Sandbox: ay
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