Episode 25  ·  12m 50s

X-Ray, Ultrasound, CT, or MRI — How an ER Vet Decides What to Use

Dr. Michael LoSasso, DVM & Julie Schwenzer Frisco Emergency Pet Care
Diagnostic imaging Ultrasound CT scan MRI Emergency diagnosis
"When seconds matter, the right diagnostic tool can reveal what the eyes can't see — sometimes it's the difference between guessing and knowing."
— Julie Schwenzer

Episode summary

Dr. LoSasso walks through the four imaging tools available in modern emergency veterinary medicine, and how far each has come since he started practicing. Film X-rays and dip-tank development gave way to digital radiography, which he says has dramatically improved image quality and let him get a "stat read" from a remote radiologist in about an hour — a process that used to mean mailing film and waiting a week. Ultrasound machines have shrunk from desk-sized consoles to something that fits in a briefcase, while delivering far more diagnostic information than he expected when he first started using them.

"I generally will honestly look with ultrasound first because it's the fastest, it's the least expensive to the client, and it's the least stressful for the patient — I don't have to ask them to do something they don't want to do."
— Dr. Michael LoSasso, DVM

His default starting point on a critical patient is ultrasound. On a hit-by-car dog struggling to breathe, a probe on the chest can immediately show whether there's air trapped outside the lung, pulmonary hemorrhage, or bruising within the lung tissue — without needing to position a distressed animal for an X-ray. In the abdomen, ultrasound reveals something X-ray simply can't: when a collapsed dog with pale gums has a bleeding abdominal tumor, the surrounding blood makes everything on an X-ray blur into one uniform gray mass, since most soft tissue is fluid-density to begin with. Ultrasound, by contrast, can clearly show a distinct mass floating inside that pool of blood.

But ultrasound has real limits too — it's essentially useless for bone, where X-ray remains the clear winner, and the two are frequently used together rather than one replacing the other. CT earns its place for areas that are difficult to evaluate any other way, especially the skull, where overlapping bone structures make X-ray hard to interpret and ultrasound can't penetrate at all; he cites a recent case diagnosing an abscess behind a dog's eye that only CT could catch. MRI is reserved for the nervous system — brain, spinal cord, intervertebral discs — where it substantially outperforms CT, which Dr. LoSasso says catches only 5-10% of brain tumors in dogs.

Ultrasound-guided procedures

Beyond diagnosis, ultrasound can guide a needle directly to a hard-to-reach target — Dr. LoSasso describes the board-certified radiologist who works with his hospital using this technique to safely drain prostatic abscesses and even pancreatic cysts, precise work he says would be far too risky to attempt blind. Specialized needles designed to show up clearly on ultrasound let a skilled operator watch the needle tip in real time as it reaches its target.

Questions answered in this episode

The following questions are answered by Dr. LoSasso in this episode, drawn directly from the conversation. These are real clinical answers from a practicing emergency veterinarian with 30+ years of experience.

