Selecting IR Parameters

How do you set the IR parameters to achieve desired image contrast?  
The approximate signal intensity (S) from an IR pulse sequence is given by the following equation: 
where K is a scaling factor and [H] is the spin density. S may be either positive or negative, depending on whether the tissue Mz is pointing in the +z or −z direction at the end of the TI interval.  When using magnitude-reconstructed IR (the most common method), the absolute value (|S|) should be used. For phase-corrected IR maintaining the sign of S is important, as +S values are displayed brighter than −S ones.

The effect of TE is readily apparent, appearing in the multiplier term e−TE/T2. If TE is short compared to T2, the exponential term converges to unity and the T2 sensitivity is minimized. Conversely, if TE is the same or longer than T2, then this term becomes large and T2-weighting of the IR sequence is accentuated.

The effects of TR and TI on image contrast are not immediately obvious. You will recall that in SE imaging, the TR value controls T1-weighting through a term of the form e−TR/T1. Even though there is a similar exponential term in the IR equation above, there is a second term (−2e−TI/T1) that also contains T1. In general, therefore, the TR value does not tightly control T1-weighting in IR sequences like it does in SE sequences. T1-weighting is primarily determined by the TI value selected. The only real adjustment needed for TR is to make sure it is long enough to allow enough slices to be acquired and sufficiently long to allow enough tissue recovery to generate a usable signal.

We will cover how to select TI for various imaging scenarios in the next several Q&As. In particular we will see that judicious selection of TI will allow you to null the signal from a given tissue, such as fat, white matter, gray matter, or CSF as shown below.
Inversion time (TI)
By judicious choice of TI, selective nulling of fat (STIR sequence, TI=180), white matter (TI=400), gray matter (TI=650) or CSF (T2-FLAIR, TI=2500) is possible.

Advanced Discussion (show/hide)»

References
     Bydder GM, Young IR. MR imaging: clinical use of the inversion recovery sequence. J Comput Assist Tomogr 1985; 9:659-675.
     Hornak JP. The Basics of MRI (www.cis.rit.edu/htbooks/mri/), 2013. (A derivation of the signal intensity equation for IR given above can be found there).
Related Questions
     What is the inversion recovery pulse sequence?
     What is phase-sensitive IR and why does it have a gray background?