RADIO FREQUENCY COILS OF MRI

I had previously posted about static magnetic field this part the RADIO FREQUENCY COILS will continue the description of MRI ,hope you all will like it ,there could be typing error please correct it on the comment bar The static field does not produce any MR signal it is actually produced by use of two types of coils (transmitting coils )they are the transmitter and the receiver coils ,they generate and receive electromagnetic fields at the resonant frequency of the atomic nuclei within the static magnetic field ,this process gives the name resonance to magnetic resonance imaging. Because most atomic nuclei of interest for MRI studies have their resonant frequency in the radiofrequency portion of electromagnetic spectrum (at typical field strength of MRI),these coils are called radio frequency coils ,these coils can be evaluated on the same criteria as static field :uniformity and sensitivity .The radio frequency coils sends electromagnetic waves that resonate at a particular frequency ,as determined by the strength of magnetic field into the body ,disturbing the equilibrium state the process is known as excitation .When atomic nuclei are excited they absorb the energy of the radiofrequency pulse, But when the radiofrequency pulse ends the atomic nuclei go to equilibrium releasing energy absorbed during excitation ,this releasing energy are detected by the receiver coils the process is called as reception .The detected electromagnetic pulse defines raw MR signal

To explain it in a much simpler let me explain the way as I understood let me put it like this ,when I throw a ball in air I apply some potential energy to it by that I can give some energy to the ball ,now after the energy is lost the ball will come to equilibrium ,the energy present in the ball is given to the environment ,what happens in the atom is RF coils give potential energy and as the process of equilibrium is obtained the atom losses its energy to the surroundings ,here the difference with the ball and atom is we keep a receiver coil to extract all the energy i.e. the receiver coils the energy received depends on the distance from the sample being measured so the RF coils are generally placed immediately around the head in FMRI in both surface and volume coil arrangement. Now let me introduce you all to two other concepts of surface coil and volume coil. As I am from electrical background I would elaborate on this concept a bit more, as simple as possible The concept of surface coil begins with single loop (indicator-capacitor circuit) an additional resistance is also provided. As you all know that capacitor has a property of charging and discharging now when connected with an inductor due to rapid charge and discharge between capacitor and indicator it provides an oscillating current that can be tuned with the frequency of our interest, what happens here for the capacitor to charge we need a slow rising current which is accomplished by using an inductor what happens here to the total circuit is it will yield us a frequency which is f=1/2*(pi)*sqrt(L*C) by varying capacitance and inductance we can control the frequency ,it is similar to a radio receiver which uses similar circuit to receive station ,since receiver resonates at a particular frequency stations which are far off will not excite or give response ,the value of R should be small as possible so that only station is picked .In MRI the hydrogen atom will have a frequency which can be taken by the receiver coil ,the frequency adjustment is done by inductance and capacitance and Resistance should be small so that energy of only required area is captured .Because of close proximity towards brain surface coil it produces a high image sensitivity so it is generally used in FMRI to target one specific region of brain ,since the imaging depends on the distance from the surface coil ,whenever whole volume imaging is required it is inappropriate to use this

Now let us move to a second type of coil that is the volume coil which provides uniform spatial coverage throughout a large volume, the basic concept is that of surface coil (LC circuit) but here this LC circuit is replicated around a cylindrical surface to achieve uniform distribution of energy within the enclosed volume coil and sometimes referred as birdcage coil. As volume coil is father from the brain so sensitivity in the region of brain is less

A compromise approach has been made to get the best features of both the coil types, use volume coil for exiting and surface coil for receiving the MR signal .if multiple receiver coils is used arranged in a overlapping pattern known as phased array. Sensitivity of radiofrequency coil is proportional to strength of magnetic field generated within the coil by unit current. To obtain quantitative measurement of coil sensitivity, Quality factor is defined as ratio of maximum energy stored and total energy dissipated per period

Quality factor (Q)=sqrt(INDUCTANCE)/RESISTANCE*sqrt(CAPACITANCE)

Minimizing resistance will boost coil sensitivity