The use of nuclear magnetic resonance in cardiology –

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					                                           Bratisl Lek Listy 2005; 106 (10): 333 – 336                                              333




                                                             REVIEW


            The use of nuclear magnetic resonance in cardiology – a
                comparison of Slovak Republic and the world

                                       Uhliar R, Strbova J, Neuschl V, Satko M

                 Hospital of Ministry of Inferior of the Slovak Republic, Bratislava, Slovakia.bll@fmed.uniba.sk


            Abstract

            Since beginning, NMR examination is fully established in experimental work and common clinical praxis.
            Its extensive use in cardiology is determined by non-invasive character and relatively short examina-
            tion time. After solving predominately economical problems, NMR examination will rapidly develop
            even in Slovak Republic (Tab. 2, Fig. 2, Ref. 29).
            Key words: nuclear magnetic resonance, cardiology.


    Since its description in 1946, nuclear magnetic resonance        magnetic moment (nuclei with a non-zero spin) are essential.
(NMR) was used to obtain physical and chemical information           These nuclei rotate like flywheel and gain a non-zero magnetic
about a molecule. From 1977, it has been used in medicine in         moment.
magnetic resonance imaging; firstly in central nervous system             Hydrogen nuclei are the best for NMR – because their fre-
imaging and later also in other specializations. Prof. Felix Bloch   quency belongs to radiofrequency.
and Prof. Edward Purcell shared the Nobel Prize in 1952, for the          Protons can be imagined as small magnets with magnetic
discovery and description of the behavior of atomic nuclei within    moments oriented entirely accidentally, if they are not within an
a strong magnetic field. An increasing importance of NMR in          external magnetic field. If protons are inserted into an external
medicine is highlighted by the Nobel Prize in medicine and physi-    magnetic field, according to the quantum theory, their magnetic
ology, awarded in 2003 to Prof. Lauterbur for the discovery of       moments will organize and precess around the axis of a mag-
coding the spatial information into NMR signal and to Prof.          netic field B0 in two possible orientations: parallelly and anti-
Mansfield for the discovery of mathematical analysis of mag-         parallelly to the external magnetic field (Fig. 1). These two orien-
netic signals.                                                       tations have different potential energies.
    In the eightieth, NMR was introduced into clinical praxis.            To make it easier, for the description of a set of magnetic
Cardiovascular NMR (CMR) has become an important part of the         moments of the protons, the sum of their vectors, vector of mag-
evaluation of the inborn and acquired heart diseases. This non-      netization M, is used.
invasive imaging technique is currently used also in atherosclero-        If protons are within the magnetic field in a state of equilib-
sis of coronary arteries, and myocardial perfusion and function.     rium, they precess in two possible directions so the vector of
    In Slovak Republic, the first experimental NMR images were       magnetization has the same direction as the external magnetic
performed at the Institute of Measurement Science, Slovak Acad-      field. If this direction is marked z, the vector M will posses only
emy of Science, by Prof. Frollo and his colleagues in 1981. Dur-     constituent Mz; in a coordinate xy perpendicular to the direction
ing ninetieth, also a whole-body tomograph TMR-96 was con-           of magnetic field its constituent Mxy will be zero.
structed at this institute for the study of new imaging methods.          If the equilibrium is externally disrupted, vector of magneti-
                                                                     zation will helically divert from the axis. After the external af-
Principles of NMR imaging
                                                                     Hospital of Ministry of Inferior of the Slovak Republic, Bratislava,
    Tissues of living organisms are formed from many molecules,      Slovakia
most of which are composed of atoms of hydrogen and carbon,          Address for correspondence: R. Uhliar, MD, NsP MV SR, Fr. Krala
in less extend of oxygen, nitrogen, phosphorus and other ele-        14, SK-812 72 Bratislava 1, Slovakia.
ments. For NMR phenomenon, atomic nuclei with a non-zero             Phone: +421.2.50941111
334                                          Bratisl Lek Listy 2005; 106 (10): 333 – 336




