Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

sure, owing principally to the relative radiosensitivities of the tissues at the time ofexposure.The preimplantation stage begins with the union of the sperm and egg and continuesthrough day 9 in humans, when the zygote becomes embedded in the uterinewall. During this period, the two pronuclei fuse, cleave, and form the morula andblastula.The conceptus is extremely sensitive during the preimplantation stage andradiation damage can result in prenatal death. During this period the incidenceof congenital abnormalities is low, although not completely absent. Embryosexhibit the so-called all-or-nothing response, in which, if prenatal death does notoccur, the damaged cells are repaired or replaced to the extent that there areunlikely to be visible signs of abnormalities even though radiation may have killedseveral cells.Several factors, including repair capability, lack of cellular differentiation, andthe relatively hypoxic state of the embryo, are thought to contribute to its resistanceto radiation-induced abnormalities. During the first few divisions, the cells areundifferentiated and lack predetermination for a particular organ system. If radiationexposure were to kill some cells at this stage, the remaining cells could continuethe embryonic development without gross malformations because they are indeterminate.However, chromosomal damage at this point may be passed on andexpressed at some later time. When cells are no longer indeterminate, loss of even afew cells may lead to anomalies, growth retardation, or prenatal death. The mostsensitive times of exposure in humans are at 12 hours after conception, when thetwo pronuclei fuse to the one-cell stage, and again at 30 and 60 hours when the firsttwo divisions occur.Chromosomal aberrations from radiation exposure at the one-cell stage couldresult in loss of a chromosome in subsequent cell divisions that would then be uniformthroughout the embryo. Most chromos.omal loss at this early stage is lethal.Loss of a sex chromosome in female embryos may produce Turner's syndrome.The woman may not know she is pregnant during the preimplantation period,the time at which the conceptus is at greatest risk of lethal effects. Animal experimentshave demonstrated an increase in the spontaneous abortion rate after dosesas low as 50 to 100 mGy (5 to 10 rad) delivered during the preimplantation period.After implantation, doses in excess of 250 mGy (25 rad) are required to induce prenataldeath. The spontaneous abortion rate has been reported to be between 30%and 50%.OrganogenesisEmbryonic malformations occur more frequently during the period of majororganogenesis (2nd to 8th week after conception). The initial differentiation ofcells to form certain organ systems typically occurs on a specific gestational day.For example, neuroblasts (stem cells of the CNS) appear on the 18th gestationalday, the forebrain and eyes begin to form on day 20, and primitive germ cells areevident on day 21. Each organ system is not at equal risk during the entire periodof major organogenesis. In general, the greatest probability of a malformation in