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Today we know clearly the dangers of x-ray and know the procedures to be followed to ensure that its usage is in the safest manner possible. Still we are bombarded by a massive amount of information in the news media concerning the dangers of radiation, often relative to the dangers of minimal amounts of radiation. As a result of this, we find interesting concepts relative to radiation exposure that have developed in the minds of Veterinarians, Animal Health Technicians, students in schools of Veterinary Medicine and the public in general. It is the purpose of this section to briefly describe the dangers of ionizing radiation and to carefully list the techniques that are available to us today to ensure that radiographs are produced in the safest manner possible. In addition it is important to understand the significance of the exposure received while assisting in radiography of animals. These levels of radiation must be placed in the proper perspective relative to the level of background radiation that we all receive and relative to permissible limits established by advisory governmental agencies.
The major objective of a Veterinary practice that uses ionizing radiation in diagnostic radiology should be obtaining the maximum diagnostic information with the minimal exposure to all people involved in the radiographic examination including the owner or handler of the animal examined.
We gain information relative to radiation effects from a number of sources. Unfortunately not all of these sources can be considered totally
reliable. Since the evaluation of radiation effects can be subjective, it is possible to hear widely differing opinions.
In looking at the information made available to the general public let us begin with an example that clearly demonstrates how easily we are influenced by the news media. Mammographic examinations have been used in the diagnosis of mammary carcinoma in women for approximately 15 years. The effect of the wide spread knowledge that a past president's wife, Mrs. Ford, and a past Vice-President's wife, Mrs. Rockefeller, were diagnosed as having breast cancer caused a steady increase in the number of mammograms until 1974 and 1975. In 1976 a paper was published indicating that perhaps more cancers would be induced by the use of 126 mammography than would be detected by this type of examination. The news media gave wide coverage to this information. Currently (1980) there are fewer mammographic examinations performed in the United States than were performed in 1971 even though the level of exposure to each patient was reduced by a factor of 10 during this time through the conversion from non-screen studies to screen type examinations. (Gray, 1979)
The public has developed a paranoia to the effects of exposure by ionizing radiation. While there is little doubt of the effects of massive exposures, information concerning the significance of minimal exposure is harder to find. Because of this a portion of this section will explore both the effects of massive exposures as well as examine low level exposures such as might be encountered in routine diagnostic radiology.
Another example of news media influence is the reporting of the Three Mile Island incident in 1979. Secretary Califano, of what was then called the Department of Health, Education and Welfare, reported that residents within a 50 mile radius received a total dose of 3,500 - 4000 person-rems during the period of March 28 to April 7, the worst days of the accident. The article did not define the "person-rem" that was received by the 2.2 million people living with the 50 mile radius of the accident. If we divide the number of person-rems by the number of residents living in the area, we find that each person received in this period of time an additional 1.75 millirems (3,500 personrems/2,200,00 persons) as a result of the accident. In assessing the significance of this level of radiation, you should remember that each person living in the U.S. receives approximately 200 millirems each year in background radiation. During this significant 11 day period in March and April of 1979 each resident of this 50 mile radius received an additional 6.0 millirems in background radiation (an increase of 3% ).
It is estimated that the additional radiation level to this select population will cause 1 to 10 additional deaths in the remaining life times of these 2 million persons. The news media failed to point out that these 1 to 10 new cancer deaths as a result of the nuclear accident would be in excess of the already expected 3,800 cancer deaths per year in this population. (Gray, 1979)
It is certainly not safe for us to hide behind certain reports of radiation exposure in the field of veterinary radiology that suggest that occupational exposure to x-rays in veterinary practice is unusually low. These reports are often based on low average usage. In one study the average workload was 2.5 mA min each week and the highest work workload was 30.0 mA min. It was the low number of exposures made which caused the reports of low radiation exposure and not the practice of good radiation safety since 90% of the practices were failing to limit the primary beam by beam limiting devices, wear adequately protective aprons and gloves, use radiation monitors or use anesthesia or tranquilizers where possible (O'Riodan, 1968).
