HIER - Congress Company

eduRAD syllabus 69
28
Diffusion imaging
Apparent diffusion coefficient (ADC) values as measured
with DWI are inversely related to cellularity, i.e. increased
cellularity restricts diffusion and reduces ADC values.
As increased cellularity is considered an indication of
high grade tumour, it is expected that ADC may be useful
for radiological tumour grading. Findings, however,
are inconsistent and conflicting, with some authors
reporting decrease of ADC in high grade tumours, as
would be expected with increased cellularity (3), but
others reporting increased ADC (4). The source of such
discrepancies may, at least partially, be found in tumour
heterogeneity. High grade tumours contain (micro)necrotic
components in which diffusion is increased. Furthermore,
diffusion is also increased in the peritumoural vasogenic
oedema.
MR spectroscopy
Proton MR spectroscopy shows a correlation of the
choline (Cho) peak with cell density and the lipid peak
with proliferation (5). With increasing tumour grade there
is generally an increase in the Cho/Creatine (Cr) ratio, a
reduction in N-acetylaspartate (NAA) and an increase in
the lactate/lipid peak (5-8). MR spectroscopy may be of
particular use in the grading of oligodendrogliomas, in which
contrast enhancement and increased rCBV ratios may be
observed both in low and in high grades (7-9).
Guiding neurosurgical intervention
Among the commonly performed neurosurgical interventions
we can distinguish diagnostic and therapeutic procedures.
During either intervention damage to eloquent brain regions
needs to be avoided. While eloquent brain regions can
readily be identified on the basis of anatomical landmarks
in the normal brain, such landmarks may be obscured in the
presence of brain tumour with considerable mass effect.
Advanced MR imaging techniques may be used to provide
such information preoperatively. Furthermore, advanced MR
imaging may also be used to identify the optimal target for
diagnostic procedures.
MR perfusion and spectroscopy
Tumour grading is based on the highest malignancy grade
identified within a tumour. Especially with stereotactic
biopsy, sampling error is a real issue, when the tumour is
under graded if the most malignant part of the tumour is
not biopsied. With conventional MR imaging, stereotactic
biopsy is generally targeted at the enhancing part of the
tumour, which does not necessarily correspond with the
most malignant part of the tumour. Such sampling errors
may be avoided with the use of MR perfusion imaging or MR
spectroscopy, with which the most vascular or malignant
regions are readily identified.
I n s c h r i j v e n v i a w w w . c o n g r e s s c o m p a n y . c o m
o f w w w . r a d i o l o g e n . n l
Figure 1. Combined fMRI and DTI-tractography indicating
displacement of both the primary motor cortex and the corticospinal
tract by the tumour mass effect.
Functional and diffusion tensor MR imaging
The aim of neurosurgical therapy is maximum tumour
resection, while at the same time avoiding new functional
deficit. In cases of tumour localisation in or near eloquent
brain areas, such the motor cortex or language areas,
additional advanced imaging may be advantageous to guide
the neurosurgical approach, shorten surgery duration and
obtain prognostic information prior to surgery.
Functional MR imaging (fMRI) is used increasingly to assess
the relationship between functionally eloquent cortex and
brain pathology. Such information is particularly useful when
normal anatomy is obscured by tumour mass effect or in
cases of cortical plasticity. In an elegant study of 39 brain
tumour patients, Petrella et al. demonstrated that treatment
plans were altered based on information obtained with
fMRI in 19 patients (10). Most notably, out of 9 patients
who were considered inoperable based on information from
conventional imaging, 7 were in fact considered operable
upon assessment of the additional fMRI findings.
While fMRI provides valuable information on eloquent
cortex, with diffusion tensor MR imaging (DTI) the anatomy
and involvement of white matter tracts may be evaluated.
Inadvertent transection of white matter tracts during surgery
leads to severe neurological deficit. DTI-tractography offers
attractive visualisation of the major white matter tracts such
as the corticospinal tract and the arcuate fasciculus (figure
1), and offers valuable preoperative information on their
relationships with the brain tumour to be resected (11-13).
As well as providing such anatomical information, colour
coded eigenvector maps obtained with DTI can be used
to categorise involvement of the white matter tracts
by brain tumour (14). Four patterns of such involvement
can be distinguished, indicating whether a tract is only
displaced but not infiltrated, is altered due to vasogenic
oedema, is infiltrated by tumour or is completely destroyed.