Guest guest Posted February 28, 2005 Report Share Posted February 28, 2005 Magnetic resonance imaging http://www.answers.com/topic/magnetic-resonance-imaging-1 & method=6 A special radiology technique designed to image internal structures of the body using magnetism, radio waves, and a computer to produce the images of body structures. In magnetic resonance imaging (MRI), the scanner is a tube surrounded by a giant circular magnet. The patient is placed on a moveable bed that is inserted into the magnet. The magnet creates a strong magnetic field that aligns the protons of hydrogen atoms, which are then exposed to a beam of radio waves. This spins the various protons of the body, and they produce a faint signal that is detected by the receiver portion of the MRI scanner. A computer processes the receiver information, and an image is produced. The image and resolution is quite detailed and can detect tiny changes of structures within the body, particularly in the soft tissue, brain and spinal cord, abdomen and joints. An MRI is painless and has the advantage of avoiding x-ray radiation exposure. There are no known risks of an MRI. The benefits of an MRI relate to its precise accuracy in detecting structural abnormalities of the body. Patients with heart pacemakers, metal implants, or metal chips or clips in or around the eyes cannot be scanned with MRI because of the effect of the magnet. Metallic chips, materials, surgical clips, or foreign material (artificial joints, metallic bone plates, or prosthetic devices, etc.) can significantly distort the images obtained by the MRI scanner. Similarly, patients with artificial heart valves, metallic ear implants, bullet fragments, and chemotherapy or insulin pumps should also not have an MRI. Claustrophobia can be a problem. For an MRI, patients lie in a closed area inside the magnetic tube. Some patients experience a feeling of claustrophobia. In 2003 the Nobel Prize in Physiology or Medicine was awarded to the American Paul C. Lauterbur (1929-) and the Briton Sir Peter Mansfield (1933-) " for their discoveries concerning magnetic resonance imaging. " The presentation speechs was given by Professor Hans Ringertz, Chairman, The Nobel Assembly at Karolinska Institutet, who recounted the history of MRI, as follows. Felix Block and Edward Mills Purcell first demonstrated the physical phenomenon of nuclear magnetic resonance in 1946. These discoveries were awarded a Nobel Prize in Physics in 1952. Magnetic resonance occurs in magnetic fields between atomic nuclei and electromagnetic waves of radio frequencies. Atomic nuclei have a magnetic moment and in the magnetic field, their spin depends on the strength of the field. The direction of magnetization resulting from the magnetic moments can change. This happens when the nuclei are in resonance with radio waves of the same frequency as the frequency of their own rotation. In the same way the nuclei can send back radio waves, when there is a change in the direction of the magnetic moment. Quote Link to comment Share on other sites More sharing options...
Guest guest Posted March 1, 2005 Report Share Posted March 1, 2005 Also, CT is a very low-risk procedure. * You will be exposed to radiation when undergoing a CT. However, it is a safe level. * The biggest potential risk is if you need to get a contrast (also called dye) injection. This can help distinguish normal tissues from abnormal tissues. It also helps to distinguish blood vessels from other structures such as lymph nodes. * Like any medication, some people can have a bad reaction to the contrast. The chance of a fatal reaction to the contrast is about 1 in 100,000. Those at increased risk may require special pretreatment and should have the test in a hospital setting. Anyone who has had a prior contrast reaction or severe allergic reaction to other medications, has asthma or emphysema, or has severe heart disease is at increased risk for a contrast reaction and is referred to a hospital x-ray department for the exam. * Any time an injection is done into a vein, there is a risk of the contrast leaking outside of the vein under the skin. If a large amount of contrast leaks under the skin, in rare cases, this can cause the skin to break down. CONTRAST MATERIALS ARE DRUGS All contrast agents used in radiography are potentially harmful and should be used knowledgeably. Contrast materials must be considered drugs because they are NOT biologically inert, pharmacologically inactive, and efficiently and innocuously excretable. [5] " From a purely pharmacologic point of view, contrast media should be regarded as among the safest of all intravascular drugs. Nevertheless, the high utilization rate of these substances must invariably be associated with significant numbers of reactions even at very low reaction incidence. " [16, p. 171] RADIOGRAPHERS SHOULD BE ALERT TO IMPLICATIONS OF USING CONTRAST MATERIALS Radiographers and physicians involved in contrast procedures should frequently review product package inserts for the specific contrast used during a procedure. The appropriate contrast material for a procedure depends on the area to be examined, the patient's age, weight, general physical condition, state of health, and the route by which it should be eliminated after the procedure.