As you can see in the image below, the MRI machine consists of different layers. Each of these layers represents different magnets that we use for imaging.

If we look at the MRI machine from the front, the patient is placed horizontally inside it as shown in the picture.

The patient inside the MRI machine

The difference between the MRI machine and other imaging devices such as X-ray or ultrasound is that the signal used to create the image in the MRI comes from inside the patient’s body, so for accurate imaging, we need to know which part of the body the signal sent came from. To do this, we use the Cartesian coordinate axes or the x, y, and z axes and divide the image along three axes.

The z component, which is placed along the patient’s body, and its direction is from head to foot, is used for axial imaging. In this type of imaging, a wide range of organs and body structures such as the brain, spine, abdomen, chest, and pelvis are examined. Also, the y component provides coronal images. These images examine the anatomy of the body from the front to back or back to front view. Finally, the x component is used to prepare sagittal images. This plane divides the body into left and right parts. It goes without saying that the z-axis is the longitudinal axis (longitudinal plane) and the xy plane, which is perpendicular to the z-axis, is called the transverse plane.

In MRI imaging, a concept called “nuclear magnetic resonance” (Nuclear Magnetic Resonance or NMR) is used, and by applying a very large magnetic field, we induce resonance in certain atoms inside the patient’s body. If you are wondering what atom is used for this, the answer is hydrogen atom, because there are a lot of atoms in question in the human body and its spin is also opposite to zero.

As you know, about 75% of the human body is water and each water molecule consists of two hydrogen atoms and one oxygen atom. Hydrogen and oxygen atoms are connected to each other through polar covalent bonds and electron sharing, But this sharing is not done equally.

Due to its greater electronegativity, the oxygen atom is more attractive to electrons, and for this reason, it pulls the electron cloud towards itself. This asymmetry in the distribution of electrons causes polarity in the water molecule. Oxygen atoms, due to having a negative partial charge, and hydrogen atoms, due to having a positive partial charge, form the two poles of the water molecule.

We can think of the patient as a box full of hydrogen atoms that exist completely randomly in the absence of a magnetic field, moving around in no particular direction. The speed of movement of hydrogen atoms depends on the temperature of a person’s body, that is, the higher the temperature of a person, the faster the speed of movement of atoms, and the lower the temperature of a person’s body, the speed of movement of atoms is lower. These atoms are affected by the external magnetic field due to having a magnetic moment. This state is similar to the position of the compass needle in the direction of the earth’s magnetic field; Therefore, by applying an external magnetic field, the hydrogen nuclei are placed in a certain direction.

The MRI machine consists of a main magnet, gradient coils (these coils play an important role in creating the spatial magnetic field), radio signal (RF) coils, and a computer system. In general, we can think of MRI as a large magnet that creates a magnetic field called B0.

Typically, the B0 value in MRI machines is between 1.5 and 3 Tesla (about 300,000 times stronger than the Earth’s magnetic field and 30,000 times stronger than the magnetic field on a refrigerator magnet). Most of the hydrogen nuclei are placed in the direction of the magnetic field after being placed in it, but the direction of some nuclei is also opposite to the direction of the external magnetic field. From the point of view of quantum physics, hydrogen atoms exist in both states, but the important point in this section is that the number of hydrogen atoms that are in the direction of the B0 magnetic field is greater than the number that is in the opposite direction.

We subtract the field resulting from the nuclei placed in the direction of the magnetic field B0 from the total field of the nuclei placed against the direction of the B0 field and call it Bnet. If we consider the z-axis of the human body placed in the MRI machine, the Bnet will be aligned with the z-axis as shown in the image below. As you can see, the B0 and Bnet fields are parallel to each other and in the positive direction of the z-axis. Note that the magnet is in a low energy state when aligned with the magnetic field. To put it more simply, small magnets tend to align with the z-axis when placed in the magnetic field B0.

Positive hydrogens, in addition to being in the direction of the magnetic field of the MRI machine, rotate around their central axis like a spinning top. This movement is called nuclear spin and the speed or frequency of this rotational movement depends on the magnitude of the magnetic field B0. The bigger the B0 field is, the higher the frequency of the forward movement of the hydrogen atom will be. In MRI imaging, we should not look at hydrogen atoms individually, but we should pay attention to the total magnetization vector created by them and the change of this vector due to the change of magnetic fields in the MRI machine. Therefore, we replace the hydrogen atoms with the total magnetization vector (Bnet) along the z-axis.

