In High Field MR

During recent years much effort has been put into detecting and classifying disease states, and MR imaging has been a very important tool in this respect. Up to now, increasing the gradient strength has been the only strategy for meeting the increasing demands of advanced diagnostic imaging applications. This strategy is limited by physical, economic, and medical considerations: increasing gradient strength is technically difficult, and is associated with significant hardware costs and with the risk of inducing unwanted side effects such as peripheral neurostimulation.

The main advantage of high-field MR is the improved signal-to-noise ratio, which scales approximately linearly with field strength from 1.5 T to 3 T. This signal can be used to generate more accurate spatial representation or to speed up imaging times, depending on the specific application. Higher field strengths change tissue contrast parameters. T1 relaxation time is increased by approximately 30 %, whereas T2 and T2* relaxation times are decreased by about 15% [37]. Increasing the field strength from 1.5 T to 3 T also doubles chemical shift and susceptibility. MR spectroscopy benefits considerably from the improved spectral resolution possible with high-field MR, and images acquired on 3 T MR demonstrate enhanced sensitivity to the blood oxygen level-dependent (BOLD) effect [77]. On the other hand, tissue heating induced by radiofrequency power is a disadvantage with the use of3T MR. In fact, specific RF absorption rate approximately quadruples when field strength increases from 1.5 T to 3 T. The most recent attempt to circumvent these limitations is parallel imaging MRI.

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