X-Message-Number: 768
Date: 26 Apr 92 02:09:36 EDT
From: Brian Wowk <>
Subject: hi-res brain scanning

To: >INTERNET: 
 
        Regarding David Stodolsky's recent questions about x-ray holography  
of the brain, I don't think such a modality would provide useful information. 
 
        Aside from the technical difficulties, holography at best provides  
only limited external views of an object.  Anatomical structures in such  
images would be hopelessly overlapped.  Only computed tomography (CT) could  
reconstruct the internal details of cellular connectivity. 
 
        As I reported earlier, radiation dose rises sharply with resolution  
in CT (although it is 4th power relationship, not 6th power as I erroneously  
stated).  A typical clinical head scan with 1mm resolution requires a patient  
dose of about 3 rads.  This means that CT images with 1 micron resolution  
(the resolution required to view synaptic connections) would expose the brain  
to some 3 trillion rads!  Such a scan could only be performed on a frozen  
brain (to prevent chemical diffusion), and would take about 100 years due to  
heat dissipation constraints. 
 
        Finally, there is the fact that such a scan would not reveal anything  
about the chemcal state of synapses, which is probably just as critical to  
memory as connectivity.  Indeed, the lack of molecular information is the  
greatest weakness of non-invasive tissue characterization techniques.  How  
could we posibly recover personal identity from an x-ray scan when we  
couldn't even recover the genome? 
 
        I trust this question has been laid to rest. 
 
 
                                                --- Brian Wowk 

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