[11] The latest nonionic iodine contrast agents have low osmolality and demonstrate significant risk reduction for patients. These new contrast materials require us to re-evaluate all contrast procedures. While radiographers do not independently select the type of contrast for any particular patient's radiographic exam, the nature of the procedures places most radiographers in immediate and continuous contact with patients after they receive contrast materials. Radiographers are the first to recognize and respond to a patient's reaction to contrast material. Radiographers have input into departmental protocol and purchasing. For these reasons as well as professional interest, radiographers should often review current information about contrast materials. Some patients receiving contrast studies deserve special consideration. As with other ionizing radiographic examinations, the radiographer should confirm that the patient is not pregnant before beginning a contrast procedure. Because of inconclusive human data, nursing mothers should convert to bottle feedings for 24 hours following iohexol (iodine contrast) injection.[12] PURPOSE OF CONTRAST MATERIALS * The purpose of contrast materials is to increase differential attenuation between a soft tissue structure and surrounding tissue. * The purpose of double contrast procedures is to coat the surfaces of a body structure with positive contrast and fill the lumen with negative contrast. POSITIVE AND NEGATIVE CONTRAST MATERIALS Negative Contrast Materials Negative contrasts appear dark on the radiographic image because they decrease the tissue density. X-Ray photons have little attenuation as they pass through an area filled with air so more photons reach the film. This creates a " dark " area on the radiograph. Room air is most often used as a negative contrast material because of the increased risk of emboli when using oxygen and the tendency of carbon dioxide to decrease local tissue pH. Negative contrast may be used alone (example: myelograms, arthrograms) or in combination with a positive contrast material (examples: upper GI, BE with air). When a positive and a negative contrast material are used together, it is called a " double contrast " study. Positive Contrast Materials Positive contrasts appear white or light on the radiographic image because they increase the tissue density. X-Ray photons are attenuated by the contrast material's molecules, casting a " shadow " on the film to represent areas filled with the contrast material. The linear attenuation coefficient of a contrast material is proportional to the mg of iodine or barium per mL of solution. Examples of positive contrast materials include barium sulfate and iodinated contrast materials. PATIENT PREPS Patient prep information should include descriptions of the preparations necessary before the procedure, purpose of the test, mechanics of the procedure, patient's role, approximate time required, explanation of equipment, and any requirements following the procedure. In some states, patient prep may also include a consent form for iodine contrast procedures. Radiographers or receptionists should give clear directions to outpatients in a variety of formats. Talking slowly with the patient, giving them written directions to take home, and having the patient repeat the instructions are ways to be sure the patient understands. Nurses are usually responsible for prepping inpatients. Patient preps before contrast procedures have several functions: * clean GI tract for better visualization of structures * decrease chance of nausea / vomiting and subsequent aspiration * allow more concentration of contrast material in body part Examples of patient prep for GI procedures include[3]: * special diets o low residue foods for several days before the procedure o increased water and transparent liquids 24 hours before the procedure, to promote bowel cleansing and decrease chance of dehydration o NPO 8 to 10 hours before the procedure o chewing gum and smoking are discouraged because they stimulate gastric secretions which could dilute the contrast material and increase the chance of aspiration. Chewing gum and smoking also increase the amount of air in the GI tract. * cathartics o laxatives o may be oral or suppositories o should encourage patients to increase intake of fluids to prevent dehydration * enemas o tap water or soapy water * pre-meds o to reduce anxiety, anesthetize area, reduce peristalsis, fill area with contrast, etc. HOCM v. LOCM According to McClennan [2] low osmolality nonionic compounds should replace high osmolality contrast materials because of their reduced osmolality, reduced chemotoxicity, and increased hydrophilicity. The Katayama data show a conventional contrast adverse reaction rate of 12.66% compared to 3.13% for nonionic contrast agents.