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The Life History of an MS Lesion
For this discussion we will be talking about the classic MS lesion which is often called a T2 Hyperintensity. This is the lesion that is most associated with the "Relapses" in the disease. MS does its damage by causing the nerves in localized areas in the brain and spinal cord to lose their protective sheaths, called myelin. At first, when the myelin is being attacked, the body brings a higher blood supply to the attacked area or areas to fight the attack, and they becomes swollen and inflamed. These areas now become "lesions." (A "lesion" is simply an area of abnormality in the brain tissue.) If the areas being attacked are large enough they can be seen on an MRI. At this point, when they are inflamed and blood-engorged, they are called "active lesions" because the inflammation is actively attacking the nerve cells. At first the nerves themselves haven't changed much and they appear (and have the same density) as the healthy areas around them. The body attempts to repair the damage that is being done and sometimes these areas try to re-myelinate, with varying success. They may disappear from the next MRI. They aren't perfect in their function, but the areas may return to a normal appearance.
If the nerves do not re-myelinate and the damage continues, for a long time the lesions sit as scars. These scarred areas contain damaged and dying cells, their blood supply shrinks, and they become dense--more dense than the normal brain around them. These are the classic MS "plaques" and are considered old lesions. They show up as the bright areas, also called "hyperintense," most of us have seen in pictures and on our films. If the attack on the myelin sheath is too strong for the immune system to repair, more and more myelin disappears and the area of nerves eventually dies. Then it completely scars and contracts. The blood flow is decreased to that area and the body tries to reabsorb it. The area then becomes "less dense" than the surrounding normal nerve tissue. After a longer time the scar can reabsorb completely and the area becomes "empty." It's called a black hole.
How the MRI Shows The Different Stages of MS Lesions
When you image these lesions with an MRI you can see different things, depending on the technique, the age (stage) of the lesion, the power of the MRI, and whether contrast is used. The first MRI image is done without contrast. Several different techniques are used in obtaining the images, including two called T1-weighted and T2-weighted. The first pass of the MRI will show old lesions that are big enough to be seen by the power of that MRI machine. WE KNOW that many lesions in MS are too small to be seen in some MRI's. As each new generation of MRI machines becomes available, it is more powerful and more able to show up areas of damage than the previous generation. If the newer, more powerful MRI with a 3-Tesla magnet is used, more lesions will be seen (by at least 25%) than with the older 1.5-Tesla machines. (Please note that "Tesla" refers to the strength of the MRI magnet, and should not be confused with the "weights" of images, here meaning T1 or T2.)
The "classic" old, scarred, mature MS lesion or plaque is somewhat oval in shape, will have well-defined borders and will appear in the white matter. The "classic" MS lesion will also have its long axis perpendicular to the ventricles (the large fluid-filled spaces) of the brain. Characteristic places (but not the only places) are peri-ventricular, juxtacortical, the corpus callosum, the cerebellum and the cervical spine. Also, important and often very symptomatic lesions are found in the brainstem and the thoracic spine. The spinal cord ends at the bottom of the thoracic spine, so there is no such thing as a lumbar spinal cord lesion. The scarred lesions will be evident as light, bright (hyperintense) areas on the T2 images. These are the classic MS lesions or "plaques." But, with just the regular MRI image one CANNOT say if it is old and dormant or if it has active inflammation in or around it.
Now the very old, mostly dead areas where nerve fibers have died will be seen as less dense (empty) spaces or "black holes" on the T1 images. If there are many of these empty areas, the brain will eventually contract and shrink around them. This will be depicted as a loss of brain volume. This is also known as brain atrophy. It is particularly seen in the progressive types of MS and after many manys of disease. In brain atrophy there will be an increased space between the skull and the brain. The interior area of CSF will be larger. Also the deep folds in the brain (called sulci) will appear widened.
Black holes are areas where a good number of the nerve fibers have been lost. But, not all of them are completely dead. New techniques are showing that black hole can heal and disappear, leaving some scarred tissue behind.
The Need For Contrast
For the most part all visible lesions can be seen without contrast. They show up as T2 Hyperintensities just like all lesions. The problem is that the neurologist can't tell if a lesion is old or new. So new activity will be missed and the MRI may be erroneously dismissed as "unchanged" or "no progression." In the inflammation of new lesions there is a breach in the blood-brain barrier. Wherever there is a break in the blood-brain barrier, the contrast will leak into the brain tissue and the areas will "highlight" or "enhance." They show up as even brighter than the brain around them and brighter than an old, scarred lesion. So new lesions will appear as "enhancing," or "active." Also, older hyperintense lesions that have undergone a new attack at their adges or margins. This is also called reactivation. Lesions that have reactivated may show an even brighter enhancing rim or ring. The appearance of an "enhancing ring or rim" is especially characteristic of MS. When you compare the regular MRI to the contrast MRI you can see the increased brightness of this reactivated, old lesion. New lesions with active inflammation willy typically show up for 4 to 6 weeks before they scar down and become "old" lesions.
