Inversion recovery sequences

Inversion recovery pulse sequences are a type of MRI sequence used to selectively null the signal for certain tissues (e.g. fat or fluid).

Inversion recovery can also generate heavily T1-weighted images and was originally developed for this purpose.

Physics

Basically, an inversion recovery (IR) pulse sequence is a spin echo pulse sequence preceded by a 180° RF pulse. The preparatory pulse inverts longitudinal magnetization (M_z), namely, it flips M_z to its negative value, -M_z. Tissues regain M_z at different longitudinal (T1) relaxation rates determined by their T1 relaxation times. The spin echo 90° readout pulse is applied at the exact time when longitudinal magnetization reaches the null point for the tissue we wish to suppress.
The time elapsed between the preparatory 180° pulse and the 90° excitation (slice selection) RF pulse is termed time to inversion (TI) (figure 1).

By choosing the appropriate TI, suppression of different tissues is possible:

• short tau inversion recovery (STIR): fat is nulled
• fluid attenuated inversion recovery (FLAIR) or double inversion recovery (DIR): fluid is nulled
• phase-sensitive inversion recovery (PSIR): no nulling

Image reconstruction

Virtually all IR spin echo sequences use magnitude reconstruction for the final image. What this means is that pixel intensity reflects only the magnitude of longitudinal magnetization, disregarding polarity; absolute values (i.e. absolute distances from the null point) are used.
The little-used phase-sensitive inversion recovery (PSIR) reconstruction method, by contradistinction, takes polarity into account, rendering pixels with negative M_z values darker and vice versa.

Advantages

• twice the dynamic range of T1W spin echo sequences; due to the 180° inverting pulse, longitudinal relaxation goes from -M_z to M_z
• using a short TI, longer T1 values will contribute to T2 contrast since inverted values are also rendered as positive when using magnitude reconstruction (see above) 
• can be performed on a device with any magnetic field strength
• relatively insensitive to magnetic field inhomogeneity (cf. SPIR/SPAIR)
• relatively low susceptibility to metal, particularly useful for imaging patients with orthopedic hardware
• can be used with fast (turbo) spin echo sequences, thereby reducing scan times

Drawbacks

• 180° preparatory pulses lengthen scan times
• higher specific absorption rate (SAR) due to additional 180° pulses
• potential signal-to-noise ratio (SNR) decrease due to tissue suppression
• increase in flow-related artifacts
• inversion recovery technique is not specific to a certain tissue (i.e. not chemical suppression). For this reason, STIR should not be done after gadolinium injection, as tissues that take up gadolinium may exhibit T1 shortening into fat range and be inadvertently nulled along with fat