Multicore MRI Method

Hi, Habr.

I will talk about multi-core medical magnetic resonance imaging - one of the many areas of MRI development. I will touch on the features of the method, the necessary technical solutions, the application and the prospects.

For a start, a little excursion into the basics of MRI.

The Basics of MRI

The process of the MRI can be described in the following steps:

1. The object under study is placed in a constant magnetic field of a large, as a rule, superconducting main magnet . The field strength of this magnet is indicated by $$$B_0$and the axis along which it is directed is designated by the axis Z. It is the field $$$B_0$determines how much Tesla this MR scanner has. In clinics tomographs with 1.5 T and 3 T are used. MRI tomographs with ultra-high fields - 7 T, 9.4 T, are found only in research institutes.
2. The nuclei of some chemical elements have their own non-zero magnetic moment. The presence of a magnetic moment in a nucleus is caused by the inherent quantum property of particles — spin . Under the influence of the field $$$B_0$The magnetic moments of the nuclei are oriented parallel (most) and antiparallel (a smaller part) to the lines of force. Together, these nuclei give the object macroscopic magnetization in the direction along the Z axis.
   
   
   
   
   

   In addition, the cores precess . So far, the precession does not affect the overall magnetization, since the phases of all nuclei are randomly distributed and the components of their magnetic moments perpendicular to the Z axis cancel each other out. Precession frequency - Larmor frequency , depends only on the magnetic field strength $$$B_0$and the properties of the nucleus - its gyromagnetic ratio.
   

   $$

   $$\ omega_0 = B_0 \ cdot \ gamma,$$
   Where $$$\ omega_0$- Larmor angular frequency of the precession of the nucleus, [rad / s];
   $$$B_0$- magnetic field strength, [T];
   $$$\ gamma$Is the gyromagnetic ratio of the core, [rad / (T $$$\ cdot$c)].
   The gyromagnetic ratio is defined as $$$\ gamma = \ mu / h$where $$$\ mu$- own magnetic moment of the atom, [A $$$\ cdot$m $$$^ 2$]; $$$h$- Planck's constant $$$h = 6,626 070 040 (81) \ cdot10 ^ {- 34}$J $$$\ cdot$with.
   
   

   Today, medical tomography is based on work with hydrogen atoms, the nucleus of which is an ordinary proton. Atoms of various chemical elements in the same field will precess at different frequencies. Atoms are interesting for multi-core MRI $$$^ {23} Na$, $$$^ {31} P$, $$$^ {13} C$, $$$^ {19} F$, $$$^ {17} O$, $$$^ {129} Xe.$
   

   
   

   Larmor frequencies of some atoms, MHz

   Atom	Gyromagnetic ratio, MHz / T	Field strength $$$B_0$T
   		1.5	3	7	9.4
   $$$^ {1} H$	42,58	63.87	127.73	298.04	400.22
   $$$^ {23} Na$	11.26	16.89	33.79	78.83	105.86
   $$$^ {31} P$	17.24	25.85	51.71	120.65	162.01
   $$$^ {13} C$	10.71	16.06	32.13	74.96	100.66
   $$$^ {19} F$	40.05	60.08	120.16	280.36	376.49

   
   

   From these data one can understand the possible problems of a multi-core MRI. The cells of other atoms are very different from the frequency of hydrogen; this requires equipping the tomograph with a second set of electronics to work with the RF signal. On the other hand, the frequency of fluorine-19, on the contrary, is close to the frequency of hydrogen, and therefore difficulties arise with the differentiation of their signals. Ultra-high fields can be used to solve this, in which the frequency step becomes narrower.

3. Transmitting RF coil (antenna) creates a magnetic field pulse $$$B_1$rotating in the XOY plane. Here the phenomenon of resonance occurs, if the frequency of rotation of the field coincides with the Larmor frequency, then the nuclei rotate to the XOY plane and synchronize the phases of rotation. If the duration of the RF pulse is such that the magnetic moments of most nuclei are reoriented into the XOY plane, then the pulse is called 90-degree . After a 90-degree pulse, the macroscopic magnetization of an object rotates in the XOY plane with a frequency equal to the Larmor frequency of the nucleus.
   

   
   

   In receiving RF coils, this rotating magnetization induces a voltage — a signal (decay) of free induction . Recession, because the relaxation from this state occurs and this special magnetization is lost. Relaxation occurs in two ways. Transverse relaxation , with time constant $$$T_2$, associated with the loss of synchronization of the phases of rotation of atoms. Longitudinal relaxation , with time constant $$$T_1$associated with the return of the orientation of the magnetic moments of the nuclei along the field $$$B_0$.
   

   
   
   
   
   

   In general, to obtain some information about the object is enough. The signal will contain integral, averaged information about these cores of the object. For example, in the frequency spectrum of a signal, one can see chemical shifts — changes in the Larmor frequency due to the interaction of atoms in a chemical compound. This is the basis of NMR spectroscopy, a method used by chemists to analyze the chemical composition of an object.
   

   
   

   In this post, I'll talk a little more about RF coils and their features in multi-core MRI.
   

RF coil designs

The transmitting (Tx) RF coils are tasked with effectively transmitting a pulse of a given frequency and creating a uniform magnetic field perpendicular to the Z axis. It is interesting that the loss of the RF pulse in the system is enormous. From a few kilowatts of power amplifiers, only tens of watts reach the coils. Therefore, RF coils are made electrically resonating at a given frequency. The design of the RF coil also imposes limitations and anatomy. In MRI studies, often only part of the body is examined — the head, chest, knee, etc. The transmitting coil for the study of the whole body is usually built into the tomograph itself, and for the study of individual parts of the body it is represented by separate modules.

