When the positrons arrive at thermal energies, tey begin connecting with electrons either by annihilation, wich produces two 511 keV photons that are anti-parallel in the positronís frame, o through development of a hydrogen-like orbiting couple known as positronium. A its foundation condition, psitronium has two types’ of- ortho-positronium, were the spins of the electron and positron are parallel, ad para-positronium, were the spins are anti-parallel. Hwever, Pra-positronium decomposes again through self- annihilation those end up in creation of two anti-parallel 511 keV photons. Otho-positronium self-annihilates by the emission three photons.
Te photons spread along nearly collinear paths, wth the degree of noncollinearity relying on the momentum of the positron and electron when they annihilated. Prting from collinearity is around a single degree or less and is not into considerations. Te extent to which the positrons move before annihilating is positron range. Te significance of the scope relies on positron energy, wich differs in different isotopes. Hwever, psitron scope is much smaller than the resolution of most scanners, s it is not a serious source of error and usually no tin consideration.
Te limit positron scope and the non-collinearity of the annihilation photons give rise to an inherent positional in absolute not present in the conventional single photon emission techniques. Hwever, oher features of PET are in discussion below more than offset this disadvantage. Te only two possibilities of interactions at 511 keV are photoelectric concentration and Compton scatter. I photoelectric absorption, a atom concentrates a photon and in the process, a electron is let out from one of its bound shells. Te chance of photoelectric concentration faster with increasing atomic amount of the concentrator atom, ad reduces faster with enlarging photon energy.
Hwever, i water, te probability of photoelectric absorption decreases with roughly the 3rd power of the photon energy and is negligible at 511 keV. Dring an ordinary connection, te photon connects with an outer shell electron such that its way is in deflection and some of its strength is lost. Mst of the scattering photons encounter reduction of energy and a related transformation of direction are scattered out of...
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