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How do we change image contrast? What are the effects of parameter changes? This section explores image contrast and image acquisition methods.

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What is STIR? Briefly describe how it works.

STIR (Short TI Inversion Recovery) is an inversion recovery pulse sequence with specific timing so as to suppress the signal from fat.

An inversion recovery pulse sequence is a spin echo pulse sequence preceded by a 180° RF pulse.

An inversion recovery pulse sequence.

The action of the RF pulse is to invert the starting magnetisation, which then returns to its equilibrium value (M0) according to T1 relaxation.

A 180° RF pulse.

The time between 180° preparation pulse and the 90° excitation pulse (TI) is chosen at the time

ln(2) T1fat

so that when the 90° pulse is applied, there is no longitudinal magnetisation to be flipped into the x-y plane from fat—at the null point of fat. Fat will reach its null point faster than white matter, grey matter, water or oedema, and so an image of these structures can be generated. At 1.5 T the appropriate TI to null fat is about 140 msec.

Mz recovery in STIR.

Suppression of any tissue is possible with appropriate TI chosen so that the 90° pulse is applied at the null point of that tissue. For example, make the TI long and you have the FLAIR pulse sequence, which suppresses signal from fluid.

Mz recovery in FLAIR.

A STIR image, revealing cancer lesions within spinal vertebrae at 1.0 T.

A FLAIR image, revealing periventricular lesions more clearly because the adjacent fluid signal is suppressed.

Further reading on this topic:
Books: MRI From Picture to Proton p39-41, 92-93, Q&A in MRI p239-241, MRI The Basics p72-74, 173, 259
Online: MRItutor, e-MRI

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