A.V. Protopopov – Post-graduate Student, Moscow Institute of Physics and Technology (State University) 

(Dolgoprudniy, Moscow region); Federal Scientific Clinical Center of Children's Hematology, Oncology and Immunology n.a. Dmitry Rogachev (Moscow) E-mail: proto.alex@hotmail.com 

M.V. Gulyaev – Ph.D. (Phys.-Math.), Senior Research Scientist, Faculty of Fundamental Medicine,  M.V. Lomonosov Moscow State University E-mail: mihon-epsilon@ya.ru

Yu.A. Pirogov – Dr.Sc. (Phys.-Math.), Professor, Faculty of Physics, M.V. Lomonosov Moscow State University;  National Research Nuclear University MEPhI, Engineering Physics Biomedical Institute E-mail: yupi937@gmail.com

Traditional methods of visualization of pathologies in magnetic resonance imaging are based on empirical methods of rendering image contrast, which do not allow to determine particular physical parameter that is responsible for the pathology. The physical parameters are proton density, spin-spin interaction, and local magnetic gradients caused by tissue structure. Application of algorithms, developed previously by the authors, each physical parameter may be selected from the gradient-recalled echo (GRE) signal. The aim of the present work was to study correspondence between physical parameters and known pathologies. The experiment was done at strong-field scanner Bruker BioSpec 70/30 USR (7T) on small animals in vivo.

Historically, a method that uses the principles of modern GRE sequence was first described in the year 1980 and was called "Spin warp NMR imaging" or "NMR imaging of spin distortions". This method has received the new name of the FLASH that is an abbreviation of "Fast Low-Angle Shot" or "quick small angle shot". The main difference of this sequence from other used at that time techniques was a quite new rephasing spins mechanism inside a voxel. Instead of using 1800 pulses, the sequence FLASH uses only apply gradients for rephasing spins. The GRE technique is sensitive to local magnetic gradients of tissues within a single voxel. This feature means that relaxation curve measured in GRE sequence contains more information about tissue structure than other thechniques, using standard spin echo sequences. In particular, the experimental study made on small animals in vivo and reported in the present publication showed that the method of mathematical analysis and special fitting routines open a possibility to determine volumetric properties and locate pathologies in the animal brain, as well as any other cavity filled with a liquid substance, the volume of which exceeds the dimensions of a voxel.

1. Edelstein W.A., Hutchison J.M., Johnson G., Redpath T. Spin warp NMR imaging and applications to human whole-body imaging // Phys. Med. Biol. 1980. V. 25. P. 751–756.
2. Haase A., Matthaei D., Hanicke W., Frahm J. Dynamic digital subtraction imaging using fast low-angle shot MR movie sequence // Radiology. 1986.  V. 160. P. 537–541.
3. Frahm J., Merboldt K.D., Hanicke W. Direct FLASH MR imaging of magnetic field inhomogeneities by gradient compensation // Magn. Reson. Med. 1988. V.6. P. 474–480.
4. Fernandez-Seara M.A., Wehrli F.W. Postprocessing technique to correct for background gradients in image-based R2* measurements // Magn. Reson. Med. 2000. V. 44. P. 358–366.
5. Dahnke H., Schaeffter T. Limits of detection of SPIO at 3.0T using T2* relaxometry // Magn. Reson. Med. 2005. V. 53. P. 1202–1206.
6. Hernando D., Vigen K.K., Shimakawa A., Reeder S.B. R*2 mapping in the presence of macroscopic B0 field variations // Magn. Reson. Med. 2012.  V. 68(3). P. 830–840.
7. Protopopov A. Relaxation model and mapping of magnetic field gradients in MRI // Applied Magnetic Resonance. 2017. V. 48(3). P. 255–274.
8. Protopopov A. Structural analysis of relaxation curves in MRI // Applied Magnetic Resonance. 2017. V. 48(8). P. 783–794.
9. Protopopov A. Izmerenie vremeni poperechnoj relaksacii T2 v MRT-posledovatel'nostyah gradientnogo ehkha // Biomedicinskaya radioehlektronika. 2018. № 4. S. 4–8.
10. Protopopov A. Physical parameterization in MRI // WSEAS Transactions on Biology and Biomedicine. 2018. V.15. P. 35–39.
11. Hahn E.L. Spin echoes // Physical Review. 1950. V. 80(4). P. 580–594.