Tvs15T

In the past 10-15 years, 1.5 T systems have been the most commonly used field strength in everyday clinical use. The advances in technology and the increased availability of higher fields have opened the door to a variety of exciting improvements in clinical and research application of MRI. In particular, 3 T systems have continued to gain wide acceptance as one of the main field strengths used for clinical and research studies [22]. The reasons for this acceptance are many.

One of the main advantages of high field imaging is the improvement of the signal to noise ratio (SNR). It has been shown [22] that the signal is expected to increase by a factor of 4 at 3 T, with respect to 1.5 T. Unfortunately, the noise also increases by a factor of 2. As a consequence, there is a twofold improvement in SNR, which may have profound clinical implications. (1) Shorteningof data acquisition times. It is possible to decrease the total data acquisition times by a factor of 4 at 3 T, while maintaining a SNR comparable to that obtained at 1.5 T. This time reduction shortens the overall exam length and could minimize patient motion artefacts. (2) Improving spatial resolution, while keeping acquisition times similar to 1.5 T. Spatial resolution improves differently according to the acquisition approach used. Therefore, depending on the structures in exam and the imaging sequences used, it is possible to improve SNR to optimize the visualization of relevant details of interest.

Another effect observed with high field imaging is an increasing susceptibility. fMRI is probably one of the best examples of converting artefacts into useful physiological information [22]. Signal changes due to BOLD effects are directly proportional to the magnetic field strength. The higher the field strength, the more sensitive the sequence will be to BOLD effects. Assuming the changes in the deoxyhaemoglobin concentration remain identical, the percentage signal changes are expected to increase twofold at 3 T when compared with that obtained from 1.5 T. However, since the absolute signal changes are small (~10% with a 3 T system), appropriate task paradigms must be planned.

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