[5] These data show dramatic reductions in incidences of nausea, heat sensation, vomiting, itching, urticaria, flushing, vascular pain, hoarseness, sneezing, coughing, chest pain, abdominal pain, palpitation, facial edema, rigor/shiver, dyspnea, and sudden drops in blood pressure. " High osmolality and viscosity are the major characteristics of water-soluble contrast media that are responsible for the hemodynamic, cardiac, and subjective effects ... These effects include vasodilation, heat, pain, a variety of hemodynamic effects, and an osmotic diuresis. " [16, p. 2] These actions cause patient dehydration. PHYSIOLOGIC EFFECTS OF IODINATED CONTRASTS [1, 8, 15] Neurologic System During IV injections, the blood-brain barrier usually prevents contrast material from contacting delicate nerve cells. During intrathecal injection, there is no barrier to protect the nerve cells. Because the central nervous system is sensitive to changes in electrical potential of cell membranes, and ionic contrasts can change electric potentials, ionic contrasts can cause seizures if introduced into the subarachnoid space around the brain. Radiographers and physicians had to depend on gravity and patient position to keep the contrast in the spinal canal and away from the subarachnoid space around the brain. Nonionics do not cause a change in electric potential. Most facilities no longer use any ionics for myelography or intrathecal injections.[15] Patient conditions which affect the blood-brain barriers, like brain metastases, place the patient at higher risk for seizures. Seizure rates after IV contrast are rare in the general population (0.01%), but are significantly higher in patients with brain metastases (6 - 19%). [16, p.16] Effects reported on the neurologic system include headache, vertigo, photomas, taste perversion, anxiety, blurred vision, motor/speech dysfunction, paresthesia, somnolence, stiff neck, hemiparesis, syncope, and nystagmus.[12] Unconsciousness and rarely coma and convulsions are described in other literature. Reports of transient blindness usually resolve completely in a few moments, but on occasion last 48 hours. IODINATED CONTRAST REACTIONS Generalized Contrast Reactions [3, 11, 16] Comparisons between studies are difficult because there are no universal symptom categories for minor or mild, moderate, and severe or major contrast reactions. The following is a compilation of several resources. a. Minor Reaction - occur in 5% of injections [16] Symptoms include: - metallic taste in the mouth - warm flushed feeling - nausea and vomiting - sweating - light-headedness - perioral dysesthesia - pain at injection site - urticaria - headache - salivary gland swelling - few, scattered hives Radiographer Response: - reassure patient - make patient comfortable with cool cloth - avoid aspiration - some references suggest interrupting the contrast injection if flushing or nausea occurs[16] - alert MD of symptoms - MD may give Compazine for nausea - MD may give Benadryl for hives b. Moderate Reaction - occur in 0.022% of injections Symptoms include: - pain at the injection site - urticaria - hives - persistent and intense minor symptoms - dyspnea - hypotension - chest pain Radiographer Response: - reassure patient - make patient comfortable - alert MD of symptoms - MD may give Compazine for nausea - MD may give Benadryl for hives - MD may give Tagamet, Zantac, or cimetidine for urticaria c. Major Reaction - occur in 0.0025% of injections Early symptoms include [11]: - itching at injection site - sneezing, coughing - bronchospasm - apprehensiveness - persistent and intense minor symptoms - nausea, vomiting - paresthesia - diarrhea Later symptoms include [3]: - edema of face, hands, and other body parts - subclinical pulmonary emboli - choking, wheezing, dyspnea, cyanosis, bronchospasm - decreasing blood pressure, bradycardia, weak rapid pulse, shock - pulmonary edema - cardiac arrhythmia, MI - dilated pupils - convulsions - paralysis - renal failure - coma - death Radiographer Response: - recognize need for action, stop procedure - know how to call for help and how to call a code - monitor vital signs - keep code cart accessible and updated - know location of emergency drugs and supplies - know location and how to use oxygen - know and begin CPR if necessary - assist patient during seizure activity if necessary - MD may give oxygen, inhalation therapy of Alupent, Brethaire, or Proventil for bronchospasm - MD may give epinephrine for bronchospasm - MD may give IV fluids (for shock), Solu-Cortef, Solu-Medrol (antihistamines and corticosteroids are useless initially - hour lapse), dopamine (for tachycardia), atropine (for bradycardia), diazepam (Valium), etc. Nausea and Vomiting The most likely reason for nausea and vomiting is stimulation of the chemoreceptor trigger zone which controls the vomiting center and is located in the floor of the fourth ventricle. This area has no blood-brain barrier. Nausea and vomiting have been substantially reduced when using nonionics.[15, 16] > > Quote Link to comment Share on other sites More sharing options...
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