5.1. The image with the Terra 7T Siemens Healthcare Magnet encouraged the work of this article.
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 field, 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 field is reached. The current continues to flow in a closed loop, without a significant drop. The resulting property is that the magnetic field is always present. The construction of superconducting magnets is considered to be extremely expensive, and cryogenic helium is expensive and difficult 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 fibers of one of the superconductors, NiTi, Nb3Sn, Va3Ga or MgB2, are inserted into the copper conductors. Copper acts as an insulator at low temperatures, relative to the zero resistance of the microsial fiber 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 field, a balanced disturbance of the fundamental field 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 configuration. More precisely, the basic design of the z-direction gradient is Helmholtz's pair of coils: two loops with currents flowing in opposite directions.
Helmholtz coils produce a gradually changing field, which is zero in the magnetic isocenter, but increases linearly outward, in both directions + z and -z. When this is added to the constant field B0, 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 field (RF), used in MR, is denoted by B1. It must be normal to the main magnetic field B0. It is produced by special RF windings. RF windings can be transmitters, receivers, or both. If the oscillation B1 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. B1 is only switched on for short periods of time (several milliseconds), called "RF pulses". By adjusting the magnitude or duration of these B1 pulses, the nuclear spin system can rotate at variable angles of rotation, such as 900 and 1800.
5.7. A superconductor, unlike a conductor, conducts electricity indefinitely, without energy losses. This is an important characteristic of these materials and a challenge for their use. They do not lose electricity! For the main magnets of MR scanners are used: niobium-titanium (NbTi), Tc = 10K, B0 ~ 15T (Since 1960), niobium-tin (Nb3Sn), Tc = 18,30K 254.80C / -426.7F), B0 ~ 25T to 30T (Since 1960), vanadium-gallium (V3Ga), Tc = 14.20K, B0 ~ 19T, magnesium-diboride (MgB2), Tc = 390K (-234 0C / -389 0F) (Since 2001). It can be noticed that out of four superconductors, three are low-temperature and one is high-temperature.
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 = 390K 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 significant 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, Tc = 160 0K, 1945. described perovskite, 1962. made the first 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. 2001. Described by MgB2.
5.8. This study confirmed, as important, that room temperature superconductors, although discovered, are not in the field 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 scientific method by which they could be turned into useful, application, which should be eliminated. It is estimated that research to achieve thesis extremly expensive. Scientific optimism gives hope that it is a matter of time before they become applicable, and that the practical benefits of their discovery will be obtained.