Fig. 1. Distribution of NMR machines in Slovak Republic at the end     Fig. 2. Distribution of NMR machines in Czech Republic at the end
of 2004.                                                               of 2004.


fection disappears, magnetization will recover possessing direc-       Use of NMR in cardiology
tion of axis z, whilst the velocity of this recovery is determined
by relaxation times of external processes, causing disruption of            Besides an established CNS examination, musculoskeletal
equilibrium (Fig. 2).                                                  system and gastrointestinal track, NMR in cardiology develops
    One of the methods, enabling to divert the vector of magne-        rapidly. This technique has won predominantly in determination
tization from equilibrium, is addition of energy; so a weak alter-     of structural changes like inborn heart defects, ischemic heart
nate magnetic field constant in a perpendicular direction is ap-       disease (extend, localization, functional disorders, complications),
plied. It is important for the transmission of energy that the fre-    myocardial diseases (for example determination, localization and
quency of the field corresponds to the frequency of precession         severity of hypertrophic cardiomyopathy), valve diseases (iden-
(Larmor frequency). It is useful for the clinical use that this fre-   tification and quantification of regurgitation), pericardial diseases
quency belongs to radiofrequency (RF).                                 and so on. NMR enables even the examination of patients with
                                                                       an artificial valve, and intra a para-cardial masses (35).
NMR principles in tissue imaging                                            In NMR examination in cardiology, several established stan-
                                                                       dard procedures are used. In so called single phase multi slice,
     In order to gain an image it is necessary to evaluate the re-     during one heart beat several layers of examined myocardial area
laxation times of individual tissue constituents, because their        are demonstrated. This approach is suitable for the evaluation of
value is related to the image contrast.                                morphologic changes. In contrast, multi phase uses demonstra-
     Therefore a total NMR signal is determined by the set of          tion of one layer during several heart beats. It enables the evalu-
Larmor frequencies and phase shifts (phase shift may be consid-        ation of heart function and chambers and large vessels perfu-
ered as a periodic function with its own spatial frequency) re-        sion. Combination of both procedures gives an image loop in
lated to tissue density and tissue relaxation times, i.e. times T1     one of demonstrated heart plain, which markedly increases the
or T2. This signal is converted from analogous to digital form,        quality of evaluation of heart function and perfusion. During heart
which can be imagined graphically in the form of aligned lines or      and large vessels imaging also special sequences demonstrating
matrix called k-space. Each layer has own k-space. Axis x and y        moving protons with very high or low signal are used. Therefore,
in the k-space represent directions coded by frequency and phase.      according to the signal of the blood moving in heart chambers,
     Two-dimensional Fourier transform of k-space produces the         this method is called the method of either white or black blood.
final image. There is no relationship between the date of k-space
and the image. Each point in k-space contains spatial informa-
tion on the whole object. Points near the center of k-space (areas     Tab. 1. A comparison of NMR, CT and ultrasound examination in
                                                                       Slovak Republic.
of coded low frequencies) determines general appearance and
image contrast, whilst border points (areas of coded high fre-         Type of                       Number of           Number of
quencies) contains coded details of image borders, important for       machine                       machines            examinations
the discrimination.
     It may be simplified that in NMR, individual image points         ultrasound                       251               1 572 440
                                                                       CT                                47                 223 940
are determined by accumulation of hydrogen nuclei and their
                                                                       NMR                               13                  27 986
characteristics in contrast to CT, where tissue absorption coeffi-
cients are most important for the ionized radiation.                   Institute of Health Informative Statistic, 2003
                            Uhliar R et al. The use of nuclear magnetic resonance in cardiology…                                   335

Tab. 2. Number of NMR and Number of NMR machines/million inhabitants.

