By H. Narkam. Salisbury State University.
Hybrid segmentation techniques have also been proposed by some researchers (Chen & Metaxas kamagra super 160 mg mastercard, 2000) buy kamagra super 160mg amex. These approaches seek to exploit the local neighbourhood information of region-based techniques, and the shape and higher level information of boundary-based techniques. Extraction and Selection of Characteristic Features “Characteristic features” are a group (vector) of quantitative indices extracted after processing the patient data. The aim of feature extraction and selection is to reliably discriminate between pathological and physiological structures or to characterise a pathological structure (e. Characteristic features may be derived from measurements of mean image intensity or image texture, shape analysis, and so on. It is often desirable to reduce the dimension of the vector of characteristics extracted from patient data (Jain, Duin, & Jianchang, 2000). The selection of the most robust character- istics allows maximisation of classification accuracy and minimisation of system complex- ity. Ideally, a method for feature selection would examine all 2N (N: the number of character- istic features) different combinations of characteristic features and would find the optimal combination that satisfies the previous criteria. The disadvantage of this exhaustive search technique is its high computational cost which may be prohibitive even for relatively small values ofN. Statistical methods, including ANalysis Of VAriance (ANOVA), have been used for feature selection in a number of applications. ANOVA compares the means of two or more groups of data and returns the probability for the null hypothesis that the means of the groups are equal (Furlong, Lovelace & Lovelace, 2000). Other techniques, based on heuristic or random search methods, attempt to compensate computational complexity with classification accuracy. Feature selection is usually achieved through the following steps: • Production of a candidate feature subgroup • Evaluation of the eligibility of the candidate subgroup through an appropriate evaluation function • Procedure termination after satisfaction of a predefined criterion • Confirmation of the validity of the selected feature subgroup Methodologies such as Genetic Algorithms (GAs) may be applied to feature selection problems. GAs are adaptive heuristic search methods which may be used to solve complex pattern recognition and classification problems (Goldberg, 1989). Copying or distributing in print or electronic forms without written permission of Idea Group Inc. As the natural populations evolve according to the principles of natural selection and “survival of the fittest” first laid down by Charles Darwin, so by simulating this process GAs are able to evolve solutions to real-world problems, if they have been suitably encoded (Holland, 1975). Feature selection through GA-based search along with an ANN classifier has been used to classify “difficult-to-diagnose” microcalcifications from mammography (Dhawan, Chitre, Kaiser-Bonasso, & Moskowitz, 1996). The use of similar procedures has also been reported for the recognition of skin tumours and endothelial cells (Yamani, Khiani & Farag, 1997). Classification The role of the classification module is to decide about the inclusion of the feature vectors appearing at its input into a category or class, among a set of predefined classes. The development and implementation of the classification module may be based either on supervised or unsupervised training methods. In supervised training, a set of characteristic vectors is considered, known as training set. For each vector in the training set the class to which it belongs, is known a priori. The vectors in the training set are used to train the classifier, which can subsequently classify any vector appearing to its input into one of the available classes (Duda, Hart & Stork, 2000). In unsupervised training a set of characteristic vectors is examined and the class each vector belongs to is unknown. Classification of vectors in the available categories is done automatically during the formation of the classifier.
