Læknablaðið - 15.03.1984, Page 42

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LÆKNABLADIÐ
radiation from a coil is imposed on such a
group of protons in a magnetic field, then
there will be a strong interaction or resonant
effect when the frequency of the oscillator is
equal to the precessional frequency of the
protons. NMR refers to the resonant absorp-
tion and re-emission of radio waves shown by
such nuclei. The nuclear signal picked up
following an RF pulse is known as the free
induction decay (FID). The magnitude and
length of this FID is determined by the so-
called relaxation times which can be regarded
as a measure of the molecular motion of the
protons. The signal following an RF pulse has
an initial size which is proportional to the
density of protons and it then gradually dies
away as the disturbed protons which are
initially all moving in phase return or relax
back to their original position exchanging
energy with their surroundings and between
themselves as they do so. The T, relaxation
time is the time that it takes for them to return
to their original position and the T2 relaxation
time is related to the time that it takes for the
nuclear motions to get out of step with one
another. Different RF pulse sequences have
been devised which are able to generate
signals dependent on the three principal NMR
parameters either singly or in combination.
These sequences are of two types. On one the
protons are disturbed intermittently and a
signal collected whilst they return to their
original positions. This includes the repeated
free induction decay sequences where the
resulting image is weighted by proton density
(p); the inversion recovery sequences where
the resulting images have a strong T, depen-
dence and the spin echo sequences which have
a high T2 dependence. In the second type of
sequence the protons are repeatedly disturbed
by a stream of RF pulses applied in rapid
succession so that a continuous signal is
generated. This is called a steady state free
precession sequence and the intensity of the
NMR signal is given by the expression px
T2/T,.
LOCALISATION OF THE NMR SIGNAL
Since the resonant frequency is proportional
to the strength of the magnetic field, if the field
is not constant over the sample but has a
uniform gradient in one direction then the
resonant frequencies of protons in different
regions of the sample will differ dependent
on whether they are in a high or low field
region. Each volume element in the sample is
therefore labelled by having a different reso-
nant frequency for the protons contained
within it. The resulting complicated nuclear
signal can be digitised and frequency analysed
in a computer using a technique known as
Fourier Analysis. The distribution of the fre-
quencies so obtained is a one-dimensional
projection of the proton NMR signal onto the
gradient direction. By applying gradients
along different directions at univorm angular
increments to cover a total of 180° these
different projections can be combined to give
a two-dimensional cross-sectional image of
the object using reconstruction algorithms
similar to those employed in conventional CT-
scanning. The spatial resolution obtainable is
limited by the dimensions of the smallest
volume element whose NMR signal can be
detected whilst still maintaining a usable
signal to noise level. By increasing the field
gradients images of a restricted portion of the
body can be obtained with a smaller pixel size
and therefore improved spatial resolution.
Unless the imaging time is also increased
however there is an inevitable reduction in the
signal to noise ratio leading to loss of contrast.
SELECTION OF THE POSITION AND
THICKNESS OF THE IMAGING PLANE
Because the radio-frequency radiation from
the transmitter coil cannot be collimated into
a narrow beam like X-rays on account of its
greater wavelength; there is a fundamental
difference in the method of selecting the body
plane to be imaged. With NMR imaging this is
achieved by a variety of methods which
confine the collection of data to the desired
region. One method is to isolate a slice by
applying two oscillating magnetic fields which
effectively destroys the NMR response except
from a central null region where the fields
cancel out. The position of the selected plane
as well as its orientation and thickness can be
altered by adjusting the current fed to the
coils. Alternatively, each RF pulse can be so
tailored that, within the field conditions only a
selected strip of protons within the sample
will be excited.
COMPONENTS OF AN NMR IMAGING
SYSTEM
AIl NMR scanners have the same basic
components; a large magnet, within which are