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1、Magnetic Resonance Imaging (MRI)nIntroductionnThe ComponentsnThe TechnologynPhysics behind MRnConclusionPresented bySalwa Aziz & Christina Derbidge IntroductionnWhat is MRI?nMagnetic resonance imaging (MRI) is a spectroscopic imaging technique used in medical settings to produce images of the inside
2、 of the human body.nMRI is based on the principles of nuclear magnetic resonance (NMR), which is a spectroscopic technique used to obtain microscopic chemical and physical data about molecules nIn 1977 the first MRI exam was performed on a human being. It took 5 hours to produce one image. Introduct
3、ionnHow Does it Work?nThe magnetic resonance imaging is accomplished through the absorption and emission of energy of the radio frequency (RF) range of the electromagnetic spectrum. The Components:nA magnet which produces a very powerful uniform magnetic field.nGradient Magnets which are much lower
4、in strength.nEquipment to transmit radio frequency (RF).nA very powerful computer system, which translates the signals transmitted by the coils. The MagnetnThe most important component of the MRI scanner is the magnet:nThe magnets currently used in scanners today are in the .5-tesla to 2.0-tesla ran
5、ge (5,000 to 20,000- gauss). Higher values are used for research.nEarth magnetic field: 0.5-gaussThe Magnet (cont.)nThere are three types of magnets used in MRI systems: nResistive magnets nPermanent magnetsnSuper conducting magnets (the most commonly used type in MRI scanners).nIn addition to the m
6、ain magnet, the MRI machine also contains three gradient magnets. These magnets have a much lower magnetic field and are used to create a variable field. The TechnologynHow Does It All Work?nSpin:nThe atoms that compose the human body have a property known as spin (a fundamental property of all atom
7、s in nature like mass or charge).nSpin can be thought of as a small magnetic field and can be given a + or sign and a mathematical value of multiples of .nComponents of an atom such as protons, electrons and neutrons all have spin. The Technology (cont.)nSpin (cont.):nProtons and neutron spins are k
8、nown as nuclear spins.nAn unpaired component has a spin of and two particles with opposite spins cancel one another.nIn NMR it is the unpaired nuclear spins that produce a signal in a magnetic field. The Technology (cont.)nHuman body is mainly composed of fat and water, which makes the human body co
9、mposed of about 63% hydrogen.nWhy Are Protons Important to MRI? npositively charged nspin about a central axisn a moving (spinning) charge creates a magnetic field.nthe straight arrow (vector) indicates the direction of the magnetic field.The Technology (cont.)nWhen placed in a large magnetic field,
10、 hydrogen atoms have a strong tendency to align in the direction of the magnetic filed nInside the bore of the scanner, the magnetic field runs down the center of the tube in which the patient is placed, so the hydrogen protons will line up in either the direction of the feet or the head. nThe major
11、ity will cancel each other, but the net number of protons is sufficient to produce an image. The Technology (cont.)nEnergy Absorption:nThe MRI machine applies radio frequency (RF) pulse that is specific to hydrogen.nThe RF pulses are applied through a coil that is specific to the part of the body be
12、ing scanned. The Technology (Cont.)Resonance (cont.) The gradient magnets are rapidly turned on and off which alters the main magnetic field.nThe pulse directed to a specific area of the body causes the protons to absorb energy and spin in different direction, which is known as resonance Frequency (
13、Hz) of energy absorption depends on strength of external magnetic field.The Technology (cont.)nThe resonance frequency, 0, is referred to as the Larmor frequency.The Technology (cont.)nImaging:nWhen the RF pulse is turned off the hydrogen protons slowly return to their natural alignment within the m
14、agnetic field and release their excess stored energy. This is known as relaxation. nWhat happens to the released energy?nReleased as heat ORnExchanged and absorbed by other protons ORnReleased as Radio Waves.The Technology (cont.)nMeasuring the MR Signal:nthe moving proton vector induces a signal in
15、 the RF antennanThe signal is picked up by a coil and sent to the computer system. the received signal is sinusoidal in nature nThe computer receives mathematical data, which is converted through the use of a Fourier transform into an image.The ImagePhysics of MRIIt is an interplay ofnMagnetismnReso
16、nancenFig: 1. A) The top spinning in the earths gravity. The gravity tries to pull it down but it stays upright due to its fast rotation. B) A charge spinning in the magnetic field Bo. nFig: 2. A) The protons spinning in the nature, without an external strong field. The directions of spins are random and cancel out each other. The net magnetization is nearly 0. B) In the presence of a large external magneti
