RU2063702C1 - Method of magnetoresonance tomography and device for its accomplishment - Google Patents

Abstract

FIELD: medical equipment, in particular, nondestructive diagnosis. SUBSTANCE: the method of magnetoresonance tomography consists in the fact that the object to be examined is placed in a static magnetic field, having preliminarily injected substances containing electronic paramagnetic centers, the object is exposed to the effect of r.f. pulses, which excite nuclear resonance signals, and pulse gradients of the magnetic field in the X-, Y-, Z-axes; spin echo signals from the object nuclear spins are recorded; the image is reconstructed in the selected section, and visualization of the reconstructed image is accomplished; the object under examination is also subjected to the action of r.f. pulses of excitation of electronic states, varying the basic frequency of the r.f. pulse and identifying the pathologic nature, depending on the frequencies at which reaction of the object on the change of the image is registered; the mentioned action is accomplished just before the action of r.f. pulses; during the action of r.f. pulse the static magnetic field is decreased, and, according to variation of the magnitude of its level, variation of the image is registered for identification of the pathologic nature. The method is accomplished by means of a device using a cluster of solenoid coils, control unit of basic magnetic field coil current, magnetic field pulse gradients unit, whose output is connected to the cluster of solenoid coils, passive commutator, whose output via an amplifier and signal transducer is connected to the control computing unit, whose three outputs are connected to the visual representation unit, magnetic field pulse gradients shaping unit and shaping unit of r.f. pulses of spin echo signal excitation, respectively, and an electronic state exciting unit using a frequency synthesizer with digital control and r.f. pulse power amplifier, basic magnetic field level control module; the cluster of solenoid coils additionally uses an r.f. inductance coil for excitation of electronic states and an inductance coil for excitation of electronic states and an inductance coil for decreasing the static magnetic field. EFFECT: possibility of identification of pathologic tissues. 10 cl, 8 dwg

Description

 The proposed solution relates to reconstructive computed tomography and can be effectively used for medical diagnostics and non-destructive testing of dielectric materials.

 A known method of magnetic resonance imaging (MRI tomography), which consists in placing the test object in a constant magnetic field, exposing it to radio pulses and pulses of magnetic field gradients to excite nuclear magnetic resonance signals, digital recording of these signals, mathematical reconstruction of the image in the selected section with using computers and visualization of the obtained image (see, for example, Soroka P. M. "Introscopy based on NMR", Moscow, Energoatomizdat, 1986. Zhuo 3.Kh. et al. "Tomography based on f NMR with Fourier transform ", TIIER, 1982, v.70, p. 10, see also: V. Utkin," Method of tomography based on nuclear magnetic resonance ". USSR author's certificate N 1543317, CL G 01 N 24 / 08, priority from 06.23.87).

In the most common modification of the known method in practice, the following pulses are used to excite signals from the object under study:
90-degree radio frequency pulse with the orientation of the vector perpendicular to the direction of the magnetic field in the presence of a selective gradient of the magnetic field along the Z axis,
phase-coding magnetic field gradient along the Y axis (cyclically changing in amplitude after the start of the next 90-degree radio pulse),
180 degree pulse or use alternating gradients to form a spin echo signal,
frequency-coding magnetic field gradient along the X axis with simultaneous digital registration of the echo signal.

 The indicated sequence forms a single structural impulse, these impulses are given periodically, i.e. a stream (sequence) of pulses is formed, each of which has the structure indicated above. The object’s responses to the influence of each structural impulse (echo signals) are accumulated line by line in the computer memory, forming a digital working matrix, then it is converted into an image matrix by the two-dimensional Fourier transform (see the technical description "MAGNAVIEW MR-Imaging system", Instrumentarium company, Finland, June 1989 year).

 The described method is closest in its technical essence to the proposed technical solution and therefore is selected as a prototype.

