Absolute metabolite quantification by in vivo NMR spectroscopy: IV. Multicentre trial on MRSI localisation tests

Big GABA: Edited MR spectroscopy at 24 research sites

Magnetic resonance spectroscopy (MRS) is the only biomedical imaging method that can noninvasively detect endogenous signals from the neurotransmitter γ-aminobutyric acid (GABA) in the human brain. Its increasing popularity has been aided by improvements in scanner hardware and acquisition methodology, as well as by broader access to pulse sequences that can selectively detect GABA, in particular J-difference spectral editing sequences. Nevertheless, implementations of GABA-edited MRS remain diverse across research sites, making comparisons between studies challenging. This large-scale multi-vendor, multi-site study seeks to better understand the factors that impact measurement outcomes of GABA-edited MRS. An international consortium of 24 research sites was formed. Data from 272 healthy adults were acquired on scanners from the three major MRI vendors and analyzed using the Gannet processing pipeline. MRS data were acquired in the medial parietal lobe with standard GABA+ and macromolecule- (MM-) suppressed GABA editing. The coefficient of variation across the entire cohort was 12% for GABA+ measurements and 28% for MM-suppressed GABA measurements. A multilevel analysis revealed that most of the variance (72%) in the GABA+ data was accounted for by differences between participants within-site, while site-level differences accounted for comparatively more variance (20%) than vendor-level differences (8%). For MM-suppressed GABA data, the variance was distributed equally between site- (50%) and participant-level (50%) differences. The findings show that GABA+ measurements exhibit strong agreement when implemented with a standard protocol. There is, however, increased variability for MM-suppressed GABA measurements that is attributed in part to differences in site-to-site data acquisition. This study's protocol establishes a framework for future methodological standardization of GABA-edited MRS, while the results provide valuable benchmarks for the MRS community.

N-Acetyl peak in MR spectra of intracranial metastatic mucinous adenocarcinomas

Absence of N-acetylaspartate (NAA) is one important diagnostic criterion of MR spectroscopy (MRS) that may suggest that an intracranial mass lesion is a metastasis. We report two cases of histopathology-confirmed intracranial metastatic mucinous adenocarcinoma, which predominantly showed a large metabolite peak at 2.0 ppm, mimicking an NAA peak of normal brain tissue. This finding could be of help in the interpretation of MRS in cases of intracranial enhancing mass lesions, metastases or gliomas.

Reducing voxel bleed in Hadamard-encoded MRI and MRS

The point spread function (PSF) of Hadamard encoding deviates from its ideal profile due to practical (as opposed to intrinsic) reasons. Finite radiofrequency (RF) pulse length and gradient strength cause slice profile imperfections that lead to cross-talk ("voxel bleed") as large as 17% for a 1-KHz bandwidth, 5.12-ms RF pulse under 3 mT/m. This could adversely affect localization and quantification, and consequently clinical usefulness. A simple modification of the Hadamard RF pulse synthesis that exploits its unique ability to encode noncontiguous slices is proposed and shown to markedly improve the PSF. Computer simulation, in vitro and in vivo experiments confirm the theoretical derivation of voxel bleed reduction from approximately 17% to below 5% per Hadamard-encoded direction.

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

Magnetic resonance spectroscopy (MRS) in the investigation of cancer at The Royal Marsden Hospital and The Institute of Cancer Research

Developments in magnetic resonance spectroscopy (MRS) at The Royal Marsden Hospital and The Institute of Cancer Research are reviewed in the context of preceding developments in nuclear magnetic resonance (NMR) and MRS, and some of the early developments in this field, particularly those leading to human measurements. The early development of technology, and associated techniques for human measurement and assessment will be discussed, with particular reference to experience at out institutions. Applications using particular nuclei will then be described and related to other experimental work where appropriate. Contributions to the development of MRS that have been published in Physics in Medicine and Biology will be discussed.

The effect of respiratory motion on CSI localized MRS. Cooperative Group on MR Applications to Cancer

A theoretical model to assess the effect of respiratory motion on chemical shift imaging (CSI) data of an extended sample is described. Data from the model are shown to agree with those acquired experimentally from phantoms. In CSI, Fourier bleed and ghost artefacts are of particular significance when the tissue of interest is adjacent to other tissue with much higher concentrations of metabolites, such as chest wall muscle adjacent to breast tumour tissue. We have evaluated contamination due to Fourier bleed and ghosting due to subject motion separately. The results demonstrate that the amount of signal contamination due to motion is relatively independent of the ratio of sample width to voxel width and of the period of motion, but has a strong dependence on the number of phase-encoding steps and the amplitude of the motion.

