(link to beginning of article)Because each type of tissue has different percentages or concentrations of these ions (i.e. water) then each tissue will show up with a different shade of gray. So depending on the MR sequence everything from areas of high water content which can look almost white called high signal to dark or black areas that have low water content called low signal areas and everything in between.
Now there are three different MRI sequences most commonly used in Dogs to target and enhance these gradations of gray to even better differentiate tissues. The T1, T2, and a Fat Suppressed image called STIR (an acronym that will only confuse you, so I wont delineate it here). The “T’s” primarily reflect different “Time” variables relative to the time between the Magnetic Pulses (which creates those banging sounds) and when the image is taken. Not to confuse the issue more, just know that each sequence makes, by contrasting one tissue from another, the target tissue easier to visualize in the final image.
So with that let me give you an anecdote regarding an MRI study and a conversation with a Pet owner and their Veterinary Neurologist I had last week. I was waiting for a patient to arrive when I received a call from a concerned pet owner who explained that their Chiweeny (Dachshund Chihuahua mix) was in ICU at one of the specialty clinics here in the San Francisco Bay Area. She further explained that her pup had suddenly lost the use of his rear legs and was in a lot of pain as well. Having been to yelp apparently she read that I deal with a lot of paralyzed dogs. I have spent 35 years researching and refining a non-surgical technique or protocol that has proven very successful over the years with neurologic pets who still retain “Deep Pain Sensation” (more on this later). She went on to explain that they would be performing an MRI around noon that day. I told her that that was good and to please have them send me the MR images as soon as they can.
Instead of receiving the emailed images I received a call from the attending Veterinary Neurologist. The doctor proceeded to explain rather insistently, and seemingly a bit annoyed with my uninvited involvement, that her patient needed surgery immediately. She explained that he had a 60% occlusion of his spinal cord and that he had a fragment of disc compressing his spinal cord at lumbar 3 and 4 (low back). I was thinking...wow I’ve never seen a 60% occlusion of the spinal cord in a dog. So in my mind I was pretty sure that I knew the answer to the first question I would ask her once I was able to get a word in edgewise. "So this patient has obviously lost deep pain sensation, right?" I expected her to say absolutely, in which case I would have said, “what are you waiting for talking to me, get that dog into surgery immediately!” To my confused amazement she responded that he did in fact retain deep pain sensation. I was confounded by that answer.
What does losing deep pain sensation mean in these paralyzed dog cases. Losing deep pain sensation is serious and unfortunate condition where the spinal cord is literally being crushed inside the spinal canal. Since the nerve pathway or tract (spinoreticular tract) that transmits this sensation is well protected, deep within the spinal cord, it's the last thing to go when the spinal cord is being significantly compromised. So even when a dog is completely paralyzed without any feeling in the rear legs they can still retain this sensation in many cases. In fact what I have seen in my career is that most of the time and in most of these cases they retain this sensation. Which is a very good sign to me, in that they are potentially good candidates for my therapy. In such cases surgery is not time critical and conservative measures or therapies can be instituted with far better outcomes, clinically for the patient and financially for the pet owner, than surgery in my opinion based upon 35 years of dealing with these paralyzed dog cases. However, when they have lost deep pain sensation, it is a very bad sign and is a time critical condition that requires surgical decompression as soon as possible. A very similar condition in humans is called cauda equina syndrome.
So how could there be a 60% occlusion of this dog's spinal cord and yet the patient respond to the noxious input involved with hard squeezing of a knuckle of one of the paralyzed paws? 60% occlusion and retaining deep pain sensation were incompatible. One or the other had to be wrong. So to stop the argument that this dialog had devolved into, I asked her to please forward me the MR study, which she kindly did.
Subsequently, I called the clinic back and asked for the Neurologist to discuss what I considered good news for her patient. I was told that now she was in surgery. I thought immediately that perhaps she didn’t want to discuss it with me inasmuch as she apparently had the time in her schedule to perform surgery for the patient of issue. Perhaps she had another patient ready to go, so I cant say for sure if she just didn't want to spend any more time talking with me.
The moral to this story is that it is always best practice to get a second opinion before you put your beloved companion under the knife or at least have a Veterinary Radiologist have a look to confirm what the Neurologist is saying.
Why is my dog paralyzed?