Imagen por Resonancia Magnetica Cadiovascular
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This document provides an overview of the physical principles of magnetic resonance imaging (MRI). It explains that MRI works by detecting the magnetic resonance signal of hydrogen protons in the body when they are excited by radio waves within a strong external magnetic field. When a radio frequency pulse is applied that matches the protons' Larmor frequency, it causes the protons' magnetization to be deflected from its equilibrium alignment with the magnetic field. As the magnetization then precesses and relaxes back to equilibrium, an MRI signal is emitted and detected which can be used to construct images. The document outlines the basic physics involved, including precession, Larmor frequency, excitation, and relaxation processes.
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Imagen por Resonancia Magnetica Cadiovascular
- 10. x Authors index Prof. D r. WARREN J. M ANNI NG Depart ment of Medicine Cardiovasc ular Division Beth Israel Deaconess Med ical Center 330 Brookline Avenue Boston, MA 02215, USA Ur. ,. TIM MARCUS Dept. of Clinical Physics and Informatics University Hospital VU De Boclelaan 1117 1081 IIV Amsterdam, Th e Netherlands Dr. OLAF MOttLI NG Cardi ac Imaging Research Fellow Department of Medicine, Grosshadcrn Campus University of Mu nich Marchioninistrasse IS 81377 Munich. Germa ny Priv.-Doz. Dr. EIKE NAGEL German Hearl Inst itute Berlin Cardiology - CMR Augustenburger Platz I 13353 Kerl in. Germa ny Professor Dr. STEFAN N f.U8AU ER Department of Cardiovascular Medi cine John Rad cliffe Hospital Headley Way Headington Oxfo rd OX3 9DU, United Kingd om Dr. MICHA EL N EUSS German Heart Inst itute Berlin Cardiology - CMR Augustenburge r Platz 1 13353 Berlin. Germany Dr. INGO PAETSCH German Heart Institute Berlin Cardiology - CMR Augu stenburger Platz 1 13353 Berlin , Germany Prof. Dr. FRANK E. RA DEMAKERS University Hospital Gasthuisbc:rg Department of Cardiology Herestraar 49 3000 Leuven, Belgium JANINA RERAKOWSKI German Heart Institute Berlin Cardiology - CMR Augustenburger Platz I 13353 Berlin, Germany Professor Or. ALRERT C. VAN ROSSUM Department of Cardiology VU University Medical Center De Boelelaan 1117 1081 HV Amsterdam , The Netherlands Prof. Dr. MAYTlIEM SAEED Department of Radiology, Schoo l of Medicine University of California San Francisco 50S Parnassus Ave, L-308 San Francisco. CA 9~ 143-0628. USA Dr. BER NIIARD SCliNACKEN8URCo Philips Medical Systems Rbmgenstr. 24-26 22335 Hamburg, Germany Dr. JORG SCItWITTER Senior Co nsultant Division of Cardio logy Ca rd iovasc ular MR Center Raemistrasse 100 8091 Zur ich, Switze rlan d Professor Dr. UDO SECItTEM Division of Cardiology Department of Med icine III Robert Bosch Medi cal Center Auerbachstrasse 110 70376 Stuttgart, Germany Priv-Doz, Dr. ELMAR SPONT RUP Department of Diagn ostic Radiology RWTII Aachen Pauw elsvrrasse 30 52057 Aachen, Germ any Prof. Dr. MATTIIIAS STUBER Johns Hopkins University Schoo l of Medicine IHOC ~243 60 1 North Caroline Str. Baltimore, Mil 2 1287-08~5. USA Dr. ANIA WAGNF.1t Division of Cardiology Department of Medi cine III Robert Bosch Medical Center Auerbachstrasse 110 70376 Stuttgart, Germa ny Dr. DOMI NIK WEIStlAUl'T Institute of Diagnostic Radiology University Hospital Raemistru sse 100 8091 Zurich. Switzerland Dr. NORBERT M. WILKt: University of Flor ida Cardiovascular MR and cr Center Jacksonville/Gainesville. Florida, USA Dr. W. YO NG KI M Aarhus University Hospital Skejby Sygehus Brendstrupgardsvej 100 8200 Arhus N, Denmark
- 11. IPart AI Basics
- 12. 1Physical principles of MR imaging BERNHARD$CHNACKENBURG ~ Magnetic resonance (MR) 1 In orde r to prese nt a short but eas ily under- stood description of the physical principles, the following text uses an extremely simplified model. Reference to further literature can be found at the end of Chapte r 4. The basis for MR imaging is the magnetic resonance of atomic nucl ei, which was demon - stra ted experi mentally by Felix Bloch and Ed- ward Purcell, working ind ependentl y, as early as 1946 (Nobel Prize 1952). In medical diagn ostics, the resonance signal from the hydrogen nucleus is used to create images. Hydro gen is present in tissue fluids and fat, and is therefore plent iful in the human body. The nucleus of the hydro- gen atom consis ts of only one pa rt icle: the pos i- tively cha rged proton. Due to its inh erent rota- tion (known as spin) the proton creates a mag- netic field like that of a small bar magnet (Fig. J.l). In biological tissue, these magnet ic fields usually have arbitrary, randomly distrib- uted di rect ions, so that there is no external magnetic effect. However, in an external mag- netic field (Bo) these magnetic fields align themselves with the extern al field like compass needles. In equilibrium, the sum of all the mag- netic fields of the protons produces a macro- scopic magn etizati on ' , but thi s canno t be de- I More correctly: Nuclear Magnetic Resonance (NMR), but the 'N' is omitted in medical applica- tions. 2 The behavior of individual atomic nuclei, which has been greatly simplified in this account. can really only be described with the aid of quantum mechanics. However, as even very small volume elemen ts contain a very large number of atomic nuclei, the total effect of all nuclei - the macro- scopic magnetization - can be considered. This total effect can be treated with the more easily un- derstood methods of classical physics. tected as lon g as it is pa rallel to the external magnetic field. If the eq uilibrium of this mag- neti zation is dist urbed, so that its orientatio n is at an angle to the orie ntation of the main field Bo, the magneti zat ion will per form a rotation movement (precession, Fig. 1.2). The freq uen cy of the precession; is the Larmor frequency (wo= i揃xBo). It is propor tional to the strength of the magn etic field Bo, as the gyromagnetic ratio J' is a consta nt (i揃= 42.6 MHz!T for protons). As a result, there is a different Larmor frequency, and consequently a different MR signal, for dif- ferent field strengths. The strength of the mag- netization depends on the density of protons in the tissue. The external, static magnetic field is usually produced by a hollow cylindrical super- conducting magnet, with the patient lying with- in it. In this case, the magn eti c field is aligned with the longi tudinal axis of the cyli nder (z- axis). The deflection of the magnetization from its equilibrium position, which is needed for detection, is obtai ned by applying a high- or radi o-frequency (RF) pulse. This process is also referred to as excitation. The freq uency of the RF pulse must be the same as the Larmor fre- quency Wo (the resonance requirement ) as Fig. 1.1. The nucleus of the hydrogen atom is positively charged and rotates about its own axis. Due to the rotation, the electrical charge moves in a circle and creates a mag- netic field, similar to that of a bar magnet, which is oriented alonq the axis of rotation