 The disadvantage of the prototype method in relation to medical diagnostics is that it does not allow for confident enough to distinguish between normal and pathological tissues, which significantly reduces diagnostic capabilities, this disadvantage is due to the fact that normal and pathological tissues, although they have different chemical composition and different composition of paramagnetic impurities (free radical centers, metal enzymes, etc.), however, these tissues are very similar precisely in those physical characteristics that are used in It is the prototype. The tomogram of the prototype shows the internal structure of the studied objects, but the image reflects the internal difference in the studied medium only in proton density and relaxation times. Pathological changes in tissues (especially at the initial stage), as a rule, take place at the chemical level without changing the proton density and relaxation times. In the analogue method, it is not possible to selectively select those areas in the biological medium that are similar in proton density and relaxation times, but are saturated with various free radicals and metal enzymes.

 Therefore, the known method does not allow to distinguish between sites of objects having a small difference in proton density and a small difference in relaxation times, in which, however, the chemical composition and composition of paramagnetic impurities are different. It is known that the same relaxation times can be provided by different paramagnetic impurities; such shortcomings significantly limit the diagnostic possibilities in medical practice.

 The technical effect achieved in the proposed method consists in the ability to isolate and distinguish parts of tissues that are normal and pathological. In addition, the proposed method in some cases can either significantly reduce the accumulation time of the tomogram, or use magnetic fields of lower intensity and save energy.

 The indicated technical effect is achieved by the fact that in the method of MR imaging, which consists in placing an object in a constant magnetic field, exposing the object to radio pulses of excitation of nuclear magnetic resonance (NMR) signals and pulsed magnetic field gradients along three mutually perpendicular axes, recording spin echo signals , reconstructing the image in the selected section and visualizing the reconstructed image, additionally affect the object with a high-frequency excitation pulse x states. This effect is performed immediately before exposure to a radio pulse exciting the NMR signal. In addition, it is proposed in the research process to change the frequency of the newly introduced pulse and to identify the nature of the pathology depending on what frequency of the excitation pulse of one or another electronic state is detected by the presence of the reaction of the object. Further, a variant of the method is proposed in which, during the action of the newly introduced high-frequency pulse, a constant magnetic field pulse is additionally introduced, which reduces the intensity of the total constant magnetic field by this time.

 And finally, a variant is proposed in which substances containing electronic paramagnetic centers are injected into the studied object (patient’s body) before the MR tomogram is accumulated, which are affected by an excitation pulse of electronic states in order to change the local proton polarization.

 Comparison of the newly proposed method of MRI tomography with known analogues and prototype allows us to state that the distinguishing features introduced (the presence of additional high-frequency effects on electronic states and its placement in time immediately before the excitation of the spin echo) were not previously known and, thus, the proposed MR method tomography meets the criterion of "novelty."

 The introduced additional effect provides resonant excitation of electronic spin states of only certain molecules of metal enzymes, free radical centers, or other paramagnetic centers required for diagnostic purposes.

 From the foregoing, it follows that the proposed method of MRI tomography, based on a change in the polarization of nuclei due to the excitation of electronic states, has a different intensity distribution of the images obtained than is typical for the prototype and analogues. Therefore, it provides an opportunity to receive fundamentally new information about the vital activity of living objects.

 The introduction of the above distinguishing features allows us to achieve a qualitatively new technical effect, which consists in the possibility of identifying areas of tissue that are in pathology, despite the fact that these tissues are similar to similar characteristics of normal tissues in terms of proton density and a number of other physical characteristics. Thus, the proposed method meets the criterion of "inventive step".

 A device for MR imaging is known, comprising a block of electromagnetic coils having a main magnetic field coil, a radio frequency coil and coils for linear and non-linear magnetic field gradients along the X, Y, Z axes, a current control unit of the main magnetic field coil associated with the electromagnetic coil block , a control unit for the gradients of the magnetic field, the output of which is connected to the block of electromagnetic coils, a passive switch connected to a radio frequency coil, the output of which is connected in series with the amplifier an amplifier, an analog-to-digital converter, a buffer memory unit, and a computing unit associated with a visual display unit, a trigger pulse generating unit that is connected to a spin echo excitation signal generating unit connected via a power amplifier unit with a passive switch, and to a pulse gradient generating unit including a multi-channel digital-to-analog converter connected to a computing unit, the output of which is connected to the input of the gradient control unit by the magnetic-field (see Fig. technical description "MAGNAVIEW MR-Imaging system", Instrumentarium company, Finland, June 1989).

