MRI Imaging Techniques: Spin Echo, Inversion Recovery, and Gradient Recalled Echo, Slides of Radiology

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Basic M R I

Imaging Methods

Outline

I. Review of Spin Echo Imaging II. Inversion Recovery (IR) III. Gradient Recalled Echo IV. Three Dimensional (Volume) Techniques V. Fast Imaging Techniques VI. Echoplanar Imaging

Other techniques not necessarily covered in this lecture (but in others): Parallel imaging, flow-related imaging techniques

Hahn Spin Echo

The concept of Spin Echo production was developed by Hahn. Spin echoes are sometimes referred to as Hahn spin echoes.

The Hahn Spin Echo technique consists of a 90 o flip, followed by an interpulse delay time, t, a 180 o^ flip, followed by a second interpulse delay

(t). A “spin echo” occurs at echo time TE (2t) following the initial 90 o^ flip.

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Hahn Spin Echo (continued)

The Hahn Spin Echo was developed in 1950 for use in nMR long before the advent of MRI. The Hahn Spin Echo was used for the measurement of T 2 values. (^) docsity.com^5

Spin Echo Imaging (continued)

Remember, the value of the repetition time (TR) and the echo time (TE) can be varied to control contrast in spin echo imaging.

For general spin echo, TR is used to mediate T 1 contrast, and TE is used for T 2 contrast.

“Long” TR, “short” TE  Proton Density Weighting (want to minimize both T 2 , T 1 contrast) “Long” TR, “medium” TE  T 2 Weighting “Medium” TR, “short” TE  T 1 Weighting

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Spin Echo Images

T1 weighted PD weighted T2 weighted

TR 510 TE 14 2min 7sec for 17 slices

TR 4500 TE 15eff (ETL7) 2min 39sec for 24 slices

TR 4500 TE 105eff (ETL7) 2min 39sec for 24 slices

Inversion Recovery (IR) (continued)

The basic IR pulse sequence consists of a 180o “inversion pulse” inserted before whatever sequence you usually choose for a particular contrast. The standard 90o^ RF excitation pulse of your sequence follows the inversion pulse after an inversion time TI.

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The remainder of whatever sequence that you choose goes here.

Inversion Recovery (IR) (continued)

First, application of the 180o^ RF pulse inverts the macroscopic magnetization. During the inversion time, the macroscopic magnetization shrinks along the negative Z axis, eventually passes through Z = 0 and regrows along the positive Z axis toward thermal equilibrium. Before the macroscopic magnetization is fully relaxed, the 90o^ RF pulse flips the partially relaxed longitudinal magnetization into the transverse plane in order to measure the signal induced in an RF coil.

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Animation of Inversion Process for STIR

Transverse magnetization for shorter T 1 tissue is suppressed.

Transverse magnetization for longer T 1 tissue is retained.

Inversion Recovery (IR) (continued)

In many inversion recovery imaging sequences, the 90o^ pulse is followed by a 180o^ pulse in order to produce a spin echo at time TE following the 90o^ pulse.

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FLAIR: Inversion time: 2.5sec (CSF is suppressed) TR 10sec TE 119msec (ETL7) 3min 49sec for 19 slices

IR Image Comparison

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STD T 2 weighting

FLAIR: fluid attenuated IR (T 2 -weighted spin echo)

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IR Image Comparison

STD T 1 weighting STIR: short tau inversion recovery

STIR image demonstrating high signal surrounding the distal radius fracture indicating marrow edema and hemorrhage (McAlinden, P.S., et al., Imaging 2003; 15: 180-192).

STIR is an inversion recovery sequence used to null fat.

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Gradient Recalled Echo (continued)

GRE differences from spin echo:

1. The creation of the echo is accomplished solely by gradient magnetic fields.
2. Since 180o^ RF pulses are not used to create the echo, the deposition of RF energy in the patient is lower (less heating of patient tissues).
3. Static inhomogeneity of the magnet and inhomogeneity caused by magnetic susceptibility of patient tissue are NOT corrected by gradient recalled echo techniques.

T2 contrast strictly becomes T2* contrast with GRE.

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Gradient Recalled Echo (continued)

4. The initial flip angle is frequently chosen to be less than 90o^. The flip angle in gradient recalled echo techniques is called a. Selection of a as less than 90o^ prevents severe saturation of the spin population when TR is short for rapid imaging. The optimum value of a for a particular TR and tissue having spin lattice relaxation T 1 is called the Ernst angle.

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-TR T cos(a^1 e) =