Improving MRI contrast

Key points

Technique	Principles	Advantages	Disavantages
STIR	Inversion-recovery with TI adapted to suppress the fat signal (according to its T1) Usually combined with fast spin echo sequences	Good fat signal suppression Low dependency on magnetic field heterogeneities	Not specific to tissue but to T1 (fat, hematoma, lesion enhanced by Gadolinium…) (Long acquisition times in standard STIR)
Fat Saturation	Selective RF pulse centered on the resonance frequency of fat + spoiler gradients +/- selective inversion associated with adapted TI	Fat suppression after Gadolinium injection Fat selective No modification in the contrast of the other tissues	Requires homogenous field: less effective in wide FOV or if there are magnetic susceptibility artifacts SAR increased TR and acquisition time increased
Selective excitation	Combination of RF pulses at intervals adapted to water/fat dephasing to separate magnetization of the water and fat Only the water supplies the signal	Less sensitive to field heterogeneities Faster Efficacy increased if the pulse sequence is more complex	The longer the preparation, the longer the acquisition time

Contrast agent	Principles	Advantages / Applications	Contraindications Adverse effects
Gadolinium chelates	T₁ reduction +++ T₂ reduction	Enhanced T1-weighted signal Perfusion imaging Well tolerated	Allergy, pregnancy Renal/liver failure: Nephrogenic systemic fibrosis
SPIO. USPIO	Superparamagnetic iron oxides: T2* effect	SPIO: liver USPIO: ganglion
Manganese chelates	T₁ effet	Liver and biliary imaging