2D approach to quantitation of inversion-recovery data

Functional Magnetic Resonance Spectroscopy of Prolonged Motor Activation using Conventional and Spectral GLM Analyses

Background

Functional MRS (fMRS) is a technique used to measure metabolic changes in response to increased neuronal activity, providing unique insights into neurotransmitter dynamics and neuroenergetics. In this study we investigate the response of lactate and glutamate levels in the motor cortex during a sustained motor task using conventional spectral fitting and explore the use of a novel analysis approach based on the application of linear modelling directly to the spectro-temporal fMRS data.

Methods

fMRS data were acquired at a field strength of 3 Tesla from 23 healthy participants using a short echo-time (28ms) semi-LASER sequence. The functional task involved rhythmic hand clenching over a duration of 8 minutes and standard MRS preprocessing steps, including frequency and phase alignment, were employed. Both conventional spectral fitting and direct linear modelling were applied, and results from participant-averaged spectra and metabolite-averaged individual analyses were compared.

Results

We observed a 20% increase in lactate in response to the motor task, consistent with findings at higher magnetic field strengths. However, statistical testing showed some variability between the two averaging schemes and fitting algorithms. While lactate changes were supported by the direct spectral modelling approach, smaller increases in glutamate (2%) were inconsistent. Exploratory spectral modelling identified a 4% decrease in aspartate, aligning with conventional fitting and observations from prolonged visual stimulation.

Conclusion

We demonstrate that lactate dynamics in response to a prolonged motor task are observed using short-echo time semi-LASER at 3 Tesla, and that direct linear modelling of fMRS data is a useful complement to conventional analysis. Future work includes mitigating spectral confounds, such as scalp lipid contamination and lineshape drift, and further validation of our novel direct linear modelling approach through experimental and simulated datasets.

Integrated data acquisition and processing to determine metabolite contents, relaxation times, and macromolecule baseline in single examinations of individual subjects

Absolute quantitation of clinical (1)H-MR spectra is virtually always incomplete for single subjects because the separate determination of spectrum, baseline, and transverse and longitudinal relaxation times in single subjects is prohibitively long. Integrated Processing and Acquisition of Data (IPAD) based on a combined 2-dimensional experimental and fitting strategy is suggested to substantially improve the information content from a given measurement time. A series of localized saturation-recovery spectra was recorded and combined with 2-dimensional prior-knowledge fitting to simultaneously determine metabolite T(1) (from analysis of the saturation-recovery time course), metabolite T(2) (from lineshape analysis based on metabolite and water peak shapes), macromolecular baseline (based on T(1) differences and analysis of the saturation-recovery time course), and metabolite concentrations (using prior knowledge fitting and conventional procedures of absolute standardization). The procedure was tested on metabolite solutions and applied in 25 subjects (15-78 years old). Metabolite content was comparable to previously found values. Interindividual variation was larger than intraindividual variation in repeated spectra for metabolite content as well as for some relaxation times. Relaxation times were different for various metabolite groups. Parts of the interindividual variation could be explained by significant age dependence of relaxation times.

MR spectroscopy quantitation: a review of time-domain methods

In this article an overview of time-domain quantitation methods is given. Advantages of processing the data in the measurement domain are discussed. The basic underlying principles of the methods are outlined and from them the situations under which these algorithms perform well are derived. Also an overview of methods to preprocess the data is given. In that respect, signal-to-noise and/or resolution enhancement, the removal of unwanted components and corrections for model imperfections are discussed.

Myocardial high-energy phosphate metabolism in heart transplant patients is temporarily altered irrespective of rejection

A reliable, sensitive, non-invasive alternative for transvenous endomyocardial biopsy in detecting cardiac allograft rejection is desirable for optimal management of heart transplant patients. To establish whether (31)P magnetic resonance spectroscopy can become a non-invasive tool for detecting cardiac allograft rejection, the cardiac high-energy phosphate metabolism of human heart transplants was serially examined in 13 patients by means of (31)P MRS from post-operative day 13 to day 294, and compared with histologic evaluation of endomyocardial biopsies. Biopsy scores of 2 or higher, according to the Working Formulation criteria of Billingham et al., were considered to indicate rejection. Logistic regression, which was corrected for differences between the individual patients and the time after transplantation, showed no significant correlation between the occurrence of histologically detected rejection and the PCr:ATP ratio. However, using an analysis of variance, the PCr:ATP ratios of non-rejecting cases obtained within 50 days after transplantation (mean: 27 +/- 11 days) appeared to be significantly different from those obtained after post-operative day 50 [0.95 +/- 0.17 (n = 25) vs 1.17 +/- 0.17 (n = 32), mean +/- SD; p < 0.01]. No significant difference was observed between the PCr:ATP ratios obtained 100 days after transplantation (mean: 162 +/- 52 days) and the PCr:ATP ratios in the hearts of healthy volunteers [1.18 +/- 0. 18 (n = 19) and 1.23 +/- 0.17 (n = 6), mean +/- SD, respectively; p = 0.55]. The PCr:ATP ratio in transplanted human hearts is not a sensitive indicator for the detection of early acute human cardiac allograft rejection. This may be due to a temporarily altered high-energy phosphate metabolism early after transplantation irrespective of rejection.

Metabolic changes in reflex sympathetic dystrophy: a 31P NMR spectroscopy study

The lower leg skeletal muscles of 11 patients affected by reflex sympathetic dystrophy were investigated at rest by 31P nuclear magnetic resonance spectroscopy at a fieldstrength of 1.5 T. The results were compared with similar investigations of unaffected lower leg muscles of patients and volunteers. A significant increase was observed for the average tissue pH of the muscles of affected legs as deduced from the chemical shift of the resonance for inorganic phosphate. The average inorganic phosphate/phosphocreatine ratio of these muscles was also increased. The impairment of high energy phosphate metabolism, as deduced from the NMR data, may be caused by cellular hypoxia or diminished oxygen utilization, which would agree with previous findings that oxygen extraction is reduced in extremities affected by reflex sympathetic dystrophy.