Læknablaðið - 15.03.1984, Blaðsíða 41

LÆKNABLADID 70,107-117,1984
107
Brian A. Worthington
THE CLINICAL POTENTIAL OF NMRIMAGING
SUMMARY
Nuclear Magnetic Resonance (NMR) is a new
non invasive method of studying the internal
structure of the body which completely avoids
the use of ionizing radiation and appears to be
unassociated with any adverse effects. The
principles underlying the technique are ex-
plained and an indication is given of its great
versatility in studying pathology by reference
to the preliminary findings in a study of over
700 patients. Much work i^now being directed
towards both defining the'optimal technique
for a given pathological condition which
requires a better understanding of the factors
controlling tissue contrast, and exploring the
specificity of the information which can be
derived from the images so as to determine
the extent to which precise tissue identifica-
tion will be possible.
INTRODUCTION
Within the electromagnetic spectrum there
are several windows available for imaging the
body consisting of bands of radiation which
can penetrate human tissue. Nuclear Magnetic
Resonance (NMR) imaging utilises radio-fre-
quency radiation in the presence of a magne-
tic field to produce cross-sectional images of
the body which portray the distribution den-
sity of hydrogen nuclei and parameters relat-
ing to their motion in cellular water and lipids.
As with CT, the display of soft tissue detail is
at a premium and with NMR there is the
added advantage of being able to manipulate
the contrast between different tissues in order
to highlight pathological changes by altering
the pattern of radio-frequency pulses which is
applied. In NMR imaging there are no specifi-
cally directed photons or specially aligned
This article is based on a paper read to the Icelandic
Radiological Society on December, 6th, 1982 by professor
B. S. Worthington, B.Sc., M.B.B.S., D.M.R.D., F.R.C.R. All
enquiries and correspondence should be addressed to:
Division of Radiology, Queen’s Medical Centre, Notting-
ham, NG7 2UH, England.
detectors. Choice of imaging plane is achieved
by methods which restrict data collection to
the desired region; this means that choice of
slice thickness and position can be achieved
without patient movement and that the addi-
tional perspective of direct sagittal and coro-
nal views is possible as well as the more
conventional transverse sections (1). This avo-
ids the time and radiation dose penalty associ-
ated with reformatted projections in compu-
ted tomography. NMR has the further advan-
tages of being non-invasive; avoiding the use
of ionizing radiation; penetrating bone witho-
ut significant attenuation and it appears to be
without hazard. The phenomenon of nuclear
magnetic resonance (NMR) was discovered in
1945 and from it derives the discipline of
NMR spectroscopy which is now widely used
in the study of matter at the molecular level.
Only relatively recently has it been applied as
a method for imaging the body, but there has
now been progression from a period where
work was only carried out on laboratory
prototypes to the present widespread installa-
tion of commercial machines for clinical evalu-
ation (2, 3, 4).
PRODUCTION OF THE NMR SIGNAL
Almost all of the images to date have been of
the nuclear magnetism of the hydrogen nucle-
us, or proton, which is a particularly favour-
able nucleus from the NMR standpoint becau-
se it gives a relatively high signal and it has a
high abundance in biological tissues. Protons
have associated with them both a spin and a
magnetic field and can be regarded as tiny bar
magnets spinning about the vertical axis.
When placed in a uniform magnetic field this
axis is both tilted and caused to rotate with a
precessionel movement like a tiny gyroscope.
The frequency of this precession is directly
proportional to the applied magnetic field and
for protons in a field of 0.15 tesla is 6.39 MHz.
If a pulse of oscillating radiofrequency (RF)