By VIN Community Contributors
John Daugherty DVM
Imaging in veterinary medicine has advanced greatly since the first radiographs (x-rays) were taken of pets just decades ago. Now there is a multitude of imaging tests available to help diagnose and treat diseases in our pets. These tests include radiography (x-rays), ultrasound, CT (or CAT) scans, and MRI scans. Each of these tests has its own advantages and disadvantages, and will provide the veterinarian with different information. Many of these tests are available in general veterinary practices, but some, such as CT and MRI scans, are most often performed by specialists in large referral practices.
As with all tests, none of them is perfect for every situation. The following is a brief description of different imaging tests.
Radiograph, or x-ray. Photo by Dr. John Daugherty
A radiograph, commonly called an x-ray, is a black and white two-dimensional image of the interior of a body. An image is generated by passing radiation through a particular structure or area, such as the chest or a limb, and the image is then captured. The traditional way of recording the image is on specific x-ray film that senses how much radiation passes through the structure and reaches the film, much like photographic film captures light. The denser a tissue is (such as bone), the whiter the image is on the film. Less dense structures, such as air in the lungs, allow almost all of the x-ray energy to pass through to the film, turning that area black.
Recently, many practices have begun to upgrade to digital radiography. The principles are similar, but the images are captured on a digital recording device and displayed on a computer screen. No x-ray film is used. These images are easy to store as well as to transmit to other hospitals, or to copy to send home with pet owners. These systems are more expensive than the traditional film systems, but more and more practices are installing them as costs gradually move downward.
Regardless of whether the images are on film or digital, radiography is the most common and readily available imaging test in veterinary practice. It is used to evaluate the size and shape of organs such as the heart and lungs, as well as to demonstrate fractures (broken bones), some foreign objects, fluid accumulations, and many more abnormalities that may aid in diagnosis. It is also the most affordable imaging test, and is most often done prior to any of the other imaging options.
There is a subcategory of x-ray studies that use contrast dyes that show up on radiographs to highlight certain structures. The most familiar of these is the barium series, in which either a liquid or a paste containing barium is given orally or by enema to a patient to highlight a part of the gastrointestinal (GI) tract. Because some objects do not show up on radiographs (things like plastic, cloth, toys, rubber, etc.), barium can help diagnose obstructions or blockages. Barium shows as bright white on radiographs, so if it reaches a certain point in the GI tract and stops abruptly, we can infer that there is something blocking its progress. Sometimes we can also see a foreign object outlined by the barium trying to get around it.
The reason it is called a barium “series” is because a series of x-rays is taken at timed intervals as the barium goes through the stomach and intestines to the large intestines. The amount of time the series must be continued depends on what is found, but it can take up to 24 hours to complete in some cases, so usually the pet is hospitalized for the day. Some patients may even need to return to the hospital the following day for follow-up.
A barium study can be done in any clinic that has radiograph equipment and barium dye. However, if the pet is vomiting so much that the barium is vomited, the test may not be useful.
Another example of a contrast study is the intravenous pyelogram (IVP) in which a dye is injected intravenously. This dye is filtered from the blood by the kidneys and excreted through the urinary tract, so a series of images is taken to show its progress through the kidneys and into the ureters and bladder. This can be helpful to demonstrate structural kidney diseases, ureters that are abnormally large or do not enter the bladder in the correct place, and for certain bladder diseases.
There are numerous other contrast studies that can be done, including cystograms, a dye placed in the bladder through the urethra via catheterization; myelography, a dye around the spinal cord; and many others.
Unlike radiographs, no radiation is used in an ultrasound. An ultrasound uses sound waves. The ultrasound waves move out from the wand and either become absorbed into organs, pass through them, or are reflected (echo) back. Depending on how many sound waves are absorbed or reflected, an image of the internal organs is formed that can be seen on a monitor.
Ultrasound is painless and does not require anesthesia or even sedation in most cases. For an ultrasound evaluation to be done, the pet needs to have the hair shaved from the area to be evaluated, as hair will interfere with the images.
This test is typically done after blood tests, x-rays, or a physical examination indicates a possible problem. It is useful for evaluating things like abdominal organs, eyes, and the reproductive system. As with people, it can be used during pregnancies. There is a specific ultrasound called an echocardiogram that is used to visualize the heart and blood vessels as well as the valves of the heart.
Ultrasound can “see” some things that can’t be visualized on radiographs. For example, if the abdomen is filled with fluid, the organs can’t be distinguished on traditional x-rays because fluid and tissue have the same density. However, they appear quite different from each other on an ultrasound image, so we can see through the fluid. It is also useful, for the same reason, for seeing inside an organ such as the heart or liver.
