Radiography of Animals

Radiographs are made using a specialized type of vacuum tube that produces x-rays. The tube current is measured in milliamperes (mA), and voltage is measured in kilovolts (kV). The kilovolts and milliamperes together determine the strength and number of x-rays produced and along with exposure time comprise the 3 exposure factors that can be adjusted on most x-ray machines:

• Kilovoltage potential (kVp) is the highest potential voltage achieved at any given kilovolt setting. Higher kilovolt settings produce x-rays of a higher energy, such that a higher percentage of the x-rays penetrate the subject being radiographed. There is also a decrease in the percentage difference in absorption between tissue types, which results in decreased contrast on the final image.

  High kilovoltage potential techniques are most useful for studies of body regions with many different tissue densities (eg, thorax). Higher kilovoltage potential techniques are also often used for larger and thicker animals. Increasing kilovolts is not a linear function, and small increases in kilovoltage potential settings can substantially increase the number of x-rays penetrating the patient. However, for a number of reasons relating to the production and absorption of x-rays, this effect is much less dramatic above 85 kVp.

• Increasing the milliampere setting on the machine increases the number of x-rays produced. The energy spectrum of the x-ray beam is essentially unchanged, as are the relative numbers of x-ray photons penetrating tissues of different densities such as bone, soft tissue, and fat. However, the amount of exposure produced on the image is related to the total number of photons reaching it. Therefore, increasing milliamperes increases image contrast.

  Changes in milliampere settings are relatively linear. Increased contrast is desirable when tissue densities are similar (eg, soft tissue components of the musculoskeletal system). However, increasing milliamperes generally results in more heat loading on x-ray tubes, thus limiting exposure times and decreasing tube life as well as increasing radiation exposure to the patient.

• The third major parameter in making a radiographic exposure is exposure time. Increasing the exposure time increases the number of photons produced and hence the "darkness" of the image. For exposures in the general diagnostic range, this is a linear function. As is the case with increasing milliamperes, increasing exposure time generally results in greater heat loading of the x-ray tube than increasing kilovoltage potential, once again potentially shortening tube life and increasing radiation exposure.

All 3 of the above parameters are interdependent, with exposure time and milliamperes so much so that the term milliampere-seconds (mAs) is usually used to indicate the product of these 2 factors. Increasing the milliamperes and decreasing the exposure time by a proportional amount results in a radiograph less likely to be degraded by motion. As a rule, it is best to minimize the exposure time but maintain an appropriate milliampere-seconds and scale of contrast. Increasing kilovoltage potential also increases the number of photons penetrating the subject and darkens the image. This effect can be used within limits to correct an underexposure. The converse is likewise true.

When correcting a previously unsatisfactory image, underexposure or overexposure should be corrected for by adjusting the milliampere-seconds when examining areas of high contrast or by adjusting the kilovoltage potential when examining areas of low contrast. This will maintain the same relative contrast for that anatomical area while adjusting the image darkness. If the image is overexposed or underexposed, this is often accomplished through adjusting the milliampere-seconds by approximately 50% or the kilovoltage potential by approximately 15%.

In general, with the widespread use of filmless (digital) radiographic systems, mild changes in kilovoltage potential and milliampere-seconds tend to have minimal effect on the resultant images due to the wide exposure latitude of these systems.

Establishing a technique chart to take radiographs makes it easy for the operator to arrive at a technique by simply correcting a standardized protocol for the size of the animal being examined and the anatomical area under consideration. It also ensures that radiographs of the same anatomical region will have a consistent appearance from animal to animal. A technique chart must be made for each machine, but the settings are often programmed into the system itself. The operator need only enter the species, body part, and thickness, and the machine automatically sets the technique. This is convenient and decreases mistakes in technique; however, the settings might need to be altered to suit the specific equipment, film-screen (detector) speed, and viewer’s preferences (eg, contrast level).

