High-field MR, with its intrinsically higher SNR, can be a platform for new diagnostic applications or can serve to improve existing methods. However, its use is not unfettered by limitations.

In fact, the very high spatial resolution and shorter acquisition times permitted by the greater SNR result in a loss of image quality to a level below that which can be achieved with 1.5 T systems. Given the high SNR of 3.0 T MR systems, neuroradiologists can choose between imaging the brain in the same time taken with a 1.5 T unit, thus obtaining better quality images (by increasing the matrix and/or reducing slice thickness and/or FOV) and generating images of the same quality

Fig. 3.12. Single volume spectroscopy (PRESS TR/TE 2000/30) acquired at 3.0 T (a) and 1.5 T (b). In a the increased chemical shift improves spectroscopic resolution, resulting in broader separation of the peaks of the different metabolites in the Glx area

as those of a 1.5 T system in a shorter time, thus increasing patient comfort or reducing the dose of contrast medium administered.

Resolution quality tends to be privileged in clinical practice, with acquisition of a larger number of slices of reduced thickness, use of smaller FOV and broader matrices, and acquisition times similar to those of 1.5 T systems (Fig. 3.3).

A compromise is generally sought between resolution and speed (e.g. by reducing the number of excitations) depending on the clinical problem and the type of patient [34].

The high SNR of high-strength magnetic fields can also be obtained with 1.5 T devices using the latest hardware (surface receiver coils used singly or in arrays; high-yield gradients to reduce TE) and software (new sequences able to optimize the contrast/noise ratio) or longer examination times. However, this cannot be done in the case of unstable or poorly cooperative patients, or it maybe impossible for reasons of cost (reduced patient flow). In addition, although matrix size can be increased within 1.5 T systems, the attendant reduction in voxel size yields a more granular image.

Lastly, the higher SNR of high-field MR systems is a resource that the neuroradiologist can exploit in clinical practice, since the higher the SNR, the greater the scope for adjusting protocols.

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