Problems and expediencies in human 31P spectroscopy. The definition of localized volumes, dealing with saturation and the technique-dependence of quantification

Several technological problems in in vivo localized spectroscopy of metabolism are discussed in the context of comparing data obtained by different means. Deficiencies in spectroscopy localization methods can produce spectra that are dominated by artefactual signals derived from outside of selected volumes. Such artefacts are not usually correctly accounted for by representations of the profiles of the transverse magnetization alone. Selected sensitive volumes should be defined in terms of the size of tissue contributing the major fraction of signal to an observed spectrum, which is the integrated response from the sample including any phase cancellation effects. Phase cancellation in one-dimensional localization techniques employing excitation by an RF field with uniform phase distribution and surface coil detection such as depth resolved surface coil spectroscopy, chemical shift imaging (CSI) and rotating frame zeugmatography (RFZ) can significantly alter the effective radius of the sensitive volumes depending on the sample distribution and the extent of the homogeneous region of the magnet. Also, discrete spatial sampling in RFZ and CSI can radiate signal artefacts of around 25% into adjacent elements depending on the location and distribution of signal sources. Acquisition delays between excitation and detection and partial saturation are other major sources of systematic error. Saturation factors for metabolites are not easily obtainable on localized volumes during clinical exams on an individual basis, but may be expediently obtained as larger-volume tissue-averages. Better documentation of saturation effects, acquisition delays and localized volume sizes is needed to compare and validate clinical results and performance.