What is in this lecture?

Obtaining MR spectra in vivo comes with its own set of challenges. This lecture will cover the basics of spectral localization and water suppression and, of course, shimming, since a homogeneous magnetic field is central to obtaining quality spectra. As basic techniques which are applicable to single voxel acquisition are general to two and 3D acquisitions, we will focus on the single voxel in this presentation.
Localisation: Since biological specimens are rarely composed of a single tissue and we usually want to obtain spectral measures from particular areas of the body, it is essential to have methods by which we can acquire spectra from distinct locations. We can either do this using surface coils for spatial localization or, more usually, by using gradients to edit and confine the coherent signals to the area of interest. Localisation with surface coils is less precise but advantages may include silent acquisition, acquisition from surfaces or areas not suitable for linear gradients, and reduced relaxation effects. Gradient localization techniques require consideration of factors such as pulse bandwidths and profiles, echo times and chemical shift artefact. Improvements in spectrometer and RF design have meant that there are a range of options that one can use. A basic understanding of the ramifications of the choices one can make is essential to be able to choose the best option for the desired outcome. We will deconstruct the components of the PRESS sequence to see how it all works.
Water suppression: The large water signal in proton spectroscopy, which is four to five times the order of magnitude of the metabolite signals, creates dynamic range problems for the spectroscopist. Suppressing the signal is crucial and this can be done in a number of different ways (but the main key to good water suppression is good shimming). We will discuss the basic principles of water suppression and show some examples of bad, good and excellent water suppression.
Shimming: What is shimming and why does it matter? Obtaining quality spectra relies on having a homogeneous magnetic field across the volume of acquisition. The black art of shimming has been improved greatly over the years and is now a largely automated process. Understanding how and why it can go wrong is key.
This lecture will also touch upon other important aspects of MR spectroscopy such as bandwidth, echo times and chemical shift artefact.

Acknowledgements

The author acknowledges support from the Australian National Imaging Facility, a National Collaborative Research Infrastructure Scheme.