This presentation will enable you to describe the current state of magnetic resonance spectroscopy in the detection of pathology; understand the drawbacks and problems associated with the use of MRS; and understand how MRS could be further improved to allow better diagnostic and research utility.
At the end of this presentation you should be able to
1. Describe the current state of magnetic resonance spectroscopy in the detection of pathology.
2. Understand the drawbacks and problems associated with the use of MRS
3. Understand how MRS could be further improved to allow better diagnostic and research utility.
Magnetic resonance spectroscopy uniquely allows measurement of metabolic information non-invasively in tissues. MRS, as applied on the modern MR spectrometer, is quantifiable and the robustness and reliability of the approach is constantly improving through better hardware and software and through the use of consistent approaches to acquisition and analysis [1]. MRS is now indicated for radiological performance by the ACR-ASNR for 22 separate diagnostic situations [2].
Although there are several disorders where complete absence of a particular metabolite allow the use of MRS as a single diagnostic, most disorders are more complex, resulting in multiple biochemical changes, each of which may not provide a clearly significant outcome. This is further complicated by the nature of MRS itself, which is intrinsically insensitive and which is only capable of detecting a relatively small number of metabolites [3].
The future of MRS has been described as lying in coupling MRS with other approaches to improve the power of diagnostic ability. While this is undoubtedly true, more power can also be wrung from MRS through the development of approaches such as:
1. Use of multi-dimensional fast acquisition spectroscopy approaches. These include 2D approaches like COSY, chemical shift imaging and CEST.
2. Use of multivariate analyses which take into account metabolic patterns.
3. Use of sparse/non-uniform sampling regimes to reduce acquisition time and improve relative signal to noise.
4. Spectral editing to allow better detection of metabolites not resolved by one-dimensional MRS.
5. Use of standardized acquisition and analysis methods in clinical practice.
While one dimensional spectra remain the gold standard acquisition method, rapid development in sequences has allowed acquisition of two-dimensional correlation spectra in clinically feasible time frames. These include, for example, COSY (correlation spectroscopy), TOCSY (total correlation spectroscopy; [4]) and multi-dimensional J-resolved spectra [5]. These approaches, while potentially highly useful, currently remain research only tools which are not available to the vast majority of users.
Multivariate and pattern recognition approaches allow us to extract the maximum amount of information from MRS data. These are commonly used methods in metabolomics, although even there, their application could be significantly improved. The oncology community is largely driving the application of these approaches in MRS in vivo but, as yet, no distinctly advantageous method has emerged into clinical use.
Advantageous use of sparse sampling relies on the possession of reliable a priori data to inform the optimum non-uniform sampling approach. Compressed sensing may have some utility in MRS [6] but it largely relies on having sufficient signal to noise in the first place and this is often not the case in MRS. These approaches are very much in their infancy in MRS but have already shown some utility in MRS of hyperpolarized nuclei where signal to noise is less of an issue than the shortness of the available acquisition time [7] or for fast collection of water reference data in chemical shift imaging [8].
Spectral editing is now used more frequently in clinical practice due to the better availability of approaches such as MEGA-PRESS [9]. It allows more robust and reliable measurement of metabolites such as GABA [10], lactate [11] and glutathione [12]. Multiple spectral editing approaches have been used and it is likely that the more practical and more widely implemented ones will be the first to be more broadly validated and accepted [13].
Proper analysis of the utility of MRS in diagnostics is hindered by the low sample sizes typically contained in individual studies. This is where standardized acquisition parameters combined with multisite collection of data could greatly inform our understanding of the reliability of the method [14, 15].
Further advances are required in modelling, statistics, and spectral acquisition and processing to fully realise the potential of MRS in the diagnosis of disease, deregulation and damage.
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