MRI Sequences

Inversion Recovery, STIR and FLAIR

by Denis Hoa

Type of sequence Philips Siemens GE Hitachi Toshiba
IR IR
 IR TSE
IR/IRM
 TurboIR/TIRM
IR
 FSE-IR
IR
 FIR
IR
 Fast IR
STIR STIR
 STIR TSE
STIR
 Turbo STIR
STIR
 Fast STIR
STIR
 Fast STIR
STIR
 Fast STIR
FLAIR FLAIR
 FLAIR TSE
FLAIR
 Turbo FLAIR
FLAIR
 Fast FLAIR
FLAIR
 Fast FLAIR
FLAIR
 Fast FLAIR

Generic diagram

 

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Inversion-recovery is a magnetization preparation technique followed by an imaging sequence of the spin echo type in its « standard » version .
 The sequence starts with a 180° RF inversion wave which flips longitudinal magnetization Mz in the opposite direction (negative). Due to longitudinal relaxation, longitudinal magnetization will increase to return to its initial value, passing through null value.
 To measure the signal, a 90° RF wave is applied to obtain transverse magnetization. The delay between the 180° RF inversion wave and the 90° RF excitation wave is referred to as the inversion time TI.
 As longitudinal regrowth speed is characterized by relaxation time T1, these sequences are weighted in T1. Inversion-recovery also increases weighting of the associated imaging sequence (spin echo or gradient echo of varying speeds).
 With this type of sequence, certain tissues have a negative signal. In terms of display, two possibilities exist :
 

  • Either signal magnitude (amplitude in relation to 0) used for gray scale display: the more absolute the value of the tissue signal (positive or négative), the stronger it will be.
  • Or the gray levels will be distributed from the negative signal values to the positive values (with a null signal background that will be gray rather than black): this is the « true » display type.

 

Another property of inversion-recovery sequences is linked to the choice of TI: if a TI is chosen such that the longitudinal magnetization of a tissue is null, the latter cannot emit a signal (absence of transverse magnetization due to the absence of longitudinal magnetization). The inversion-recovery technique thus allows the signal of a given tissue to be suppressed by selecting a TI adapted to the T1 of this tissue .

 

Inversion-recovery can be combined with sequence types other than the standard spin echo. In particular, it can be used with fast spin echo sequences, to save considerable time, as inversion-recovery requires relatively long TR to allow magnetization the time to regrow. Iinversion-recovery also serves as magnetization preparation for gradient echo sequences, to weight them in T1.

 

STIR sequences

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In the standard STIR sequence, the spin echo sequence is completed by a previous 180° inversion pulse. Fat has a short T1. Hence by choosing a short TI of 140 milliseconds, the fat signal can be suppressed . The combination of short TI inversion-recovery and fast spin echo sequences reduces acquisition time to acceptable limits for clinical practice.
 The advantage of these sequences is that they offer a fat signal suppression technique with low sensitivity to magnetic field heterogeneities or to the effects of magnetic susceptibility in the presence of metal (orthopedic prostheses in osteoarticular imaging for example). They can be used with T1 or T2 weighting (particularly in spin echo sequences where fat appears as a hypersignal).
 This technique must not be used to suppress a fat signal gadolinium injection because gadolinium–enhanced tissues have a shortened T1 and may be erased by short TI inversion-recovery (which is not specific to tissue but to its relaxation time T1).

 

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FLAIR sequences

 

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The aim of a FLAIR sequence is to suppress liquid signals by inversion-recovery at an adapted TI. Water has a long T1. Nulling of the water signal is seen at TI of 2000 milliseconds. . As in the case of the other inversion-recovery sequences, an imaging sequence of the fast spin echo type is preferable to compensate the long acquisition time linked to long TR.
 These sequences are routinely used in cerebral MRI for edema imaging.

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