                                        Number of NMR machines                  Number of NMR machines/
                                                                                /million inhabitants

                  2000          2001           2002          2003            2000           2001           2002          2003

Australia           90                                                       4,74
Austria             88            94            108           110              11          11,7           13,4
Belgium
Canada              76           130                          147              2,5           4,2
Czech Republic      17            19             22                                          1,7           1,9           2,2
Denmark             29                           46            49              5,4                         8,6
Finland             51            57             65            67              9,9            11          12,5
France             156           141            163                            2,6           2,4           2,7
Germany            405           451                                           4,9           5,5
Greece                                           25                                                        2,4
Hungary             20            20             25                             2             2            2,5
Island               3             4              5             5            10,7            14           17,4
Ireland
Italy              435           497            605                            7,5           8,6          10,4
Japan                                          4501                                                       35,3
Korea              254           322            375           430              5,4           6,8           7,9
Luxemburg            2             2              2             5              4,6           4,5           4,5
Mexico              28           110            112                            0,3           1,1           1,1
Slovak Republic      6             7             11                            1,1           1,3             2
Spain              194           231            258                            4,8           5,6           6,2
Sweden
Switzerland         93            93            103                          12,9          12,9           14,1
Turkey                                          207           207                                            3
Great Britain      236                          234           210                4                           4
USA               2286          2342                                           8,1           8,2

OECD HEALTH DATA 2004,                                                        OECD HEALTH DATA 2004,
3rd ed                                                                       3rd ed


     Another procedure in NMR examination in cardiology is the       ever, it must be noticed that all metal items used in surgical and
angiography of large vessels and coronary arteries. It is special    vessel interventions during past 15 years are not a contraindica-
angiographic sequence time of flight, which uses the properties      tion for a NMR examination. The disadvantage, when this kind
of protons moving in external magnetic field, therefore adminis-     of an item is present in organism, lies in a fact, that these items
tration of contrast agent is not necessary, which is favorable. In   create marked artifacts in magnetic field, disabling NMR evalu-
this way an image is produced, where the blood posses a very         ation.
high signal and simultaneously in all other tissues the signal is
lowered. If we add the phase shift method, it is possible to non-    Possibilities of NMR examination in Slovak Republic and the
invasively evaluate and quantitatively analyze perfusion of indi-    world
vidual examined vessels. In NMR coronary artery angiography,
it is possible to use a contrast agent gadolinium. It is resonance       It is estimated, that worldwide total 22 000 experimental and
contrast agent, which is a minimal allergen compared to iodide       clinic institutes exist, where approximately 60 millions of NMR
contrast agents. In such a non-invasive imaging of coronary ar-      examinations are performed per one year. According to the Insti-
teries, catheterization is not necessary. However, this approach     tute of Health Informative Statistic, 13 NMR machines were in
is only experimental and it is not used in common clinical praxis.   Slovak Republic at the end of 2003, which is on average 2.6 to 1
     In eliminating image discrepancies resulting from heart move-   million of inhabitants (Tab. 1). Resulting from a comparative
ments, synchronization of examination with an ECG is neces-          table, Slovakia is behind many industrialized and surrounding
sary. If, from various reasons, the examination is not performed     countries (Tab. 2). According to preliminary data, 17 NMR ma-
during hold inspiration, synchronization with a breathing curve      chines were in Slovak Republic till the end of 2004. Many of
is also necessary.                                                   them are older then 7 years and their technical equipment does
     Indications, contraindications and limitation of NMR in car-    not meet present requirements for image quality. However, even
diology are described in another study. In summary, an absolute      some “more rich” countries like France do not reach the com-
NMR contraindication is an implantation of a pacemaker, co-          mon number of NMR machines pre 1 million of inhabitants in
chlear implant, myostimulators, neurostimulators and so on. How-     Europe. From all NMR machines in Slovakia, only NMR in NsP
336                                             Bratisl Lek Listy 2005; 106 (10): 333 – 336

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