The total magnetization vector is the quantity we want to measure in MRI imaging, but it is not possible to measure this vector along the z-axis and parallel to the B0 field, why? Because the B0 field is large enough to affect the measurement of its magnetization vector. How do we measure the total magnetization vector? To do this, we place this vector perpendicular to B0 and measure it. How do we place its magnetization vector perpendicular to the main magnetic field? We can do this with the help of the second magnetic field called radio frequency pulse (RF).

An RF signal is not really a radio wave, but electromagnetic energy with a frequency in the radio wave spectrum. The RF pulse changes at a frequency equal to the frequency of the forward motion of the hydrogen atoms. When the frequency of the forward movement of hydrogen atoms and the frequency of the RF pulse become equal, two things happen. After entering this signal, the positive hydrogens are directed to another plane (the plane perpendicular to the B0 field) and rotate around their axis at the same time and in phase with each other. The rotation angle from the z-axis depends on the magnitude and duration of the RF signal radiation.

As the positive hydrogens or protons go to the plane perpendicular to B0, the longitudinal magnetization will change. In general, most protons are placed in the direction of the B0 field; But by giving them some energy, the protons can be aligned perpendicular to B0. This is not the whole story; By energizing the protons in the form of an RF pulse, they begin to move forward simultaneously with each other and in phase. As a result of this energy, the magnetic moment of protons (hydrogen atoms) is transferred to a plane perpendicular to the main magnetic field B0 or ​​xy plane.

Therefore, the RF pulse does two things:

• transfers the total magnetic field to the xy plane;
• Hydrogen nuclei move forward in phase with a specific frequency and rotate around it like a spinning wheel with a specific angle to the central axis. As a result, the total magnetic field moves in the xy plane and perpendicular to the z-axis.

So far we have understood that in the MRI machine, unlike other imaging machines, protons inside the patient’s body are used to take the image. These protons are hydrogen atoms with a partial positive charge, which after being placed in the direction of the main magnetic field B0, are placed in the same direction and parallel to it (in the direction of the z-axis). Next, by applying a smaller field called the RF pulse, the protons are aligned perpendicular to the z-axis and in the xy plane; Then we place a small coil in the MRI machine. According to Faraday’s principle of induction, changing the magnetic field induces an electric current.

According to the Faraday induction principle, whenever the magnetic flux passing through the coil changes at a certain rate, a voltage of a certain value is induced in the coil. The amount of induced voltage depends on the speed of change of magnetic flux, the number of turns of the coil, and the area of ​​each turn. If the coil is in a closed circuit, the induced voltage can induce a current in the coil, the amount of which depends on the resistance of the circuit and the amount of the induced voltage. In the MRI machine, the current induced in the coil is used to create the image.

As we said, the total magnetic field (total magnetization vector) moves in the xy plane and perpendicular to the z-axis. According to the movement of the magnetization vector in the XY plane, we can measure a signal. This vector moves in the xy plane due to the RF pulse. The important point is that the frequency of the RF pulse must be equal to the frequency of the forward movement of hydrogen atoms. As the frequencies become equal, magnetic resonance occurs.

To better understand the concept of resonance, let’s examine a simple example together. We have a game device called “trampoline” which is made of thick fabric and on which acrobatic movements can be performed. Let’s say you and your friend are jumping up and down on a trampoline. If you jump on a trampoline alone, you will go up to a certain height, h. Now, if your friend jumps at the same time as you, the height of your jump will be greater than h. In other words, synchronizing your jump with your friend will increase your jump height. Note that the height of the jump will increase if you jump at the same time as other people.

For hydrogen atoms, when the RF pulse is irradiated, a state similar to jumping on a trampoline occurs. Only when the frequency of the forward movement of hydrogen atoms and the frequency of the RF pulse are equal to each other, the hydrogen atoms move in phase and the angle of magnetization starts to change. As we mentioned, angle changes depend on RF pulse duration and amplitude. After transferring the total field or the total magnetization vector to the xy plane and creating the necessary signal, we cut off the RF pulse radiation. The received signal is created due to the progressive movement of the magnetization vector with a frequency equal to the frequency of the RF pulse. At first glance, the generated signal looks like the image below; But in practice, the generated signal is not like this, because the RF pulse radiation is not continuous and after a certain period of time, it is interrupted.

What happens in practice is that the hydrogen atoms move in phase with the frequency of the RF pulse, and after it is interrupted, they move out of phase. In this case, the magnetization vector in the xy plane and as a result, the generated signal, become smaller and smaller. The orange graph plotted on the signal is called the free-induced attenuation curve or T2.

