A. Marcus. Emporia State University.
This will not be further discussed when the x-rays are used for diagnostic purposes generic valtrex 1000 mg online hiv infection when undetectable, but is important for x-ray crystallography buy valtrex 500 mg on-line hiv infection symptoms timeline. We are not going to describe all the technological developments with regard to the control of the exposure time – and equipment for the different types of examinations cheap valtrex 1000 mg on-line hiv infection rate vancouver. Thus, in the case of mammography the maximum energy is low (below 30 kV) whereas in skeletal and abdominal examinations the energy is larger, between 60 to 85 kV. Another aspect is that the radiation dose in an examination should be kept as low as possible. Several developments – using intensifying screens have reduced the exposure (see below). Absorption and scattering in the body The x-ray picture is based on the radiation that penetrates the body and hit the detector (flm). The details in the picture are due to those photons that are absorbed or scattered in the body. Since both the absorption and the scattering depend upon the electrons in the object (body) we can say that; “the x-ray picture is a shadow-picture of the electron density in the body. Since x-ray diagnostic uses low energy radiation only the ”photoelectric effect” and the “Compton scattering” contribute to the absorption. The photoelectric effect occur with bound electrons, whereas the Compton process occur with free or loosly bound electrons. Both processes vary with the radiation energy and the atomic number of the absorber. Photoelectric effect – variation with photon energy For the energy region in question – and for atoms like those found in tissue the photoelectric cross- section varies with E–3. Photoelectric effect – variation with atomic number The variation with the atomic number is quite complicated. For an energy above the absorption edge, the cross-section per atom varies as Z4 (i. It can be noted that the K-shell energy for all atoms in the body (C, N, O, P, and Ca) is below 4 keV. Compton effect – variation with photon energy For the energy range used for diagnostic purposes the Compton effect is rather constant – and de- creases slightly with the energy. Compton effect – variation with atomic number The Compton process increases with the electron density of the absorber. This implies that the absorption in bones (with an effective atomic number of about 13) is much larger than that for tissue (with effec- tive atomic number of about 7. For energies below about 30 keV the absorption is mainly by the photoelectric effect. In this energy region it is possible to see the small variations in electron density in normal and pathological tissue like that found in a breast. It can be noted that due to the strong dependence of the photoelectric effect with the atomic number we fnd the key to the use of contrast compounds. Thus, compounds containing iodine (Z = 53) or barium (Z = 56) will absorb the low energy x-rays very effciently. The Compton process varies slightly with the energy in this range – and is the dominating absorp- tion process for energies above 50 keV. In Rayleigh scattering the photon interacts with a bound electron and is scattered without loss of energy. In Thomson scattering the photon interacts with a free electron and the radiation is scattered in all directions. The two elastic scattering processes accounts for less than 10 % of the interactions in the diagnostic energy range. The purpose for discussing these details about absorption and scat- tering is to give some background knowledge of the physics of the x-ray picture. It is differential attenuation of photons in the body that produces the contrast which is responsible for the information. The attenuation of the radiation in the body depends upon; the density, the atomic num- ber and the radiation quality. In mammography one are interested in visualizing small differences in soft tissue – and we use low energy x-rays (26 – 28 kV) to enhance the tissue details. In the case of chest pictures the peak energy must be larger because the absorbing body is very much larger – and some radiation must penetrate the body and reach the detector. It is the transmitted photons that reach the detector that are responsible for the picture. The detector system A number of different detectors (flm, ionization chambers, luminescence and semiconductors) have been used since the beginning of x-ray diagnostic. The x-ray picture was created when the radiation was absorbed in the flm emul- sion consisting of silver halides (AgBr as well as AgCl and AgI). In the usual morning meeting the doctors were often gath- ered in front of the “light box” to discuss the patients (see illustration). Consequently, in order to increase the sensitiv- ity, intensifying screens were introduced. The screen is usually a phosphor scintillator that converts the x-ray photons to visible light that in turn expose the flm. The introduction of intensifying screens was made already in 1896 by Thomas Alva Edison. He introduced the calcium tungstate screens which were dominating up to the 1970-ties. We do not intend to go through the technical details with regard to intensifying screens – nor to the many technological details within x-ray diagnostic. In order to ensure that the photoelec- tric effect is dominant lower energies are used. Energies lower than 30 kV are used for mammog- raphy – which is very effective for seeing details in soft tissue. However, this energy range is only useful for tissue thicknesses of a few centimeter. Mammography X-ray tube In mammography the goal is to see the contrast between different den- sity of soft tissue, fat and blood ves- sels without use of contrast media. The x-ray energy is between 25 and 30 kV in order to ensure that the photoelectric effect is dominant. This also result in absorption of ra- diation and an increase of the patient dose. Detector 181 Examples Tumor It is sometimes very convincing to see a mammogram like that shown to the right. It is also amazing that we can see details like this in soft tissue without using contrast media to enhance the difference in electron density. To the left is a modern digital picture whereas the other is a flm-based mammography. Implants Muscle It is obvious, even for the layman, that the presence of breast implants does interfere and makes it more diffcult to obtain good information with mammography. The presence of implants affects the way mammograms are done, since additional views are needed during routine screening mammography to visualize all of the breast tissue. The lesson to learn from this is that implants could be an impediment to cancer detection.
