By recruiting a larger amount of protons, a high magnetic field strength supplies more signal, in keeping other scan parameters constant. Put in simple words, 3 T scanners have the following three advantages over conventional magnets: (1) they increase signal, spatial resolution or scan time, respectively, by keeping other parameters constant; (2) they enhance susceptibility effects (T2* effects), which are useful for assessing blood products and for the blood oxygenated level dependent (BOLD) technique (on which fMRI is based); and (3) they increase the chemical shift effect, which is the basis of MR spectroscopic techniques.
As shown in the previous paragraph, subjective assessment of conventional MR imaging of gross anatomy supplies information for differential diagnosis, but neither quantification nor preclinical diagnosis is possible. On the other hand, increasing spatial resolution with 3T does not supply additional information concerning changes which might be useful for quantification or preclinical diagnosis. For instance, by keeping constant other scanning parameters, voxel volume can be reduced by half, which is far from the spatial resolution that is theoretically required for imaging microstructures.
Three-tesla scanners, by enhancing T2* effects, might be more sensitive in depicting deposits of iron and calcium, which are frequent in neurodegenerative disorders; however, no work has been carried out to assess how this theoretical advantage may be useful in clinical routine, keeping in mind that quantification and sensitivity are the ultimate aims. Biochemical and structural changes in AD are associated with functional impairment, which can be investigated through fMRI. Functional MRI benefits from increased SNR and T2* effects: the BOLD technique is more sensitive and sampling frequency can be increased, allowing more accurate event-related experiments.
Enhancing chemical shift phenomenon increases spectral resolution, which is the capability to discern two metabolites in MRS. Biochemical alterations occur early in disease before atrophy, and change more rapidly than morphology. Hence MRS may represent a sensitive and specific tool for assessing early disease and early response to therapy. Potential drawbacks of high-field magnets are represented by magnetic field inho-mogeneity in specific regions that are close to air interfaces, such as in the temporal lobe region, which is close to petrous bone and sphenoid sinuses.
Three-tesla scanners allow advanced imaging of degenerative diseases to be performed faster than conventional 1.5 T magnets, thus potentially allowing the assessment with combined techniques . High-field systems enhance the capabilities of advanced imaging techniques, thus increasing the sensitivity and specificity for early diagnosis and quantification of therapy response. Furthermore, 3 T scanners might potentially be useful for transferring results obtained from group studies in experimental settings to routine clinical settings.
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