Adapting equipment and sequences for ultra high fields
The physical modifications arising from ultra high fields must be considered as a whole, as most of these are interdependent.
The theoretical increase in the signal-to-noise ratio will improve image quality and may heighten spatial resolution or acquisition speed compared to the possibilities at 1.5 T.
The effects of modifications in relaxation times are such that sequences have to be adjusted to comply with slower longitudinal relaxation (relative signal loss), shortened TE due to reduced T2, preparation of magnetization to restore a T1 contrast (Inversion, MDEFT: Modified Driven Equilibrium Fourier Transform). More than the signal-to-noise ratio, it is the contrast -to-noise ratio that provides an objective comparison of the examinations performed with different field B0. Increased Gadolinium/tissue contrast allows the dose of the injected contrast agent to be reduced.
Time-of-flight MRA will benefit from better tissue saturation at long T1 and contrast with the circulating blood.
The quantity of RF energy deposited at ultra high fields is such that SAR needs to be taken into account in sequence parametering, unlike 1.5 T where this is rarely limiting. Several options are available to reduce SAR: increasing TR, reducing the number of slices, the flip angle or length of echo train.
Parallel imaging is another good way of reducing the number of acquisitions, and thus of RF pulses, all the more so since the acceleration factors can be higher at ultra high fields than at 1.5 T (better signal thus fewer limitations of high acceleration factors in relation to the signal-to-noise ratio).
Several lines of research are in progress to reduce SAR:
- MRI with short magnet (i.e. a full-body emitting coil covering a smaller volume and delivering less RF energy)
- Emitter/receiver surface coils
- Parallel technique RF emission (shorter, better targeted pulses, correction of inhomogeneities)
- Sequence optimization
Chemical shift artifacts of the first type can be reduced by increasing the receiver bandwidth (with a drop in signal-to-noise ratio). Phase and phase opposition sequences, based on the second type of chemical shift artifact need to be adapted as the TE are different.
The bigger frequency gap between fat and water protons, makes suppression of the fat signal by saturation easier and more homogeneous.
Spectroscopy also benefits from this better discrimination between metabolites (spectral resolution and better signal-to-noise ratio).
Accentuating the effects of magnetic susceptibility may be disadvantageous at ultra high fields (image distortions and signal loss). These artifacts are diminished by reducing the TE and making the sequence less T2*sensitive, by choosing segmented rather than single shot sequences, reducing voxel size and increasing bandwidth. Parallel acquisition techniques offer a considerable gain in image quality, making it possible to perform sequences of the echo planar type and ultra fast gradient echo type, even at a very high field.
Finally, this heightened sensitivity to magnetic susceptibility has advantages for detecting hemorrhages, first pass perfusion imaging and functional MRI with BOLD contrast (better T2*sensitivity).
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