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During this stage the clinical dently of medical knowledge and other diagnostic reasoning process is characterized by lines of skills cheap benzac 20 gr overnight delivery acne 38 weeks pregnant. Instead purchase cheap benzac on-line acne wash with benzoyl peroxide, it suggests a stage theory of the devel- reasoning consisting of chains of small steps com- opment of medical expertise order benzac 20 gr mastercard skin care cream, in which knowledge monly based on detailed, biomedical concepts. An acquisition and clinical reasoning go hand in hand example of detailed reasoning is given in Table 10. This fourth year medical student is dealing with a case theory of medical diagnosis is essentially a theory of pancreatitis. His initial hypothesis set contained of the acquisition and development of knowledge gallbladder and pancreas disease. Apparently, this structures upon which a student or a physician student is entertaining the hypothesis of biliary tract operates when diagnosing a case. The development of clinical reasoning expertise 115 finding about the patient’s stools affects this hypoth- occurs. These are the personal, social, medical, hered- esis and decides that this is not the case. Next, three itary and environmental factors that affect health in a items later, he combines the information acquired positive or a negative way, or which affect the course and concludes that there is no inflammation (caus- of a specific disease. The second component is the ing this obstruction) (step 1), hence, no cholecystitis fault – that is, the pathophysiological process that is (step 2), hence the biliary tract must be obstructed taking place in a specific disease, represented in by something else, a stone for instance (step 3), or a encapsulated form. The third component consists of carcinoma (step 4), which might be the case because the consequences of the fault – that is, the signs and the patient has lost weight (step 5). Contrary to (advanced) novice knowledge them to make direct lines of reasoning between dif- networks, illness scripts are activated as a whole. The more often After an illness script has been activated, no active, these direct lines are activated the more these con- small-step search within that script is required; the cepts cluster together, and students become able to other elements of the script are activated immedi- make direct links between the first and last concept ately and automatically. Therefore, people whose in such a line of reasoning, skipping intermediate knowledge is organized in illness scripts have an concepts. We have labelled this process ‘knowledge advantage over those who have only semantic net- encapsulation’, a term that refers to the clustering works at their disposal. While solving a problem, a aspect of the process and can account for the autom- physician activates one or a few illness scripts. Boshuizen & Schmidt 1992, quently the illness script elements (enabling condi- Schmidt & Boshuizen 1993). Many of these concept tions and consequences) are matched to the clusters have (semi-) clinical names, such as micro- information provided by the patient. Illness scripts embolism, aorta insufficiency, forward failure not only incorporate matching information volun- or extrahepatic icterus, providing a powerful teered by the patient, they also generate expectations reasoning tool. Encapsulation of biomedical knowl- about other signs and symptoms the patient might edge results in the next stage of development of have. Activated illness scripts provide a list of phe- clinical reasoning skills, in which biomedical nomena to seek in history taking and in physical knowledge has been integrated into clinical knowl- examination. At this stage, students’ clinical reasoning pro- the script is instantiated; expected values are substi- cesses no longer involve many biomedical concepts. The instantiated script yields findings and clinical concepts that have the status a diagnosis or a differential diagnosis when a few of hypotheses or diagnoses in their reasoning pro- competing scripts remain active. However, if needed, this encapsulated bio- script activation by an experienced physician, deal- medical knowledge can be unfolded again, for ing with the same clinical case as the student in instance when dealing with a very complicated Table 10. Van de Wiel et al (2000) showed that he heard about the patient’s medical past and psy- experts’ clinical knowledge structures subsumed chosocial circumstances (summarized in the proto- biomedical knowledge. Rikers et al (2005) demon- col) was combined with the presenting complaint strated that in expert clinical reasoning, biomedical and activated a few competing illness scripts: pancre- knowledge is also activated, operating in a sort of atic disease, liver disease and abdominal malignancy stand-by mode. In addition, network type of knowledge organization to another hethoughtofcardiomyopathyasaneffectofexces- type of structure, which we refer to as ‘illness scripts’. The first com- col he seemed to monitor the level of instantiation of ponent refers to enabling conditions of disease; that is, every illness script. Except for gallbladder disease, no the conditions or constraints under which a disease new scripts were activated. Earlier, he had received information about enabling conditions such as mental problems and alcohol abuse.

