. 3rd ed. St. Louis, MO: Elsevier Mosby; 2005:30-31.
6. Han CM, Hurd CD. Radiation safety. In: Practical Diagnostic Imaging for the Veterinary Technician. 3rd ed. St. Louis, MO: Elsevier Mosby; 2005:41-43.