Sandbox: ay

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 the 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 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 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:

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. N.B. : Not the best sequence for assessing lesions close to brain ventricles both will appear bright.

DWI

Fat: Muscle Fluid Moving blood Bone Air Brain : -Gray matter: -White 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