Relationship of imaging parameters and main magnetic field strength
| Imaging Property | Relationship with Main Magnetic Field Strength | Observed Trend in Practice |
|---|---|---|
| SNR | SNR α B0 | SNR increases linearly with B0; however, a linear increase may not be achievable in the presence of inhomogeneity and changing tissue relaxation4,5 |
| Resolution | Resolution α n0.33 When B0 is increased by a factor of n | In practice, further increases in resolution may be achieved because of improvements in detector and gradient design and enhanced contrast-to-noise ratio for some types of imaging sequences acting in conjunction with increased field strength |
| T1 (in sec) | T1 = 1/(0.35 + 0.64 × B0−0.7) | This is a heuristic approximation (in SI units) for cortical gray matter4; generally T1 values for different tissues lengthen and converge with field strength |
| T2* (in sec) | T2* = 1/(7 + 3.5 × B0) | This is a heuristic approximation (in SI units) for cortical gray matter4,97,98 |
| Apparent T2 | Decrease with B0 | Theoretically, field strength has little effect on the spin-spin relaxation governing intrinsic T299; however, due to effects such as spins diffusing through susceptibility microgradients, apparent T2 shortens for some spin-echo sequences98,100,101 |
| Sensitivity to susceptibility | Δϕ α B0 × TE | In practice, higher B0 enables the use of shorter TEs to maintain the same phase effect; coupled with higher SNR, this results in improved resolution and speed |
| Spectral resolution in MRSI | ΔF α B0 | Although the separation of spectral peaks increases with field strength, increased B0 inhomogeneity within a voxel will cause broadening of peaks, counteracting this benefit; better shimming and smaller voxel volumes are required to prevent peak broadening |
| CSL error | CSL α B0 × section thickness/(RF bandwidth) | In MRSI, this results in substantial shifts in selected volumes for metabolites resonating at different frequencies; high bandwidth RF pulses, such as adiabatic pulses can mitigate CSL error |
Note:—Δϕ indicates the phase difference between 2 tissues; ΔF, frequency separation between metabolite peaks; CSL, chemical shift localization; SI, International System of Units.