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In addition cheap benzac 20 gr without a prescription skin care gift baskets, lung scan interpretations airspace-?lling in?ltrate at the left lung base seen on the initial can be variable purchase line benzac skin care equipment wholesale. When this patient’s scan was reread the next chest radiographs ure 12B and 10B) cheap benzac american express acne keloidalis nuchae cure. Pulmonary emboli occlude left basal segmental pulmonary arteries (contrast-?lling defects) (arrow). Nonetheless, lung scans are useful in the 27% of patients who One additional question is whether the ready availability of have either “high probability” or “normal” scans. Debilitated, bed-bound, or hospitalized patients are likely to have a positive D-dimer assay simply on the basis of their underlying medical condition. Radiology 2000;215: D-dimer in the diagnosis of pulmonary embolism: A randomized 535–542. Arch Intern computed tomography scan in patients with suspected pulmonary Med 2006;166:181–187. He had visited another doctor On neurologic examination, there was mild wasting of his one month earlier for back pain, and naproxen was prescribed thigh muscles. He delayed coming to the hospital until a ished light touch and pin prick below the waist and umbili- friend was able to drive him to the city from the upstate com- cus. He had Chest, thoracic, and lumbar spine radiographs were obtained no prior medical problems. He could support his weight standing, but needed to hold on to a support while walking. The brisk lower extremity re?exes re?ect a spinal therefore typically involves contiguous vertebrae and is associated cord lesion (upper motor neuron), as opposed to a lumbar radic- with intervertebral diskspace narrowing ure 6). The main diagnostic considerations include spinal cord compression due to a neoplasm (most likely metastatic) or in- fection (either pyogenic or tuberculous osteomyelitis) or, if an anatomical lesion is not found, transverse myelitis. The pa- tient’s spinal cord lesion is incomplete since it spares some sen- sory modalities, some muscle strength, and bowel and bladder function. There is also loss of height of the T10 verte- bral body with a bone outgrowth extending from the superior end-plate ure 5). Lesions that are con?ned to the vertebrae and spare the disk are charac- teristic of neoplastic disease. The intervertebral disk is eroded and there is involvement of adjacent vertebral body end plates. Normally, the pedicles and spinous process look like an owl’s eyes and beak (T7 and T8). Since the left hilum appears prominent, this pa- In this patient’s radiographs, there are several indirect tient might have a postobstructive pneumonia due to a hilar signs of volume loss: elevation of the left hemidiaphragm, shift mass. However, increased lung opacity may also be due to a of the trachea, and lateral and superior displacement of the left reduction of intrapulmonary air, which is known as atelecta- hilum ure 8). No hilar mass is seen on pression of lung tissue adjacent to a mass or pleural effusion. However, the most clinically important cause of atelectasis the lateral view shows a well-de?ned triangular opacity at results from obstruction of a major (lobar) bronchus by an the anterior-superior mediastinum. This is the collapsed left up- endobronchial tumor, aspirated foreign body, or mucus plug. Its well-de?ned margin represents the displaced left This is called resorption atelectasis because air trapped interlobar ?ssure ure 9). A soft tissue mass extends from T8 to T10 Increased lung opacity does not always occur with lobar at- with encroachment on the spinal canal. The most direct sign Bronchoscopy and biopsy revealed non-small-cell lung of lobar atelectasis is the displacement of an interlobar ?ssure. The patient was treated with radiation to the spine Nonspeci?c signs of volume loss include shift of the mediastinum, and lung. Unfortu- elevation of the diaphragm, and compensatory hyperin?ation of nately, his lower extremity weakness did not improve. Alveolar air is resorbed into the bloodstream and a small quantity of ?uid remains in the collapsed airspaces. The Signs of volume loss include tracheal shift (arrowhead), superior and sharp inferior margin represents the displaced left major ?ssure. Thoracic pain is more or, occasionally, a herniated nucleus pulposus (intervertebral worrisome than lumbar pain because metastatic disease and in- disk). If there are signs of such as malignancy or infection (osteomyelitis) is suspected.

