X-Message-Number: 3344 From: Brian Wowk <> Date: Mon, 24 Oct 94 00:27:41 CDT Subject: SCI.CRYONICS Brain scanning, reply to Yvan Bozzonetti Yvan Bozzonetti: > I fear B. Wowk mix anything, first, QED is the general theory of > electromagnetism and is never used for computing complex situations. Are you suggesting that QED is not used in the real world? On the contrary, QED is *the* theory that predicts how photons such as x-rays interact with electrons in matter! X-ray interactions of great biological importance (such as electron-positron pair production during clinical radiotherapy) cannot be understood without QED. Even the Klein-Nishina differential cross sections for Compton scattering require relativistic quantum mechanics (the precursor to full QED) for their derivation. In any case, I do not want to engage in a history lesson on radiation transport theory. The important point is this: The mechanisms by which x-rays interact with matter are exactly and quantitatively understood in both theory and experiment to many decimal places. > Second, Bragg scattering in about diffraction by a crystal network of > atoms, the monochrmaticity produced by that system is similar to the one > generated by a prism in visible light. This far from what a hologram needs. > So the subjects are not the same. Indeed, the subjects are not the same. I was not talking about holography when I referred to Bragg scattering. I was responding to your assertion that my knowledge of x-ray interactions was based on experiments with polychromatic beams. > There may be an understanding problem: May be I have not sufficiently > stressed than Xray brain scanning was essentially a cryonics tool. That is, > it must be used at low temperature when the brain is a solid body. There, I > am sorry, but you cannot take as model the atomic properties of a gas or a > liquid. Not only atoms are locked in molecules (mostly big ones for > interesting cases) but they are anchored in the solid. Recall the Mosbauher > effect, where an entire crystal absorbs the push of a single photon. Even > at atom border electrons are not knocked out in a solid as in a gas. No. This is not correct! Chemical binding only affects the energy levels of valence electrons by a few electron volts. This is not going to have any effect on interactions with keV x-rays. Also, the Mossbauer effect is a *nuclear* effect, and does not occur with electrons. In summary: whether a brain is liquid or frozen is not going to have any dramatic effect on its x-ray dosimetry. Yvan, normally I don't make arguments from authority, but because this argument is becoming tedious I'm now going to. I have a Master's degree in medical physics (soon Ph.D.), which makes me an expert in medical imaging and the biological effects of ionizing radiation. I am telling you that nanometer-scale resolution of intact brains (frozen or unfrozen) by any form x-ray imaging is not practical because of the large energy deposition that would result. To argue otherwise you must a) Provide an estimate of the number of x-ray quanta of a certain energy that must pass through the brain to provide an image of such resolution. b) Provide a calculation (using a formula from the literature) showing that the fraction of quanta which deposit energy in the brain is small enough to cause minimal heating. --- Brian Wowk Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=3344