On the other hand, it is not good at seeing through air or bone, so it does not replace radiography but rather is complementary to the information we can get from radiographs. It is common to do both x-rays and ultrasound in order to get a complete picture of what is going on with a patient.
Because the equipment is quite expensive, not all veterinary hospitals have an ultrasound machine. However, in those cases, they can often arrange for a traveling ultrasonographer to come to the hospital and do the evaluation. If this is not an option, then the pet can be referred to a specialist at a referral practice. Ultrasound images are different than x-ray images, and in some cases, it may be preferable for a veterinarian who is experienced in obtaining and reading ultrasound images to do the evaluation, or in some cases, to have a board-certified veterinary radiologist do the procedure.
As with radiography, there is also a subset of ultrasound imaging tests called contrast ultrasonography in which a material that is visible to ultrasound waves is injected as the image is being watched on the screen. These procedures are usually performed by a specialist.
Computer Axial Tomography (CAT or CT scan)
In this test, the patient is placed on a table that moves through the scanner, which sends x-ray beams through a structure from a variety of angles. Sensors then detect them and send the signals to a computer that interprets the information and displays an image that looks like a slice of the body. As the table moves slowly through the scanner, a new slice is imaged at short intervals, each one a little further back or forward from the previous slice. Because multiple images are being taken over a period of time, the pet must be anesthetized in order to keep it from moving while going through the scanner.
These images are white, black, and various levels of gray. Depending on how dark or light the gray is, a radiologist can see how well tissue absorbs the x-ray beam, and can thus identify abnormal tissue. It can differentiate tissue better than a radiograph. It is especially useful for evaluating structures in the lungs, as well as the brain and spinal cord.
As with the other imaging tests, there is a subset called contrast CT, in which a contrast dye is injected into the patient to highlight certain structures and make them stand out on the CT images.
Due to high cost of the equipment, CT scans are more expensive than radiography or ultrasonography.
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI) is one of the newest forms of imaging in general use. It uses both magnetic fields and radio waves to create images, so it is less invasive to the body than x-rays. A strong magnet – up to 40,000 times as strong as the earth’s magnetic field - briefly “magnetizes” the cells in the body while images are taken. An MRI can clearly show the interior of an organ. As with CT scans, pets must be anesthetized because they have to remain motionless.
This MRI shows a dog's spine. Photo courtesy of Dr. William Blevins.
Also as with CT scans, an MRI is useful for imaging certain structures, and contrast studies are used for some evaluations. Each test has its advantages and disadvantages, and the specialist performing the scan will choose one over the other depending on what exactly what is being evaluated.
Although an MRI is sometimes the only way to get a correct diagnosis, it is also the most expensive of the imaging tests currently available. There are several reasons for this, including the cost of the machine (as much as $1 million or more), as well as the cost of the facility needed to house the machine. Since it generates such powerful magnetic fields, it must be housed in a specially shielded area in order to protect the rest of the building from the magnetic waves (which can ruin computers and medical equipment in seconds). The cost of operating the equipment is also substantial. A board-certified veterinary radiologist most often reads the images.
In spite of the expense, MRI scans are now available to nearly all patients, as more and more referral facilities are using this equipment. Once thought of as an exotic test in veterinary medicine, it is now becoming quite common for pets to have an MRI scan as part of the diagnostic process.
The Future of Imaging
As technology continues to advance, new options are becoming available to veterinary patients. Among these is a growing field called interventional radiology, in which images are not only used for diagnosis, but can be used to target specific organs or tumors for therapy. Most of us are familiar with cardiac catheterization and angioplasty in human medicine. This is an example of interventional radiography.
In veterinary patients, there are many examples. One of the most exciting is the development of new cancer treatments. For example, some tumors can be visualized with contrast fluoroscopy (basically moving x-ray picture) or ultrasonography, and a catheter can be advanced directly into the blood vessels that supply the tumor while the radiologist guides it by watching the screen. Chemotherapy drugs can then be injected directly into the tumor, destroying or damaging it with much lower amounts of toxic drugs because it is being targeted only at abnormal tissue.
Other relatively new technologies that will become more common in the near future are 3-D ultrasonography, and 3-D reconstruction of images from CT and MRI scans. A computer takes multiple images and builds a realistic 3-D image from them. This can be extremely helpful in planning complex surgeries, and is already being used in some referral hospitals for small animal patients.
This is an exciting time in veterinary medicine, and the advances will continue to improve the care that we can provide to our patients in the future.