The nervous system is made up order benzac with paypal acne antibiotics, predominantly benzac 20gr low price acne face, of tissue that has the special property of being able to conduct impulses rapidly from one part of the body to another discount 20 gr benzac fast delivery acne dark spots. The specialised cells that constitute the functional units of the nervous system are called neurons. Within the brain and spinal cord, neurons are supported by a special kind of connective tissue that is called neuroglia. Nervous tissue, composed of neurons and neuroglia, is richly supplied with blood, but lymph vessels are not present. Most neurons give off a number of short branching processes called dendrites and one longer process called an axon. In a dendrite, the nerve impulse travels towards the cell body whereas in an axon the impulse travels away from the cell body. Peripheral nerves are made up of aggregations of axons (and in some cases of dendrites). During its formation, each axon (and some dendrites) comes to be associated with certain cells that provide a sheath for it. The cells providing this sheath for axons lying outside the central nervous system are called Schwann cells. Axons lying within the central nervous system are provided a similar covering by a kind of neuroglial cell called an oligodendrocyte. Deep to the neurilemma, most nerve fbres also have a second sheath that is rich in lipids. Within the central nervous system, it always terminates by coming in intimate relationship with another neuron, the junction between the two neurons being called a synapse. Outside the central nervous system, the axon may end in relation to an effector organ (e. Neurons vary considerably in the size and shape of their cell bodies (somata) and in the length and manner of branching of their processes. The shape of the cell body is dependent on the number of processes arising from it. The most common type of neuron gives off several processes and the cell body is, therefore, multipolar. Another type of neuron has a single process and is therefore described as unipolar. One of the divisions represents the axon; the other is functionally a dendrite, but its structure is indistinguishable from that of an axon. Depending on the shapes of their cell bodies some neurons are referred to as stellate (star shaped) or pyramidal. Apart from structural differences neurons can also be classifed on the basis of their functions. A classifcation of types of nerve fbres to be seen in peripheral nerves; and the arrangement of neurons giving origin to various functional types of nerve fbres has been discussed in chapter 43. These descriptions will be essential in understanding some aspects of the structure of the brain. Sections through the spinal cord or through any part of the brain show certain regions that appear whitish, and others that have a darker greyish colour. It is the refection of light by myelin that gives this region its whitish appearance. The arrangement of the grey and white matter differs at different situations in the brain and spinal cord. In the spinal cord and brainstem the white matter is on the outside whereas the grey matter forms one or more masses embedded within the white matter. In the cerebrum and cerebellum there is an extensive, but thin, layer of grey matter on the surface. Deep to the cortex there is white matter, but within the latter several isolated masses of grey matter are present. Such isolated masses of grey matter, present anywhere in the central nervous system, are referred to as nuclei. As grey matter is made of cell bodies of neurons (and the processes arising from or terminating on them) nuclei can be defned as groups of cell bodies of neurons. Aggregations of the cell bodies of neurons may also be found outside the central nervous system. Sensory ganglia contain neurons that give off processes that form the afferent fbres of peripheral nerves.