Outbreaks of passerine salmonellosis are typically observed in the vicinity of supplementary feeding stations in garden habitats buy discount valtrex 1000mg on-line hiv infection nejm. Salmonellosis outbreaks have also been reported in colonial nesting birds buy 1000mg valtrex mastercard hiv aids infection rates for south africa, such as gulls and terns buy generic valtrex on line hiv infection prevention drug. Children, the elderly, and people with weakened immune systems are at greatest risk of developing severe disease. Geographic distribution Found worldwide but most common in areas of intensive animal husbandry, especially in pigs, calves and poultry reared in confined spaces. Eradication programmes have nearly eliminated salmonellosis in domestic animals and humans in some countries but wild animal Salmonella spp. In general infection is transmitted by infected hosts, their faeces or contaminated inanimate objects. How is Salmonella Direct contact with infected faeces and through ingesting water and food transmitted to animals? In mammals, the bacteria can be transmitted from an infected female to the foetus, and in birds, from an infected adult to the egg. How does Salmonella Spread by infected animals which shed the bacteria into the environment in spread between groups their faeces. How the infection spreads between and within herds and flocks is not fully understood due to the difficulties of detecting clinical signs in animals infected with Salmonella spp. How is Salmonella Most commonly transmitted by handling and ingesting contaminated water transmitted to humans? Also transmitted through direct contact with infected animals and their faeces, particularly those of reptiles, chicks and ducklings, but also of livestock, dogs, cats, adult poultry and cage birds. Infected livestock may develop enteritis and septicaemia and commonly show signs of diarrhoea, dehydration, depression, abdominal pain and rapid weight loss. Clinical signs usually last for 2-7 days but death can occur within 24-48 hours in some species. Clinical signs may include ruffled feathers, lethargy, diarrhoea and increased thirst. Infection in humans often causes gastroenteritis but a wide range of clinical signs may be seen and death can occur in severe cases. Humans may suffer from fever, abdominal pain, diarrhoea, nausea and sometimes vomiting. The elderly, children and those with weakened immune systems may suffer from severe dehydration and more severe illnesses, such as septicaemia. In general, contact and seek assistance from human and animal health professionals immediately if there is any illness in people and/or livestock. An outbreak may mean that many humans and animals have been exposed to a common contaminated food item or water source. Diagnosis Isolation of the causative agent by health professionals is needed for a definitive diagnosis. Faeces or blood cultures are used for isolating the bacterium in humans, and in animals and birds, faeces, rectal swabs and/or caecal contents are required. If the whole carcase cannot be submitted, submit the intestine, and if possible, the liver and heart. Place the foil-wrapped specimens in tightly sealed plastic bags, and ship them frozen. After an abortion, samples should be collected from the placenta, vagina and foetal stomach. Whole eggs, egg shells and shell membranes can also be cultured for bacteria providing that the egg fragments have not been subjected to environmental conditions that would destroy the bacteria. Transmission of bacteria from animals to humans and between captive animals can be more easily prevented and controlled. Monitoring