MRI sequences
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1];Associate Editor(s)-in-Chief: Ahmed Younes M.B.B.CH [2]
Overview
- MRI is basically a huge magnet that emits energy (Radio-frequency pulse) into the body.
- Radiofrequency pulse causes the protons in H+ atoms to spin in different directions from which it used to spin.
- When the pulse stops .. the protons go back to spinning in the normal direction .. it releases energy.
- As tissues vary in a number of protons in it .. the energy emitted differ from tissue to tissue.
- Interpreting this energy using certain techniques enables us to represent every tissue in a unique density.
We are going to discuss some of the most commonly used sequences and when to use each one of them.
This video simplifies the concept of T1 and T2 relaxation times and their application in MRI.
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MRI Sequences
- An MRI sequence is a number of radio-frequency pulses (from the machine) and gradients that result (from protons in the body) in a set of images with a particular appearance.
- Each sequence gives different tissues different intensities and best used in assessing certain pathology.
T1 weighted imaging:
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Tissue densities reflex T1 which is the longitudinal relaxation time of the Net Magnetic Vector (NMV).[1]
When using T1 weighted imaging .. the tissues give the following densities:
- Fat: bright
- Muscle: gray
- Fluid: dark
- Moving blood: dark
- Bone: dark
- Air: dark
- Brain:
- Gray matter: gray
- White matter: bright
T1 is best used in assessing the anatomy as the image resembles the tissue macroscopically.
T1+Contrast (gadolinium)
- Injecting contrast material (gadolinium) increases T1 signal from moving blood and thus allows detection of highly vascular lesions.
- Tissues have the same densities as in T1 except that moving blood is bright.
- Useful in assessing hypervascular lesions (e.g. hemangiomas, lymphangiomas)
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T2 weighted imaging:
Tissue densities reflex T2 which is the transverse relaxation time of the Net Magnetic Vector (NMV).[2]
When using T2 weighted imaging .. the tissues give the following densities:
- Fat: bright
- Muscle: gray
- Fluid: dark
- Moving blood: dark
- Bone: dark
- Air: dark
- Brain:
- Gray matter: gray
- White matter: bright
Most pathologies have increased fluid content of the tissue as a part of the inflammatory process. Thus, lesions appear brighter. Used as in T1 weighted imaging in assessing the anatomy & most lesions in the body.
Important note:
- T2 weighted imaging is not the best sequence for assessing lesions close to brain ventricles both will look bright.
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Diffusion-weighted imaging (DWI):
DWI specifically detects the motion of protons in water molecules.
When using DWI weighted imaging .. the tissues give the following densities:
- Fat: low signal
- Muscle: gray
- Fluid: dark
- Brain:
- Gray matter: gray
- White matter: hypodense compared to gray matter
Fluid restricted areas appear bright. So, it’s most useful in assessing ischemia (e.g. stroke)
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Fluid Attenuation Inversion Recovery (FLAIR):
This sequence attenuates signals from fluids (e.g CSF) and thus is helpful in detecting lesions normally covered by CSF (in brain and spinal cord)
Tissues acquire the same densities as T2 weighted imaging except for that fluid appears dark.
- Fat: bright
- Muscle: gray
- Fluid: dark
- Bone: dark
- Air: dark
- Brain:
- Gray matter: gray
- White matter: darker than gray matter
Best used in assessing lesions near ventricles the lesion can be easily discriminated from CSF.
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Proton density weighted imaging:
It relies primarily on the density of the protons. So, Tissues with higher density give brighter signals.
When using PD-weighted imaging .. the tissues give the following densities:
- Fat: bright
- Muscle: gray
- Fluid: bright
- Bone: dark
- Air: dark
- Hyaline cartilage: gray
- Fibrocartilage: dark
Excellent in assessing joints as they can discriminate between fluid, hyaline cartilage & fibrocartilage.
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Short Tau Inversion Recovery (STIR)
Similar to FLAIR sequence, STIR suppresses signals from fat tissue.[3]
STIR can not be used post gadolinium injection as gadolinium has T1 in the same range of fat & eventually signals from it will be attenuated.
When using STIR imaging .. the tissues give the following densities:
- Fat: dark
- Muscle: darker than fat
- Fluid: very bright
- Bone: dark
- Air: dark
- Brain:
- Gray matter: gray
- White matter: darker than gray matter
Most useful in assessing fluid filled spaces.
This video summarizes some important aspects and uses of common MRI sequences.
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