 A disadvantage of the known device is that it does not provide the ability to confidently distinguish between areas that are normal and pathological. This disadvantage is due to the fact that the device implements the prototype method mentioned above, while normal and pathological tissues differ in many cases in chemical composition, i.e. by the concentration and presence of certain electronic paramagnetic centers.

 The technical effect achieved by the proposed device that implements the above method is the ability to identify pathological tissues.

 The proposed device contains a block of electromagnetic coils, having a coil of the main magnetic field, a radio frequency coil and coils for linear and nonlinear gradients of the magnetic field along the axes X, Y, Z. Further, it contains a current control unit of the coil of the main magnetic field associated with the block of electromagnetic coils, magnetic field gradient control unit, the output of which is connected to a block of electromagnetic coils, a passive switch, an amplifier, an analog-to-digital converter, a buffer unit are connected in series molecular memory and computing unit. The computing unit is connected to the visual display unit, the unit for generating intervals of the excitation of the echo signals and the unit for forming the gradients of the magnetic field. The block for generating intervals of excitation of echo signals is connected to the input of the circuit from a series-connected programmable pulse generator, transmitter control unit, and transmitter. The transmitter output is connected to a passive switch. The unit for forming magnetic field gradients through a multi-channel digital-to-analog converter and amplifier unit is connected to the input of the control unit for magnetic field gradients.

 In addition to the listed features common with the prototype, an electronic state excitation unit is introduced into the proposed device, the input of which is connected to the output of the computing unit, and the output is connected to the block of electromagnetic coils.

 Comparison with the prototype and with other known devices shows that the proposed device is characterized by the presence of a block of excitation of electronic states and its connections with other blocks. Thus, the proposed device meets the criterion of "novelty."

 The introduction of the above distinguishing features allows you to implement additional selective effects (see the method described above) and thereby achieve the desired technical effect of the ability to identify pathological tissues.

 In addition, options for constructing device blocks are offered.

 The electronic state excitation unit contains a high-frequency pulse generator module containing a frequency synthesizer with digital control of the set frequencies and a radio pulse power amplifier, the control input of which is connected to the output of the synthesizer, while the control input of the synthesizer is a common input of the electronic state excitation block.

 In the General case, the block of excitation of electronic states may additionally contain a module of current pulses, the input of which is connected to the output of the computing unit, and the output is connected to the block of electromagnetic coils. The proposed solution contains a digital-to-analog converter and a current pulse generator, the control input of which is connected to the output of the digital-to-analog converter, while the input of the digital-to-analog converter is a common input of the electronic state excitation unit.

 The block of electromagnetic coils additionally contains a high-frequency inductance for exciting electronic states, and also (in the general case) additionally contains an inductive winding to reduce the constant magnetic field.

 The essence of the proposal is illustrated by drawings.

 In FIG. Figures 1–7 show plots of pulses and an echo signal supplied to an object, which are presented in the required sequence in time (t-time) and are cyclically repeated when the digital matrix is accumulated.

 In FIG. Figure 1 shows the level of a constant magnetic field supplied to the object, which in the general case, due to the supply of a current pulse, can be reduced by several times by the period of supply of an electronic state excitation pulse.

 In FIG. Figure 2 shows a plot of a high-frequency pulse J (rf) for selective action on electronic states.

 In FIG. Figure 3 is a plot of a 90-degree radio frequency pulse J (rf) for rotating the nuclear magnetization vector.

 In FIG. Figure 4 shows a plot of the gradient of the magnetic field Gx along the X axis, this gradient provides the frequency coding of the components of the studied region (index i), necessary to restore the image from one of the coordinates.

 In FIG. Figure 5 shows a plot of the magnetic field gradient Gyj along the Y axis; the magnitude of this gradient cyclically changes after the next radio pulse is applied. Gradients along this axis provide phase coding (index j), which is necessary for reconstructing the image from the second coordinate.

 In FIG. Figure 6 shows the plot of the magnetic field gradient Gz along the Z axis, the gradient along this axis is used to highlight the layer under study in the selected section (fixing the third coordinate).

 In FIG. 7 shows the NMR plot of the signal Sj (t), which is fed to an amplitude-to-digital converter for digitally recording the next row of the original image matrix.