and surveillance Recording the incidence of outbreaks can identify trends in salmonellosis infections and evaluate the feasibility of control programmes. Monitoring of outbreaks in animals and humans can also help assess the contribution of animals to human illness. Disease can be reduced by good hygiene and optimal animal husbandry and by minimising stressful events. Rodent control will help prevent/reduce transfer of bacteria from rodents to animals. Fence stream banks and watering holes to limit access by livestock to water contaminated by faeces from infected animals and to reduce animals contaminating water courses. Provide clean drinking water in separate watering tanks located away from potentially contaminated water bodies. Do not chlorinate natural water bodies as this will have an adverse effect on the wetland ecosystem. During a herd outbreak, animals carrying bacteria should be identified and either isolated and treated, or culled. Vaccination can reduce the level of colonisation and shedding of the bacteria into the environment, as well as clinical disease. Re-test treated animals several times to ensure that they no longer carry Salmonella spp. Antibiotics may help with overcoming an outbreak but will not eliminate carriers, and transmission of bacteria from an infected adult to the egg or foetus may result in new outbreaks and disease spread. Maintain low densities of livestock to reduce cycles of salmonellosis within populations. Rotate the locations of feeders to help avoid accumulation of faeces and contamination of particular areas. This often happens when: - existing wetlands receive wastewater discharges - agricultural fields receive manure and slurries as fertiliser - development of landfill, livestock, and poultry operations are proposed. Ensure that waste, sewage wastewater, and wastewater discharges are properly treated, secure and contained away from livestock, poultry and wetlands: - wastewater should be stored in lagoons and treated for a combined period of 20 days to eliminate bacteria e. People with weakened immune systems should avoid contact with reptiles, young chicks and ducklings. Good personal hygiene: - wash hands thoroughly with soap and warm water: before preparing and eating food; after handling raw food; after going to the toilet or after/before changing a baby’s nappy; after contact with animals and (especially) reptiles or contact with items they have touched; after working outside; and frequently if you have symptoms such as diarrhoea. It is important to drink plenty of fluids as diarrhoea or vomiting can lead to dehydration and loss of minerals. The prevalence of bacteria in most wild bird populations is generally low although large-scale mortalities of birds using feeding stations have become common in the United States and also occur with some frequency in Canada and Europe. Effect on livestock Many infected animals will not show any clinical signs at all and disease is uncommon in healthy, unstressed adult birds and mammals. In mammals, clinical disease is most common in very young, pregnant or lactating animals, and often occurs after a stressful event. Outbreaks in young ruminants, pigs and poultry can result in a high morbidity rate, and sometimes, a high mortality rate. Effect on humans Salmonellosis is common in humans and is a major cause of food-borne illness throughout the world. Infection often causes gastroenteritis but a wide range of clinical signs may be seen and death can occur in severe cases. The incidence and severity of the disease is higher in younger children, the elderly and those with weakened immune systems.