Therefore buy kamagra super 160mg overnight delivery, potential neuroprotectants must be tested at realistic time points in pre- clinical studies generic 160mg kamagra super free shipping, but at time intervals longer than minutes. A drug that is efficacious in animal models only if given immediately after arterial occlusion is unlikely to be of benefit. For example NXY-059, a novel nitrone, is effective when admin- istered 3 to 6 hours following recirculation in transient focal MCA occlusion models51 and at 4 hours in permanent focal MCA occlusion models. LATE OUTCOME DEFINITION Preclinical studies have commonly used histological endpoints to assess therapeutic efficacy. For example, MK-801 appeared to reduce infarct size at 3 days following ischemic insult but the benefit was not significant after 4 weeks. Such mechanisms may include stimulation of neuronal sprouting and protection against retrograde neuronal death. Recent preclinical studies have employed complex behavioral tasks as endpoints for determining whether the treatment in question will aid in the reduction of ischemia- related disability. However, in some patients the penumbra may only account for a small percentage of the total infarct volume. To increase the likelihood of detecting a drug effect, clinical studies should target patients with sufficiently large penumbrae. In comparison to the rodent brain, the human brain contains a higher pro- portion of white matter (including axons) that may not be salvageable using thera- peutic agents that only target gray matter. It is therefore possible that potentially successful neuroprotectants have failed due to the inclusion of patients with white matter injuries in clinical trials. This is particularly important for Phase II clinical trials that provide data critical for a decision to proceed with or defer a large Phase III efficacy study. Because the blood–brain barrier (BBB) is often damaged by the ischemia to variable © 2005 by CRC Press LLC extents at different times, some drugs normally excluded from the brain may still be able to reach target tissue, but this is likely to be variable. Drugs that can cross the BBB may in some conditions have preferred access, depending on diffusion and vascular stasis; to cross the BBB, a drug must be lipid- soluble and have a molecular weight below 500 Da. Because no gene or drug targeting strategies are clinically available, drug testing is now limited to small lipid-soluble drugs that represent only 2% of all potential candidates for drug development. Thus, directly sampling the area of brain targeted for drug levels may be a critical control to evaluate whether access into the critical region is possible. After this critical initial point is established, mech- anisms of action may be thoroughly assessed by histological and behavioral outcomes. Testing these drugs at doses below those required for efficacy in animal models is less likely to be successful in humans. Although a drug exhibits neuroprotective properties after a single dose, multiple doses over the period in which the infarct is evolving may or may not increase its clinical effi- cacy. If given by constant intravenous infusion, a lipid-soluble drug will accumulate in the cerebral tissues faster than a hydrophilic drug and will take longer to clear from the tissues. Other considerations include the receptor-binding properties of the drug that will determine the loading dose and the need for maintenance infusion, the clearance and volume of distribution of the drug, and its therapeutic index. For example, the more potent NMDA antagonists produce psychomimetic effects that might preclude the drug from administration over days to weeks. Moderate increases in blood pressure, however, could be beneficial in improving blood flow and local perfusion. Less than half of the published clinical trials utilized validated outcome measures and only 17% indicated primary endpoint. In addition, although they are not yet widely employed, scales that incorporate quality-of-life assessment and pharmacoeconomic analysis can be used as secondary outcome measures. The use of a dichotomous division of a continuous scale may help determine whether a patient has achieved a clinically significant benefit.
Soft lenses offer increased com- fort and the advantage of extended wear; some can be worn continuously for up to one week generic 160 mg kamagra super fast delivery. While oxygen passes freely through soft lenses purchase kamagra super 160mg amex, bacterial contamina- tion and other problems can occur, requiring replace- Nearsightedness (myopia) ment of lenses on a regular basis. It is very important to follow the cleaning and disinfecting regimens prescribed because protein and lipid buildup can occur on the This illustration compares the difference between a normal lenses, causing discomfort or increasing the risk of eye shape and light refraction versus a myopic eye. Contact lenses offer several benefits over Group) glasses, including: better vision, less distortion, clear peripheral vision, and cosmetic appeal. In addition, con- tacts will not fog up from perspiration or changes in tem- RADIAL KERATOTOMY Radial keratotomy (RK), the first of these procedures made available, has a high asso- perature. It was first developed in Japan and the Soviet Union, and was introduced into the United States in Refractive eye surgery 1978. The surgeon uses a delicate diamond-tipped blade, For people who find glasses and contact lenses a microscope, and microscopic instruments to make sev- inconvenient or uncomfortable, and who meet selection eral spoke-like “radial” incisions in the non-viewing criteria regarding age, degree of myopia, general health, (peripheral) portion of the cornea. The PRK procedure manently changing the shape of the cornea so that light flattens the cornea by vaporizing small amounts of tissue rays focus properly on the retina. As of early 2001, only two performed on an outpatient basis and generally take 10- excimer lasers are approved by the FDA for PRK, although 30 minutes. It is important to make sure GALE ENCYCLOPEDIA OF GENETIC DISORDERS 783 the laser being used is FDA approved. Photorefractive ker- Prognosis atectomy can treat mild to moderate forms of myopia. The Glasses and contact lenses can (but not always) cor- cost is approximately $2,000 per eye. Refractive surgery can LASER-ASSISTED IN-SITU KERATOMILEUSIS Laser- make permanent improvements for the right candidates. It is recommended for moderate to sion and severity of myopia cannot be changed, some envi- severe cases of myopia. They include reducing LASIK uses lasers and a cutting tool called a microker- close work; reading and working in good light; taking fre- atome to cut a circular flap on the cornea. The flap is quent breaks when working at a computer or microscope flipped back to expose the inner layers of the cornea. The for long periods of time; maintaining good nutrition; and cornea is treated with a laser to change the shape and practicing visual therapy (when recommended). Eye strain can be prevented by using sufficient light Risks for reading and close work, and by wearing corrective lenses as prescribed. Everyone should have regular eye All of these surgical procedures carry risks, the most examinations to see if their prescription has changed or if serious being corneal scarring, corneal rupture, infection, any other problems have developed. In addition, a study pub- important for people with high (degenerative) myopia lished in March 2001 warns that mountain climbers who who are at a greater risk of developing retinal detach- have had LASIK surgery should be aware of possible ment, retinal degeneration, glaucoma, or other problems. The lack of oxy- gen at high altitudes causes temporary changes in the Resources thickness of the cornea. Optometric Management of Nearpoint 20/20 vision, it is important to have realistic expectations Vision Disorders. The Myopias: Basic Science and Clinical ducted by the National Eye Institute between 1983 and Management.
