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CT or MRI? When to use each and why

Both are cross-sectional imaging modalities that provide images of slices of the patient

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The dilemma over whether to use CT or MRI is fairly common. While there are many cases that could be imaged via either modality, experts say there are specific instances where one is preferred over the other.

Use MRI for meningitis, encephalitis, fibrocartilaginous embolism, malignancy of liver tumours, muscle tears and strains, and cranial cruciate rupture, says Tony Pease, DVM, MS, DACVR, chief veterinary medical officer of the Western Veterinary Conference Oquendo Center in Las Vegas. For fragmented medial coronoid processes, dental disease, 3D reconstructions, and small bone fractures, use CT, he advises.

“In general, MRI can see more medically treated lesions, but CT and MRI can see lesions that are amenable with surgery,” Dr. Pease says. “However, gastrointestinal lesions are questionable. The motion of the gastrointestinal tract makes large artifacts, as does metal (e.g. microchips), which is not a factor with CT as opposed to MRI.”

MRI is preferred whenever disease of the central nervous system is suspected, says Nathan C. Nelson DVM, MS, DACVR, clinical associate professor of radiology at the North Carolina State University College of Veterinary Medicine.

“MRI has excellent ability to image soft tissue structures and is able to differentiate grey matter, white matter, nerves, and cerebrospinal fluid to a much greater extent than CT,” Dr. Nelson says. “In any case, where brain disease is suspected—such as the acutely seizing dog or the dog with sudden behaviour change—MRI is the test of choice. The cause of these symptoms might be a brain tumour, inflammatory disease, or a vascular event, and while CT might be able to identify some of these abnormalities, it is not nearly as sensitive as MRI.”

“A majority of the time, the choice between CT and MRI divides along whether the area being imaged has a high tissue contrast or a low tissue contrast between neighbouring tissues,” says Seth Wallack DVM, DACVR, founder and chief executive officer (CEO) of the Veterinary Imaging Center of San Diego. “An example of high contrast is in the lung, where the alveoli contain air next to the blood vessels, which are soft tissue attenuating. Examples of low tissue contrast between neighbouring tissues are the brain, eye, muscles, tendons, and ligaments surrounding a joint, joint fluid, and the adjacent cartilage.”

Diseases of the spinal cord also are best imaged with MRI, as CT is able to define margins of bone, Nelson says. However, it provides little detail about diseases within the spinal cord itself and is less sensitive than MRI in detecting problems that cause spinal cord compression, he adds.

“In the case of a dog presenting with thoracic or pelvic limb deficits, where the cause might be due to intervertebral disc extrusion or a tumour of the spinal cord, MRI is the best imaging option,” Nelson says. “CT is reasonably sensitive in detecting acute disc extrusions in chondrodystrophic dogs, such as dachshunds, and is sometimes used as a first-line test in that population because it is much faster than MRI, but in other cases, MRI is the first imaging choice.”

Both CT and MRI are cross-sectional imaging modalities that provide images of slices of the patient; however, bone and lung details are much better with CT, while soft tissue contrast is much better with MRI, says Wilfried Mai, DVM, PhD, MS, DECVDI, DACVR, a professor of radiology at the University of Pennsylvania School of Veterinary Medicine.

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Improvements in equipment 
MRI units differ in the strength of magnetic field employed, rated in units termed Tesla (T), says Nathan C. Nelson DVM, MS, DACVR, clinical associate professor of radiology at the North Carolina State University College of Veterinary Medicine.

“Previously, 1.5T MRI units were the strongest available for veterinary patients, but many 3T magnets are now coming online,” Dr. Nelson says. “The stronger magnetic field provides faster examinations with greater detail than lower strength units.”

These stronger units allow advanced imaging sequences that provide biochemical information about specific body parts, though many of these sequences are still mostly used in an experimental rather than clinical setting. For example, high-field magnets can perform imaging sequences that can determine the health of joint cartilage, he says.

Other sequences (spectroscopy) can be used on masses in the brain, and by analyzing the biochemical makeup of those masses, can determine if they are more likely neoplastic or inflammatory in nature.

Newer CT machines are much faster than previous generations, with some able to image an entire dog in seconds.

“These high-speed units have allowed imaging of the heart (cardiac-gating) without any motion artifact at all,” Nelson says. “Fast CT units also remove the necessity of anesthesia, with many dogs and cats able to be imaged with only sedation or in some cases without any sedation at all.”

“CT acquisition times are much shorter—a few seconds to scan an abdomen or thorax, while they are much longer for MRI—45 to 90 minutes depending on the indication and body region,” Dr. Mai says.

Both require patients to remain still, but some fast CT scans can be performed under sedation, while MRI always requires general anesthesia. “Also, MRI is substantially more expensive than CT,” he adds.

MRI is used for brain and spinal cord diseases, as well as peripheral nerve diseases, as it provides high-quality soft tissue details of the spinal cord, meninges, nerve roots, and paraspinal tissues, Mai says.

“The exception to this would be maybe the diagnosis of traumatic vertebral/skull fractures or luxations, where CT may be better, though even in these cases, CT may overlook soft tissue injuries that may be diagnosed with MRI,” he says.

MRI is also superior to CT in several musculoskeletal soft tissue lesions, such as shoulder tendonitis and in the stifle for cruciate ligament tears or meniscal tears. “Again this is due to the excellent soft tissue contrast of MRI, whereas

CT has poor soft tissue contrast and is better for primarily bone lesions,” Mai explains.

