About Magnets and Superconductors of Mr Scanners

The topic of this paper are parts of modern MR devices, in which the magnet windings are located. MR scanner magnets are made of four types of electromagnetic windings: 1) The main magnet, made of superconducting material, creates a variable magnetic eld; 2) X coil, made of a resistive material, creates a variable magnetic eld, horizontally, from left to right, across scanning tube; 3) Y coil creates varaing magnetic eld, vertically, from botom to top; 4) Z coil creates varaing magnetic eld, longitudinally, from head to toe, within scanning tube.Superconductors, which create the main magnetic eld, should be cooled by liquid helium and liquid nitrogen. Main magnets made of superconductors should use cryostat, with cooling vessels with liquid helium and liquid nitrogen, thermal insulation and other protective elements of magnet system. The types of magnets that exist in the basic congurations of MR scanners are analyzed. Scanners in the form of a closed cylindrical cavity create their own, magnetic, elds by passing current through the solenoid, which is held at the temperature of the superconductor. The superconductors used exclusively are: niobium-titanium (NbTi), niobium-tin (Nb 3 Sn), vanadium-gallium (V 3 Ga) and magnesium-diboride (MgB 2 ). Only magnesium diboride is a high temperature superconductor, with a critical temperature Tc = 39 0 K. The three remaining superconductors are low temperature. New high-temperature superconductors have been discovered, as well as room-temperature superconductors. Newly discovered superconducting materials are not used in MR scanners. The magnet structure of the MR scanner is complex. The resonant frequency changes at each point of the eld in a controlled manner. The windings of the main magnet made of superconducting material in the form of microsial bers are built into the copper core. The nonlinear gradient eld is created by windings of conductive material. It is added to the main magnetic eld. Thus, the resulting magnetic eld is obtained.

elements of magnet system. The types of magnets that exist in the basic con gurations of MR scanners are analyzed. Scanners in the form of a closed cylindrical cavity create their own, magnetic, elds by passing current through the solenoid, which is held at the temperature of the superconductor. The superconductors used exclusively are: niobium-titanium (NbTi), niobium-tin (Nb 3 Sn), vanadium-gallium (V 3 Ga) and magnesium-diboride (MgB 2 ). Only magnesium diboride is a high temperature superconductor, with a critical temperature Tc = 39 0 K. The three remaining superconductors are low temperature. New high-temperature superconductors have been discovered, as well as room-temperature superconductors.
Newly discovered superconducting materials are not used in MR scanners. The magnet structure of the MR scanner is complex. The resonant frequency changes at each point of the eld in a controlled manner. The windings of the main magnet made of superconducting material in the form of microsial bers are built into the copper core. The nonlinear gradient eld is created by windings of conductive material. It is added to the main magnetic eld. Thus, the resulting magnetic eld is obtained.