Automatic exposure control (AEC) is a system in which the operator sets the kilovoltage potential and milliamperes, and the machine terminates the exposure at the appropriate time. If used properly, this system results in nearly identical image exposures between animals. However, appropriate kilovoltage settings are needed, and consistent animal positioning is critical. Identical positioning between animals is required to achieve identical images. Placing the heart versus the lungs over the AEC sensor will result in radically different radiographs.

AEC is probably most effective when large numbers of images are being done of the same anatomical area by the same personnel. AEC is not typically used in most veterinary applications because of the wide variation in body sizes and conformation of the animals being imaged. 

X-ray machines are equipped with collimators that allow adjustment of the size of the beam to fit the size of the area being radiographed. This decreases the amount of scatter radiation generated, improving image contrast and detail. Scatter radiation is also the major source of radiation exposure to operators, so proper collimation is important to decrease this risk. In addition, proper collimation is required for digital reconstruction algorithms to work properly. 

When a radiograph is made, some of the x-rays are scattered. When the object being radiographed is ≥ 10 cm thick (15 cm for digital systems), scattering becomes a problem by causing unwanted exposure of the x-ray film. A grid, which is a thin plate made up of alternating thin strips of lead and plastic, can be placed between the patient and the film to decrease exposure of the film. The ability of a grid to remove scattered radiation is measured by the grid ratio. The grid ratio is determined by the height of the lead strips divided by the distance between them. A grid with an 8:1 ratio will eliminate more scattered radiation from exposing the film than will a grid with a 6:1 ratio if both have the same number of lead lines per centimeter.

Digital radiographic imaging is now ubiquitous. Historically, radiographic images were more commonly stored on specially optimized film. However, even the best silver halide film is relatively insensitive to x-rays. For that reason, the x-ray film is usually placed between specially designed phosphorescent screens—panels composed of microscopic phosphorescent crystals embedded in a plastic matrix that directs propagation of the phosphorescent light toward the film. These screens are much more sensitive to x-rays than is film.

When the x-ray strikes a crystal, it causes the crystal to phosphoresce, and the light exposes the film secondarily. This process of recording the x-ray image is much more efficient than using film alone and markedly decreases radiation exposure to the subject (sometimes by a factor of 100 or more) and the operator. It also decreases the amount of scatter radiation recorded on the image. The screens and film are contained in a lightproof cassette, which is transparent to x-rays.

Although digital imaging systems have not yet been able to match radiographic film for spatial resolution, they typically produce images that have much better contrast resolution, which offsets the slightly decreased resolution by increasing the conspicuousness of structures on the image. Today, it is difficult to find film cassettes and screens for medical radiography sold by primary vendors. A darkroom is not required for digital image capture, which is now the standard in veterinary radiography, so darkrooms will not be discussed. For information on darkroom procedures, please see a text dedicated to veterinary radiography.

Digital image recording systems do not require the use of film, screens, or processing chemicals. These systems have several advantages over conventional film radiography:

• Radiographs cannot be lost if adequate data safeguards are used.

• There is no need for film storage and its attendant space and environmental requirements.

• The process allows for manipulation and enhancement after the image has been recorded.

• Images can be transmitted electronically to a remote location for immediate interpretation (eg, teleradiology).

• The images are generally available more quickly, usually nearly instantaneously.

• There is no need for a darkroom.

These digital systems can be divided into 2 categories: computed radiography (CR), in which a semiconductor plate contained in a cassette is exposed in the usual fashion and then read electronically inside a special reader that detects the magnitude of electrostatic charge on each of the semiconductor elements within the plate; and direct digital radiography (DR), in which a cesium iodide scintillator array absorbs the x-rays, producing a light pulse detected by a large array (millions) of amorphous silicone photodiode/transistor elements.

In both systems, the electrical output from each of the detector elements is proportional to the number of x-rays that strike the detector element and is mathematically quantifiable, hence the term “digital images.”

