*S*) from an IR pulse sequence is given by the following equation:

*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

**(the most common method), the absolute value (|S|) should be used. For**

*magnitude-reconstructed IR***maintaining the sign of**

*phase-corrected IR**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*) that also contains T1. In general, therefore, the

^{−TI/T1}*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.

*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.

### 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?*