4 November 2005. The development of MRI to scan living brain for amyloid lags behind the impressive progress made recently with the PET-PIB approach (see ARF related news story), but interest remains high because MRI alone holds out the promise of high-resolution imaging of individual plaques. The use of MRI with novel contrast agents presents one promising approach being pursued by Takaomi Saido’s group (see ARF related news story). Taking another tack, last year Clifford Jack, Joseph Poduslo, and colleagues at the Mayo Clinic in Rochester, Minnesota, and the University of Minnesota Medical School described an in-vivo MRI microimaging technique that resolved single plaques in the brain of AD mice, in the absence of any contrast agents (Jack et al., 2004). This technique depends on the iron content of plaques to distinguish the deposits from surrounding tissue.
Now, Jack and colleagues have followed up with further validation of their imaging technique. In a new study just out in the Journal of Neuroscience, they show that MRI of live APP/PS1 double transgenic mice can reveal an age-related increase in plaque number that correlates with the gold standard of thioflavin S histological detection. Their results demonstrate the utility of MRI for the non-invasive tracking of amyloid plaques in mice, a technique that may speed evaluation of anti-amyloid therapies. For humans, though, this kind of MRI still faces several considerable hurdles.
In the new study, Jack gathered four imaging data sets on double transgenic AD mice at 3, 6, 9, 12, and 24 months of age. In-vivo MRI was compared to three in-vitro measures performed on fixed brain: ex-vivo MRI, thioflavin S staining, and tissue iron staining. Image analysis on spatially registered data allowed the researchers to compare the methods at the level of detecting individual plaques.
On MRI, amyloid plaques show up as dark spots in the cortex and hippocampus, and an increase in both the size and density of the spots with age was apparent. Individual spots could be matched precisely with thioflavin S reactive plaques, as well as iron staining. Although MRI detected fewer plaques than did thioflavin S staining, the increase in MRI signal with aging from 9 months on was proportional to plaque load detected by histological methods. The lower sensitivity of MRI meant that the plaques could first be detected between 6 to 9 months, much later than the 10 weeks of age or earlier for thioflavin S staining of postmortem brain, but after that time, the increase in plaque number was easily measured. MRI could detect plaques of roughly 35 micrometer diameter and larger.
MRI in fixed brain sections (ex-vivo MRI) was consistently more sensitive than the in-vivo measures, even though the imaging conditions were identical. One reason for this is that even very slight movement, like the heart beating and the animal’s breathing, hampers high resolution. Nonetheless, within its detection limits, in-vivo imaging should be useful for longitudinal studies, with the caveat that the imaging depends on the amount of iron, not amyloid, in plaques. The linear relationship between MRI signal and thioflavin S staining shown in this study might not hold under conditions where the iron/amyloid ratio in plaques was changing, for example, if amyloid was removed while iron stayed behind.
Extending this technique to humans would mean reducing the scan times (1 hour and 40 minutes in these experiments), and improving methods to reduce motion artifacts. In addition, sensitivity remains a big question. The method only visualized 20 percent of total plaques in the older APP mice, animals that carry much more brain amyloid than the average human. Other techniques that combine contrast agents with MRI may turn out to be more practical for clinical use.—Pat McCaffrey.
Jack CR Jr, Wengenack TM, Reyes DA, Garwood M, Curran GL, Borowski BJ, Lin J, Preboske GM, Holasek SS, Adriany G, Poduslo JF. In vivo magnetic resonance microimaging of individual amyloid plaques in Alzheimer's transgenic mice. J Neurosci. 2005 Oct 26;25(43):10041-8. Abstract