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The unit for the quantity equivalent dose is the sievert (Sv). Thus the absorbed dose (in Gy) averaged over an entire organ and multiplied by a dimensionless factor, the radiation weighting factor, gives the equivalent dose. The radiation weighting factor is a dimensionless constant, the value of which depends on the type of radiation. The variation in the magnitude of the biological effects due to different types of radiation is described by the "radiation weighting factor" for the specific radiation type. This variation in effect is due to the differences in the manner in which the different types of radiation interact with tissue. (In terms of the older system of radiation quantities and units previously used, 1 Gy equals 100 rad, or 1mGy equals 0.1 rad.)Įquivalent dose - The biological effects of an absorbed dose of a given magnitude are dependent on the type of radiation delivering the energy (i.e., whether the radiation is from x rays, gamma rays, electrons (beta rays), alpha particles, neutrons, or other particulate radiation) and the amount of radiation absorbed. Absorbed dose is measured in joules/kilogram, and a quantity of 1 joule/kilogram has the special unit of gray (Gy) in the International System of quantities and units. Absorbed dose is thus measured in terms of energy deposited per unit mass of material. Absorbed dose is the energy deposited in a small volume of matter (tissue) by the radiation beam passing through the matter divided by the mass of the matter. This section describes the basic dosimetry quantities used to indicate patient doses during CT.Ībsorbed dose - The fundamental quantity for describing the effects of radiation in a tissue or organ is the absorbed dose. Order with caution.The unique radiation exposure conditions that exist in computed tomography (CT), during which thin slices of the patient are irradiated by a narrow, fan-shaped beam of x rays emitted from the x-ray tube during its rotation around the patient, have required the use of special dosimetry techniques to characterize the radiation doses to patients and to monitor CT system performance. The tension between the risk and benefit of CT scans is high and must not be taken lightly. And, well, for others they are simply ordered as a substitute for a good ‘ole physical exam. Some providers see CT scans as the key to preventing a medical malpractice lawsuit. They provide clarity in situations of uncertainty and help prevent misdiagnosis. CT scans offer unprecedented aid in diagnosing our patients. These studies place us as medical providers in a sticky situation. The article calculates that an astonishing one out of every 270 forty year-old women undergoing CT angiography will develop cancer related to the procedure. A 2009 study published in the Archives of Internal Medicine attributed 29,000 future cancers caused by the 70 million CT scans performed annually in the United States. Rebecca Smith-Bindman, a radiology specialist, reported that “More than 10 percent of patients each year are receiving very high radiation exposures”. Just how much cancer are all these CT scans causing? Last week, the New York Times reported that CT scans provide three-fourths of American’s radiation exposure accounting for 1.5 percent of all cancers in the United States. 90 mSv of radiation, the exposure level reached with some abdominal CT scans, is equal to the background radiation of living for over a 30 year period. Obese patients are at even greater risk of radiation exposure with imaging procedures as higher levels of radiation are necessary to obtain clear films. Experts argue that rather than the 8 mSv of radiation the FDA reports with an abdominal CT, patients are actually exposed to an average of 31 mSV, equivalent to nearly 1600 chest X-Rays. They believe radiation exposure is even higher with your average CT. This amount equates to an astounding 2.7 years worth of the level of radiation naturally experienced in the environment. According to the FDA, a CT scan of the abdomen exposes the patient to an amount of radiation 400 times that of a chest X-Ray. In 2009, the FDA published a Radiation Dose Comparison identifying the exact amount of radiation exposure from the average dental X-Ray to a CT scan of the abdomen. Just how much radiation are our patients getting with an average CT scan? What effect will this have on their future health? Belly pain? That’ll probably warrant imaging as well.
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Coming to the ER with a headache and no prior migraine history? You can probably expect a head CT.