For thoracic disease in general, and in particular for the lungs, CT is preferred, as respiratory and cardiac motions complicate MRI acquisition in the thorax, air in the lungs generates a poor signal, and a number of artifacts deteriorate image quality. “CT on the other hand provides fast imaging and with high resolution of the lung parenchyma. It also allows assessment of the mediastinal, cardiovascular, and pleural structures,” Mai says. “A common indication of thoracic CT is to rule out pulmonary metastatic disease, as CT is much more sensitive than radiography for this diagnosis.”

Specific case use: CT versus MRI
The decision to use CT or MRI may not always be easy, but there’s no doubt some cases require one modality over the other. Seth Wallack, DVM, DACVR, founder and CEO of the Veterinary Imaging Center of San Diego, offers some tips on choosing.

Cervical spinal cord evaluation: MRI

A myelogram or CT myelogram can be performed, and there are a few instances where a CT myelogram is called for, but 99 percent of cervical imaging in dogs and cats is best served by an MRI. Cervical CT without a myelogram is a poor choice, as there is very little fat around the spinal cord in this region, making cord evaluation very difficult. This doesn’t mean it’s wrong to do cervical radiographs first, particularly in an older dog or cat where a vertebral tumour or multiple myeloma may be present or in cases where discospondylitis is suspected.

Lung evaluation: CT

MRI is starting to be performed for lung assessment in people, but CT remains the go-to choice. CT image acquisition is much faster than MRI, resulting in less motion artifact. Further, the total CT study is completed in one-tenth of the time of an MRI. In fact, with some of the multi-slice CT scanners (i.e. 16 slice or greater), a complete dog or cat thorax can be imaged in five to 25 seconds once the dog or cat is positioned, so the entire study can be performed under sedation (the caveat being the animal must be able to safely undergo sedation). An MRI assessment of the lung requires general anesthesia and may take an hour or more. It’s easy to see how sedated CT evaluation of the dog or cat lung will have a transformative effect in veterinary medicine over the next five to 10 years.

Nasal passage evaluation: CT

The no-brainer choice. Again, MRI can be performed to evaluate the nasal passages. However, CT is so much quicker and provides such incredible anatomical detail that when a veterinarian is dealing with an uncomplicated nasal passage case, particularly in an older animal or one with unilateral epistaxis, he automatically should be thinking about a CT scan. In fact, CT is so much better for nasal passage evaluation that I rarely recommend skull radiographs, particularly now that nearly all nasal/skull CTs can be performed under sedation. Regarding the caveat about uncomplicated nasal passage cases, I’m specifically referring to no brain signs. If brain signs (i.e. recent seizures, mentation, or behavioural changes) are present, an MRI may be more appropriate, since it can look at both the nasal passages and the brain on the same study. That said, CT provides a great evaluation of the cribriform plate, which is the barrier between the nasal passages and brain.

Brain imaging: MRI

MRI is the safe choice here. However, contrary to what most veterinarians are taught in school, CT does have a place in brain imaging. Veterinarians need to know, and explain to the pet owner, the pros and cons of CT brain imaging. Two of the biggest factors to take into account are what is being imaged and the cost difference. For what is being imaged, pituitary evaluation for a tumour can be done with either MRI or a contrast CT. The pituitary gland enhances very well on CT, so it is easily visualized for a mass. In Cushing’s cases where the veterinarian is worried about a pituitary tumour—macroadenoma—a contrast CT is very acceptable.

Shoulder imaging: MRI

CT can provide some of the information, but when it comes to soft tissue evaluation, MRI is king. The shoulder is a complex joint with numerous tendons, ligaments, the joint capsule, and osseous structures. MRI can display them all, and different sequences can be used to highlight specific tissues. So when ultrasound isn’t identifying the cause for the shoulder issue but the bone scan pinpoints the shoulder as the problem, shoulder MRI is the next step.

Emergency imaging: CT

This is a big up-and-coming area that many in emergency medicine are pushing for two reasons, the first being speed. Second, there’s a saying, “Nothing hides from a CT,” meaning everything is visible on a CT, whereas on other emergency modalities (X-rays and ultrasound), some things can be missed due to artifact or animal position at the time of the study. We’ll see where this goes over the next few years, but I suspect CT use in emergencies is only going to increase.

CT and MRI?

It’s uncommon for an animal to have both a CT scan and MRI of the same area, but there are cases that sometimes call for both.

Animals with spinal cord tumours or involving the vertebrae may have both a CT and MRI, as the radiation therapist needs information from both modalities to exactly identify tumour margins and to input the data into planning software, Nelson says.

“Animals with spinal fractures also may have both examinations, as the MRI provides information about spinal cord damage, but the CT is used to help guide surgical planning and stabilization of the vertebrae,” he adds.

CT is the best test for advanced imaging of thoracic and abdominal disease, and MRI is generally unable to image structures that move or are adjacent to highly mobile structures, he says.

“Due to respiratory movement, most abdominal disorders are imaged first with CT,” Nelson says.
“It is common for CT to be used for surgical planning in dogs with large abdominal tumours, such as those affecting the liver, as it is able to quickly identify the vascular supply to the tumour and what portion of the liver is affected. CT also is ideal for imaging pulmonary diseases, such as pulmonary tumours or searching for small pulmonary nodules.”

CT is also the best test when imaging osseous structures, as it is able to provide much finer bony detail than MRI, and also can cover a much larger body region, he adds.

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