Posted on March 5, 2017
X-ray, Ultrasound, MRI and CT: Which Imaging Study is the Best Choice for Your Pet?
Why has your veterinarian recommended an MRI scan rather than a plain ole’ x-ray of your dog’s back? Why a CT scan rather than an ultrasound of your kitty? Nowadays, when it comes to diagnostic imaging, there are a number of options to choose from. Each of them caters differently to visualizing abnormalities. Knowing the merits and limitations of each imaging option allows your veterinarian to recommend the best choice for your dog or cat.
Of all commonly used imaging modalities, radiographs have been around the longest. High-energy radiation is passed through the body to create a radiographic image. In the past, images were transcribed onto x-ray film. Nowadays, most veterinarians use digital sensors to capture the images. Digital radiographs are preferred because, not only can the images can be enhanced, there’s no need to mess with the nasty chemicals necessary to develop x-ray film.
Radiographic images are great for looking at bones, particularly when wanting to identify fractures or arthritic changes. Chest x-rays can detect pneumonia, enlarged lymph nodes, tumors that are relatively sizeable, and heart enlargement. Radiographs of the abdomen can be useful for finding prostate gland enlargement, most bladder stones, enlargement of the liver or spleen, abnormal gas patterns within the bowel, and some gastrointestinal foreign bodies (those that contain mineral or metal). It’s quite common for veterinarians to choose radiographs as a first diagnostic imaging test. If they don’t reveal an abnormality, a different type of imaging study may be recommended.
In some states, California being one of them, it is required that only the animal be in the room when radiographs are taken so as to avoid repetitive human radiation exposure. Depending on the finesse of the x-ray technician in using gentle restraint devices as well as the malleability of the patient, sedation may be necessary. In states where such radiation exposure laws don’t exist, a technician or two wearing lead-lined gloves and aprons typically restrain the fully awake patient in position when the x-ray is taken.
Ultrasound resembles sonar in that sound waves are emitted to retrieve information. In the case of ultrasound, the sound waves are emitted from a hand held device and information about how they pass through or are blocked by the various tissues they encounter is converted into electrical signals that are then interpreted by a computer and transformed into an image. Ultrasound is a wonderful modality for evaluating abdominal organs such as the spleen, gall bladder, liver, and kidneys. It’s also great for evaluating the heart (referred to as an echocardiogram).
Unlike x-rays that provide silhouette images of structures, ultrasound allows a look inside various organs. For this reason, ultrasound is a commonly used diagnostic tool. Where ultrasound is limited is in evaluating bony abnormalities or things within the chest cavity other than the heart. This is because sound waves are blocked by air (the lungs are filled with air) and bone.
Unless an animal is quite anxious, wiggly, or fractious, ultrasound can be usually be performed with nothing more than gentle restraint. Hair gets in the way of creating a good ultrasound image, so the study site must be shaved. Not to worry, the hair coat will grow back good as new in approximately three months time (a small price to pay for valuable information).
Computerized tomography (CT)
CT imaging is a moving x-ray machine. The x-ray and recording device rotate around the patient. The recording device sends data to a computer that compiles an image. Like a loaf of bread, the image contains multiple slices. The data is then constructed so that the person interpreting the CT scan can view the images one slice at a time or compiled into the whole loaf.
A CT scan is a good choice when an abnormality is detected that an x-ray cannot find. For example, small growths within the bone or lungs that are too subtle to be seen on an x-ray will appear crisp and clear on on a CT scan. Given that the CT machine tends to be noisy and scary and the animal must lie perfectly still for several minutes, general anesthesia is invariably required.
Magnetic Resonance Imaging (MRI)
MRI imaging uses magnetic field and high frequency radio waves to create an image. The beauty of the MRI scan is twofold. First, it creates greater soft tissue (as opposed to bone) detail than any of the other imaging studies. Secondly, it does so without any radiation exposure for the patient. MRI is an ideal choice when evaluating soft tissue structures such as muscles, tendons, ligaments, solid organs (liver, spleen, kidneys), and the spinal cord.
As with CT scanning, MRI imaging requires general anesthesia so that the animal will lie completely still in a very noisy machine over the course of 30 minutes to an hour.
Have any of these imaging studies been used for your pet? If so, what was discovered?
Nancy Kay, DVM
Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
Become a Fan of Speaking for Spot on Facebook
Please visit http://www.speakingforspot.com to read excerpts from Speaking for Spot and Your Dog’s Best Health. There you will also find “Advocacy Aids”- helpful health forms you can download and use for your own dog, and a collection of published articles on advocating for your pet’s health. Speaking for Spot and Your Dog’s Best Health are available at http://www.speakingforspot.com, Amazon.com, local bookstores, and your favorite online book seller.