The purpose of this chapter is to show that by combining two inversion recovery (IR) sequences you can create an image that is very sensitive to changes in T1. We have previously shown that an IR sequence provides twice the T1 weighting as a standard fast spin echo (FSE) sequence because the maximum slope of the IR T1 tissue filter is twice the maximum slope of the SE T1 filter. By combining two IR T1 tissue filters you can create a filter with an even steeper slope. A steeper tissue filter slope means that smaller changes in the tissue property result in larger changes in signal. This is what is wanted to detect small changes from normal.

Two IR T1 tissue filters are shown in the plot. Use the controls to change the inversion times for each curve independently. TR is fixed at 10000 ms for both curves. This TR is long enough to ensure full recovery for long TIs. For inversion times targeted to null tissues with T1s similar to or less than grey matter a TR of 5000 suffices but for TIs targeted to simple fluid a longer TR is needed.

Click the SIR filter checkbox. A green curve appears that is the result of subtracting the longer TI IR tissue filter from the shorter TI IR tissue filter. Note that TI 1 is intended to be the shorter inversion time and TI 2 is intended to be the longer inversion time. The green curve is called the Subtraction Inversion Recovery (SIR) filter. The maximum slope of the SIR filter is located at T1 times between the tissues that are nulled by the two TIs. This range of T1 values is called the middle domain (mD).

The slope of the SIR filter in the middle domain is approximately twice as steep as either of the individual IR T1 filters and four tines steeper than the FSE T1 filter (not shown). (The change in signal is approximately twice that of the component IR T1 filters across the same middle domain.) The SIR sequence is created by acquiring an FSE IR sequence with TI 2 and subtracting it pixel by pixel from an FSE IR sequence with TI 1. The limtation is that this increased weighting is only valid for tissues with T1 values in the middle domain. The filter is less useful for tissues with T1 values below that nulled by TI 1 and above that nulled by TI 2. The T1 contrast is positive in the middle domain and negative outside of it.

Choose TI 1 such that white matter is nulled and TI 2 such that grey matter is nulled (these are actually the default values). This filter is very sensitive for changes in the T1 of white matter and in theory would be useful for targeting white matter pathology. Normal white matter will be dark. Pathologic changes in the white matter result in increased T1 and increased signal. This sequence would NOT be as sensitive for detecting pathologic changes in the grey matter. As grey matter T1 increases signal decreases. Such an image would be difficult to interpret: increased signal in the white matter is pathologic and decreased signal in the grey matter is pathologic. That is confusing!

If the middle domain is widened the range of T1 values with positive T1 contrast increases, but the slope of the filter decreases meaning that small changes in T1 are more difficult to see.

Likewise, if the middle domain is narrowed the range of T1 values with positive T1 contrast decreases, but the slope of the filter increases meaning that small changes in T1 are easier to see.

If you wanted a T1 weighted sequence very sensitive for small changes in white matter T1 you could decrease TI 2 so that it is closer to TI 1. Normal white matter would appear dark. Even very changes from normal would appear bright. If the changes in T1 were large enough that the T1 increased to a value greater than that of the tissue nulled by TI 2, the signal change would not be as great. So the trick is to choose TI 2 close enough to TI 1 so that the fitler is steep, but not so close such that pathology increases T1 past the maximum value of the filter. It actually turns out that this is not so critical. We will show in another chapter that even if a "lesion" has a T1 on the negatively sloped part of the filter it will be etched in high signal so it can still be seen.

The SIR fitler results in twice the weighting of an IR filter and four times the weighting of an FSE filter. We can do better.

If you click the AIR filter checkbox a purple curve will appear which is the sum of the two IR T1 filters. The AIR filter is not useful clinically and is shown to demonstrate that it is never zero and is relatively flat for most middle domains that are not too wide. An AIR image is created by acquiring an FSE IR sequence with TI 2 and adding it pixel by pixel from an FSE IR sequence with TI 1. Once you are convinced of this you should uncheck this box to prevent the plot from becoming too cluttered.

Now click the checkbox. The pink line is the divided Subtraction Inversion Recovery (SIR) filter and is the SIR filter divided by the AIR filter. This filter is nearly five times steeper than the SIR filter in the middle domain, 10 times steeper than the IR T1 filter, and 20 times steeper than the FSE T1 fitler (not shown). A dSIR sequence is obtained by dividing an SIR image by an AIR image. As with the SIR sequence, the narrower the middle domain the steeper the slope of the filter. Remember, a steeper filter allows you to detect smaller changes in tissue T1.