Electromagnetic Field in MRI

Haoqi Yu

Physics 2702

Tatiana Seletskaia

MRI is an abbreviation of magnetic resonance imaging, it is first discovered by two physicists in 1947, Felix Bloch and Edward Mills Purcell. MRI is an imaging device that uses both powerful magnetic fields and radio frequency to produce detailed pictures organs and structures inside the human body. It is mainly used to detect the oscillations of hydrogen atoms. The magnetic field strength in an MRI machine is measured in Tesla (T), majority of clinical MRI are performed at 1.5 or 3T. The MRI machines produces an extremely strong magnetic field up to 50,000 times that of the Earth’s magnetic field and electromagnet of similar strength would be able to pick up a car. Static magnetic fields are created by the flow of direct current electricity,

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The human body is made up of 70% of water, which is largely composed of water molecules. A water molecule consists of two atoms of hydrogen and one atom of oxygen. MRI relies on the magnetic properties of a hydrogen atom to produce images. Hydrogen being the simplest element in the periodic table, it consists of just a single proton for its nucleus, with no neutrons and is orbited by a single electron. A way to model a proton is to think of it as a sphere composed of positive current loops that are stacked to one another.  Like the earth, proton has a spin, a north and south pole, and it spins around its magnetic pole. As a positive spinning charged particle, it produces a magnetic dipole moment. Normally, the proton is oriented randomly so there is no overall magnetic field, it will just spin regularly. When it is placed in an electromagnetic field, the proton will start precessing, at a rate depending on the field strength of the MRI system. The components of the MRI system include the primary magnet, gradient magnet, radiofrequency (RF) coils, and the computer system. The primary magnetic field refers to the strength of the static permanent field, hydrogen atoms align parallel or antiparallel to the primary field (B0), this is called longitudinal magnetization. The main purpose of this RF pulse is to disturb the protons which are precessing. A greater proportion of the hydrogen protons aligns to the direction parallel to the primary magnetic field or low energy state than antiparallel or high energy state. The net magnetic vector is in the direction of the primary magnetic field. The proton spin around the long axis of the primary magnetic field is called precession, the precession rate is called the Larmor frequency. When protons precess together, this is known as in phase, when protons process separately, that is known as out of phase. The frequency changes in proportion to the magnetic field strength at 1.5T is 63.9MHz.

The gradient coils generate secondary magnetic field over the primary field, they are located within the bore of the primary magnet. They are arranged in opposition to each other to produce positive and negative poles, the arrangement of these gradient coils gives MRI the capacity to image directionally along x, y, z axis. Gradient magnets alter the strength of the primary magnetic field, thereby changing the procession frequencies between slices. The RF coil is used to transmit a second magnetic field, which results in a disturbance of the proton alignment. Some low energy parallel protons flip to a higher energy state, decreasing longitudinal magnetization. Protons then become synchronized and precess in phase. Thus, a net magnetization vector turns towards the transverse plane to the primary magnetic field. The radio frequency coil is used to receive signals to create images as protons resume their normal state in the primary magnetic field prior to transmission of the RF pulse. After the RF pulse, protons flip back to their low energy state parallel to the primary magnetic field.

MRI machines uses static magnetic fields in the range of 200 to 3000mT. Static magnetic fields are generated by permanent magnets, wherever electricity is used in form of direct current through superconductors. The signal with the body increases as the field strength of the static field increases. The MRI machine gets the image from the hydrogen body in our body, when the body is in the MRI, it activates the hydrogen atoms. It puts them in a state where there now susceptible to forms of energy, when radio frequency energy is added, the hydrogen atoms are in a high-energy state. When hydrogen atoms are in the high-energy state, they are going to try to decrease its energy level and release it to the surrounding structures. Hydrogen atoms in fat have a different frequency in terms of its ability to release radio frequency energy, so the MRI image is bright and dark.

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