The Difference Between Diagnostic Radiology, Radiation Therapy and Interventional Radiology

radiation therapy

At first glance, these three disciplines within veterinary medicine seem pretty much the same, but at the Animal Medical Center, diagnostic radiology, radiation therapy, and interventional radiology represent three different groups of veterinarians with three very different sets of background and training. What ties these three disparate groups together is their use of radiation to diagnose and treat disease.

Diagnostic Radiology
These days you are more likely to find a Department of Diagnostic Imaging in a hospital than a Radiology Department. Radiology is an older term, used when x-rays were the only testing modality using radiation available in medicine. Today, AMC’s Diagnostic Imaging Service uses not only traditional x-rays but also ultrasound, computed tomography (CT) scans, and magnetic resonance imaging (MRI) in the diagnostic evaluation of patients. AMC’s Diagnostic Imaging Service also has a fluoroscopy unit, which is like a video x-ray. This machine allows us to watch bodily functions like blood flow or swallowing in real time. To see an example of fluoroscopy, read about Molly the Ganaraskan. Every veterinarian at AMC depends on our diagnostic imaging team for their expertise in imaging sick pets and helping us to make an accurate diagnosis.

Radiation Therapy
A very accurate description, AMC’s Radiation Oncology Service uses radiation to treat tumors. Specifically, we have a linear accelerator (linac), a giant machine that creates various types of radiation depending on patient needs. Our state-of-the-art linac can make electrons for superficial treatments, produce high energy pinpoint beams for stereotactic radiosurgery, and stereotactic body radiation therapy. Diagnostic Imaging’s CT scanner interfaces with Radiation Oncology’s 3-D computer planning system. The interface allows the linac’s multileaf collimator to sculpt the radiation beam to precisely target the tumor being treated. The veterinarians working in radiation therapy have training in both the physics of radiation as well as the management of cancer.

Interventional Radiology
Specialists in interventional radiology use minimally invasive techniques to make image-guided diagnoses and also deploy high tech treatments for a variety of diseases. Using a range of techniques which rely on the use of images generated by diagnostic radiology equipment such as fluoroscopy, ultrasound, CT scan, or MRI imaging, the interventional radiologist precisely targets various organs with treatments such as stents, occluders and medications. Watch a video where AMC’s interventional radiology team uses fluoroscopy to close off abnormal blood vessels in the liver. The veterinarians in our Interventional Radiology Service have diverse backgrounds in surgery, internal medicine plus specialized training to use minimally invasive equipment.

Linked together by their use of radiation as a diagnostic and therapeutic tool, diagnostic radiology, radiation therapy and interventional radiology are just a few of the highly trained specialists at AMC working to make sick pets healthy again.

CT Versus MRI: Battle of the Big Machines

Veterinarians at The Animal Medical Center depend on high tech equipment to make diagnoses and monitor treatment success. Two commonly used pieces of high tech equipment are the CAT scan or CT (Computed Tomography) and MRI (Magnetic Resonance Imaging). Often, if I recommend a CT, pet owners will ask if an MRI would be better. I checked with one of AMC’s board certified radiologists, Dr. Anthony Fischetti, to help dispel any myths about which test is the best. He says “both are equally as good, but which test is used depends on the body part being imaged and the type of resolution required to optimally image that body part.”

Big Machines at AMC
Computed tomography was introduced to human medicine in the 1970s. The AMC acquired its first CT scanner about 10 years later and is currently using its third scanner, a high-powered 64-slice CT scanner. Magnetic resonance imaging became commercially available in the 1980s and The AMC installed its first MRI machine in 2002 and upgraded that machine in 2006 for a more powerful model. To give you a comparison of the frequency of use of these tests, in 2007, a total of 73 million CT scans were performed on humans. In 2013, 700 CT scans and 600 MRI exams were performed – just at The AMC!

Starting at the Top
Imaging the head is a particularly good example of why we need both a CT scanner and an MRI machine at The AMC. The brain is composed of soft tissue and the boney skull is clearly hard tissue. When our neurologists want an image of the brain to determine the cause of seizures, they choose an MRI because it produces images with exquisite detail of soft tissues comprising the brain. An MRI can show minute changes in both types of brain tissue, the grey and white matter. But if an internal medicine specialist suspects the cause of a bloody nose to be a tumor in the nasal passages, they choose a CT scan, not only for its speed, but for its ability to show changes in the bones composing the nose and nasal passages. Because computed tomography is part computer, the images it creates are easily manipulated into a variety of views and even three dimensional reconstructions. The image you see to the right shows a reconstruction of the skull of a dog with a jaw tumor.

