Posted by danf on November 3, 2000, at 16:18:02
In reply to Re: Bipolar Disorder - 30% more neuronal synapses ?, posted by SLS on November 3, 2000, at 14:38:03
Scott,
thanks for the citations & the discussion.
PET resolution.. seems to be a few mm. much larger than the micron size of neurons. PET imaging is based on metabolic markers & not on physical things like walls of nerves, etc
Resolution, kind of does not matter as PET resolution is about 6 orders of mangitude less than the size of nerve fibers in the brain. a few mm vrs a few microns.
MRI also is based on things like water content & other elemental content of tissue, not on an exact physical characteristic like the color of the tissue. MRI is similar. a few mm resolution.
There is nothing bad about this, just the limitations present.
So PET scanning gives an index of metabolic activity of the nerve tissue. an increased index means more vesicle activity /presence, not necessarily more nerves. Because this is a metabolic measurement & not a physical one, things in the diet, like alcohol, sugar, chemicals & drugs & maybe grapefruit may have an effect.
in calculating grey brain tissue volume, multiple 2 dimensional cuts are taken & measured & a 3 dimensional volume calculated. There are always problems with quantative calibration of the machines from day to day. Supply voltages vary slighty from day to day. The detectors gradually become less sensitive over time. None of these things are major. They do make it difficult to measure with a high degree of precision, changes of a few percent. There is always the error of measurement of some thing which is a few mm thick, as in... is it 10 mm thick or 11mm thick ( a 10% difference ), etc
Both of these testing devices are used more for qualitative ( is something present & what does it look like ) imaging than for quantatitive imaging ( how much is there ).
For the brain grey matter volume study, it does not really matter. If we accept the 3% as real, what does this mean ? does it mean that there are more nerves or does it mean the nerve cells are 3% larger ?
The testing method does not tell us so we have to guess which is more likely... Since we know Li affects membrane permeability, it is a reasonable hypothesis that the cells get slightly larger.
These are just the normal kinds of questions, one asks ones self when reading a published report.
poster:danf
thread:48076
URL: http://www.dr-bob.org/babble/20001102/msgs/48139.html