D.V. Volkov - Student, Faculty of Physics, Lomonosov Moscow State University E-mail: mdanf1@gmail.com M.V. Gulyaev - Ph. D. (Phys.-Math.), Research Scientist, Faculty of Fundamental Medicine, Lomonosov Moscow State University E-mail: mihon-epsilon@ya.ru O.S. Pavlova - Post-graduate Student, Faculty of Physics, Lomonosov Moscow State University E-mail: oleuf@mail.ru N.V. Anisimov - Dr. Sci. (Phys.-Math.), Senior Research Scientist, Faculty of Fundamental Medicine, Lomonosov Moscow State University E-mail: anisimovnv@mail.ru Y.A. Pirogov - Dr. Sci. (Phys.-Math.), Professor, Faculty of Physics, Lomonosov Moscow State University E-mail: yupi937@gmail.com

The problems of ¹⁹F MRI studies of perfluorocarbon (PFC) emulsions at magnetic fields 0.5T and 7T are consi-dered. Interest in these compounds is caused by their application in biology and medicine as drugs, blood substitutes, MRI contrast agents. Two factors such as a large gyromagnetic ratio, which is smaller than for proton by only 6 per cent, and 100 % natural content are favor for ¹⁹F NMR signals registration. For MRI it is important that the living tissues do not contain fluorine, so there is no background signal, which is typical for proton MRI. Specificity of ¹⁹F NMR and MRI studies is that the width of ¹⁹F NMR spectra of PFC compounds can reach several hundred ppm, in other words, these spectra are much wider than the proton ones. This make difficulties for the implementation of pulse sequences (PS), providing broadband spin system excitation. Therefore, PS optimization for MRI should be done within existing resources of hardware - the strength of gradient system, which determines the minimum value of delay in PS; the transmitter power, which determines the minimum duration of RF pulses; the speed of analog-to-digital converter, which determines the speed of data acquisition. The problem is that the requirements to MR equipment increases at higher fields. In this paper, it is noted that technical means of medical model 0.5T MR scanner are sufficient for the realization of a wide variety methods for ¹⁹F MRI visualization of PFC emulsions. At the same time, equipment of the research 7T MR scanner model allows implementation only those methods in which RF excitation is applied only for a part of ¹⁹F NMR spectrum. In this case, slice-selective (2D) spin system excitation is ineffective, and it is offered a slice selection that is performed in forms of data selection from 3D scanning. Since ¹⁹F NMR signal consists of a large number of spec-tral lines, at the time of registration of the gradient echo the signal amplitude would greatly decrease due to interference. Therefore, the most effective method of registration of the 19F signals is a spin echo (SE) pulse sequence, which avoids interference effects by reversing the motion of spins. If the transverse relaxation time T2 is sufficiently large, it is possible to speed up the scanning process N times in N-fold stimulation of SE signals in combination with variation of the phase-coding gradient amplitude, which is known as RARE. It is shown that the emulsion of Perftoranum®, which T2 is about few tens of milliseconds, good images are obtained by scanning 3D-RARE method with N = 8. The latter value was chosen experimentally for the minimum achievable spacing between refocusing pulses TE, which amounted to 5.5 ms. In this article the application of this technique for in vivo studies on small animals is described.

 

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