If there are no objective and explicit criteria for evaluating the medical records discount valtrex hiv infection ppt, the infor- mation contained in them is open to misinterpretation from the observer order valtrex with visa hiv infection long term effects. It has been shown that when performing implicit chart reviews discount 1000 mg valtrex otc antiviral used to treat flu, researchers subcon- sciously ﬁt the response that best matched their hypothesis. Researchers came up with different results if they performed a blinded chart review as opposed to an unblinded review. Explicit reviews are better and can occur when only clearly objective outcome measures are reviewed. Even when the outcomes are more objective it is better to have the chart material reviewed in a blinded manner. The Hawthorne effect was ﬁrst noticed during a study of work habits of employees in a light bulb factory in Illinois during the 1920s. It occurs because being observed during the process of making measurements changes the behav- ior of the subject. In the physical sciences, this is known as the Heisenberg Uncer- tainty Principle. If subjects change their behavior when being observed, the out- come will be biased. One study was done to see if physicians would prescribe less expensive antibiotics more often than expensive new ones for strep throat. In this case, the physicians knew that they were being studied and in fact, they prescribed many more of the low-price antibiotics during the course of the study. After the study was over, their behavior returned to baseline, thus they acted differently and changed their clinical practices when being observed. This and other observer biases can be prevented through the use of unobtrusive, blinded, or objective measurements. Misclassiﬁcation bias Misclassiﬁcation bias occurs when the status of patients or their outcomes is incorrectly classiﬁed. If a subject is given an inaccurate diagnosis, they will be counted with the wrong group, and may even be treated inappropriately due to their misclassifaction. For instance, in a study of antibiotic treatment of pneumonia, patients with bronchi- tis were misclassiﬁed as having pneumonia. Those patients were more likely to get better with or without antibiotics, making it harder to ﬁnd a difference in the outcomes of the two treatment groups. Patients may also change their behaviors or risk factors after the initial grouping of subjects, resulting in misclassiﬁcation bias on the basis of exposure. Misclassiﬁcation of outcomes in case control studies can result in failure to correctly distinguish cases from controls and lead to a biased conclusion. One must know how accurately the cases and controls are being identiﬁed in order to avoid this bias. If the disorder is relatively common, some of the control patients may be affected but not have the symptoms yet. One way of compensating for Sources of bias 87 this bias is to dilute the control group with extra patients. This will reduce the extent to which misclassiﬁcation of cases incorrectly counted as controls will affect the data. Let’s say that a researcher wanted to ﬁnd out if people who killed themselves by playing Russian Roulette were more likely to have used alcohol than those who committed suicide by shooting themselves in the head. The researcher would look at death investigations and ﬁnd those that were classiﬁed as suicides and those that were classiﬁed as Russian Roulette. However, the researcher suspects that some of the Russian Roulette cases may have been misclassiﬁed as suicides to “protect the victim. Obviously if Russian Roulette deaths are routinely misclassiﬁed, this strategy will not result in any change in the bias. Outcome classiﬁcation based upon subjec- tive data including death certiﬁcates, is more likely to exhibit this misclassiﬁca- tion. This will most likely result in an outcome that is of smaller size than the actual effect. This bias can be prevented with objective standards for classiﬁca- tion of patients, which should be clearly outlined in the methods section of a study. Miscellaneous sources of bias Confounding Confounding refers to the presence of several variables that could explain the apparent connection between the cause and effect. If a particular variable is present more often in one group of patients than in another, it may be respon- sible for causing a signiﬁcant effect. For example, a study was done to look for the effect of antioxidant vitamin E intake on the outcome of cardiovascular dis- ease. It turned out that the group with high vitamin E intake also had a lower rate of smoking, a higher socioeconomic status, and higher educational level than the groups with lower vitamin E intake. It is much more likely that those other variables are responsible for all or part of the decrease in observed cases of car- diovascular disease. There are statistical ways of dealing with confounding vari- ables called multivariate analyses. The rules governing the application of these types of analyses are somewhat complex and will be discussed in greater detail in Chapter 14. When looking at studies always look for the potential presence of confounding variables and at least make certain that the authors have adjusted for those variables. However, no matter how well the authors have adjusted, it can be very difﬁcult to completely remove the effects of confounding from a study. Contimination is more commonly seen in randomized clin- ical trials, but can also exist in observational studies. In an observational study, it occurs if the control group is exposed to the same risk factor as the study group. However, there may be an environmental situation by which those classi- ﬁed as not exposed to the risk factor are actually exposed. For example, a study is done to look at the effect of living near high-tension wires on the incidence of leukemia. Those patients who live within 30 miles of a high-tension wire are considered the exposed group and those who live more than 30 miles away are considered the unexposed control group. Those people who live 30 to 35 miles from high-tension wires could be misclassiﬁed as unexposed although they may truly have a similar degree of exposure as those within 30 miles. In fact, families living 60 miles from the wires may be equally affected by the electrical ﬁeld if the wires have four times the amount of current. Cointervention occurs when one group or the other receives different medical care based partly or totally upon their group assignment. This occurs more often in randomized trials, but could be present in an observational study when the group exposed to one particular treatment also receives different therapy than the unexposed group.