MRI Basics

What is Magnetic Resonance Imaging (MRI)>It is a modality used to image the human body without using x-rays. It uses a large magnet, radiowaves and a computer.
Non-invasive medical imaging method, like ultrasound and X-ray. Clinically used in a wide variety of specialties. Advantages are excellent / flexible contrast, non-invasive, no ionizing radiation.

Magnetic Resonance
Certain atomic nuclei including 1H exhibit nuclear magnetic resonance. Nuclear “spins” are like magnetic dipoles. Hydrogen is the most common element in the body which has highest sensitivity to magnetic resonance. Hydrogen ion is positively charged.

Nuclear spin
A particle rotating upon its own axis is called Nuclear spin. Spinning particles contain a magnetic field. Human body is made up of tiny bar magnets. Spins are normally oriented randomly. In an applied magnetic field, the spins align with the applied field in their equilibrium state. Excess along B0 results in net magnetization.

Precession

Spins precess about applied magnetic field, B0, that is along z axis. During precession the tail of the vector is fixed and the head is revolving. This revolving motion is called Precession. Precessional frequency is the speed at which the hydrogen protons precesses.

Larmour Equation is f = g M

f = Frequency in Revolution /sec

M = Magnetic field strength in T

g = Gyromagnetic ratio [42.6]

Relaxation Processes

The return of M to its equilibrium state (the direction of the z-axis) is known as relaxation. There are three factors that influence the decay of M: magnetic field inhomogeneity, longitudinal T1 relaxation and transverse T2 relaxation. T1 relaxation (also known as spin-lattice relaxation) is the realignment of spins (and so of M) with the external magnetic field B0 (z-axis). T2 relaxation (also known as T2 decay, transverse relaxation or spin-spin relaxation) is the decrease in the x-y component of magnetisation.

T1 relaxation

Following termination of an RF pulse, nuclei will dissipate their excess energy as heat to the surrounding environment (or lattice) and revert to their equilibrium position. Realignment of the nuclei along B0, through a process known as recovery, leads to a gradual increase in the longitudinal magnetisation. The time taken for a nucleus to relax back to its equilibrium state depends on the rate that excess energy is dissipated to the lattice. Let M-0-long be the amount of magnetisation parallel with B0 before an RF pulse is applied. Let M-long be the z component of M at time t, following a 90 degree pulse at time t = 0. It can be shown that the process of equilibrium restoration is described by the equation

where T1 is the time taken for approximately 63% of the longitudinal magnetisation to be restored following a 90 degree pulse.

T2 relaxation

While nuclei dissipate their excess energy to the lattice following an RF pulse, the magnetic moments interact with each other causing a decrease in transverse magnetisation. This effect is similar to that produced by magnet inhomogeneity, but on a smaller scale. The decrease in transverse magnetisation (which does not involve the emission of energy) is called decay. The rate of decay is described by a time constant, T2*, that is the time it takes for the transverse magnetisation to decay to 37% of its original magnitude. T2* characterises dephasing due to both B0 inhomogeneity and transverse relaxation. Let M-0-trans be the amount of transverse magnetisation (Mx-y) immediately following an RF pulse. Let M-trans be the amount of transverse magnetisation at time t, following a 90 degree pulse at time t = 0. It can be shown that

Type of Images

• T1 weighted image
• T2 weighted image
• Proton density weighted image

T1 weighted image:
Image produced as a result of differences in T1 times of the tissue. The basis of T1 weighted imaging is the longitudinal relaxation. A T1 weighted magnetic resonance image is created typically by using short TE and TR times. TR less than T1 (typically £ 500 ms) and TE less than T2 (typically £ 30 ms).

T2 weighted image:
Image produced as a result of differences in T2 times of the tissue and it is the image made with a sequence with long TR and TE to show contrast in tissues with varying T2 relaxation times; water gives a strong signal. TR greater than T1 (typically £ 2 000 ms) and TE less than T2 (typically £ 100 ms).


