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1、M -AGNETIC R -ESONANCE I -MAGINGCONTENT PHYSICS SAFETY APPLICATIONS QUESTIONSPHYSICSTYPES OF ATOMIC MOTION The electron orbits the nucleus The electron spins on its own axis *The nucleus spins on its own axis*MRI USES THE HYDROGEN ATOM1 electron orbits the nucleus The nucleus contains no neutrons bu
2、t contains 1 protonTHE HYDROGEN NUCLEUS HAS A NET POSITIVE CHARGEHydrogen nucleus is a spinning, positively charged particleLAW OF ELECTROMAGNETISMA charged particle in motion will create a magnetic field The postitively charged, spinning hydrogen nucleus generates a magnetic fieldWHY HYDROGEN?Very
3、abundant in the human body-H20 Has a large magnetic momentMAGNETIC MOMENTThe tendency of an MR active nuclei to align its axis of rotation to an applied magnetic field MR ACTIVE NUCLEI odd # protons or odd # neutrons or BOTH e.g. Hydrogen1, Carbon13, Nitrogen15, Oxygen17, Fluorine19, Sodium23, Phosp
4、horus31 STABLE ATOMS# protons = # electrons IONS # protons # electronsWhen a body is placed into the bore of the scanner, the strong magnetic field will cause the individual hydrogen nuclei to either: A) ALIGN ANTI-PARALLEL TO THE MAIN MAGNETIC FIELD (B0) OR B) ALIGN PARALLEL TO THE MAIN MAGNETIC FI
5、ELD (B0)B0NMVAnti-parallel high energyParallel low energyNET MAGNETIZATION VECTOR An excess of hydrogen nuclei will line up parallel to B0 and create the NMV of the patientNSNSdirectionsizeThe magnetic vectorTHE NUCLEI WILL ALSO PRECESS PRECESSION Due to the influence of B0, the hydrogen nucleus “wo
6、bbles” 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”Hydrogen nucleusB0Precessional pathPRECESSION The speed at which hydrogen precesses depends on the strength of B0 and is termed the “precessional frequency” Th
7、e 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 RESONATERESONANCE Occurs when an object is exposed to an oscillating perturbation that has a freq
8、uency close to its own natural frequency of oscillationElla Fitzgerald Tacoma Narrows bridge failureRESONANCE cont Frequency of the hydrogen proton in a 1.5T magnetic field can be found in the RF band of energy in the electromagnetic spectrumRADIOFREQUENCY ENERGY Follows the Law of Electromagnetism
9、(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 B0DURING RESONANCE 1) The hydrogen atoms begin to precess “in phase” 1)2) The hydrogen atoms align with the RFs magnetic
10、field (B1) and they flip!B0B0B1B1NMVNMV flips!RFPULSEAS THE NUCLEI PRECESS IN-PHASE IN THE B1 PLANE, A CHANGING MAGNETIC FIELD IS CREATEDIF YOU PLACE A RECEIVER COIL (ANTENNA) IN THE PATH OF THE CHANGING MAGNETIC FIELD, A CURRENT WILL BE INDUCEDTHIS IS FARADAYS LAW OF INDUCTIONFARADAYS LAW OF INDUCT
11、IONA changing magnetic field will induce an electrical current in any conducting mediumCOILSUsed to: transmit pulses of radiofrequency energy receive induced voltage - MR SIGNAL increase image quality by tuning in to one body part at a timeRELAXATIONWhen the RF pulse is turned “off”, the NMV “relaxe
12、s” back to B0 (away from B1)B0B1NMVRF pulses are applied very quickly in succession - RF PULSE SEQUENCE 3 minute sequence (20 slices, axial brain) - 60 RF pulses may be appliedMR SIGNAL Collected by a coil Encoded through a series of complex techniques and calculations (magic?) Stored as data Mapped
13、 onto an image matrixTR - REPETITION TIMETime from the application of one RF pulse to another RF pulseTE - ECHO TIMETime from the application of the RF pulse to the peak of the signal induced in the coilT1 WEIGHTINGA short TR and short TE will result in a T1 weighted image Excellent for demonstratin
14、g anatomy T2 WEIGHTINGA long TR and long TE will result in a T2 weighted image Excellent for demonstrating pathologyMANY OTHER DIFFERENT TYPES OF IMAGES THAT COMBINE ABOVE AND INCLUDE OTHER PARAMETERST1 WEIGHTED IMAGET2 WEIGHTED IMAGESAFETYTHE MAGNET IS ON ALL THE TIME!OHMS LAW OF RESISTANCE V = IR
15、V = voltage I = current R = resistance R depends on the material, the length, the cross-sectional area, and the temperature of the loops of wire through which the current flows *Decreasing the temperature of the wire will decrease resistance to the flow of electricitySUPERCONDUCTING MAGNET No resist
16、ance 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!Gauss - measure of magnetic field strengthrefrigerator magnet - 150-250 G10,000 Gauss = 1TMRI - 0.2T - 1.5T 100x stronger that fridge magnetTHE STRONG MAGNETIC FI