In both systems, the data produced are processed by a computer, which generates the image on a monitor according to a previously determined processing algorithm that is specific to the region being radiographed. Processing algorithms are critical to the development of diagnostic images. In many display systems, the algorithm can be altered to provide enhancement of various features of the image.

The digital images are then stored electronically and made available to any computer with access to the image archive and a proper display program.

The difference between the 2 systems lies in the intermediate step of exposing a plate in CR, which is then placed in a reader. These plates must be replaced periodically because of wear created during the reading process. There is also the issue of whether the latent image recorded by the reader is an accurate representation of the true image. The portability of the cassettes is an important benefit in practice situations in which radiographic images are produced in multiple places. CR systems are also still less expensive than all but the simplest DR systems and generally still have higher resolution capabilities, which can be important for imaging smaller anatomical parts. However, many of CR's strengths are now matched or surpassed by DR systems. For these reasons, the vast majority of digital systems installed today are of the DR variety. 

DR systems are very complex electronically and subject to the same insults as any complex electronic system. They are particularly sensitive to shock and electronic interference. However, when properly cared for, DR systems are durable and reliable. They do not require handling of the image recording plate, which decreases wear and tear on the system. They are much faster than CR systems and do not require the intermediate step of a reading device. 

DR systems have been developed that do not require a cable to communicate between the detector and the computer processing the data into an image. The cable has been replaced by wireless communication on specified electrical magnetic frequencies that are unlikely to be interfered with by other electromagnetic devices such as cell phones and electronic equipment. Although they are still somewhat more expensive than systems incorporating a cable connection between the detector and computer, such systems are particularly suited for use in equine ambulatory practices. Images can also be sent to the storage system via a wireless connection.

The flexibility and reliability of digital radiography systems, whether wireless or wired, have improved to the point that they are universally used in human radiology departments as well as in most veterinary hospitals. As DR systems grow in capability, reliability, ease of use, and resolution while also decreasing in cost, it is expected they will eventually replace both CR and traditional film systems. Although currently DR still cannot match the spatial resolution of either standard-speed film or CR systems, newer systems continue to narrow the gap. This relatively low spatial resolution is offset to a large extent by improved contrast resolution, which is more pleasing to the eye. Because of their inherent high contrast, direct digital systems are also the choice of imaging device for very large animals.

The advent of digital imaging has led to the development of special image storage systems and formats. The data stored on computers must be protected from loss and corruption. Loss of data can be guarded against by storing identical sets of data on different computers in different geographic locations or by copying the data files to optical storage media that are then kept in a safe location.

Protection of the data from corruption is a more challenging issue. Because images stored in a digital format are easily manipulated by various computer programs, it is possible that they could be altered (accidentally or deliberately) to reflect a different situation than the actual one. For this reason, many electronic image formats are not recognized as legal documents and are not acceptable in a court of law. Because of this potential for alteration or abuse, a special medical image format (Digital Imaging and Communications in Medicine [DICOM]) has been developed and agreed on by the American College of Radiology, the American College of Veterinary Radiology, and others as the standard format for medical image generation and storage.

The key feature of DICOM formatting is the presence of a hidden header in the image file that records all manipulations of the image or the header each time the image is saved. The header also contains a large amount of information about the patient as well as production factors of the image, which must be specified before the image is created. This makes accidental or malicious manipulation of the image much easier to trace. Another and even more important benefit of the DICOM format is that it makes images easily transferable to other sites for referral interpretation or patient referral. No digital imaging system should be purchased that does not conform to the DICOM standard.

The portability of digital images and the speed and usability of the internet have led to veterinarians in private practice having much greater access to the interpretive skills of radiologists and other specialists. Specialists are consulted much more widely than ever before. This has the potential to improve the quality of veterinary practice worldwide, not only in the field of imaging but in many other specialties.

Once digital images are generated they must be stored. Standard practice is to store these images in the DICOM format on a computer hard drive using a picture archiving and communication system (PACS) program. These programs store the images and provide a display program compatible with the DICOM format. 

