Remember, MRI is an imperfect and evolving science. Images do not necessarily correlate with degree of disability, and are only mildly predictive!

T1 Hypointense Lesions Definition:
http://ms.about.com/od/glossary/g/T1_lesion.htm
A T1-weighted magnetic resonance imaging (MRI) scan shows black holes, which are areas of permanent axonal damage. These are called hypointense lesions, meaning that they display as dark areas on the MRI image. T1-weighted lesions can also be areas of edema (swelling), which are not permanent and disappear on subsequent scans.
The T1-weighted scan is part of conventional MRI technology which is used to monitor and diagnose multiple sclerosis (MS), usually in conjunction with a T2-weighted scan and with gadolinium enhancement.
Also Known As: hypointense lesions, "black holes"

T2 Hyperintense Lesions Definition:
http://ms.about.com/od/glossary/g/T2_lesion.htm
The T2-weighted magnetic resonance imaging (MRI) scan shows the total number of lesions (lesion load) and disease burden. These are hyperintense lesions, meaning that they appear as bright spots on the MRI image.
The T2-weighted scan is part of conventional MRI technology which is used to monitor and diagnose multiple sclerosis (MS), usually in conjunction with a T1-weighted scan and with gadolinium enhancement.

Gd+ Lesions Definition:

http://ms.about.com/od/glossary/g/Gd_lesion.htm

Gadolinium is a chemical compound given during MRI scans that highlights areas of inflammation. It is also called contrast material.
A gadolinium-enhanced magnetic resonance imaging (MRI) scan shows active lesions, meaning that there is a breakdown of the blood-brain barrier and inflammation is present.
Therefore, gadolinium-enhanced lesions are new it is estimated that lesions will enhance with gadolinium for six weeks or less. The gadolinium allows the doctor to tell which lesions are active and which ones are inactive, allowing the doctor to know recent disease activity and determine if a relapse is occurring.




Alternate Nomenclature

From Imaging of the Brain
by Thomas P. Naidich, Mauricio Castillo, Soonmee Cha, and James G. Smirniotopoulos

https://www.inkling.com/read/imaging-brain-naidich-1st/chapter-3/mri-signal-intensity

The following terms are used to describe MR signal intensity:

Lesions that are brighter than the tissue of reference are referred to as T1 hyperintense or T2 hyperintense, depending on the dominant contrast weighting of the image. Because shorter T1 and longer T2 values yield higher signal intensities on T1- (T1W) and T2-weighted (T2W) images, hyperintensity can also be referred to as T1 shortening and T2 prolongation, respectively.

Lesions that are less bright than the tissue of reference are designated T1 hypointense or T2 hypointense, or alternatively as causing relative T1 prolongation or T2 shortening, respectively.

Lesions not discerned separately from surrounding structures are termed T1 isointense or T2 isointense, depending on the image weighting.

Lesions that characteristically follow the signal intensity of gray matter, white matter, or CSF on all pulse sequences are described as being isointense to one of these tissues.


See also:

http://link.springer.com/chapter/10.1007%2F978-3-540-71372-2_4
MRI Atlas of MS Lesions
2008, pp 75-93
T1 Hypointense Lesions (Black Holes)

http://cni.bwh.harvard.edu/projects/image-analysis/gdc/
Guidelines To Identify Gadolinium Enhancing MS Lesions

Hope this helps!

If you are interested in slogging through an academic report on how MRI use is being refined to define MS disease severity, you might be interested in this report just out from the Harvard Medical School affiliated Partners MS Center in Boston: An Expanded Composite Scale Of MRI-Defined Disease Severity In Multiple Sclerosis: MRDSS2." http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4166046/
Neuroreport. 2014 Oct 1;25(14):1156-61
The article can be downloaded for free by following the link.

Here's another freebie. But remember, outside of a study most people aren't getting gray matter imaging, or having serial measures of brain atrophy done on the same scanner. The garden variety MRI basically gives some indication if the disease is active, something folks generally agree is not the best thing. But one person's "active" may be far more serious that another person's active, and one person's single active lesion can be doing a lot more harm than another person's six active lesions. And an MRI showing no active lesions doesn't necessarily mean the disease isn't active -- it just might be active in ways we can't image/view.
Magnetic Resonance Imaging Correlates of Physical Disability in Relapse Onset Multiple Sclerosis of Long Disease Duration
Mult Scler. Jan 2014; 20(1): 7280.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4107776/

That is so awesome Margaret! Thank you so much! I will have to take the time to read these links!
So, by just scheming through it, looks like T1 lesions are easy to define. They are just black holes and dead cells.
I think it gets more tricky when it gets to T2 and Gd+ lesions. So Gd+ lesions are brand new lesions and areas of damage. Now T2 lesions are both brand new lesions (Gd+) and old lesions that have not yet turned into black holes? Did I get that right?
So, T2 = Gd+ + old lesions that are not black holes, yet ?

I also think it is really refreshing to have you state that numerous number of lesions on the brain does not correlate with severe disability. If that is in fact true, it really is great news because many MS patients might have numerous lesions and to hear that it might correlate with severe disability can be daunting to them.

I do love pubmed articles and can't thank you enough for those links, Margaret. I think really those clinical studies and critiquing can be a valuable asset in determining treatment and figuring out what is going in the world of MS.

"So, by just scheming through it, looks like T1 lesions are easy to define. They are just black holes and dead cells.
I think it gets more tricky when it gets to T2 and Gd+ lesions. So Gd+ lesions are brand new lesions and areas of damage. Now T2 lesions are both brand new lesions (Gd+) and old lesions that have not yet turned into black holes? Did I get that right?"
Pretty close. T1 lesions can also be areas of swelling, and not permanent damage, and they can disappear. But more commonly they indicate areas where the nerve fibers have died, leaving a empty "black hole". It is possible for the brain to compensate for some of these dead areas, or even heal the areas to the extent that they once again show up as simply scarred..T2 lesions can be old "scars", or they can be new. There's no way to tell without the administration of gadolinium. Gadolinium crosses the blood-brain barrier, the "wall" that protects the brain from many circulating infections and prevents antibodies and most drugs from reaching the brain. But in MS that barrier is disrupted, and it allows the white cells to cross over and (to put it simply) attack the myelin. Since gadolinium can cross the blood-brain barrier along with the white cells, it can show exactly where the barrier is actively disrupted. You can think of the Gad as a flashlight-waving traffic cop, saying "here is the opening!" Gd+ lesions are active passages, while non-enhancing T2 lesions have been sealed back over with scars instead of cement.