Improving MRI contrast: Imaging water and fat
Learning objectives
After reading this chapter, you should be able to:
- Present the different fat signal suppression techniques
- Describe the advantages and disadvantages of STIR sequences, fat saturation, selective excitation of water
- Specify the different families of contrast agents
- Explain their action on relaxation time and contrast
- List their contraindications and main indications
- Explain magnetization transfer and how it can improve background suppression
Key points
Water and Fat
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 agents
Contrast agent | Principles | Advantages / Applications | Contraindications / Adverse effects |
Gadolinium | T1 reduction +++ T2 reduction | Enhanced T1 Perfusion imaging Well toterated | Allergy, pregnancy Renal / liver failure : Nephrogenic systemic fibrosis |
SPIO | Superparamagnetic iron oxides : T2* effect | SPIO : liver USPIO : ganglion | |
Manganese chelates | T1 effect | Liver imaging |
Magnetization transfer:
- RF pulse offset in relation to the resonance peak of water
- Saturation of protons with restricted mobility (brain, muscle, liver)
- Drop in signal due to exchanges between free and bound protons
- Does not affect fluids (LCR, blood, urine) containing no bound protons
References
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- McRobbie. MRI from picture to proton. 2003:xi, 359 p.
- NessAiver. All you really need to know about MRI physics. 1997.
- Kastler. Comprendre l'IRM. 2006.
- de Kerviler, Leroy-Willig. Fat suppression techniques in MRI: an update. Biomedicine & pharmacotherapy = Biomedecine & pharmacotherapie. 1998;52(2):69-75.
- Gandhi, Brown. MR contrast agents for liver imaging: what, when, how. Radiographics. 2006 Nov-Dec;26(6):1621-36.
- Lin and Brown. MR contrast agents: Physical and pharmacologic basics. J Magn Reson Imaging. 2007 May;25(5):884-99.
- Sadowski, Bennett. Nephrogenic Systemic Fibrosis: Risk Factors and Incidence Estimation. Radiology. 2007 Jan 31.