 In FIG. 8 presents a structural diagram of the proposed device that implements the proposed method.

The device comprises a block of electromagnetic coils 1 (Fig. 8), including a main magnet coil, a radio frequency coil and coils for creating linear and nonlinear magnetic field gradients along the X, Y, Z axes, designed to be placed in its inner space of the object under study; block 2 control the current of the coil of the main magnet, designed to maintain the intensity of the constant magnetic field of a given level; block 3 control the gradients of the magnetic field along the axes X, Y and Z; passive switch 4; an amplifier and a NMR signal converter 5, a control electronic computing unit (PC) 6, a visual display unit 7; block 8 for generating radio pulses of excitation of spin echo signals and a block for exciting electronic states 9
In FIG. Figure 8 shows a variant of the circuit in the general case, where block 9 consists of four modules: a frequency synthesizer module 10 with digital control from a computing unit, the output of this module is connected to the input of the high-frequency pulse power amplifier module 11, the output of which is connected to the block of electromagnetic coils. The computing unit also controls the module for controlling the level of the main magnetic field, containing a digital-to-analog converter 12, generating a control pulse to reduce the magnetic field and module 13. The output of module 12 is connected to the input of module 13, which ensures coordination of the magnetic field reduction pulse. This pulse from the output of module 13 is supplied either to the current control unit of the coil of the main magnet, or directly to the block of electromagnetic coils for additional inductance to reduce the magnetic field.

 In addition, in the claimed technical solution, the block of electromagnetic coils 1 further comprises an inductance for irradiating the test object with an electron resonance frequency and, in general, an inductive device for decreasing the magnetic field.

 The claimed technical solution allows the development of new ultralow-field MRI scanners in which modules 12 and 13 may be absent.

 The functioning of the method is as follows.

 The studied object is placed in a constant magnetic field B (Fig. 1).

 The action of the magnetic field on the object leads to the fact that the magnetic moments of the nuclei are mainly partially oriented in the direction of the magnetic field and nuclear polarization occurs. It is significant that the degree of orientation of the nuclear spins is small, on the order of thousandths of a percent, and accordingly the nuclear polarization is small, which also leads to the weakness of the signals of nuclear magnetic resonance and leads to the need for long-term accumulation of the tomogram.

 Next, an object is excited by an excitation pulse of electronic states (Fig. 2), which changes the local nuclear polarization due to the following.

Spins of electrons are connected by interaction with the spins of nuclei. This interaction during the stimulation of electronic transitions leads to an increase (change) in the polarization of nuclei. If the momentum of excitation of electronic states is intense, then the effect of increasing nuclear polarization due to spin-spin electron-nuclear interaction extends due to the phenomenon of spin diffusion to the region surrounding this electronic state. Moreover, if the main mechanism of nuclear relaxation is relaxation due to the spin-spin electron-nuclear interaction, then, under the selective action of this electronic state, the local polarization of protons increases by a factor of P:
P = 1+ (1/2) [μ (elek) / μ (prot)] ≃ -328
where μ (elec) is the magnetic moment of the electron, μ (prot) is the magnetic moment of the proton (see, for example, R. Ernst, D. Bodenhausen, A. Vokaun, “NMR in one and two dimensions”, ed. Mir, 1990 , p. 230).

 Practically in the framework of spectroscopy, it was found that when pumping electronic states of model solutions containing, for example, protons of organic molecules and unpaired electrons of paramagnetic centers (such as gadalinium), nuclear polarization can be increased by a factor of two (see, for example, "Magnetic resonance." Collection of reports of the XII All-Union School-Symposium on Magnetic Resonance, Perm, 1991, p. 94 97). Moreover, it should be noted that for a significant diagnostic effect (for the case of revealing, for example, pathological changes), it is enough to change the nuclear magnetization by only 20% since such an effect is already enough to change the brightness on the tomogram. For this reason, the requirements for the magnitude of the intensity of the excitation pulse of electronic states are not high.

 In the case of a living organism, we have a complex system. On the one hand, tissues have a high concentration of water, the protons of which form a system of nuclei convenient for tomographic use. On the other hand, tissues contain free radical centers, metal enzymes and other inclusions with unpaired electrons, the spins of which can be used to polarize protons.

