(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.
T1 weighted images make Fatty tissue appear lighter gray in color and make high water content tissues appear darker gray in color. This sequence is really good for seeing anatomical detail. In the picture here if you look at the bladder (which is distended due to a neurogenic bladder) it appears darker gray as does the CSF or Cerebrospinal Fluid because they have a high water content. Spinal cord appears very light gray because of the fat content of the myelin sheaths or protective nerve coverings that make up the white matter of the spinal cord, which is lipid rich (Fatty).
T2 weighted images make Fatty tissue appear darker and high water content tissue appear whiter. Anything really dark to black will be something with low water content. This sequence is really good for discerning more physiologic information especially when it comes to edema (i.e. excess fluid which often causes swelling in tissues or increased physiologic pressure in bone). In this picture you will see the bladder and CSF are lighter in color than the spinal cord itself which is darker gray. So the T2 weighted image is just the reverse in appearance of the T1 weighted image. High water content appears very lighter gray to white and fat rich tissues appear darker gray.
STIR images make Fatty tissues appear darker and high water content tissue appear even brighter gray to even bright white. In other words anything bright is fluid. This sequence is good for visualizing bone marrow edema, the fluid in joints (synovial fluid), ligament and cartilage evaluation. Also good for visualizing tumors, fractures and bone bruising. In other words as seen in the accompanying picture the bladder and CSF are very much brighter in color than even in the T2 weighted sequence.
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.
Upon reviewing the study I could clearly see a lesion dorsal to the Lumbar 3 body on the STIR sequence. However instead of being dark or black, which would have been expected when visualizing a fragment of disc material in the spinal canal, it was in fact bright white. Remember above when we discussed the STIR image, that high water content would appear white to bright white? Consequently I reasoned that this lesion wasn’t in fact a disc fragment at all but merely fluid in and around the spinal cord, possibly a bleed, certainly effusion. As you can see in the accompanying picture the lesion is bright white similar the Aorta which also has high water content. The two are termed isointense in the parlance of MRI interpretation, meaning the same signal intensity. The following picture below is that of a human spine and you can see how large a disc herniation has to be to compress the spinal cord. As you can see the disc material compressing the cord is very dark to black in color due of its low water content.
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.
I then laid down my considerations in an email to her explaining that in my humble opinion this was not a surgical candidate. I felt that in days the patient in all likelihood would be better without surgery. Sure enough as you can plainly see in this picture the patient a few days later entered the clinic I work in under his own steam with only a slight wobble in his rear legs when he walked in. I am currently rehabbing this pup's spine with NMR-T and a new form of Light Therapy.
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.
As an aside please notice in the human T2 weighted image just discussed, that the there is a lot more space within the spinal canal, as evidenced by the space surrounding the spinal cord, compared with the lack of space surrounding the dogs spinal cord on the T2 weighted image previously discussed. This is why certain breeds of dog are more prone to spinal cord compromise and sudden paralysis than humans and why they can also recover from this seemingly catastrophic condition. In other words it takes far less to cause neurologic involvement in dogs than humans because there is significantly more space around the spinal cord in humans to accommodate to intrusions into that space. Here is an MRI of a Pug with a mild disc herniation. However since the spinal cord occupies a greater volume of the space within the spinal canal even small bulges like this can cause oversized symptoms such as walking like a "drunken sailor" or frank paralysis. More on that in the next article: