X-ray, Ultrasound, MRI and CT: Which Imaging Study is the Best Choice for Your Pet?

Photo Credit: Liverpoolhls, x-ray, Flicker CC licenseWhy 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.

Radiographs (X-rays)

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?

Best wishes,

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.




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6 Comments on “X-ray, Ultrasound, MRI and CT: Which Imaging Study is the Best Choice for Your Pet?

  1. So sorry for your loss Barbara. Yes, indeed, MRI imaging is the gold standard test for spinal cord disease.

  2. Yes Jann, you are correct in terms of which imaging studies involve radiation exposure and which ones do not.

  3. One of my elderly cats, Charlie, had one eye dilated where the other eye was not. It was Sunday of Labor Day weekend and I texted my vet. She said I’d better take him to the emergency vet. So I did and they told me he was blind in that eye. I said why and made them do glaucoma test and blood pressure check. They said I needed to get the Charlie a CT scan, since x-rays wouldn’t work for the head. That branch of the emergency vet did not have a CT scan machine. So I looked up 5 other emergency vets which had the machine and called for prices and got Charlie in to one the Wednesday after Labor Day. They thought it could be caused from inflammation from a tooth or a tumor and did remove a tooth and also did a biopsy and the results came back as squamous cell carcinoma tumor wrapped around his optic nerve. The CT scan and biopsy and tooth removal and consult cost about $2k. They told me the tumor would either spread to his mouth and he’d stop eating or spread to his brain and cause seizures. November 7th that year he started circling and crying so I thought he was having a seizure. I rushed him to the emergency vet and realized it wasn’t a seizure, he just went blind in the other eye from the cancer spreading so he was totally blind. But at that point I decided to have him euthanized to avoid him having seizures.

    Another cat, Beeb, got melanoma on her ear which the local vet found out from biopsy after the ear was cut off and then did blood work and all was normal so they thought she’d be fine. Well, she stopped eating within a week and then they did a chest x-ray of the lungs and found out it had spread to her lungs. So the chest x-ray should always be done first if melanoma is suspected.

    Another cat, Minnie Mouse, was diagnosed with tracheal lymphoma from a fine needle aspirate of her mass. They thought it was contained there only so we didn’t do an abdominal ultrasound and did chemo and followed with radiation of the spot. Well months later it returned and we did do the abdominal ultrasound to see it was all over so the radiation was wasted money in this case. So for a lymphoma diagnosis, always do an abdominal ultrasound.

    Alex had an abdominal ultrasound after she stopped eating and was extremely lethargic. My vet did in-house blood work and her bilirubin was extremely high. We suspected hepatic lipidosis and not much hope but I had to force feed her and gave her metronidazole and had the bilirubin rechecked in 2 days. It went up. Not good. So I took her to University of Penn on a Saturday and had to pay more for the ultrasound since it was a weekend but they decided it was cholangiohepatitis, not hepatic lipidosis, due to the color they saw on the ultrasound. They added Denamarin to her regime and continued force feeding. After 5 days she started eating on her own and she recovered.

    Sometimes you can pay less if you just ask for a specific organ ultrasound, such as the liver. It might only cost $150 instead of $400.

    Nineteen year old Nipper’s case is a case of when not to do any further imaging. Last August 22 Nipper suddenly went blind overnight. I googled “feline sudden blindness” and the articles said it could be from a detached retina due to high blood pressure and if corrected soon enough it could be reversed. So off to my vet and the bp was indeed high, and then off to Univ of Penn internal medicine (since you can get an appointment that day with them) and they get you that day consults with ophthalmology and cardiology. The ophthalmology vet said the retina was indeed partially detached and then the cardiologist ruled out high bp from thyroid (under control with methimazole) and kidneys (blood work OK) and the heart via echo cardiogram (that was the one imaging I did have done and they don’t have to shave them for that ultrasound). So, they determined that Nipper’s high blood pressure must be caused by a tumor on either her pituitary gland (CT scan to see) or her adrenal gland (abdominal ultrasound to see). We started her on amlodipine for her high blood pressure and the next day she could see again. I researched what could be done with a tumor on either place and nothing could be done in the US for a tumor on the pituitary gland. For the adrenal gland tumor if I did have an ultrasound done to confirm that, surgery could remove that tumor, but I really didn’t want to do surgery on a 19 year old, 5 pound cat. So Nipper took her blood pressure medicine daily and had her sight another 4 months until she stopped eating on December 30, 2016 and I had her euthanized. I am so pleased she could see from taking the amlodipine.

    Sorry so long winded, but have lots of experience to share.

  4. I had a 10 1/2 year old chocolate Lab who was experiencing painful back spasms. Her vet prescribed rest and a muscle relaxer. After two courses of medication which yielded no relief, she referred us to a neurologist. After a consult and extensive physical examination, he suggested either a CT scan or MRI of her back and explained the benefits of both. The cost difference between the two was considerable, so I opted for the CT scan, which was less expensive. This turned out to be a case of penny-wise and pound-foolish, because the CT scan produced no findings other than my Lab had arthritis in her hips which would not account for the back spasms. So, we followed up with an MRI which identified a malignant tumor on her spine. Surgery and daily chemo (pill form) followed and I enjoyed another 2 1/2 years with her before the tumor grew back and with it, the ever-painful spasms. This time around, I had no alternative but to ease her struggles and let her go. I am extremely thankful for the opportunity to have had an MRI done, irregardless of the cost, as we otherwise would have never discovered the tumor.

  5. Just to clarify, ultrasound and MRI do not involve any radiation exposure to the animal, while X-rays and CT do. When my dog had a JAAR (just ain’t acting right) episode the progression of studies was from X-ray to ultrasound. Nothing was seen and she perked up in a couple of days.

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