Background
Paul Laterbur and Peter Mens eld received the Nobel Prize in Medicine in 2003, "for discoveries related to the visual representation of magnetic resonance imaging", thirty years after the publication of the results of Paul Laterbur's experiment. Graphical representation of these resultes, published on March 16, 1973, is shown on the next Figures. What is the structure and working principle of magnets used in modern MR scanners? It is an interesting and signi cant question, which deserves a comprehensive account.
MR scanner magnets are made of main magnet windings, gradient windings and RF windings. The windings for the main magnets are made of superconducting materials and the windings for radients and RF windings are made of resistant materials. The eld created by the main magnet is necessary to bring the system of nuclear spins, on which it acts, to a state in which magnetic resonance is posible. The induction strength of the main magnetic eld is the most important quantity for obtaining and for the strength of the magnetic resonance signal. Almost all modern MR scanners use a superconductor made of niobium-titanium (N i T i ), which becomes superconducting below 9,4 0 K. niobium-tin (N b3 S n ), vanadiumgalium (V 3 G a ) and magnesium-diboride (M g B 2 ) are also used. Only M g B 2 is a high temperature superconductor with critical temperature T c =39 0 K. N b3 S n is low temperature with T c =18,3 0 C and V 3 G a low temperature with T c =14,2 0 C. Gradient system produce calibrated distorsion of main magnetic eld in x-, y-or z-direction. RF coils are responsible for perturbing the nuclear spin system, while Patient coils are responsible for MR signal detection. New high temperature superconducting materials have been discovered as well as room temperature superconductors, (Tl 5 P b2 ) B a2 M g2 C u9 O 18+ , T c =28 0 C(310 0 K/55 0 K) and (Tl 5 P b2 ) B u2 M g2,5 C u8,5 O 17+, T c =30 0 C(303 0 K/58 0 F) are given as an example. The hypotesis that main magnets could be made of high temperature superconductors and room temperature superconductors has not been proven. On the contrary, it has been shown that this is, for now, imposible becouse there are many di culties in the application of newly discovered superconductors (instability in the magnetic eld, low current density, the need to make high quality and extremely expensive materials…). All practical solutions must be cooled at this time. 2.2. Superconducting magnet: an electromagnet made from superconducting windings. The windings must be cooled, during operation, below their critical temperature, at which they pass from the basic state to the superconductor state. A cryiostat is a system for maintaining a very low temperature.It is thermally insulated from the environment. A typical cryostat, used in MR scanners, consists of three vessels, located one inside the other. The outher vessel is lled with a vacuum. It acts as a thermal insulator.
Beneth this vessel is a vessel lled with liquid nitrogen. It serves as a protection, insulation, middle shield, which prevents the action of heath from the outher vessel, on the inner vessel. The inner vessel contains cryogen (cooler-liquid helium).
Radiofrequency (RF) Body Coil (transmits B 1 eld). Patient Coils (primarily to detect MR signal, some are transmit/receive). Complete gradient system is consisted of coils mounted along the inner bore of the system. Produces calibrated distorsion of main magnetic eld in x-, y-or z-direction. Radio frequency (RF) Coils. 5.2. MR scanner magnets are a set of three components: main magnet, gradient magnets (x, y, z) and RF magnets. Together, they create the preconditions for magnetic resonance imaging to occur. In this set of magnets, the main magnet stands out in size and weight, which is made of superconductors, although the main magnet can also be made of permanent magnets and resistive conductors. The other two components of the MR magnet system are made of resistive conductors. 5.3. In order to achieve the superconductivity of the material, which creates the main magnetic eld, a crystal is used, with cooling vessels for liquid helium and liquid nitrogen, thermal insulation and other elements to protect the main magnet system. These elements determine the dimensions and size of the device. Initially, nitrogen should be replenished weekly and helium monthly. This was, gradually, perfected, so that liquid helium had to be replenished every 2-3 years. There are data that, lately, zero temperature cooling systems (ZBO) have become the standard.
In the case of a superconducting magnet, the power supply is connected on both sides of the coil segment. The current through the coil increases gradually, over several hours, until the desired eld is reached. The current continues to ow in a closed loop, without a signi cant drop. The resulting property is that the magnetic eld is always present. The construction of superconducting magnets is considered to be extremely expensive, and cryogenic helium is expensive and di cult to maintain. Nevertheless, today they are the most common type of magnet found in MRI scanners. It is estimated that investing in the production of new usable superconductors would be a process that does not pay off.
5.4. The microsial bers of one of the superconductors, NiTi, Nb 3 Sn, Va 3 Ga or MgB 2 , are inserted into the copper conductors. Copper acts as an insulator at low temperatures, relative to the zero resistance of the microsial ber alloy. Supports and protects alloy windings from damage, provides mechanical strength, prevents deformations and vibrations. It takes over the conduction of electricity, if, due to a fault, the superconducting mode is lost. (Quench) 5.5. A linear gradient is added to the main magnetic eld, a balanced disturbance of the fundamental eld along the axis of the magnet (x-side-side, y-front-back, z-head-heel). The cross section of all three axes is the isocenter of the magnet. In it, the basic magnetic induction has, always, the same value.
The construction of the z-gradient is usually based on circular coils, while the transverse(x-and y-) gradients typically have a saddle winding con guration. More precisely, the basic design of the z-direction gradient is Helmholtz's pair of coils: two loops with currents owing in opposite directions.
Helmholtz coils produce a gradually changing eld, which is zero in the magnetic isocenter, but increases linearly outward, in both directions + z and -z. When this is added to the constant eld B 0 , the result is a gradual increase in the gradient along the z-axis.
With the help of gradient windings, the value of the strength of the magnetic induction at each point of the three-dimensional space of the magnet is changed in a controlled manner. The resonant (Larmor) frequency is proportional to the strength of the magnetic induction. So, it has been achieved that the resonant frequency changes, controlled in every point of three-dimensional space. This also means that the magnetic resonance signal is different at each point. The signal strength is proportional to the spin concentration at the observed point. By measuring the magnetic resonance signal, the concentration of spins in various parts of the sample can be determined. 5.6. The time-varying radio frequency eld (RF), used in MR, is denoted by B 1 . It must be normal to the main magnetic eld B 0 . It is produced by special RF windings. RF windings can be transmitters, receivers, or both. If the oscillation B 1 has a value close to the precession of the nuclear spins (Larmor frequency), the energy is deposited in the spin system, causing a change in its net magnetization. B 1 is only switched on for short periods of time (several milliseconds), called "RF pulses". By adjusting the magnitude or duration of these B 1 pulses, the nuclear spin system can rotate at variable angles of rotation, such as 90 0 and 180 0 . 5.7. A superconductor, unlike a conductor, conducts electricity inde nitely, without energy losses. This is an important characteristic of these materials and a challenge for their use. Professor Allen D. Elster states that there is, in the experimental phase, re-production of MR scanners using "high temperature" superconductors. An example is magnesium-diboride (MgB2), with a critical temperature Tc = 39 0 K and others. He envisions the use of these superconductors, in the future, to build MR scanners. This was an incentive for digression from the main topic of this paper and a review of the current state and chronology of superconductors (Fig. 12). It can be seen that a signi cant number of high temperature superconductors and room temperature superconductors were discovered. Their application in practice is far away. The question is whether and when, these superconductors will be usable because of the properties they have. From the chronology of the discovery of superconductors, the following events can be singled out as markers: 1941. nobium-nitride, T c = 160 0 K, 1945. described perovskite, 1962. made the rst commercial superconductor NbTi. At that time the use of nobium-tin, Nb3Sn, began. In 1972, the BSC theory of superconductivity (Burden-Cooper-Schiffer) was published.
5.8. This study con rmed, as important, that room temperature superconductors, although discovered, are not in the eld of research of the world's leading laboratories, as possible materials for making magnets for MR scanners. Their indisputable discoveries are imposed, for research and application. The current situation is such that there is no known scienti c method by which they could be turned into useful, application, which should be eliminated. It is estimated that research to achieve thesis extremly expensive. Scienti c optimism gives hope that it is a matter of time before they become applicable, and that the practical bene ts of their discovery will be obtained.

