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 Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=768