It is necessary that methods be available to detect and measure the amount of ionizing radiation present both in air and absorbed in body tissue. The following units have been described:
Roentgen (R) measures the quantity of ionization produced by X- or gamma radiation. This quantity is called exposure. It is that quantity of X- or gamma radiation such that the associated corpuscular emission in 0.001293 gm of air, produces in air, ions carrying one electrostatic unit of quantity of electricity of either charge. If one cc of dry air under standard conditions is held within a charged chamber, one roentgen of X- or gamma radiation will cause sufficient ionization so that 1 e.s.u. of electricity is generated by the discharge of the charged electrodes. The roentgen is based on ionization produced in air and is also defined as 2.58 x 10 4 Coulomb/ kilogram of dry air. This unit is used for ionizing electromagnetic radiation and not for particulate radiation such as alphas, betas, or neutrons.
Rad is a unit of absorbed dose for any ionizing radiation if the energy transferred to the irradiated material is 100 ergs/gram of any substance. When water and soft tissue absorb X- or gamma radiation of an energy between 100 keV and 3 meV, the absorbed dose/roentgen is between 0.93 and 0.98 rad. Therefore, the number of rads is approximately equal to the number of roentgens. The rad is used for particulate as well as electromagnetic radiation.
REM (rad equivalent man) is a unit of dose equivalent and was proposed to make allowances for the fact that the same dose in rads in living tissue from different types of radiation does not necessarily produce the same degree of biological effect. The dose in rems is equal to the absorbed dose in rads times the relative biological effects factor for the type of radiation being absorbed.
Curie is a unit of radioactivity that is related to the three preceding units of radiation. It is a unit of the quantity of radioactive material and not the radiation emitted by that material. One Curie is that quantity of material in which 3.7 x 10'° atoms disintegrate every second.
Electron Volt (eV) is the unit used to measure the energy of an x-ray. Often, this is done in thousands of electron volts (keV). Binding energies are also expressed in electronic volts.
Radiation injures tissue by ionization of vital molecules within the cell. It is heavily dependent on the dose rate of the exposure. A total dose of 600 rads administered to the whole body of a person probably would result in the death of that person. The same 600 rads would probably be survived if the dosage was administered in six 100 rads doses spread over a period of several months. This difference in response to the same total dosage recognizes the ability of the body to repair injured tissues.
Another factor strongly influencing radiation effects is the percentage of the body exposed to the radiation beam. The tumor site in an animal treated by radiation therapy often receives 500 rads administered three times each week for four weeks to a total of 6000 rads . This exposure to the entire body even in 12 equal doses would cause death. However, because the radiation was administered to a small field, the patient can repair the damage from the surrounding healthy tissue.
There are physical factors affecting radiosensitivity. One of these is the linear energy transfer, LET, which is a measure of the rate at which energy is transferred from the ionizing radiation to the soft tissue. The ability of ionizing radiation to produce a biologic response increases as the LET of the radiation increases. As the dose of radiation increases, the ability to produce biologic damage also increases. The relative effect is quantitatively described by the term, relative biologic effectiveness (RBE). RBE is the ratio of absorbed dose of x-rays or gamma rays to the absorbed dose of a certain particulate radiation required to produce an identical biological or chemical effect in a particular experimental organism or tissue .
Injury to the body by radiation occurs in two different categories: somatic and genetic. Radiation injury is also divided into early and late effects dependent on the time of detection of the injury. A third consideration in evaluation of radiation injury is whether the animal receiving the exposure is physiologically mature or is still in a stage of tissue and organ maturation. This obviously takes into account in-utero exposure. (Fig. 23-1)
Early Effects. If the whole body is exposed to a sufficient amount of ionizing radiation, there are changes in many organs of the body. As a result of either the effects to one particular organ or the interaction of
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effects in several organs, the animal as a whole shows characteristic syndromes. Some syndromes inevitably result in death while others may or may not be lethal depending on the extent of the tissue damage. The length of time prior to the appearance of the clinical syndrome, its duration, and the survival or death of the organism depend on many factors such as:
An LD50 (30) is the exposure determined through experimentation that is required to cause death within 30 days of 50°O of those animals exposed. Typical LDso (30) values for total body exposure of animals to X or Gamma radiation are listed (Table 23-1).