Siemens Head RF Coil

I will give a few examples of coil designs.

1. Coil in the form of a solenoid.

   
   
   
   
   

   A simple way to create a uniform field inside the solenoid windings. It may seem that the fields in such a coil rotating can not be done. But it is worth remembering that the vector $$$B_1$, varying according to a sinusoidal law, can be represented as a sum of two oppositely rotating components.
   

   
   

2. Saddle reel

   
   
   
   
   

3. Squirrel cage (birdcage) coil

   
   
   
   
   

   On the left there is a “squirrel cage” of the lower frequencies type, on the right - the upper ones.

   
   

   Advanced option. May be in the form of low frequencies or high frequencies. By adjusting the elements — capacitors and inductances, due to the length of the legs (rarely), the current of the required frequency has a close to ideal sinusoidal distribution in the angle and creates a uniform field. If you give it a quadrature signal, then the field $$$B_1$will be purely rotating.

   
   

4. Multi-element coils

   
   
   
   
   

   The head coil consists of shortened dipole antennas and rectangular loops.

   
   

   Constructed from several simpler antennas, built around the circumference. Dipole antennas, loop-shaped antennas, microstrip antennas, etc. can be used as elements. Here you can see how anatomy affects the design. For example, the emission wavelength of the Larmor frequency of the proton by 7 T is a whole 1 m. A typical dipole antenna should be the field length of the wavelength of the detected radiation. It is impractical to make such a long coil for examining the head, therefore the dipole antenna is shortened by adding inductors to its shoulders.
   

   
   

The function of the receiving coils can be implemented on the transmitting, receiving a receiving-transmitting coil (TxRx). Pure receiving coils (Rx) should also be resonant, but the design requirements are somewhat different. They can be made in the form of a grid of flat loop antennas. So they are located directly on the surface of the body, thereby reducing the loss of the received signal.

Siemens surface receiver coil

Fine tuning the frequency of the coils by varying the capacitance of the capacitors. Also important is the coincidence of the impedances of the coil and the path for efficient energy transfer. The coil impedance using inductances and impedance transformers leads to a standard 50 ohm.

Features of RF coils for multi-core MRI

So, to receive a signal from a hydrogen nucleus and in addition to some other element in MRI, RF coils must have different properties. How to implement it.

1. The simplest option. Make two different coils, one for hydrogen, the other for another element. Conduct a full study with a coil for a proton, remove the object and the coil, put another coil to return the object and repeat the study. Given that the MRI examination takes a lot of time and is sensitive to movement, the option is not applicable.
   

2. Make coils with double resonance. The second resonant peak can be inserted into the coil by adding an LC circuit in series. Adding additional LC circuits allows you to tune the coil to 3 or more frequencies
   

   
   
   
   
   

3. Use switches. For example, additional PIN capacitors can be bridged using PIN diodes. So, when DC voltage is applied, the adjustment circuit and the resonant frequency of the coil change accordingly.
   

   
   
   
   
   

4. Use two (or more) coils simultaneously. Each of them is tuned to its own frequency. Here there is a problem with the mutual inductive coupling between the coils. Often it is solved with the help of a special coil design. Geometry and type of antennas are selected so that the fields they create are orthogonal to each other. Other options - add a passive LC filter to each coil, which removes the signal from the other; using PIN diodes to upset the coil not currently in use.
   

   
   
   
   
   

5. Squirrel cage coil with four rings. To the usual “cage”, on the one hand and on the other side, one more “cage” is added. The internal segment operates in the same way as a conventional single-frequency coil. The outer segments together form a “squirrel cage” tailored for another frequency. This design allows the coils to resonate independently of each other.
   

   
   
   

   On the left there is a 4-ring “squirrel cage” with an external segment of the type of high frequencies, on the right - lower ones.

   
   

Conclusion

In vivo imaging and spectroscopy in MRI studies is a difficult task. The concentration of atoms other than hydrogen in the human body is quite low, because of this, the signal-to-noise ratio when working with these atoms is low. To improve the SNR, MRI with ultrahigh fields is used, but in such fields difficulties arise with field uniformity. With such Tesla, the wavelength of proton radiation is already comparable with the size of body parts.

But the use of other atoms carries valuable information about metabolism. Atoms $$$^ {23} Na$carry information about the salt balance in the cells. Living healthy cells constantly maintain a low concentration of sodium ions inside themselves at high outside using sodium-potassium pumps. This process goes with energy costs, therefore metabolic disorders are reflected in changes in the concentration of sodium ions inside the cells. Brain tumors, ischemia, strokes, bipolar disorders are associated with an increase in the concentration of sodium inside the cells and this can be seen with a multi-core MRI.

Another example is phosphorus in the form of an atom. $$$^ {31} P$. It enters important metabolites - ATP, phosphocreatinine, etc. By performing phosphorus spectroscopy in muscles, one can assess the presence of these substances and the level of metabolism in the muscles.

Spectroscopy $$$^ {13} C$is already used in NMR spectroscopy for the analysis of organic chemical compounds, but in vivo its concentration in humans is small, but the method is still applicable.

Atom $$$^ {17} O$has a low concentration in the natural state, but by saturating the air with which the person under study breathes, it is possible to build a map of its metabolic rate, which helps in the diagnosis of tumors.

But nevertheless, there is still a long way to go before the widespread use in multi-core MRI clinics and it will take 20-30 years.

Sources