The filter for spin echo T2 is . The tissue filter shows the signal due to T2 weighting for tissues of different T2s for a spin echo pulse sequence with specific fixed TE. Change the TE of the pulse sequence with the slider or text box and the graph moves from side to side providing contrast for different ranges of T2. Note that for a given TE, there are only certain ranges of T2 that can be distinguished . i.e. have different signals.
"T2 weighted" images are generally used for pathology imaging and the images are used to look for changes in T2. Disease usually lengthens T2 as a result of edema or inflammation though thee are exceptions to this rule. Sensitivity to pathology is based on the slope of the curve at the T2 of a given tissue.
To visualize anatomy, different tissue T2s must result in different signal. This is determined by the slope of the filter between the two tissue T2s. You can click on different tissues to see how much relative signal that tissue will have at a given TE. What would be a good TE for differentiating grey and white matter? For differentiating fat from muscle?
To visualize pathology, changes in tissue T2 from normal must result changes in signal. The steeper the slope of the curve at a specific T2, the more signal change there will be for smaller changes in T2. For example, to be sensitive to edema or increased fluid in the white matter, the sequence should have a steep slope around the T2 of white matter. To look for edema in the muscles, the tissue filter should have a steep slope around the T2 of muscle.
What TE would be good for distinguishing T2 changes in grey and white matter? In liver? In fat? What TE values are good for looking for T2 changes in ligaments and tendons (as in tendinopathy). Is this a different T2 than would be useful for looking at a fluid filled tendon tear (where you are looking for fluid as opposed to subtle changes in the tendon)?
Linear or log axes can again show the slope of the curve for absolute versus fractional changes in T2. The log axis is generally better for looking at the slope of the curve at TEs used in clinical practice.
As with the T1 filter, you can determine the TE which will maximize the difference in signal between two tissues. The difference in signal between tissues with T2 values of "a" and "b" is
The TE resulting in the maximum difference im signal between the two tissues can be determined by taking the first derivative and setting it to zero. Solving for TE shows that the maximum signal difference between two tissues at is .
For grey and white matter this is TE = 95 ms.
However, when looking for pathology in a specific tissue, what you really want is a TE which results in a curve with a steep slope at the T2 of that tissue. This optimal TE is determined by setting the second derivative of to zero and solving for TE.
For the linear T2 axis the steepest slope is at TE = 2T2,
For the lnT2 axis the steepest slope is at TE = T2.
This means that to create a T2 weighted sequence most sensitive to absolute changes in the white matter the TE should be twice the T2 value of white matter, or TE = 180. This is much different from the optimal TE to distinguish white matter from grey matter.
To create a T2 weighted sequence most sensitive to fractional changes of T2 in white matter the TE should equal the T2 value of white matter, or TE = 90. This turns out to be close to the optimal TE for distinguishing white matter from grey matter and is close to the TE used in most routine brain imaging. Standard T2 weighted images in the brain are therefore good for anatomy (distinguishing grey from white matter) AND for looking for pathology (changes in grey and white matter T2).