 Various effects on living tissue or part of the tissue (mechanical, chemical, radiation, etc.) lead to a change in the intensity of biochemical processes and, accordingly, to a change in the average static concentration of active free radical centers and metal enzymes. As an example, we note the works of D. A. Svistunenko and S. V. Voloshchenko, L. M. Bader (Magnetic Resonance. Coll. Reports, Perm, 1991, p. 103 105), in which they were recorded by electron paramagnetic spectroscopy resonance changes and an increase in the concentration of new paramagnetic centers during pathology and radiation exposure.

 Consequently, selective electronic pumping of various electronic states will lead to different polarization of protons in different regions.

Further, a 90-degree impulse (Fig. 3) and alternating impulse gradients along the axes X (Fig. 4), Y (Fig. 5), Z (Fig. 6) are applied to the object. This action generates a spin echo signal Sj (t), whose components are frequency and phase encoded. The spin echo signal is amplified (Fig. 7) and digitally recorded at time intervals ti. The cycle (Fig. 2-7) is repeated with a change in the gradient of Gyj and the working digital matrix is accumulated line by line:
Sij sj (ti) (2)
The accumulated digital information is further processed using a double Fourier transform:
Sij --- Aij (3)
In this case, we obtain a matrix Aij whose elements are proportional to the local nuclear polarization Pij. It is the local nuclear polarization that determines the contrast of the tomographic image. Therefore, the implementation of the proposed technical solution is carried out by the fact that periodically, before each excitation of the nuclear spin system, a selective resonant effect on the electronic states is carried out (Fig. 2), because of which around the selected electron centers Pij local polarization of the nuclei increases.

 The selectivity of the effect is ensured by the selection of the frequency of the supplied high-frequency pulses, which corresponds to the energy of quantum transitions associated with a change in the orientation of the electron spins relative to the direction of the used magnetic field for certain molecular structures with paramagnetic centers that are of interest for diagnostics.

 As a result, due to a local change in proton polarization, the resulting image has a qualitative difference from the image characteristic of analogues and prototype.

 The operation of the device is as follows.

 The investigated object is placed in the geometric center of the system of electromagnetic coils 1 (see Fig. 8). The stability of the magnetic field is maintained using the current control unit 2.

 The operation of the device is controlled by a computing unit 8, which, in particular, controls the formation of an excitation pulse of electronic states. This pulse is generated in block 11 and from its output is fed to a high-frequency winding located around the object under study in block 1.

 Then, an NMR signal excitation pulse is supplied to the object, this pulse is generated in block 10 with control from computing unit 8 and passes through a passive switch 4 to a block of electromagnetic coils.

 To ensure spatial resolution, the NMR signals are encoded by applying pulsed gradients Gx, Gy, Gz, which are formed by current pulses from block 3 with control from computing block 8.

 NMR signals are recorded using a resonant inductance in block 1 and, through a passive switch 4, are fed to an amplifier-converter 5 and then to a computer. These pulses are fed periodically with a change in the coding gradient Gyj, while the digital matrix is accumulated line by line.

 After the digital matrix is accumulated, the computing unit 6 reconstructs the image using the double Fourier transform and transfers it to the visual display unit 7. Due to the use of excitation pulses of electronic states in this technical solution, the resulting image has a different brightness distribution than the prototype.

Selective excitation of electronic states in the general case is carried out both by means of the formation of a high-frequency pulse (modules 10 and 11), and by means of the formation of a magnetic field reduction pulse (modules 12 and 13). Modules 12 and 13 are used to technically simplify the implementation of the proposed technical solution, since the frequency of electronic spin states f (EPR) is higher than the proton resonance frequency f (NMR) by about K times
K = f (EPR) / f (NMR) = μ (elec) / μ (prot) ≃ 658 (3) (3)
If, for example, a 0.04 T magnetic field is used, then the proton resonance frequency f (NMR) is 1.6 MHz, then f (EPR) is 1000 MHz. Such frequencies (of the order of 1000 MHz) are difficult to create with high intensity in biological tissues. Therefore, an impulse is supplied that reduces the magnitude of the main magnetic field. With a decrease in the field, for example, 2.5 times, respectively, the paramagnetic resonance frequency will be reduced to f (EPR) ≈400 MHz, which is more convenient for the practical pumping of electronic states.

