An assessment of artefacts in localized and non‐localized ³¹P MRS studies of phosphate metabolites and pH in rat tumours

Abstract

UA hepatomas, GH3 prolactinomas and N-methyl-N-nitrosourea-induced mammary tumours, which were subcutaneously grown in rats, have been studied by 31P MRS using non-localized pulse-acquire, image selected in vivo spectroscopy (ISIS) and one-dimensional chemical shift imaging (1-D CSI) techniques. Comparisons have been made with measurements from acid extracts of these tumour types and surrounding tissues (i.e., muscle and skin). Since muscle containing high concentrations of phosphocreatine (PCr) is often found adjacent to the tumour, we have compared the ratio of the PCr to gamma-NTP peaks in the spectra with the same ratio calculated from the acid extract data, and have used deviations between the two sets of data to assess the discrimination of the MRS localization technique to signals from the tissue surrounding the tumour. Extract data showed an average NTP content of 1.25 mumol/g wet wt for all three tumour types. PCr (at 0.42 mumol/g wet wt), was significant only in the GH3 prolactinoma whereas it was negligible in the other tumour types (< 0.1 mumol/g wet wt). There was good agreement between the ISIS PCr/gamma-NTP ratio and the extract data for all tumours. However, the 1-D CSI data showed an unexpectedly large contamination of the tumour spectrum with PCr signals from the skin which was shown by subsequent phantom experiments to be due to the curved geometry of tumour and skin rather than Fourier bleed. In pH measurements by MRS it was found that biological variability was greater than the effects of artefacts (due to either the chemical shift artefact in the ISIS technique or partial volume effects) in the localization technique. An average pH of 7.2 was observed for all tumours. By initially comparing data from different localization schemes with that from chemical extracts potential sources of error have been highlighted and show that phantom studies alone are not sufficient to fully assess the accuracy of localized MRS data.

Current practice in the use of MEGA-PRESS spectroscopy for the detection of GABA

“…In addition, the main Cr resonance is very close to the edited GABA signal; therefore chemical shift displacement issues will be negligible. The water signal has higher SNR and is more easily modeled, although chemical shift effects (Howe et al, 1993; Weinreb et al, 1985) mean that the acquired water signal (in some implementations) may be from a different location than the acquired voxel. Within the Gannet pipeline, referencing to both Cr and water is used (when water data are provided), which is useful in establishing that it is the GABA numerator, rather than the denominator, that drives observed effects as changes in NAA, Cr and water signals are all well documented in disease.…”

In vivo Magnetic Resonance Spectroscopy of cerebral glycogen metabolism in animals and humans

“…MRI readily provides anatomical and functional information whereas MRS supplies insights into tissue metabolism, including assessments of energy balance and enzyme kinetics. In vivo MRS can be performed using several nuclei such as 1 H, 13 C, 15 N, 17 O, 19 F, 23 Na, and 31 P. Each NMR active nucleus provides unique MR spectral patterns informing different aspects of physiology and metabolism, for example, 31 P MRS provides information about pH and high energy phosphates (Madden et al 1991; Petroff et al 1985; Roberts et al 1981; Howe 1993). The most abundant nuclei in living systems, hydrogen and carbon, are the basis of 1 H and 13 C MRS, respectively.…”

High-field small animal magnetic resonance oncology studies

“…Volume selection and spectral localization methods, including 3-dimensional spectroscopic localization, have been improving continuously (de Certaines et al , 1993; Arias-Mendoza and Brown, 2003), further advancing the applicability of localized 31 P MRS. Potential signal contamination due to signal contributions from skin and muscle/animal body can be reduced significantly by the application of spectroscopic localization techniques (Howe et al , 1993). …”