In some cases a newly active MS lesion may not be visible on a regular MRI because the area of nerves, though inflamed, is still pretty much intact and has normal brain density. On the regular MRI it will look like normal brain. Without contrast it won't show up and will likely be missed. When the next phase of MRI is done the contrast is injected into the bloodstream.
In the inflammation of new lesions there is a breach in the blood-brain barrier. Wherever there is a break in the blood-brain barrier, the contrast will leak into the brain tissue and the areas will "highlight" or "enhance." They show up as even brighter than the brain around them and brighter than an old, scarred lesion. So new lesions will appear as "enhancing," or "active." Also, older hyperintense lesions that have undergone a new attack right around them (also called reactivation) will show an even brighter enhancing rim or ring. The appearance of an "enhancing ring or rim" is especially characteristic of MS. When you compare the regular MRI to the contrast MRI you can see the increased brightness of this reactivated, old lesion. New lesions with active inflammation willy typically show up for 4 to 6 weeks before they scar down and become "old" lesions.
What does the word "ACTIVE" mean?
Some radiologists call new lesion inflammation "active lesions." Others may refer to "new lesions " or "actively enhancing lesions." These all refer to the same thing. Also since some new lesions heal, the MRI's can be compared to old films where they did appear formerly, leading to the conclusion that they have disappeared. In addition, between different sets of MRIs done after some time has passed, the radiologist can see an increase in old and in new activity. Right here it is important to remark that, in discussing MS, the word "active" is used in two different ways. Note: If one is speaking about the appearance of lesions on the MRI, "active" simply means that the lesion has "active inflammation" in it. However, if we are discussing a person's disease and symptoms, an active lesion means one that is currently causing symptoms.
Another word that is used in two different ways in MRI reports is the word "new." If the radiologist is comparing the current MRI to an older one, a "new" lesion will be one that shows up now that didn't show up earlier. It has formed in the time period between the two MRIs. If the radiologist is describing lesions that show enhancemant on an MRI, they might refer to these lesions as "new." The difference is in the context in which the word "new" is used.
In MS the Relapses and Remissions which are so typical of this disease occur because of the attacks on the nerves and then the body's ability to heal some or most of them. So we can often see the reflection of the disease process in the changes of the MRIs done over time.
What is the difference between the T2, T1 and the FLAIR images?
Arggghh... Okay, I was looking up for a good description and didn't readily come across an easy one. I did find a thorough discussion of the T1 and T2 topic. It is a deeply scientific matter of mathematics and physics, and trying to read it made my brain hurt! So, I will describe what I understood. Where the stuff is really technical (i.e., I don't understand it) I will just refer to the "miracle." In the technology of the MRI the complex software that interprets the signal from the magnetic receiver is largely responsible for the difference in techniques.
Remember a "lesion" is just an area of abnormality - from "any" cause. The T1-weighted images and the T2-weighted images refer to the different rates and strengths of the electromagnetic pulse that is sent through the body to the EM receiver. There are long-strong pulses, short-strong pulses, and long- and short- weak pulses, and actually many more. In a miracle the machine and the programmers use these different pulse/spin sequences to make the different tissue structures in the body stand out from each other. In 20+ years of studying the miracle they have discovered that different tissues (brain, bone, liver, blood, etc.) all show up best using different combinations of pulse techniques. They have also discovered that certain combinations of techniques show abnormalities like tumors or scars or whatever.
That's it, folks. I can't get any better than that. I do know that T1 images show the CSF to be white and the brain medium-gray and the bone black. T2 (which I think is the most commonly displayed) shows the bone to be white, the brain medium gray, and the CSF to be black. (They are flip-flops or positive/negatives of one another.) But the computer, during a miracle, compares these two and further distinguishes things that weren't really apparent on either type image alone.
The T2 technique is best for showing mature, dense, scarred lesions, where the oval lesions look brighter than the surrounding brain. This is where the term, "T2 hyperintensities" comes from. All T2 hyperintense lesions are not necessarily from MS. Other disorders like migraines, hypertension, small strokes and others can also cause them. On the other hand, the T1-weighted imaging technique is best for showing the old, reabsorbed "black holes" where the lesions once were. The FLAIR is a little easier for me. It stands for Fluid Attenuated Inversion Recovery. It is part of the T2 imaging, with a twist. It is another miracle of pulses and signals. Its purpose is to distinguish things that border on areas of fluid ( for our purposes, CSF in the ventricles). Apparently a lesion right up against any pocket of fluid, like the ventricles, can blur out and be missed. The FLAIR technique recovers information and through a miracle makes the lesion visible. There! Here is the link that made my brain hurt:
There are other techniques such as fast-SPIN, STIR, SPIN-echo, but these are techniques used to clarify tiny differences in the tissue and to make lesions stand out more clearly. That's it in a small nutshell.
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