Many of these systems can also be integrated with an electronic medical record system so the images can be directly incorporated into the patient’s medical record. Most systems also provide tools to aid in the evaluation of the images as well as aiding in distribution of the images to other sites or to the owner. PACS systems also allow images to be viewed simultaneously in different locations, such as during a consultation with a radiologist or other specialist.

Although use of PACS systems might seem to be an unnecessary expense, they make storage of digital images more secure and simpler, especially in practices where radiography is routinely used.

Image viewing and interpretation should be performed on diagnostic monitors in a dark or dimly lit room.

Board-certified imaging specialists are available to review studies from anywhere around the world through teleradiology, though the basics of imaging interpretation are important skills for any veterinarian using diagnostic imaging.

At the minimum, substantial experience and attention to detail are required to become proficient in interpretation of diagnostic images. Board-certified radiologists and diagnostic imaging specialists are highly trained veterinarians who have spent a considerable amount of time in addition to their veterinary schooling to become experts in image interpretation.

Most diagnostic imaging residencies in the United States are 3–4 years in length and require at least 1 year of a rotating internship before the residency. Many applicants also complete a speciality imaging internship, such that many residency hopefuls have spent an additional 4–6 years beyond their postgraduate veterinary training to become imaging specialists.

Regardless, imaging studies are still largely acquired by general practitioners or other specialists who might or might not choose to have the study interpreted by a radiologist. Therefore, it is imperative that even new veterinarians have a basic understanding of radiographic interpretation upon graduation such that critical disease processes (eg, GI mechanical obstruction, pneumothorax, fractures, aggressive osseous lesions) can be identified and addressed promptly. It is less common that veterinarians who are not imaging specialists will interpret CT, MRI, or ultrasonography without assistance from (or complete reliance on) an imaging specialist.

Above all else, veterinarians who are interpreting images must have solid understanding of the anatomy of interest. Imaging changes are often characterized by alterations in size, shape, location/position, opacity/echogenicity/attenuation/intensity, and margination, which represent the basis of radiographic interpretation. In addition, the extent of change, whether generalized throughout an organ or associated with other abnormalities, must be evaluated.

The presence of lesions that do not affect the entirety of an organ, such as focal enlargement of the liver or focal opacification of a lung field, is strongly suggestive of localized disease such as neoplasia. Conversely, lesions causing generalized change throughout an organ such as the liver or kidneys are most suggestive of systemic disease such as infections, endocrinopathies, or toxicities. Combinations of lesions in different locations or organs also help narrow the list of differential diagnoses. Close attention to the basic principles of interpretation and a careful systematic approach will often provide answers not readily apparent on initial examination.

Without proper anatomical knowledge, normal findings might be reported on and potentially treated as abnormal, or abnormal findings might be erroneously interpreted as normal. Both situations can have potentially deleterious effects on patient care. Therefore, it is imperative that the acquired images are of sufficient quality, including appropriate exposure factors and positioning, such that patterns of disease are more easily recognized and the anatomical region of interest is consistently and accurately depicted.

At best, rotated or improperly exposed images will add a considerable amount of time and confusion to interpreting the study. At worst, these images might not accurately represent any pathological changes, rendering the study nondiagnostic, delaying time for a potential diagnosis, and adding potential stress to the patient and unnecessary financial burden to the client.

Much like performing a thorough physical examination, a systematic approach to evaluation of diagnostic images will improve the reading skill of even very experienced individuals and ensure that lesions in areas not of primary interest or near the edge of the image are not missed.

• When evaluating thoracic radiographs, for example, it might be of benefit to always evaluate the extrathoracic soft tissues and musculoskeletal system first, followed by the pulmonary parenchyma, cardiovascular structures, and airways. In this outside-in approach, the areas that are most likely to be of interest (eg, heart and airways) are evaluated last so that other potentially clinically relevant findings are not missed.

• Alternatively, an inside-out approach can be used such that the heart and pulmonary parenchyma are evaluated first, with the extrathoracic structures evaluated last. A similar approach might be used when evaluating radiographs centered on the musculoskeletal system.

• A systems-based approach is another way to interpret images, in which groups of organs are evaluated together to identify any abnormalities.

Regardless of the method chosen, as long as a systematic approach is consistently used, the interpreter is less likely to miss lesions. With experience, diagnostic imaging specialists tend to use a more random approach to image interpretation, which can be equally effective but still leaves the interpreter susceptible to imaging interpretation biases or flaws.

It is essentially impossible to evaluate radiographs without a preexisting bias as a result of knowledge of the history, physical examination findings, and previously performed laboratory results. This bias can easily promote underevaluation of the image by focusing on only the area of interest associated with the bias. Even so, it is true that this knowledge of the history and signalment is absolutely necessary to achieve consistent and accurate interpretation of radiographic studies.

An imaging report is typically comprised of 2 or 3 of the following sections:

Plain radiographs lack sufficient contrast to evaluate many structures. Therefore, contrast procedures are used to increase the native contrast of organs and lesions and delineate them from surrounding tissues.

Contrast media are radiopaque compounds that have extremely low toxicity, although there are well-recognized hemodynamic alterations observed after administration of IV contrast agents. These consist of primarily a reflex hypotension followed by a rebound mild hypertension. In extreme cases, the hypotension can lead to vascular collapse; anaphylaxis and acute renal failure have also been reported.

This effect is thought to be primarily associated with the hypertonic nature of ionic contrast agents and is markedly less evident when modern nonionic (low osmolarity) agents are used. For these reasons, nonionic agents have almost completely replaced ionic agents as IV contrast material.

IV and intra-arterial contrast agents are generally iodine based and increase the opacity of the blood, making vascular structures visible. Iodinated contrast agents are cleared primarily by the kidneys, making the collecting system of the urinary tract visible. Orally administered agents, primarily barium sulfate–based compounds, outline the mucosa and lumen of the GI tract. Intrathecal contrast agents are also iodine based and allow evaluation of the spinal cord and meninges.

Though many of these contrast procedures have been largely supplanted by modern advanced imaging procedures, many of them are still the best way to image the structures they are designed to evaluate and should not be forgotten if advanced imaging procedures fall short or are not available based on physical proximity or financial constraints. Many contrast procedures do not require special equipment and can be performed in most veterinary practices; however, interpretation is best performed by someone with extensive experience and training in the interpretation of radiographic images.

The internet has had a huge impact on the way radiography is used in veterinary practice. Because all images are or can be made digital in one manner or another, it is possible to send images from any modality using DICOM transfer utilities or another standard image format (eg, JPEG) to a radiologist for interpretation and get a report back within minutes to a few hours.

As the scope of veterinary practice continues to expand, many veterinarians in general practice want the support of a radiologist for interpretation of radiographic images. A large percentage of board-certified radiologists practice teleradiology to some extent, and many do so exclusively. For more information, contact the American College of Veterinary Radiology (ACVR).

Considerable attention has been paid to the development, use, and implementation of artificial intelligence (AI) in diagnostic imaging interpretation. There are, however, considerable legal, ethical, and logistical issues surrounding the involvement of AI in diagnostic imaging studies that are unresolved or unclear. Nevertheless, there are multiple veterinary teleradiology companies that provide artificial intelligence-guided and -assisted image interpretation. It is likely that AI will only continue to grow within the field of radiology in the future.

For more information, see ACVR's current stance on the use of AI in veterinary imaging.

• Brown M, Brown L. Lavin's Radiography for Veterinary Technicians. 7th ed. Elsevier; 2022.

• American College of Veterinary Radiology

• Alexander K, Magestro L. Artificial intelligence. American College of Veterinary Radiology. May 7, 2021. Accessed January 15, 2024.

• Hilton R. Positioning for good limb photographs. Veterinary Information Network. 2012. Accessed January 15, 2024.

• Also see pet health content regarding diagnostic imaging.