IR-Prepped Sequences
What is meant by an "IR-prepped" sequence? How is this different from "standard" inversion recovery?
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The 180°-inversion pulse (and its associated gradients) used for conventional IR imaging can be thought of as a generic "module" that can be used in a wide range of other pulse sequences. Such modular design is commonplace in MR sequence development:
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Modular structure of MR pulse sequences
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In an "IR-prepped" sequence the Preparatory Module contains a 180°-inversion pulse. Other possibilities include spectral saturation pulses ("fat-sat", "silicone-sat"), driven equilibrium pulses, and flow or spatial saturation pulses. The effective inversion time and overall image contrast depends on the k-space ordering of the acquisition sequence (e.g. centric, linear) and where in the echo train the central lines of k-space are acquired.
The Acquisition portion can be any set of pulses (and their associated gradients) that generate a signal: spin-echo, fast spin-echo, stimulated echo, gradient echo, etc.
Recovery is a relative "dead time (TD)" when no signals are being generated and the system is allowed to return to equilibrium. This period may still contain gradient manipulations such as "spoilers" or "rewinders" to eliminate unintended phase coherences and unwanted echoes that might have developed from manipulations of spins earlier in the pulse sequence. After Recovery, the cycle repeats.
The Acquisition portion can be any set of pulses (and their associated gradients) that generate a signal: spin-echo, fast spin-echo, stimulated echo, gradient echo, etc.
Recovery is a relative "dead time (TD)" when no signals are being generated and the system is allowed to return to equilibrium. This period may still contain gradient manipulations such as "spoilers" or "rewinders" to eliminate unintended phase coherences and unwanted echoes that might have developed from manipulations of spins earlier in the pulse sequence. After Recovery, the cycle repeats.
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The conventional inversion recovery sequence we have discussed can be considered an "IR-Prepped" fast spin-echo method. If the Acquisition portion is changed to a "spoiled" gradient echo, then the image contrast will be somewhat different. Two IR-prepped gradient echo sequences (MP-RAGE and FGATIR) with different TI's are shown.
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MP-RAGE
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FGATIR
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Simplified schematic of the MP-RAGE sequence (imaging gradients have been omitted for simplicity). Typical parameters for brain imaging are TR=2000, TI=1000, and TE=3 ms.
References
Mugler JP 3rd, Brookeman JR. Rapid three-dimensional T1-weighted MR imaging with the MP-RAGE sequence. J Magn Reson Imaging 1991; 1:561-7.
Sudhyadhom A, Haq IU, Foote KD et al. A high resolution and high contrast MRI for differentiation of subcortical structures for DBS targeting: the fast gray matter acquisition T1 inversion recovery (FGATIR). Neuroimage 2009; 47:T44-T52.
Tanner M, Bambarota G, Kober T, et al. Fluid and white matter suppression with the MP2RAGE sequence. J Magn Reson Imaging 2012; 35:1063-1070.
Mugler JP 3rd, Brookeman JR. Rapid three-dimensional T1-weighted MR imaging with the MP-RAGE sequence. J Magn Reson Imaging 1991; 1:561-7.
Sudhyadhom A, Haq IU, Foote KD et al. A high resolution and high contrast MRI for differentiation of subcortical structures for DBS targeting: the fast gray matter acquisition T1 inversion recovery (FGATIR). Neuroimage 2009; 47:T44-T52.
Tanner M, Bambarota G, Kober T, et al. Fluid and white matter suppression with the MP2RAGE sequence. J Magn Reson Imaging 2012; 35:1063-1070.
Related Questions
What is the inversion recovery pulse sequence?
What is the difference between MPRAGE and MP2RAGE?
What is the inversion recovery pulse sequence?
What is the difference between MPRAGE and MP2RAGE?