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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.

MRI Sequence

• 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:

When using T1 weighted imaging .. the tissues take 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 picture resembles the tissue macroscopically.

T1+Contrast (gadolinium)

• Tissues have the same densities as in T1 except that moving blood is bright.
• Useful in assessing hypervascular lesions (e.g. hemangiomas, lymphangiomas)

T2 weighted imaging:

When using T1 weighted imaging .. the tissues take 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, tissues become brighter. Used as in T1 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 appear bright.

Diffusion weighted imaging (DWI):

DWI specifically detects the motion of protons in water molecules.

When using T1 weighted imaging .. the tissues take 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)

Fluid attenuation inversion recovery (FLAIR):

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.

Proton density weighted sequences:

• 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.

Short Tau Inversion Recovery (STIR)

• 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.