Introduction to MRI Physics.docx

***[Introduction to MRI Physics]*** **[Hydrogen]**\ -Atom with Single Proton\ -Is most Strongly affected by the magnetic field\ -Abundant in human body\ -Static Magnetic Field causes Hydrogen Protons to behave like tiny compasses\ -Most of the Hydrogen Atoms will align parallel with the  main magne...

***[Introduction to MRI Physics]*** **[Hydrogen]**\ -Atom with Single Proton\ -Is most Strongly affected by the magnetic field\ -Abundant in human body\ -Static Magnetic Field causes Hydrogen Protons to behave like tiny compasses\ -Most of the Hydrogen Atoms will align parallel with the  main magnetic field\ -The hydrogen protons are then hit with Radiofrequency waves that are targeted only to the Hydrogen protons because they are resonating at the same frequency\ -The RF waves cause the protons to spin at a particular frequency in a particular direction\ -Once the RF waves are turned off, the Hydrogen Protons release the energy and return to their resting state in the static magnetic field\ -The excess energy released by the Hydrogen Protons is picked up by the coils and sent to the scanners computer\ \ **[Basic MRI Hardware:]**\ -Gantry\ -Surface Coils\ -Pt table/transport\ -Computer Workstation\ \ **[Main Magnet]**\ -Measured in Tesla, most frequently 1.5T and 3T\ -1T=42.5MHz\ -1.5T=64MHz\ -3T=128MHz\ \ **[History]**\ -MRI first used clinically in the 1980\'s\ -Nikola Tesla was the first to describe the rotating magnetic field in 1882\ -Isidor Rabi demonstrated nuclear magnetic resonance when he described how nuclei could be flipped by an oscillating magnetic field\ -Used molecular beams to measure the magnetic moment of Lithium Chloride\ -Won the Nobel Prize in Physics in 1944\ -Felix Bloch demonstrated NMR in Water in the 1940\'s (Standford)\ -Edward Mills Purcell demonstrated NMR in Paraffin in the 1940\'s (MIT)\ \*\*\*Felix Bloch and Edward Mills Purcell made their discoveries simultaneously and independently and shared the Nobel Prize in Physics in 1952\ \*\*Their discoveries were made possible by the development of Radar Technologies during WWII\ -Raymond Damadian applied the nuclear magnetic resonance method to living organisms to detect disease in the 70\'s\ -Was the first to create the first full body MRI machine called \"Indomitable\" in 1977\ -Performed the first full body scan of a human for cancer\ -He discovered that cancer cells contain more water than healthy cells\ -\"Indomitable\" was considered too slow and clumsy\ -Took out full page ads in newspapers with the headline \"A Shameful Wrong that must be righted\"  when Paul Lauterbur and Peter Mansfield won the Nobel Prize in Physics in 2003\ -Paul Lauterbur was the first to realize a two-dimensional image could be produced using a gradient magnetic field and could be combined to create a 3D View\ -Peter Mansfield adopted a technique called \"line scan imaging\" to complete an image within minutes instead of hours.\ -Created the first image of the human body, one of his students\' fingers, which took 15-23 minutes per section\ \*\*\*Paul Lauterbur and Peter Mansfield shared the Nobel Prize in Physics in 2003, and were considered the \"Real Inventors\" of MRI\ -Other notable Scientists:\ -Jean Baptiste Fourier: Fourier Transform\ -Joseph Larmor: Larmor Equation\ -Michael Faraday: Faradays Law of Induction\ -Denis Le Bihan: DWI/DTI/fMRI\ -Charles Dumoulin: MR Angiography\ -Richard Ernst: Ernst Angle **[Magnetism:]**\ -A physical phenomenon produced by the motion of electrical charge, resulting in attractive and repulsive forces between objects\ -Faraday\'s Law of Magnetic Induction:\ -When a coil or loop of wire is exposed to a moving magnetic field, a voltage will be induced into that loop of wire\ -Flux lines run parallel to the Bore with the N end at the far end and the S end at the entrance of the gantry\ -Diamagnetic\ -No magnetic susceptibility or attraction due to evenly paired electrons\ -Does not retain magnetism\ -Examples:\ Helium, Hydrogen Molecule, Gold, Copper, Silver, Lead, Glass, Wood\ -Paramagnetic\ -Weak magnetic attraction to external magnetic field\ -Can increase the strength of the external magnetic field due to unpaired electrons\ -Does not retain magnetism\ -Examples:\ -Titanium, Platinum, Aluminum, Gadolinium Contrast, Lithium\ -Superparamagnetic\ -Exhibits attraction to an external magnetic field at a strength in between Paramagnetic and Ferromagnetic\ -A single domain magnetic particle\ -Does not retain magnetism\ -Examples:\ -Ferritin and Hemosiderin\ -Ferritin is a protein that stores iron in tissues; a repository for iron atoms\ -Hemosiderin is released by the breakdown of hemoglobin and ferritin\ -Ferromagnetic\ -Strong attraction to external magnetic field\ -Does retain magnetism\ -Examples:        \ -Steel, Nickel, Cobalt, Gadolinium Element, Iron\ \ **[-Types of MRI Magnets]**\ -Permanent\ -ALWAYS ON\ -.5T or less (variable w/room temp)\ -Magnet field runs vertical\ -Open bore\ -Considered very low maintenance; No Cryogens to replace, permanently turned on\ -\"ALNICO\": Magnetic field produced by: Aluminum, Nickel, and Cobalt; ferromagnetic materials used to create the permanent magnet\ -Very heavy\ -Superconducting\ -ALWAYS ON\ -.5T or Higher (typically 1.5 or 3T at JH)\ -Mostly Closed bore (70cm)\ -Helium must be topped off at less than 50%, checked daily\ -Used to be Nitrogen but boiled off too quickly and had to be topped off too often\ -Magnetic field produced by super-cooled electric coils\ -Can only be turned-off by Quenching\ -Emergency Quench done if life threatening or fire\ -Controlled Quench if object stuck to scanner\ -Concerns with quench: asphyxiation or frostbite\ -Resistive\ -CAN BE TURNED OFF\ -.4T or less\ -Very Heavy\ -Magnetic field created by constantly powered electric copper-wound coils and solenoids (Faraday\'s Law of Electromagnetic Induction)\ -No cryogens\ -Closed Bore, small bore size\ -Large amount of electricity needed\ **[\ Solenoid Electromagnet Components:]**\ -Bucking Coil (surrounds Main magnet Solenoid)\ -Main Magnet Solenoid\ -Solenoid Segments (between main magnet solenoid and bucking coils)\ \ **[Main Magnet Components:]**\ -Main Magnet\ -Shim System\ -Gradient System\ -RF Transmission Coils\ -RF Receiver Coils\ -Computer Subsystem\ -Computer System\ \ \ **[Magnet Shielding:]**\ -The magnetic field extends beyond the scanner\ -Measures must be taken to contain the magnetic field within the MRI room\ -Two types of Magnetic Shielding:\ 1. Passive\ -Maintains the magnetic field within the walls of the room\ -Large Steel Plates placed in the walls\ -Very large, heavy and expensive\ -Effects the homogeneity of the magnetic field\ 2. Active\ -Introduced in 1987\ -Maintains the magnetic field within the scanner\ -Components: Bucking Coils-an extra set of coils within the gantry that creates an opposing magnetic field to the main magnet