It is often a little confusing to decide which imaging modality is the best choice for each patient. Each patient, pathology, and anatomic region requires a different decision in order to determine the right answer. I get a lot of phone calls and questions about this and so I've made up this quick guide to help. As always, it is usually best to give us a call before referring a patient for imaging so we can help make this determination and ensure that the process runs as smooth as possible.
In general the primary thought behind making this determination is that CT is great for bones and things that are moving, while MRI is especially sensitive to soft tissue differences and subtle structural changes.
CT is like riding in a car, you can cover a lot of ground very quickly and observe the major structures in the area. It is also easy in this case to see things that are moving along with you, though you miss a lot of detail in the things you are passing. MRI is like walking along the same area, granted in this case it takes a lot longer to cover the same area. As you walk the things moving past you are a blur but you are able to take in many more details of the soft subtle things around you. Identifying specific characteristics that would otherwise be lost to you if you were back in the car.
Hopefully with that metaphor in mind it's easier to make sense of when and why we would choose one modality over another.
Lets take a few minutes and go through some of the areas that routinely require advanced imaging and make some distinctions about which modality we would choose. There are going to be some areas that overlap for good reasons and often it is not as simple as what you may expect.
When imaging the head we would typically choose CT to evaluate the sinuses, fractures of the skull, or acute hemorrhage. Trauma cases would often go to CT first for rapid evaluation to help stabilize the patient's condition. MRI would be utilized to evaluate the specific internal structures of the brain, the changes between grey matter white matter and CSF, the inner auditory canal, the pituitary, or brainstem.
The neck is evaluated as soft tissue versus the bony structures. Here is where those roles between MRI and CT can get reversed. MRI is typically used to evaluate the spinal cord, the brachial plexus, and the relationship of the vertebrae and disks to these structures. For these reasons MRI is also the primary tool for evaluating the rest of the spine. MRI is excellent at evaluating the soft tissue structures, glands, and musculature of the neck but in cases such as evaluating for a foreign body obstruction or migration CT would be the primary choice. CT is utilized for evaluating the vertebrae for pre-surgical measurements and planning as well as post surgical cases where there are metallic screws or plates that can create artifacts on MR images.
If we wanted to look closely at the lungs or the intestines it would be most beneficial to use CT. It is possible to evaluate these areas on MRI but it requires a lot more time to compensate for the respiratory, cardiac, and peristaltic motion. MRI does come in handy when performing imaging studies on specific organs. There are MRI protocols specifically tailored to evaluation the tissues and functionality of the pancreas, liver, kidneys, and spleen but not always at the same time. For these reasons though it is possible to perform angiography and venography on both modalities depending on what you are looking for and providing the diagnosis of conditions such as a portosystemic shunt, blood clot, or an aneurysm.
When evaluating the pelvis CT is considered to be utilized for evaluating the bony pelvis and major metastatic changes. MRI is used in this area to look at the musculature, reproductive organs, urinary bladder, and the nerves extending to the lower limbs.
When imaging the joints (shoulder, hip, elbow, stifle) MRI is the way to go. The sensitivity of MRI provides clear visualization of the joint capsule, cartilage, tendons, ligaments, bones, muscles, and lymph nodes making the results from any other modality incomparable.
Obviously enough, CT is the primary tool for evaluating the long bones such as the femur, humerus etc. Though MRI can diagnose an osteosarcoma earlier than CT which typically requires the disease to progress far enough to destroy the cortical bone layer before it can be easily visualized.
Because of the nature of MRI it is used to evaluate the pathological properties of tumours allowing specific distinctions to be made between different types of lesions.
Hopefully this has helped clear up some of the differences between these modalities and has given you some further insight into understanding the diagnostic process.
Of course there is always a lot more to understanding these modalities so here are some answers to several frequently asked questions about our imaging services.
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