The body grams may experience problems with typical academic breaks down proteins from food into individual amino tasks such as math discount 160mg kamagra super visa, reading buy cheap kamagra super 160mg, and spelling. Ten years of age seems to be an impor- sues, and are the key components of enzymes, antibodies, tant milestone for PKU patients. PKU and the human nervous system People with PKU tend to avoid contact with others, The extensive network of nerves in the brain and the appear anxious, and show signs of depression. KEY TERMS It is also interesting to note that people with PKU are less likely to display such habits as lying, teasing, and active Amino acid—Organic compounds that form the disobedience. There are 20 types of research findings are still quite preliminary and more amino acids (eight are “essential amino acids” extensive research is needed to clearly show how abnor- which the body cannot make and must therefore mal phenylalanine levels in the blood and brain might be obtained from food). Genetic profile Enzyme—A protein that catalyzes a biochemical reaction or change without changing its own PKU symptoms are caused by alterations or muta- structure or function. Mutations in the PAH gene prevent the liver from producing ade- Gene—A building block of inheritance, which quate levels of the PAH enzyme needed to break down contains the instructions for the production of a phenylalanine. The PAH gene and its PKU mutations are particular protein, and is made up of a molecular found on chromosome 12 in the human genome. Each gene is detail, PKU mutations can involve many different types found on a precise location on a chromosome. The term autosomal means that the gene for inheritance and expression of a genetic mutation. A recessive genetic trait, such as PKU, is one true or chronological age and multiplies that ratio that is expressed—or shows up—only when two copies by 100. A person with one normal and one PKU gene is Metabolism—The total combination of all of the called a carrier. A carrier does not display any symptoms chemical processes that occur within cells and tis- of the disease because their liver produces normal quan- sues of a living body. However, PKU carriers can Mutation—A permanent change in the genetic pass the PKU genetic mutation onto their children. Two material that may alter a trait or characteristic of carrier parents have a 25% chance of producing a baby an individual, or manifest as disease, and can be with PKU symptoms, and a 50% chance having a baby transmitted to offspring. Although PKU conforms to these basic genetic patterns of inheritance, the actual Myelin—A fatty sheath surrounding nerves in the expression, or phenotype, of the disease is not strictly an peripheral nervous system, which help them con- “either/or” situation. Although some PKU Nervous system—The complete network of mutations cause rather mild forms of the disease, others nerves, sense organs, and brain in the body. The more severe the PKU mutation, the Phenylalanine—An essential amino acid that must greater the effect on cognitive development and perform- be obtained from food since the human body can- ance (mental ability). Also, it must be remembered that human cells con- Protein—Important building blocks of the body, tain two copies of each type of gene. Different combina- composed of amino acids, involved in the forma- tions of any two PKU mutations tend to produce a wide tion of body structures and controlling the basic spectrum of physiological and psychological symptoms. For example, patients who receive two “severe” PKU Recessive—Genetic trait expressed only when mutations from their parents can potentially develop present on both members of a pair of chromo- more serious symptoms than people who possess a com- somes, one inherited from each parent. To further complicate the genetic picture of PKU, 920 GALE ENCYCLOPEDIA OF GENETIC DISORDERS other types of genes have been identified which seem to be responsible for the abnormal processing of phenylala- nine in brain tissue. These abnormalities add to the sever- ity of PKU symptoms experienced by patients who inherit these genes. In more detail, the association of multiple types of genes with a single condition, such as PKU, is referred to as molecular heterogeneity. Demographics One in 50 individuals in the United States have inherited a gene for PKU. Studies indicate that the incidence of this disease in Caucasian and Native American populations is higher than in African- American, Hispanic, and Asian populations.