Target Audience

The target audience for this talk is junior physicists and engineers who are interested in understanding and designing RF pulses. As RF pulses are a critical portion of a pulse sequence those who are interested in pulse sequences will also benefit.

Outcome/Objective

The information provided in this lecture provides a framework for the development of many pulses, from basic Shinnar-LeRoux (SLR) pulses to more advanced 2- or 3-dimensional adiabatic pulses. The talk is structured around how the different properties of RF pulses (flip angle, spatially selectivity, adiabaticity) relate to one-another, and how they can be used. This will enable a deeper understanding of how RF pulses operate, and help the audience in designing and choosing the proper pulses for their own experiments. At the end of this talk the audience will have a clearer understanding of the different types of RF pulses, and how to choose the most appropriate pulses for their experiments.

Methods

First, the basics of RF pulses are discussed, namely the amplitude (as measured in Tesla or micro Tesla), the duration (as measured in seconds) and the resolution of the waveform. The Bloch equation is then discussed, as this is what dictates how the magnetization and RF pulses interact. An intuitive solution to the Bloch equation is presented which provides a clear understanding of how RF pulses operate without the need for any dense mathematics. The most fundamental quality of an RF pulse, the flip angle is discussed, with emphasis that although 90 and 180 degree pulses are the most common they are certainly not the only two types of RF pulses. From here spatial selectivity is discussed, starting with how that is achieved (through the simultaneous application of RF pulses and gradients). This leads into the more complex and commonly utilized SLR transform, with practical advice given on how to design SLR pulses. Then the excitation k-space formalism is discussed, which allows for the design of RF pulses which excite a general spatial location, as well as the concept of multiband pulses which are used in simultaneous multi-slice imaging. Adiabaticity is discussed, starting with the hyperbolic secant pulse then providing a diagrammatic explanation of the Bloch equation which enables a deeper understanding for all adiabatic RF pulses. At the end all of these concepts are discussed in tandem, through the example of a 2-dimensional adiabatic inversion pulse. Some practical tips regarding implementation and simulation are given.

Discussion

RF pulses are a critical portion of every magnetic resonance experiment. The proper choice of RF pulses can distinguish between a high quality and meaningless data.

Conclusions

The concepts of RF pulse design will provide a framework for understanding broad classes of RF pulses found throughout the field. Practical tips regarding implementation, RF pulse and simulation are provided. This will help enable researchers to choose the proper RF pulses for their desired applications.

Acknowledgements

No acknowledgement found.

References

No reference found.