[Localized nuclear magnetic resonance spectroscopy in vivo. Physical principles and quantitative problems]

Recent technological developments extended the use of nuclear magnetic resonance to clinical imaging (MRI), as well as to in vivo metabolic studies on tissues and organs by means of localized spectroscopy (MRS). The quantitative determination of absolute metabolic concentrations by in vivo MRS still represents a challenge for technological efforts and biochemical investigations. In fact, the different methodologies today available for spectral acquisition from localized regions within a body organ or pathological lesion should ideally lead to a complete suppression of signals from surrounding tissues, without any appreciable signal loss from the volume of interest. Accurate assessment of deviations of the real from the ideal case represents an essential requirement for either relative or absolute quantification of in vivo localized MR spectra. A review will be presented of the most utilized MRS methodologies, together with a discussion on their potentialities and limitations. Techniques have been classified according to the use of either r.f. (B1) and/or static magnetic field (B0) gradients. The last session will be devoted to the presentation of some results obtained in our laboratory on the use of a particular test-object (constructed at the Deft Institute of Technology) for assessing signal localization efficiency provided by a surface coil in a small scale MRS/MRI equipment (for in vivo biochemical studies on small animals). The surface coil was used in conjunction with a number of sequence (t1-90 degrees-acq; 1-D "chemical shift imaging"; "depth") of either square or adiabatic r.f. pulses. The results allow the selection, on a quantitative basis, of the most appropriate pulse sequence(s) to be used for metabolic studies on superficial experimental tumours (implanted s.c. in small animals), according to the mean area and thickness of the neoplastic lesion. The latter parameters can be non-invasively assessed by previous MRI analysis of the tumour. This study was carried out in the frame of the EC COMAC-BME Concerted Research Project on Tissue Characterization by MRS and MRI.