Common STIR parameters at 3T are a TR of 4500, an inversion time of 240 and a TE of 80. If you increase TR above 4500 all the way up to 10000 (which is the maximum this page will support), you will see that maximum signals and contrast do not increase. For TR < 4500 you start to see overall signal loss and the T1 contrat between different tissues begins to decrease. It is preferable to always use the shortest TR that preserves signal and contrast.
The commonly recognized advantage of STIR is that it is a robust way to obtain fat suppression. Two common fat suppression techniques use fat selective excitation pulses to excite and then null fat (either via spoiler gradients or subsequent inversion recovery . . . these techniques are beyond the scope of this section). These techniques are vulnerable to field inhomogenities where it is difficult to execute selective RF pulses. Therefore, STIR has historically been used as an alternative to suppress fat in areas close to metal, air, or in body parts not well centered in the middle of the magnet. On newer scanners with better fields and gradients traditional fat suppression can be used in these areas. But as we will discuss, there may be reasons to continue using STIR.
The benefit of fat suppression is two-fold. First, since fat is often the brightest tissue on a T2 weighted image, removing the high signal from fat decreases the dynamic range of the image which makes more subtle areas of T2 prolongation easier to visualize. Second, there is a lot of fat in MSK imaging and making normal fat dark makes it easier to see edema/pathology within the fat.
However, STIR provides more than just robust fat suppression. As it turns out, STIR should be more sensitive to pathology than the standard FSE T2 weighted images. Most pathologies in the brain, abdominal organs, fat, muscles, and tendons result in increased proton density, increased T1, and increased T2. This is certainly true for gliosis, demyelination, soft tissue edema, and most tumors (there are exceptions like low grade gliomas, papillary renal cell carcinomas which have decreased T2, melanoma and prostate cancers which result in increased T1, and hemorrhage which results in increased T1). Remember back to the section on the FSE T1, T2, and PD weighted images. The PD and T2 filters for these sequences work against the T1 filter. INCREASED proton density results in INCREASED signal. INCREASED T2 results in INCREASED signal. But INCREASED T1 results in DECREASED signal.
With STIR, INCREASES in proton density, T1, and T2 ALL result in increased signal. Since most pathology results in increased proton density, T1, and T2; the filters work together to make pathology bright! In addition, the T1 filter for STIR is twice as steep as the FSE T1 filter, making STIR more sensitive to small changes in T1. Traditional FSE T2 sequences use a long TR to minimize the effects of T1 on tissue signal. STIR takes advantage of the IR pulse to accentuate the effects of T1 on tissue signal!
Lets look at how STIR works in different parts of the body. Its best to use the linear x scale for this discussion.
Start with fat. Check the fat check-box and choose fat from the drop down menu of tissues. Fat is normally dark on STIR because the inversion time results in the T1 filter cancelling all signal. However, fat sits at a very steep part of the T1 filter. Small increased in T1 within fat from early edema result in markedly increased signal. The weighting shows that 98% of signal change in fat is due to T1 weighting; and you thought STIR was a T2 weighted image! If there is a frank fluid collection in the fat then the signal is bright due to the T2 filter, because you imaging the fluid. But when mild inflammation results in increased signal in the fat the effects are mostly due to T1! PD and T2 weighting contributes much less, but at least these contributions are synergistic. Even so, set the second tissue drop down menu to muscle. Fat and muscle appear similar on STIR- and the difference is due to nearly equal positive T1 and negative T2 weighting!
Ligaments and tendons are dark on STIR because their T2 filter nulls the signal. These structures have very short T2 and will look dark on sequences with high TE. However, ligaments and tendons sit on a very steep part of the T2 filter. Small changes in T2 will result in a large amount of increased signal. The sequence weighting shows that changes in ligament and tendon signal on a STIR is 90% due to T2 weighting.
Muscle is generally low signal on STIR due to the T2 filter. However, it also sits on a steep part of the filter. Changes in muscle signal due to small changes in PD, T1, and T2 are 24% due to PD weighting, 11% due to T1 weighting, and 65% due to T1 weighting. Weighting is predominantly T2, but with some PD and T1 as well.
Liver is also relatively dark on STIR due to its low T2 similar to muscle (and sometimes there is fat in the liver which is suppressed on STIR). However, since liver sits on a steeper part of the T1 filter than muscle there is more T1 weighting in the liver.
Choose white matter in the tissue drop down and highlight the white and grey matter check boxes. Based on the relative sequence weightings, changes in white matter signal due to mild pathology are 34% attributable to PD weighting, 27% due to T1 weighting, and 39% due to T2 weighting. STIR in the white matter uses all three tissue properties nearly equally!
Choose grey matter in the tissue drop down box. Grey matter sits on a flatter part of the T1 filter and as you would expect, changes in grey matter signal due to subtle pathology is less T1 weighted (12%) compared to white matter (27%). There is still a nearly even split between PD and T2 weighting.
Simple fluid returns high signal from all three filters. You do not usually look for "pathology" in fluid. The exception being hemorrhage into fluid. If the hemorrhage markedly reduces the T1 and T2 of the fluid you will see decreased signal - this is not terribly common but you do see layering low signal in muscle hematomas, etc . . . Generally, fluid is seen in seromas, muscle tears, ligament/tendon tears, and abscess collections where it appears very bright.
We've just discussed where contrast comes from in STIR. Take home points include
(1) STIR is very sensitive to subtle pathology because the PD, T1, and T2 filters work synergistically to produce increased signal for most diseases.
(2) Increased signal is due to different tissue properties (PD, T2, and T2) for different tissues. STIR is not simply T2 weighted!