Keywords: magnetic resonance imaging, spin echo double resonance, indirect imaging

Images in vivo of the spatial distribution of phosphorus metabolites are of significant interest for biology and medicine. One of the most informative methods of spatial imaging is the magnetic resonance imaging (MRI). However, the obtaining of MRI for phosphorus nuclei is prevented by the low intensity of nuclear magnetic resonance (NMR) signals from ³¹P nuclei, namely, in 15 times less than from ¹H nuclei (protons) at equal concentrations. Therefore, phosphorus MRI is practically not realized, although NMR spectroscopy ³¹Р is quite developed. In the work presented a method of the spatial distribution mapping of phosphorus-containing compounds using the proton NMR with the additional irradiation at the frequency of nuclear magnetic resonance of ³¹Р nuclei is described. The technique is based on the spin echo double resonance and can be used to obtain MR images for nuclei other than protons, connected with them by the indirect spin-spin interaction with a constant J. To construct the ³¹Р image the following pulsed NMR experiment is performed: (i) the standard proton spin echo is obtained using a pulse sequence of 90˚-τ-180˚-τ-echo; (ii) the additional 180˚-pulse at the NMR frequency of ³¹Р nuclei, acting simultaneously with the 180˚-pulse at the proton frequency, is used. Under the condition τ = 1/(2J), the spin-echo from protons, interacted with ³¹Р, disappear. The difference of images obtained with and without the additional 180˚-pulse allows one to create the image of the distribution of ³¹Р nuclei interacted with protons.  

A model experiment was carried out with a trimethylphosphate sample on the homebuilt laboratory NMR- scanner operated at the 7 mT magnetic field. The radiofrequency pulses at two independent frequencies were generated using a Midrange two-channel synthesizer based on the AD9833 direct digital synthesis. The control of the synthesizer, as well as the creation of pulse sequences, collection and processing of data, are carried out using a personal computer with the NI PCI-6229 multifunctional control and registration board. All control and processing programs are created in the environment LabView.

The study has shown that in the presence of indirect spin-spin interaction ¹H-³¹P the developed method allows one to obtain the mapping of the spatial distribution of phosphorus without direct registration of NMR signals from the ³¹P nuclei with sensitivity typical for proton images.