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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.

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 tissues different intensities and best used in diagnosing specific pathology/ies.

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+C (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)

FLAIR:

Same as T2 except that fluid is dark. Fat Muscle Fluid Moving blood Bone Air Brain : -Gray matter: -White matter:

Best used in assessing lesions near ventricles the lesion can be easily discriminated from CSF.

Proton density weighted sequences:

Fat Muscle Fluid Moving blood Bone Air Hyaline cartilage fibrocartilage

Excellent in assessing joints as they can discriminate between fluid, hyaline cartilage & fibrocartilage