He says it's the fastest, least expensive for the client, and least stressful for the patient — he doesn't have to ask a critical animal to hold a position it doesn't want to hold. On a hit-by-car dog breathing hard, for example, he can put a probe on the chest and immediately tell if there's air trapped outside the lung (pneumothorax), pulmonary hemorrhage, or bruising, and check the abdomen for free fluid, all in minutes.
Dr. LoSasso explains that most soft tissue is essentially fluid-density, so when blood or other fluid surrounds an organ or mass, an X-ray just shows one uniform gray blur with no detail. Ultrasound, by contrast, can clearly show a mass floating in a pool of blood inside the abdomen — critical for diagnosing conditions like a bleeding splenic tumor in a collapsed dog with pale gums.
For bones. Dr. LoSasso says ultrasound can't show anything useful about bone, while X-ray excels at it. X-ray also gives different, complementary detail on lungs and hearts compared to ultrasound, which is why the two are frequently used together rather than one replacing the other.
CT is especially valuable for areas that are hard to image any other way — Dr. LoSasso singles out the skull, since overlapping bones make X-ray nearly impossible to interpret there, and ultrasound can't penetrate bone at all. He cites a recent case diagnosing an abscess behind a dog's eye that could only be found with CT. It's also useful for the chest, abdomen, and pelvis when more detail is needed than X-ray or ultrasound can provide.
MRI is the clear choice for the nervous system — brain, spinal cord, and intervertebral discs — according to Dr. LoSasso. It's the best modality for detecting brain tumors and spinal cord problems, though he's candid that CT only catches 5-10% of brain tumors in dogs, making MRI far more useful for that specific question, particularly in dogs with seizures. MRI machines are expensive and space-intensive, so they're typically only available at larger emergency or tertiary referral hospitals.
Dr. LoSasso describes ultrasound-guided fine needle aspiration, where a skilled operator uses specialized needles that show up clearly on ultrasound to precisely target a structure while watching the needle tip in real time. He credits the board-certified radiologist who works with his hospital for safely draining hard-to-reach targets like prostatic abscesses and even pancreatic cysts — something he says would be far too risky to attempt without that kind of real-time visual guidance.
JulieWhen seconds matter, the right diagnostic tool can reveal what the eyes can't see, and sometimes it's the difference between guessing and knowing. What role does advanced imaging play in emergency diagnosis?
Dr. LoSassoIt plays quite a big one — imaging in general plays a role in almost everything we do. Radiographs have come such a long way since I graduated in the 1900s — I remember taking X-ray on film, using dip tanks, developing our own X-rays, then automatic processors, then digital, which was new and cutting edge at the time. Now the quality is so much better than even 10 years ago, and it's easy to share with other practices and get a radiologist to look at them — guys sitting at a desk, some at home, reading X-rays that 30 years ago I'd have mailed in a huge envelope and waited a week for. Now I can get a stat read in an hour. Ultrasound has come a long way too — when I was a student it was a huge console, the size of about three filing cabinets. Now it fits in a briefcase and gives us a tremendous amount of information. I use it, as an ER vet not a radiologist, to check if the bladder is healthy, the gallbladder normal, for hearts, fluid around the heart, heart function, and lungs — which I was taught 30-some years ago you couldn't look at with ultrasound because it doesn't like air, and that's true, dry lungs give you a characteristic artifact. But wet lungs look completely different, so ultrasound is essential for congestive heart failure, pneumonia, and other fluid problems in or around the lungs.
Dr. LoSassoMore emergency hospitals and even some general practices are adding CT now. It's very effective at parts of the body we can't see well with X-ray or ultrasound — the big one being inside the head. Skulls are notoriously difficult with X-ray because all the bones and jaws overlap, making diagnosis really hard. Just two nights ago we did a CT on a dog with an abscess behind its left eye — something you can suspect but plain film won't confirm, and ultrasound can't reach because the abscess is under bone. MRI is the last technique we use — I don't have experience with nuclear medicine, that's more for oncologists. MRIs are very expensive machines, even today, and take up much more space than CT. CT is really a computer driving an X-ray machine that circles the body and builds a 3D image; MRI puts you in a chamber, lines up your atoms with a magnet, sends a magnetic wave through, and reads the displacement. It's invaluable for the nervous system — spinal cord, brain, tumors, even discs, where CT would require injecting dye around the spinal cord, essentially a myelogram. Part of why we have ours is to look at brain tumors in seizure dogs, but unfortunately CT only picks up 5 to 10% of brain tumors in dogs — most don't show well on CT at all. But for the chest, abdomen, and pelvis, which are also hard to reach with X-ray or ultrasound, it's super valuable. Most of the time though, it's going to be X-ray or ultrasound.
JulieHow does imaging guide when and where to perform an aspiration safely?
Dr. LoSassoIf you're using a needle — a fine needle aspirate, or FNA — and you can isolate what you want to sample with an ultrasound probe, skilled operators can actually see the tip of the needle; there are special needles made specifically to show up well on ultrasound. It helps guide exactly into that space. I don't do a lot of that myself, honestly never acquired the skill, but the board-certified radiologist who does a lot of our in-depth ultrasounds is amazingly good at it. We've had him drain prostatic abscesses — the prostate's hard to reach unless you know exactly where you're going — and even pancreatic cysts, which honestly terrifies me as a concept, sticking a needle anywhere near a pancreas. But with someone skilled who knows exactly where the needle is at all times, it's a really valuable technique.
JulieHow do you know which modality to use first when a patient comes in critical and you don't know right away what's wrong?
Dr. LoSassoIt depends somewhat on what you're looking for — they're complementary ways of looking inside the body. I generally look with ultrasound first because it's fastest, least expensive to the client, and least stressful for the patient — I don't have to ask them to do something they don't want to do. With a hit-by-car dog breathing hard, I can put a probe on the chest and know if there's fluid around the lungs, air between the chest wall and lung — that looks very different from looking straight into the lungs — so I can learn something similar to X-ray and know we've got a pneumothorax, pulmonary hemorrhage, or pulmonary contusions, bruising within the lungs. Looking in the abdomen with ultrasound, I can see free fluid — a lot of my collapsed older dogs that were fine until that afternoon and now don't want to walk with pale gums have bleeding abdominal tumors, and on X-ray, since most tissue is fluid density, surrounding it with blood turns everything into one gray mass with no detail. With ultrasound I can absolutely see that mass floating in a sea of blood. By the same token, for bones, X-ray beats ultrasound completely — you can't see anything about bone with ultrasound. CT is even better than X-ray for bone, but for most bone questions besides the head, X-ray does the job. X-ray also gives more information about lungs and hearts than ultrasound does, different detail, so they're frequently done together. I had an ER doctor back around 2007-2008 suggest we get an ultrasound machine, and I said I don't need ultrasound to find a bladder, I can do that myself — that's all I thought it was for at the time. Thankfully I've gotten some education since, and it's really changed how we practice medicine.

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