X-Message-Number: 3360 From: Brian Wowk <> Date: Thu, 27 Oct 94 01:51:28 CDT Subject: SCI.CRYONICS Brain Scanning, reply to Yvan Bozzonetti Yvan Bozzonetti: >I come to the number of photon estimate: >I take as an example a one dm^3 cube scanned with a 0.1 micrometer >resolution. On one cube face there will be 10^12 squares .1 micron on a >side. The information is encoded in tiny phase shifts on each wave, there >must be one wave (one photon) for each square. Doing that on three >perpendicular sides produces both, all the informations requested to define >the content of each .1 micrometer cell and a redundant data for result >checking. 3 x 10^12 have been used, even if one in ten only was not >absorbed (this is plainly wrong), 3 x 10^13 photons would travel in the >brain. If each one has a 1 keV energy, the total deposited energy is at >most: (calculation deleted) 1 keV x-rays will be completely absorbed by photoelectric interactions with the K shells of oxygen, carbon, and nitrogen within the first 1 mm of brain tissue. To penetrate a 100mm wide brain you will need at least 50keV x-rays, and even then 90% will be absorbed before reaching the other side. >>>> This absorption will occur even if your x-rays are so monochromatic that they have coherence length of kilometers. <<<< The photoelectric effect doesn't care about coherence. (*Please* consult any elementary health physics or nuclear physics textbook to learn about the photoelectric effect. You will also find there information about partial absorption windows that exist near K edges.) There is an even more serious problem with your calculation. The number of photons you have estimated is at least 6 orders of magnitude too small to make a hologram. A hologram consists of an interference pattern recorded on a photographic plate or other recording medium. To encode the three dimensional structure of a brain to 0.1 micron resolution, the interference pattern must contain 10^18 bits of information. How are you going to get an interference pattern with 10^18 bits of information by recording only 10^12 photons? Here is yet another problem: How are you going to stop 50keV photons in your detector in a short enough distance to record nanometer-scale interference patterns? Even if you could magically stop all 50keV x-ray photons within a nanometer, how would you detect only the primary ionization event and distinguish it from the shower of secondary scattered electrons (which would spread up to 100 microns away from the initial event). The only solution would be to enlarge the interfernce pattern by moving the recording film hundreds of meters away, and using film hundreds of meters wide! Give it up, Yvan. Holography with 50keV x-rays is psuedoscience! --- Brian Wowk Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=3360