Physical parameters and ultra high field
According to the Boltzmann distribution, an increase in the field is accompanied by an accentuation of the difference between the populations of parallel and antiparallel spins.
Population difference between energy levels and field B0
This results in an increase in the potential signal, which varies with the square of B0 field, counterbalanced by a linear progression of the noise.
As a result, the signal-to-noise ratio follows a linear relation with the value of the field: it is theoretically twice as high at 3.0 T as at 1.5 T.
The relaxation times of the different tissues of the body vary in function of B0.
T1 increases from 20 to 40 % for most tissues, and T2 reduces. The free water (LCR) relaxation times are only slightly modified between 1.5 T and 3.0 T. The T1 difference between blood and the surrounding stationary tissues is accentuated. On the other hand T1 between the tissues is poorer at ultra high fields.
At ultra high fields, there is a rapid increase in the quantity of RF energy deposited, proportionate to the square of the value of the field B0.
SAR and field B0
The SAR value in W/kg is of the type:
- B0 = amplitude of the static magnetic field
- B1 = amplitude of the RF pulse
- α = flip angle
- D = cyclic ratio (fraction of the duration of the sequence during which the RF waves are emitted)
- ρ = density
The difference in frequency between the various molecules is proportionate to the amplitude of the magnetic field and the chemical shift between these. Thus it is twice as high at 3.0 T as at 1.5 T. For instance, a chemical shift of 3.25 ppm between the fat and water protons produces a resonance frequency difference of 220 Hz at 1.5 T and 440 Hz at 3.0 T.
Phase and phase opposition times are modified (2.3 and 1.15 msec instead of 4.6 and 2.3 msec).
Magnetic susceptibility is proportionate to B0. These effects are far more marked at high field.
The various conductivity effects of the different tissues of the body cause heterogeneity in RF excitations. These are seen as an image with a non uniform signal and areas of varying degrees of signal loss.
These effects, which also occur to a lesser extent at 1.5 T, are mainly seen in abdominopelvic imaging (bigger explored volume and more heterogeneous dielectric characteristics).
Reducing these artifacts involves optimizing the coils and RF emission, as well as developing parallel emission techniques adapting RF emission to heterogeneous environments.
The noise present inside the MRI scanner virtually doubles when passing from 1.5 to 3.0 T, and is further increased by strong gradients. The noise must be softened for the patient (passively and actively by headphones). Some manufacturers place the superconducting magnet housing under a vacuum to provide acoustic insulation.
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