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Concept
MRI sequence
Résumé
An MRI sequence in magnetic resonance imaging (MRI) is a particular setting of pulse sequences and pulsed field gradients, resulting in a particular image appearance.
A multiparametric MRI is a combination of two or more sequences, and/or including other specialized MRI configurations such as spectroscopy.
Spin echo and Relaxation (NMR)
Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T1 (spin-lattice; that is, magnetization in the same direction as the static magnetic field) and T2 (spin-spin; transverse to the static magnetic field).
To create a T1-weighted image, magnetization is allowed to recover before measuring the MR signal by changing the repetition time (TR). This image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions, and in general, obtaining morphological information, as well as for post-contrast imaging.
To create a T2-weighted image, magnetization is allowed to decay before measuring the MR signal by changing the echo time (TE). This image weighting is useful for detecting edema and inflammation, revealing white matter lesions, and assessing zonal anatomy in the prostate and uterus.
The standard display of MRI images is to represent fluid characteristics in black and white images, where different tissues turn out as follows:
Proton density (PD)- weighted images are created by having a long repetition time (TR) and a short echo time (TE). On images of the brain, this sequence has a more pronounced distinction between gray matter (bright) and white matter (darker gray), but with little contrast between brain and CSF. It is very useful for the detection of joint disease and injury.
Gradient echo
A gradient echo sequence does not use a 180 degrees RF pulse to make the spins of particles coherent. Instead, it uses magnetic gradients to manipulate the spins, allowing the spins to dephase and rephase when required. After an excitation pulse, the spins are dephased, no signal is produced because the spins are not coherent.
Source officielle
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