CT Goes with the Flow
CT scan is a form of x-ray and can detect a special contrast agent when the agent is administered intravenously. Using an intravenous contrast agent during a CT scan (CT angiography) helps veterinarians identify abnormal blood vessels in the liver – a common congenital disorder in small breed dogs – or determine, prior to surgery, if a tumor has breached a major blood vessel. Armed with this information, surgeons can better plan their approach before they get to the operating room.

MRI has a Heart
MRI also uses intravenous contrast agents to differentiate various soft tissues in the body. The MRI image you see on the right shows a tumor of the heart in a dog following administration of a contrast agent.

Your Pet and the Big Machines
Here are some tips for pet owners whose pets require a CT scan or MRI:

  1. Expect blood tests and possibly a chest x-ray to be done before the scan. Testing helps veterinarians determine safe anesthetic protocols for your pet.
  2. Unlike when you or I receive an MRI or CT scan, you should anticipate that anesthesia will be administered to your pet. You know how hard it is to get a clear photograph of your wiggly pet. We need them to be perfectly still for imaging so that we can obtain an accurate scan.
  3. Know that it may take up to 24 hours for the radiologist to issue a final report on the scan. Waiting is hard, but reviewing images takes time and should not be rushed.


Ultrasounds (or sonograms) use sound waves to produce an image of internal organs. The same equipment used in people (for say, a pregnancy exam) is used for evaluating companion animals with various diseases, most commonly diseases in the abdomen and heart.  Our Cardiology Service performs echocardiograms (ultrasound of the heart). The Diagnostic Imaging Service uses ultrasound for all other areas of the body. AMC’s Diagnostic Imaging Service recently purchased a new machine (Hitachi Prerius) equipped with state-of-the-art technology for optimizing image quality and improving diagnostic capabilities.  The addition of this unit to our existing machines allows the radiologists to complete two exams at the same time, shortening the wait time for ultrasound exams.

The clinical questions demanded of veterinary abdominal ultrasound are fairly broad and can at times be intense. Advanced imaging (like CT or MRI) may be cost prohibitive or impractical for some veterinary applications. Ultrasounds by skilled operators identify an assortment of diseases, including small tumors, vascular invasion and vascular anomalies. All imaging exams at The AMC are performed by either a board certified radiologist or a radiologist-in-training under supervision by the radiologist.

CT Scan

The Animal Medical Center is excited about the installation of a new, upgraded CT scanner. We have replaced our previous single-slice unit with a much advanced 64-slice unit, similar to the equipment used in high-end human hospitals.

The benefits of this new scanner include decreased scan time and improved image quality. The decreased scan time reduces or eliminates the need for general anesthesia while scanning. This anesthesia-free protocol is much safer for our patients needing advanced imaging. The 64-slice CT scanner acquires more information in less time, and actually exposes the patients to less radiation than the smaller slice scan machines. Less radiation is, of course, safer for our patients.

Finally, the improved image quality of our new CT scanner allows for real-life, 3-dimensional illustrations of patient anatomy. Three-dimensional and multi-planar reconstructions of structures like the head, chest and bones provide exquisite detail for improvements in diagnosis and treatment planning.

The Animal Medical Center is one of the few animal hospitals in the world to have such a powerful machine dedicated solely to animals.

3-D reconstruction of the middle ears and temporomandibular joints of a dog with difficulty chewing and opening its mouth. The tympanic bulla and temporomandibular joint on the right are irregular in shape and size relative to the left (R denotes right). The middle ear infection was so extensive that it extended rostrally to infect the temporomandibular joint. 

3-D reconstruction of a rabbit with a bulging eyeball. Other images from this CT revealed a large pocket of infection behind the eye secondary to an infected tooth root under the eye.

Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI) provides high detail anatomic information unlike any other modality offered by our Diagnostic Imaging Service. AMC’s MRI is a high-strength, 1.5T magnet, identical to units used in imaging people. Organs like the brain and spinal cord are imaged in companion animals suffering from conditions like seizures, behavior change, back pain and paralysis.   Additionally, chronic shoulder pain and other musculoskeletal problems are optimally imaged with MRI. MRI is the single best modality for providing contrast and detail of the body. The drawback is that MRI requires long scan times and general anesthesia in order to acquire the images.

The most common indication for MRI is in the case of acute paralysis in dogs. Chondrodystrophic dogs (like dachshunds) are predisposed to acute intervertebral disc herniations, often requiring emergency surgery to decompress the spinal cord. The MRI pinpoints the exact location of a disc herniation, directing the surgeon where to cut. AMC’s MRI is available 24 hours a day, unlike other institutions providing veterinary services. The high strength of AMC’s magnet also allows us to image in a shorter amount time than other institutions. A shorter scan time means a faster diagnosis, and shorter time to surgery.

The image below is a classic example of disc herniation in a dog that acutely could not walk and had severe neck pain. Can you identify the abnormal intervertebral disc?