 The proposed technical solution provides an option in which, before the accumulation of the tomogram, substances containing paramagnetic electronic centers are introduced into the studied object (patient’s body). For example, it can be substances whose molecules are organic compounds with chromium.

 Chromium atoms contain unpaired electrons and increase the relaxation rate of the surrounding protons of water molecules in the body. The use of such compounds in the proposed method is especially promising, since the width of the EPR signals of chromium is small and, therefore, selective resonant pumping of the electronic states of chromium atoms can be ensured by a low power level. As a result, the use of contrast agents introduced into the body is of particular importance in the claimed method for the development of diagnostic methods. The use of contrast agents can significantly increase nuclear polarization and, accordingly, either use lower magnetic fields (in order to save energy consumption), or significantly reduce the accumulation time of the tomogram (increase productivity).

 Thus, the claimed method is the main one for the development of new diagnostic methods in wide medical practice and may find technical application in the future as an effective method of non-destructive testing of dielectric materials in the development of new technologies. YYY2 YYY4 YYY6

Claims (10)

 1. The method of magnetic resonance imaging, which consists in the fact that the test object is placed in a constant magnetic field, act on the object with radio pulses that excite nuclear resonance signals and pulsed magnetic field gradients along the X, Y, Z axes, record spin echo signals from nuclear spins of the object, reconstruct the image in the selected section and visualize the reconstructed image, characterized in that the object under study is additionally exposed to a high-frequency pulse waiting for electronic states, moreover, the indicated action is performed immediately before the action of radio pulses.  2. The method according to claim 1, characterized in that during the study, the filling frequency of the high-frequency pulse is changed and the nature of the pathology is identified depending on the frequencies at which the presence of the reaction of the object by image change is recorded.  3. The method according to claims 1 and 2, characterized in that during the action of the high-frequency pulse, the constant magnetic field is reduced.  4. The method according to claim 3, characterized in that the magnitude of the decrease in the constant magnetic field is changed and the magnitude of this level is fixed changes in the image to identify pathology.  5. The method according to claims 1 to 4, characterized in that previously, substances containing electronic paramagnetic centers are introduced into the test object.  6. Device for magnetic resonance imaging, containing a block of electromagnetic coils having a main magnetic field coils, a radio frequency coil and coils for forming magnetic field gradients along the X, Y, Z axes, a current magnetic field coil current control unit associated with the electromagnetic coil unit , a unit for generating pulsed gradients of a magnetic field, the output of which is connected to a block of electromagnetic coils, a passive switch connected to a radio frequency coil of a block of electronic coils, an output which is connected in series through an amplifier and a signal converter with a control electronic-computing unit, the three outputs of which are connected respectively to the visual display unit, the pulsed magnetic field gradient generation unit and the radio pulse generation unit for generating spin echo signals, the output of which, in turn, is associated with a passive switch, characterized in that it additionally introduced an electronic state excitation unit, the output of which is connected to the fourth output ulation electron-computing unit and the output unit with the electromagnetic coils.  7. The device according to claim 6, characterized in that the electronic state excitation unit comprises a high-frequency pulse generator module, including a frequency synthesizer with digital control of preset frequencies and a radio pulse power amplifier, the control input of which is connected to the output of the frequency synthesizer, while the control input of the frequency synthesizer connected to the fourth output of the control electronic computing device, and the output of the power amplifier with a block of electromagnetic coils.  8. The device according to claim 6, characterized in that the main magnetic field level control module is additionally introduced into the electronic state excitation unit, the input of which is connected to the output of the control electronic computing device, and the output is with the current control unit of the main magnetic field coil.  9. The device according to claim 6, characterized in that the block of electromagnetic coils further comprises a high-frequency inductance for exciting electronic states.  10. The device according to claim 6 and 9, characterized in that the block of electromagnetic coils further comprises an inductive winding to reduce the constant magnetic field during the excitation of electronic states, the input of which is connected to the output of the block of excitation of electronic states.

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