Clinical Techniques
MRI
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Get access- Nuclear dipoles (protons) are aligned with each other by a strong external magnetic field
- The protons rotate (precess) around their own axes in different phases
- The direction of nuclear dipole alignment is then altered by a radio frequency pulse
- The protons also precess in phase with each other in response to the pulse
- The pulse then dissipates
- The protons realign along the original magnetic field (T1 relaxation)
- They begin to rotate out of phase again (T2 relaxation)
The relaxation releases signals that are detected by receiver coils which a computer builds into images
Hot Topic
STIR and FLAIR MRI sequences
Short tau inversion recovery (STIR) sequences correlate strongly with orbital inflammation and clinical activity scores in thyroid eye disease (T2 STIR specifically)
Fluid-attenuated inversion recovery (FLAIR) sequences employ fluid suppression techniques to highlight paraventricular pathology: useful to identify demyelination
Summary Table: T1 vs T2 MRI features
| Tissue | T1 | T2 |
| Air | Black | Black |
| Bone | Black | Black |
| Calcium | Black | Black |
| CSF (ie. low protein fluid) | Dark | Bright |
| Vitreous | Black | Bright |
| White matter | Grey | Dark |
| Grey matter | Grey | Grey |
| Acute blood | Grey | Dark |
| Subacute blood | Bright | Bright |
| Fat | Bright | Moderately bright |
| Mucous | Bright | Dark |
| Long T1 times render substances hypointense (dark) on T1 MRI | ||
| Gadolinium is visible on T1 weighting |
Advantages over CT
- No ionising radiation: useful for pregnant patient or children
- High soft tissue contrast: can differentiate white and grey matter
- Vasculature can be imaged without contrast: useful if renal impairment
- No artefact from bone
- Functional imaging
- DWI: assesses movement of water molecules to characterise lesions
Functional: assesses differences in metabolic rate of tissues by detecting different oxygen concentrations
MRI features of choroidal lesions
| T1 | T2 | Contrast enhancement | |
| Choroidal melanoma | Hyperintense | Hypointense | Marked |
| Choroidal haemangioma | Hyperintense | Hyperintense | Marked |
| Choroidal osteoma | Hyperintense | Hypointense | Central core enhanced |
| Choroidal metastasis | Hyperintense | Hypointense | Marked |