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Amplified signals are supplies the electrical pulse to the transducer array at processed by the signal conditioning circuits before times decided by the clock generic benzac 20gr online acne adapalene cream 01. Transmitter voltages being digitized for computer storage and eventually range from about 5 V up to a few hundreds of volts; displayed as gray-scale images purchase 20gr benzac skin care 1920s. Piezoelectric materials decreased as the signals are processed since small sig- can tolerate field strengths of up to 1 kV mm 2 buy genuine benzac on-line skin care in your 20s. The main ultrasound systems which would give practical image intensity of an ultrasound beam is directed perpendi- depths plotted in Fig. Apodization of the acoustic Echoes originating from greater tissue depths will have aperture by shaping the electrical pulse applied to the smaller amplitudes as the sound has undergone atten- transducer reduces side lobes in the ultrasound pulse uation with distance traveled. In order to produce and grating lobes in the multi-element array where good quality images it is necessary to compensate for the width of each element also influences the magni- echo attenuation and a time varying gain control or tude of grating lobe dimensions. The process is Small echo signals ( V) undergo linear amplification also known as swept gain or time varied gain. Either analog or digital methods which use returning from tissues can have dynamic ranges look-up tables may be used. This form of signal com- of between 100 and 150 dB giving amplitude differ- pensation is illustrated in Fig. Dynamic range is usually reduced to about 20 to 30 dB which can be handled more easily by the electronics. A threshold is placed on the demodulated signals to reject Compression smaller amplitude pulses which mostly carry acoustic or electronic noise and have very little useful informa- (b) tion. This increases the In current ultrasound equipment image formation is amplitude of the smaller signals at the expense of the controlled by computer and the clinical image is Image processing 539 Scan line 100 6 bits deep x 80 60 40 y Time 20 0 60 dB 40 dB 30 dB 0 10 20 30 40 50 60 (c) Digital matrix display (a) Echo strength (dB) Transducer Figure 18. First the echo signals undergo some pro- ber in each memory location depending on dynamic cessing and are then digitized according to echo range. Echoes from located in an image matrix according to element posi- beam-1 occupy columns 1 and 2, those from beam-2 tion (matrix x-axis) and echo depth (matrix y-axis). The matrix rows (y-axis) are 256 256 memory holds a digital number represent- chosen by the transducer group position. Pixel depth influences the production of sector scans (see phased arrays displayed image contrast; more bits per pixel increases later) follows the same general principle. Typical pixel depths are: sector scans the relationship between memory loca- 6 bits or 26 giving 64 gray scales tion and echo position is by polar coordinates. The • 7 bits or 27 giving 128 gray scales angle and radius at which the echo occurs defines the • 8 bits or 28 giving 256 gray scales memory location. The digital memory (frame) of a • modern ultrasound display is typically 512 512 pix- the binary values in each pixel are converted to analog els; each pixel 6, 7 or 8 bits deep. Several of these (voltage) levels for display on a video monitor for visual frames can be acquired each second. The digital values are depth is 15 cm then each of the 512 pixels in the y-axis fed out from the matrix in sequence into a digital-to- represents 150/512 0. Echoes emanating from deep structures expe- Mechanical scanning methods previously used in ultra- rience greater signal attenuation than those from sound imaging have almost entirely been replaced by superficial structures. Diagnostic imaging equipment electronically scanned multi-element array transduc- allows the user to vary the amplification of echoes from ers. Usually, the amplification increases with depth but the user may decide not to amplify echoes • Sequenced (switched) transducer arrays (linear or from a variety of depths depending on whether that curvilinear designs) information is useful or not. Each transducer the simplest image involves only one reflecting sur- has its own advantages and disadvantages, these are face in the path of the beam but in practice of course described for each transducer type and typical appli- there is more than just one. It may happen that echoes from diffuse structures Resolution is determined by the number of scanned at depth are so small as to be indistinguishable from lines, field of view and transducer aperture already the noise level which would also be amplified. The electronic distinction between user may vary the depth of field of view by using sequenced and phased arrays is decided by the exci- zoom or expansion controls and, in sector imaging, tation pulse timing. In real-time imaging the operator is Annular array transducers differ from these depth limited, e. The continuous real-time effect may be sacri- ficed if extra image depth is required. During each transmission/receive cycle only Transducers 541 12345 12345 Linear Steered linear 3 4 4-group group Annular or curved linear Phased array Element width Line width (a) (b) Figure 18.