Updated 9 December 2002
Return to 8th International Conference Index
MRI in the Practical Evaluation of Dementia: ‘Beyond Exclusion’
By Philip Scheltens
Philip Scheltens' presentation.
This paper reviews the use of imaging techniques to aid in
the clinical diagnosis of dementia. Two approaches are distinguished.
The exclusionary approach is one in which imaging is used
to rule out diseases that would mimic or cause dementia.
Based on the literature this approach yields very little,
if any, information that was not identified clinically.
The more positive approach
uses imaging as a diagnostic tool to identify changes specific
for causes of dementia.
Any assessment of medial temporal lobe atrophy on Magnetic
Resonance Imaging (MRI) will result in a reasonably high positive
likelihood ratio distinguishing AD patients from non-demented
individuals but fails to distinguish AD patients from patients
with other dementias. For a diagnosis of vascular dementia,
imaging is necessary, although not all vascular changes fulfill
requirements of being relevant dementia. Potentially of more
importance, given the higher prevalence of AD, is identifying
concomitant vascular changes in AD that may be amenable to therapy
and may be used to identify subgroups.
Serial imaging allows an assessment of progression. Subjects
serve as their own reference and rates of change (e.g atrophy)
can then be calculated. As long ago as the early 1980s, serial
imaging studies (mainly CT) showed that when subjects are
rescanned after an interval of more than one year, rates of
ventricular enlargement were significantly greater in AD than
in normal controls. On serial MR scans, hippocampal atrophy
rates are significantly increased in AD, with a mean rate
of 4% per year compared to age-matched elderly controls (1.6%).
Manual measurements such as these are time-consuming and haven't
entered routine clinical practice. Digital subtraction of
serial registered (positionally matched) scans allows small
amounts of diffuse atrophy to be detected and displayed automatically.
Using such methods, rates of whole brain atrophy in AD (2.5
+/- 1.1 % per year) have been shown to be several times greater
than in normal ageing (0.4 +/- 0.4 % per year). Furthermore
rates of cerebral atrophy are already increased in asymptomatic
individuals at risk of familial AD who subsequently become
clinically affected. Analysis of the presymptomatic scans
suggests atrophy rates increase first in medial temporal lobe
and posterior cingulate regions but by the time a clinical
diagnosis can usually be made atrophy is already generalized.
Serial imaging is likely to be increasingly used in clinical
trials in the search for treatments that slow diseases such
as AD. Demonstration of a slowing in rates of cerebral atrophy
would provide a strong suggestion that a therapy is actually
slowing degeneration rather than simply providing short-term
symptomatic relief. If and when such therapies are found there
will be even greater interest in making an accurate diagnosis
earlier in these diseases and MRI will be used to support
View Philip Scheltens' presentation
To hear the associated audio, click on the "Play Audio" button on each slide.