Conclusion
This paper is important, primarily, because it explains the complex structure of the MR scanner magnet and the way in which the resonant frequency changes in a controlled manner at each point of the magnetic eld. The principle realization of the scanner, the cross section of the windings of a superconducting magnet with microsial bers, the principle of creating a nonlinear gradient and its role in creating the resultant magnetic eld, the chronology of the discoveries in the eld of high temperature superconductors and room temperature superconductors are presented.
The paper emphasized the importance of the discovery of high-temperature superconductors and room temperature superconductors, which were awarded Nobel Prizes, but also the lag in the application of these discoveries for practical purposes. Objective reasons for this are given.
The initial assumption that the main magnets of MR scanners are made of high-temperature superconductors and room temperature superconductors has not been proven. On the contrary, it has been shown that these superconductors cannot be used to make the main magnets of MR scanners. This remains as a possible idea for some future work, if and when, the newly discovered superconductors are brought to a level that is practically applicable. There are no new proposals for the main magnets of the MR scanner. The conception, design, acquisition, analysis and interpretation of data are, on the whole, the contribution of the author. This is, on the whole, individual research work.
The author agrees that issues related to the accuracy or integrity of any part, even those in which the author is not personally involved, should be investigated, resolved and the resolution documented in the literature.

Figure 2
Proton nuclear magnetic resonance zeugmatogram of the object described in the text, using the four relative orientations of the object and gradients as in Figure 1 (zeugmatogram = "what is used to connect") Representative cross section of a typical superconducting magnet (designs vary). Liquid helium chambers are green-blue.

Figure 4
Representative cross section of a typical superconducting magnet (designs vary). Liquid helium chambers are green-blue.

Figure 5
Representative cross-section of a superconducting scanner, showing the nested arrangement of "coils". Both superconducting and resistive coils are shown. Two different types of patient coils are also illustrated: a spinal chain receiver only, and a knee carrier / receiver Figure 6 Representation of the principle realization of the MR scanner magnet Figure 7 Cross section of the windings of a superconducting NbTi magnet with microsial bers embeded in the Cu core.
Page 13/15 Figure 8 Gradient windings for the three basic directions Gradients x and y act only to create a "tilt" of the zcomponent B0 Figure 9 Magnetic eld produced by z-windings. Currents ow through two loops in opposite directions (Helmholtz pair). The gradient eld (large red arrows) is zero in the middle, but grows stronger in the z + and zdirections.
Page 14/15 Figure 10 Diagram showing how the z-gradient eld is added to the base eld B0, to obtain a eld that increases linearly from -z to + z Figure 11 RF eld B1 is normal to B0 Figure 12 Perovskite structure with general chemical formula ABX3. The red spheres are X atoms (usually oxygen), the blue spheres are B atoms (smaller metal cations, such as Ti4+), and the undistorted cubic structure; the symetry is lowered to orthorhombic, tetragonal, or triangonal in many perovskites Figure 13 Superconductor timeline. BSC (Burden-Cooper-Schiffer) superconductors are shown as green circles, cuprates as blue diamonds, and iron-based superconductors as yellow squares