Imagen por Resonancia Magnetica Cardiovascular

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·xBo). It is propor tional to the strength of
the magn etic field Bo, as the gyromagnetic ratio
J' is a consta nt (i·= 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