Læknablaðið - 15.03.1984, Blaðsíða 48

110
LÆKNABLADID
situated gradient coils and immediately sur-
rounding the part to be imaged are appropria-
tely sized RF transmitter and receiver coils
(Fig. 1). Ancillary equipment is required to
generate and analyse the NMR signals from
which the final image is reconstructed. With
our system sections 1 cm. in thickness are
obtained in a time of 2 minutes and are
displayed on a 128x 128 matrix (Fig. 2). As
with CT the dynamic range of data is such that
it cannot be encompassed on a single grey
scale whose levels can be appreciated by the
human visual system. It is therefore necessary
to adopt a windowed display of the data with
variations possible in both its width and level.
In the majority of images shown in this
review, which were obtained using steady
state free precession sequences, tissues such as
fat with a high density of mobile protons are
displayed at the white end of the grey scale,
whereas cortical bone with a correspondingly
low density is shown at the black end of the
scale. By making alterations to the pulse
sequence used dramatic changes can be pro-
duced in the appearance of tissues which have
precisely the same proton density and relaxa-
tion times as is shown in Fig. 3. With our
modified SSFP sequence regions with a T,
relaxation time greater than 800 milliseconds
give no signal so that CSF in the ventricular
system appears black despite having a high
proton density. By using our unmodified SSFP
pulse sequence, however, so that iong T|
values now give a signal the ventricular CSF
appears white. The sensitivity of our multiple
pulse sequences to motion also modifies con-
trast by the selective removal of signal from
moving protons so that large blood vessels
appear black.
CRANIAL SCANNING
In cranial scanning comparison with CT shows
that expanding lesions such as intrinsic tu-
mours produce recognisable displacements
and deformity of the ventricular system. More
significantly in diagnosis, however, are the
pattern of altered tissue density and texture
together with the zone of transition between
the lesion and the adjacent normal brain (5).
Whilst all the intrinsic neeoplasms which we
have studied were shown by NMR scanning,
in a few cases the separation of tumour
margins from surrounding oedema was less
clear than on contrast enhanced CT scans. On
the other hand several low grade infiltrating
gliomas have been better seen with NMR than
CT emphasising its greater sensitivity. The
multiplanar facility allows precise volumetric
assessment and is useful in accurate localisati-
on, as for example in small high convexity
tumours. The lack of significant artefacts from
adjacent bone and air containing structures as
occurs with CT should be stressed; and this is
of particular value when studying tumours
within the posterior fossa (6). Like other
workers (4) we have noted that certain combi-
nations of relaxation times may lead to there
being poor or even no contrast between a
neoplasm and the surrounding brain when
using one spin sequence; whereas a striking
contrast is present when using another sequ-
ence in which the resulting signal is weighted
differently by the relaxation times (Fig. 3).
The investigation of pituitary and juxta-
sellar mass lesions can be particularly difficult
because clinical manifestations such as visual
failure may occur when the Iesions is small.
Appropriate management requires precise
localisation and a distinction between the
possible pathologies. The multiplanar facility
of NMR is valuable in defining the extra-sellar
extension of adenomas and in establishing
their topographical relationship to adjacent
structures (Fig. 4). Follow-up studies to assess
the affect of radiotherapy or drug therapy on
tumour size are simple to carry out. An empty
sella syndrome is readily identified as a cause
for an enelarged pituitary fossa with a single
midline sagittal section. Using flow dependent
sequences the presence of fast mooving blood
within a juxta-sellar aneurysm can be demon-
strated thus allowing a precise diagnosis to be
made (Fig. 8).
In the diagnosis of acoustic neuroma (Fig. 5)
the absence of signal from bone has meant
that the images are unaffected by artefacts
and that small intra-canalicular tumours can
be visualised directly. The multiplanar facility
allows assessment of both the tumour volume
and its relationship to the ventricular system,
the brain stem and tentorial hiatus.
Cerebral infarction within the hemispheres
can be shown as well with NMR as with CT
and in the brain stem NMR has been shown to
be clearly superior (7). Intra-cranial haemato-
mas which have a relatively short T, relax-
ation time and a long T2 relaxation time are
well shown (Fig. 6). After cranial trauma NMR