Over the past two decades the contribution of MRI to medical practice has been unrivalled, and a review of the breakthroughs achieved with this in vivo imaging technique would be beyond the scope of this chapter. Research and development in the field of MR imaging have been fuelled by the constant need, among other things, for better pictures of the brain and spinal cord with a higher signal-to-noise ratio (SNR) in order to better visualize and quantify the pathological changes imputable to white matter diseases. While increasing the number of acquisitions (number of excitations, NEX) can improve the SNR of the images, this comes at a cost of time. In fact, the SNR is proportional to the square root of the number of acquisitions. Therefore, in order to double the SNR, the acquisition time will be four times longer. Although in theory this seems interesting, in clinical practice longer acquisition times are prohibitive, and could result in an increased likelihood of image degradation due to motion artefacts, especially in the case of patients having trouble lying still in the scanner for relatively long periods of time.
The main advantage of 3.0 T over lower field MR scanners is a better SNR, which increases roughly linearly with the strength of the magnetic field. Therefore, the SNR of a 3.0 T MRI scanner is theoretically twice as much as the SNR obtained at 1.5 T. Consequently, imaging at 3.0 T enables higher resolution scans with higher imaging matrices and/or thinner slices to be obtained that permit visualization of more detailed anatomical structures while keeping the scan time virtually unchanged. These advantages come at a trade-off of an increased sensitivity to field inhomogeneities and changes in relaxation times, which in turn produce changes in image contrast. At comparable acquisition times, images obtained at 3.0 T have a higher quality with an improved resolution than images obtained at 1.5 T. Alternatively, 3.0 T MRI can be used to obtain acceptable images, similar to those obtained at 1.5 T, but at a fraction of the time, thus reducing potential motion artefacts and improving patients' comfort.
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