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核磁共振成像原理课件(英文)
M -AGNETICR -ESONANCE I -MAGING CONTENT PHYSICS SAFETY APPLICATIONS QUESTIONS PHYSICS TYPES OF ATOMIC MOTION The electron orbits the nucleus The electron spins on its own axis ***The nucleus spins on its own axis*** NET MAGNETIZATION VECTOR An excess of hydrogen nuclei will line up parallel to B0 and create the NMV of the patient THE NUCLEI WILL ALSO PRECESS… PRECESSION Due to the influence of B0, the hydrogen nucleus “wobbles” or precesses (like a spinning top as it comes to rest) The axis of the nucleus forms a path around B0 known as the “precessional path” PRECESSION The speed at which hydrogen precesses depends on the strength of B0 and is termed the “precessional frequency” The precessional frequency of hydrogen in a 1.5 Tesla magnetic field is 63.86 MHz The precessional paths of the individual hydrogen nucleus’ is random, or “out of phase” WE NEED THEM TO BE “IN-PHASE” OR TO RESONATE… RESONANCE con’t Frequency of the hydrogen proton in a 1.5T magnetic field can be found in the RF band of energy in the electromagnetic spectrum RADIOFREQUENCYENERGY Follows the Law of Electromagnetism (charged particles in motion will generate a magnetic field) Magnetic field known in MR as B1 Applied as a “pulse” during MR sequences The RF pulse is applied so that B1 is 90? to B0 MR SIGNAL Collected by a coil Encoded through a series of complex techniques and calculations (magic?) Stored as data Mapped onto an image matrix T1 WEIGHTED IMAGE T2 WEIGHTED IMAGE SAFETY THE MAGNET IS ON ALL THE TIME!!! OHM’S LAW OF RESISTANCE SUPERCONDUCTING MAGNET No resistance to flow of electricity Coils of wire surrounded by cryogen bath (Helium) at -273? C No external source of energy required Magnetic field present ALL THE TIME!!! THE STRONG MAGNETIC FIELD OF THE MAGNET CAN TURN THE FOLLOWING INTO DANGEROUS PROJECTILES: wheelchairs oxygen tanks I.V. poles I.D. tags keys coins scissors trauma boards sandbags safety pins THE CHANGING MAGNETIC FIELDS CAN DO DAMAGE TO: THE FOLLO
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