It is important to note that each tissue in the body has a unique *T2 diagram and is different from other tissues. The free-induced damping plot of water is very slow with respect to time, whereas the free-induced damping plot of bone or tissue is very fast with respect to time. You might ask yourself what it means if this chart is slow or fast compared to time; The graph of fast damping with respect to time means that the amplitude decreases rapidly with respect to time; But in the slow damping diagram, the amplitude decreases slowly and very slowly with respect to time. By using these differences, we can create the necessary contrast in the captured images.

The above process occurs simultaneously with another independent process. At the same time as the magnetization decreases in the xy plane, the longitudinal magnetization increases along the z-axis. The decrease of magnetization in the xy or *T2 plane occurs much faster than the increase of magnetization along the z-axis. As you can see in the diagram below, over time the magnetization increases along the z axis. Finally, there comes a time when the hydrogen atoms are completely out of phase and the magnetization in the xy plane becomes zero. In this case, the magnetization vector has no component in the xy plane and has only one component along the z-axis. Note that the time that the magnetization is completely aligned with the z-axis (T1) is much larger than *T2.

Note again that these two processes occur completely independently of each other; That is, by knowing the T2 for a specific tissue in the body, we cannot easily obtain the T1 of that tissue, because these two quantities are completely independent of each other. Don’t forget that we can only measure the signal perpendicular to the main magnetic field, B0; Therefore, to measure its magnetization, we must make it perpendicular to B0.

Now we have reached the stage where we can do MRI imaging. To do this we need two separate parameters that use the differences of T2 and T1; These two parameters are called “Time of Echo” (TE) and “Time of Repetition” (TR). Consider two separate tissues in the body, each of which has protons aligned along the z-axis. Now we irradiate the RF pulse to two tissues. Protons inside each of the tissues perform forward movement in the plane perpendicular to the main field B0. Further, with hydrogen atoms being out of phase, the magnetization in the xy plane decreases in the time *T2.

The time interval between the applied radio pulse to excite the hydrogen atoms and the measurement of the signal resulting from their spin is called the echo time. In other words, TE represents the time it takes for the MRI signal to be measured after the initial excitation. By giving more time, the phase inhomogeneity and the difference between the two tissues increases. At the same time, two textures acquire longitudinal magnetization or magnetization along the z axis with different tunes. Finally, the magnetization vector of two tissues is placed along the z-axis. By irradiating the second RF pulse, we can once again place the protons of the two tissues in the xy plane. The time between the first RF pulse and the second RF pulse is called the repetition time or TR.

In the previous section, we said that we can consider the MRI machine as a big magnet; But the main question is how to create a magnetic field with a magnitude of 1.5 to 3 tesla in MRI. By increasing the B0 magnetic field, the received signals from different tissues are amplified, as a result, the obtained image will be of better quality. The MRI machine can create a magnetic field of up to 20 tesla. Do not forget that reaching this amount was not easily achieved.

Early MRIs used permanent magnets to create a magnetic field, but these magnets could only create a magnetic field up to 0.5 tesla; Therefore, the created images did not have an interesting quality. Next, researchers used electromagnets instead of permanent magnets to achieve a stronger magnetic field; But electromagnets cannot create a magnetic field as large as 1.5 Tesla; Because large magnetic fields require high currents that melt ordinary wires.

To solve this problem and have high currents, researchers used superconducting coils. Temperature affects conductive materials so that their resistance decreases as the temperature decreases. But superconductors have a unique feature; Their resistance becomes zero at a temperature close to -273 degrees Celsius or absolute zero. In this case, the electric current in a loop made of superconducting material can flow forever. In reality, the superconducting coil in the MRI machine does not directly require any external electrical power; Rather, the coils only need to be kept cool by spending some energy, in which case the MRI magnet will stay on permanently.

The energy required to operate the MRI machine for a year is about 130,000 to 140,000 kilowatt hours. Niobium-titanium is one of the most common superconducting materials in MRI, in such a way that 80% of niobium-titanium extracted from the earth is used to make MRI devices. As we said, superconductivity occurs at a very low temperature, so we need a very advanced cooling system to reach this temperature.

When engineers built early MRI machines, they kept superconducting wires inside a liquid helium bath at -269 degrees Celsius; But the problem with doing this was that helium evaporates quickly, so it was necessary to constantly refill the container containing liquid helium, which cost about $26,000 a year. To solve this problem, the researchers equipped the MRI machine with a vacuum chamber and placed liquid helium inside it.