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Prof. D r. WARREN J. M ANNI NG
Depart me nt of Med icin eCardiovasc ular DivisionBeth Israel Deaconess Med ical Center330 Brookline AvenueBoston , MA 02215, USA
Ur. ,. TIM MAR C US
Dept. o f Clinical Physics and Informat icsUniversity Hospital VUDe Boclelaan 11171081 IIV Amsterdam, Th e Netherlands
Dr. OLAF MOttLI NGCa rdi ac Imaging Research FellowDepar tment of Medicine, Grosshad crn CampusUn ivers ity of Mu nichMarchioninistrasse IS8 1377 Munich . Germa ny
Priv.-Doz. Dr. EIKE NAGELGerman Hearl Inst itut e Ber linCa rdiology - CMRAugustenburger Platz I13353 Kerl in. Ger ma ny
Professor Dr. STEFAN N f.U8AU ER
Department of Card iovascula r Medi ci neJohn Rad cliffe HospitalHeadley WayHead ingtonOxfo rd OX3 9DU, Un ited Kingd om
Dr. MI CHA EL N EUSS
German Heart Inst itute BerlinCa rdiology - CMRAugustenburge r Platz 113353 Berlin. Germany
Dr. INGO PAETSCHGerman Heart Inst itute BerlinCard iology - CMRAugu stenburger Platz 113353 Berl in , Ger many
Prof. Dr. FRANK E. RA DEMAK ERSUniversity Hospital Gasthuisbc:rgDepartment of Cardio logyHerestraar 493000 Leuven, Belgium
JANINA RERAKOWSKIGerman Heart Institute BerlinCa rd iology - CMRAugustenburger Platz I13353 Berlin, Germa ny
Professor Or. ALR ERT C. VAN ROSSUMDepartment of CardiologyVU Univers ity Medical CenterDe Boelelaan 11171081 HV Amsterdam , The Netherl ands
Prof. Dr. MAYTlIEM SAEEDDepartment of Radi ology, Schoo l of Med icineUniversity of California San Francisco50S Parnassus Ave, L-308San Francisco . CA 9~ 143-062 8. USA
Dr. BER NIIARD SCliNACKEN 8URCoPhilip s Medical SystemsRbmgenstr. 24-2622335 Hamburg, Germany
Dr. JORG SCItWITTERSenior Co ns ultant Division of Cardio logyCa rd iovasc ular MR Ce nterRaemi strasse 1008091 Zur ich, Switze rlan d
Professor Dr. UDO SECItTEMDivision of CardiologyDepartment of Med icine IIIRobert Bosch Medi cal CenterAuerbachstrasse 11070376 Stuttga rt, Germa ny
Priv-Doz, Dr. ELMAR SPONT RUPDepartment of Diagn ost ic Rad iologyRWTII AachenPauw elsvrrasse 3052057 Aachen , Germ any
Prof. Dr. MATTIIIAS STU BE RJohns Hop kins UniversitySchoo l of MedicineIHOC ~243
60 1 North Ca roline Str.Balti more, Mil 2 1287-08~5. USA
Dr. ANI A WAGNF.1tDivision of CardiologyDepartment of Medi c ine IIIRobert Bo sch Medical CenterAuerbachst rasse 11070376 Stuttga rt, Ge r ma ny
Dr. DOMI NIK WEIStlAUl'TInstitute of Diagnostic Radiolog yUniversity HospitalRaemi stru sse 100809 1 Zurich. Switzerland
Dr. NORBERT M. WILKt:University of Flor idaCardiovascular MR and cr CenterJacksonville/Gainesville. Florida, USA
Dr. W. YO NG KI M
Aarhu s University Hosp italSkejby SygehusBrend strupgar dsvej 1008200 Arhus N, Denmark
IPart AI Basics
1 Physical principles of MR imagingBERNHARD$CHNACKENBURG
~ Magnetic resonance (MR) 1
In orde r to prese nt a shor t but eas ily und erstood description of the physical princip les, thefollowi ng text uses an ext remely simplifiedmodel. Reference to further literature can befound at the end of Cha pte r 4.
The basis for MR imagi ng is the ma gnet icresona nce of atomic nucl ei, whic h was dem on stra ted experi me ntally by Felix Bloch an d Edward Purcell, working ind ependentl y, as early as1946 (Nobel Prize 1952). In med ical di agn ost ics,the resonan ce signa l fro m the hydrogen nucleusis used to create images. Hydro gen is present intissu e fluids and fat, and is ther efore plent ifulin the human body. The nucleu s of the hydrogen ato m co nsis ts of only one pa rt icle: the pos itively cha rged proton. Due to its inh erent ro tation (known as spin) the prot on creates a magnetic field like that of a small ba r magnet(Fig. J.l). In biological tissu e, these magnet icfields usually have arbi trary, randomly di stributed di rect ions, so that there is no externalmagnet ic effect. However, in an exte rnal magnetic field (Bo) these ma gnetic fields alignthem selves with the exte rn al field like co mpassneedles. In eq uilib rium, the sum of all the magnetic fields of the protons produces a macroscopic magn etizati on ' , but thi s canno t be de-
I More correctly: Nuclear Magnetic Resonance(NMR), but the 'N' is omitted in medical applications.
2 The behavior of individual atomic nuclei, whichhas been greatly simplified in this account. canreally only be described with the aid of quantummechanics. However, as even very small volumeelemen ts co ntain a very large number of atomicnuclei, the total effect of all nuclei - the macroscopic magnetization - can be considered. Thistotal effect can be treated with the more easily understood methods of classical physics.
tected as lon g as it is pa ra llel to the externalmagnet ic field . If the eq uilibrium of th is magneti zation is di st urbed, so that its orientatio n isat an ang le to the orie nta tion of the main fieldBo, the ma gneti zat ion will per form a rotationmovem ent (precession, Fig. 1.2). The freq uen cyo f the precession ; is the Larmor frequency(wo= i·xBo). It is propor t ional to the strength ofthe magn et ic field Bo, as the gyromagnetic ratioJ' is a consta nt ( i·= 42.6 MHz!T for protons). Asa result, there is a d ifferent Larmor frequency,and conseque ntly a d iffere nt MR signal, for different field streng ths . The st reng th of the magnet ization depends on the density of proton s inthe tissu e. The external, static magnetic field isusually produced by a hollow cylindrical su perconducting magnet, with the patient lying wi thin it. In th is case, the magn eti c field is alignedwith the longi tudi nal axis of the cyli nde r (zaxis) . Th e deflection of the magnetization fromits equilibrium position, which is needed fordetection, is obtai ned by applying a high - orradi o-frequency (RF) pu lse. This process is alsoreferred to as excitat ion. The freq ue ncy of theRF pulse mu st be the same as the Larmor freq uency Wo (the resonance requi rem ent ) as
Fig. 1.1. The nucleus of the hydrogen atom is positivelycharged and rotates about its own axis. Due to the rotation,the electrical charge moves in a circle and creates a magnetic fi eld, similar to that of a bar magnet, which is orientedalonq the ax is of rotation