A constrained Gauss-Seidel method for correction of point spread function effect in MR spectroscopic imaging

Magnetic resonance spectroscopic imaging is limited by a low signal-to-noise ratio, so a compromise between spatial resolution and examination time is needed in clinical application. The reconstruction of truncated signal introduces a Point Spread Function that considerably affects the spatial resolution. In order to reduce spatial contamination, three methods, applied after Fourier transform image reconstruction, based on deconvolution or iterative techniques are tested to decrease Point Spread Function contamination. A Gauss-Seidel (GS) algorithm is used for iterative techniques with and without a non-negative constraint (GS+). Convergence and noise dependence studies of the GS algorithm have been done. The linear property of contamination was validated on a point sample phantom. A significant decrease of contamination without broadening the spatial resolution was obtained with GS+ method compared to a conventional apodization. This post-processing method can provide a contrast enhancement of clinical spectroscopic images without changes in acquisition time.

Tumour phospholipid metabolism

Following the impetus of early clinical and experimental investigations, in vivo and in vitro MRS studies of tumours pointed in the eighties to the possible significance of signals arising from phospholipid (PL) precursors and catabolites as novel biochemical indicators of in vivo tumour progression and response to therapy. In the present decade, MRS analyses of individual components contributing to the 31P PME (phosphomonoester) and PDE (phosphodiester) resonances, as well as to the 1H 'choline peak', have reinforced some of these expectations. Moreover, the absolute quantification of these signals provided the basis for addressing more specific (although still open) questions on the biochemical mechanisms responsible for the formation of intracellular pools of PL derivatives in tumours, under different conditions of cell proliferative status and/or malignancy level. This article is aimed at providing an overview on: (a) quantitative MRS measurements on the contents of phosphocholine (PCho), phosphoethanolamine (PEtn) and their glycerol derivatives ģlycerol 3-phosphocholine (GPC) and glycerol 3-phosphoethanolamine (GPE)[ in human tumours and cells (with particular attention to breast and brain cancer and lymphomas), as well as in normal mammalian tissues (including developing organs and rapidly proliferating tissues); (b) possible correlations of MRS parameters like PEtn/PCho and PCho/GPC ratios with in vitro cell growth status and/or cell tumorigenicity; and (c) current and new hypotheses on the role and interplay of biosynthetic and catabolic pathways of the choline and ethanolamine cycles in modulating the intracellular sizes of PCho and PEtn pools, either in response to mitogenic stimuli or in relation to malignant transformation.

Absolute metabolite quantification by in vivo NMR spectroscopy: I. Introduction, objectives and activities of a concerted action in biomedical research

By utilizing achievements and results of two previous concerted research projects on magnetic resonance imaging and spectroscopy (MRS), the EU BIOMED 1 Concerted Action on "Cancer and brain disease characterization and therapy assessment by quantitative MRS" was specifically aimed at: 1) developing at a multicentre level harmonized methodologies and protocols for quantitative and reproducible MRS measurements, as a basis for validating these procedures in well controlled clinical and experimental conditions; and 2) providing multicentre critical reviews on the present understanding of the significance of MRS parameters as possible new markers of diagnosis, prognosis and response to therapy. The programme comprised the following main areas of collaborative research and multicentre evaluation: a) development of methods and protocols for quality assessment, calibration and absolute metabolite quantification in in vivo localized, volume-selective MRS; b) design and validation of a new method for assessing localization performance in spectroscopic imaging (MRSI); c) interlaboratory comparison of different methods of signal processing and data analysis, for improving signal quantification in vivo and in vitro MRS spectra; d) quality assessment of high resolution MRS analyses of biological fluids; e) protocol for assembling a pilot data base of MR spectra of tumour extracts for pattern recognition analysis; f) multicentre review on evaluation of the significance of MRS parameters in monitoring lipid metabolism and function in cancer; and g) multicentre review on evaluation of drug pharmacokinetics and metabolism using MRS. The main results and conclusions of four multi-centre trials on items (a), (b) and (c), which involved 24 teams, are reported in the accompanying papers of this series.