Proton density weighted image
Image that is produced as a result of differences in proton densities of the tissues. An image produced by controlling the selection of scan parameters to minimize the effects of T1 and T2, resulting in an image dependent primarily on the density of protons in the imaging volume. Images are generated by choosing TR greater than T1 (typically £ 2 000 ms) and TE less than T2 (typically £ 30 ms).

Production of X-Rays

X-Rays

X rays are a form of electromagnetic radiation with wavelengths that range from about 10 −7 to about 10 −15 meter. No sharp boundary exists between X rays and ultraviolet radiation on the longer wavelength side of this range. Similarly, on the shorter wavelength side, X rays blend into that portion of the electromagnetic spectrum called gamma rays, which have even shorter wavelengths.

X rays have wavelengths much shorter than visible light. (Wave lengths of visible light range from about 3.5 × 10 −9 meter to 7.5 × 10 −9 meter.) They also behave quite differently. They are invisible, are able to penetrate substantial thicknesses of matter, and can ionize matter (meaning that electrons that normally occur in an atom are stripped away from that atom). Since their discovery in 1895, X rays have become an extremely important tool in the physical and biological sciences and the fields of medicine and engineering.

Production of X rays


The method by which X rays were produced in Roentgen's first experiments is basically the one still used today. An X-ray tube consists of a glass tube from which air has been removed. The tube contains two electrodes, a negatively charged electrode called the cathode and a positively charged target called the anode. The two electrodes are attached to a source of direct (DC) current. When the current is turned on, electrons are ejected from the cathode. They travel through the glass tube and strike a target. The energy released when the electrons hit the target is emitted in the form of X rays. The wavelength of the X rays produced is determined by the metal used for the target and the energy of the electrons released from the cathode. X rays with higher frequencies and, therefore, higher penetrating power are known as hard X rays. Those with lower frequencies and lower penetrating power are known as soft X rays.

X-ray Tube Components:




Cathode: The cathode acts to excite electrons to the point where they become free from their parent atom and are then able to become part of the electron beam. The cathode acts as a negative electrode and propels the free elections, in the form of an electron beam, towards the positive electrode (the anode).

Anode: The anode acts as a positive electrode, attracting the free electrons and accelerating the electrons through the electromagnetic field that exists between the anode and cathode. This acts to increase the velocity of the electrons, building potential energy. The higher the kV rating, the greater the speed at which the electrons are propelled through the gap between the cathode and anode. The electrons then impact a target (most commonly made of tungsten, but this target can also be molybdenum, palladium, silver or other material). This causes the release of the potential energy built up by the acceleration of the electrons comprising the electron beam. TMost of this energy is converted to heat and is radiated by the copper portions of the anode. The remainder is refracted off of the target in the form of high energy photons, or x-rays, forming the x-ray beam.

Glass envelope: The above components are sealed into a glass envelope. This allows for gases and other impuritites to be pumped out of the tube, creating the vacuum necessary for proper performance. The x-ray creation process must occur in a vacuum so as not to disrupt the electron beam, and also to allow for proper filament performance and durability.

Filament: The filament is actually a very tightly wound coil of wire. The electrons are produced by thermionic effect from a tungsten filament heated by an electric current. The filament is the cathode of the tube. The high voltage potential is between the cathode and the anode, the electrons are thus accelerated, and then hit the anode.

Focusing Cup: The focusing cup is a shallow depression in containing the filament. The filament emits electrons, all of which have a negative charge. Since negative repels negative, the electrons that have been emitted have a tendency to diverge. As this is counterproductive in x-ray tubes, the focusing cup is a negatively charged housing that "encourages" the electrons to stay together. Essentially, the force that causes the electrons to repel each other is overpowered by the repulsive force of the focusing cup and the electrons tend to converge rather than diverge. Focusing cup acts as an electromagnetical lens and the electron beam is condensed on the focal spot.

Tungsten Target: Tungsten is the most commonly used target material in the anode because it has a high atomic number which increases the intensity of the x-rays, and because it has a sufficiently high melting point that it can be allowed to become white hot. During operation, the tungsten target can get as high as 2,700 degrees centigrade. In many cases, the tungsten target is surrounded by copper - the high heat capacity of copper improves the dissipation of heat.

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