Inversion recovery sequences

Inversion recovery pulse sequences are a type of MRI sequence used to produce images with heavy T1-weighting or to selectively null the signal for certain tissues (e.g. fat or fluid).

The basic part ofBasically, an inversion recovery (IR) pulse sequence is a spin echo pulse sequence preceded by a 180-degree° RF pulse. The preparatory pulse inverts longitudinal magnetisation (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 beginningexact time when longitudinal magnetisation reaches the null point for the tissue we wish to suppress.The time elapsed between the preparatory 180° pulse and the 90° readout pulse is termed time to inversion (TI).

By choosing the appropriate TI, suppression of the pulse sequence, inverting the net magnetisation vector. After a pre-determined delay, known as the inversion time (TI), the rest of the pulse sequence (e.g. spin echo) occurs.

Spin echo inversion recovery

After the initial 180-degree inversion pulse, the usual spin echo or fast spin echo sequence can take place. Depending on the inversion time chosen specificdifferent tissues can be nulled, such that no signal is returnedpossible:

• STIRShort tau inversion recovery (STIR): fat is nulled
• FLAIRFluid attenuated inversion recovery (FLAIR) or DIRdouble inversion recovery (DIR): fluid is nulled

This techniqueInversion recovery can also be used to generate profoundly T1 weighted images.

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 magnetisation, 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 accentuatednegative M_z values darker and vice versa.

Advantages

These sequences offer several 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

Disadvantages

• 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. For this reason, STIR should not be used with gadolinium injection, as tissues with T1 shortening owing to gadolinium uptake may be inadvertently suppressed along with fat