Four syndromes have been described following exposure to ionizing radiation. The levels of exposure vary greatly, as do the clinical signs observed.
Doses of 100,000 rads cause molecular death since massive doses of this nature presumably cause an inactivation of many enzyme systems which are needed for basic metabolic processes of the cells and tissue. Death occurs during or immediately after irradiation.
Doses in excess of about 10,000 rads produce death within a day or two after exposure. Terminal signs of hyperexcitability, incoordination, respiratory distress, and intermittent stupor suggest damage to the nervous system.
Doses of 900-10,000 rads cause most animals to die between 3 to 5 days after exposure. Deaths during this period are attributable to a gastrointestinal syndrome and relate to loss of fluids from the gastrointestinal tract and invasion of microorganisms and toxin absorption through the damaged gastrointestinal mucosa.
Doses of 300-900 rads may or may not cause death, depending upon the species. If death results, it is due to damage to the hematopoietic tissue and usually occurs between 10-15 days after exposure. A marked decrease in the level of leukocytes is noted, permitting secondary infections such as pneumonia.
Death generally does not occur from partial body irradiation. Extremities can withstand much higher doses than the blood-forming organs or the gastrointestinal tract. Exposure to the limbs is important because of the possible repeated exposure of the skin and the chance of eventual development of squamous cell carcinoma of the skin. Exposure of this type can occur to the veterinarian or assistant who fail to utilize their diagnostic radiographic equipment in the manner prescribed. Improper use of a fluoroscopic unit could easily cause partial body irradiation of such a nature than chronic effects such as squamous cell carcinoma would result.
Late Effects. Late effects from radiation are the least understood of the effects of radiation exposure. It is relatively easy to measure the effect of high levels of acute exposure by observing the immediate death of the animal or rather obvious clinical signs. But the delayed effects of an acute exposure or the consequences of low level exposure over an extended period of time aren't as easily identified. Results are believed to include:
A recent study of the causes of death of men served in the U.S. Army as radiological technologists during World War 11, revealed that there was no significant difference in the number of deaths resulting from cancer in this group who received chronic exposure to low levels of x-rays than was found in a control group of medical, laboratory, and pharmacy technologists who received no level of radiation. (Jablon and Miller, 1978)
The effects on man of diagnostic and low level therapeutic radiation (less than 300 rads to the bone marrow) was also recently investigated. No statistically significant increase in leukemia was found in these patients when matched with two control groups. It was noted that the amounts of radiation were administered in small doses over long periods of time as might be found in routine medical care or as might be experienced by an animal health technician assisting in diagnostic radiology studies (Linos, A. et al., 1980).
A recent study of low levels of whole-body gamma radiation, 100 rads and 20 rads to the dog, demonstrated the importance of irradiation during organ development. Exposure at this time induced abnormalities of growth and development related to the skeletal system, the kidney, and the eye. Perinatal irradiation of the kidney at 55 days post coitus or 2 days post partum was-associated with renal dysplasia and significant renal disease in later life. Retinal dysplasias were also induced in the developing eye and were thought to be responsible for significant visual impairment with age. Malignant lymphoma was diagnosed in some of the irradiated dogs with half of these having been exposed at 55 days post coitus. Testicular-neoplasms were also detected in relatively young irradiated male dogs especially those exposed at 55 days post coitus. The importance of exposure at the perinatal period was clearly shown in this study. The doses delivered to the dogs were at least 100 to 1000 times higher than those which might be delivered in routine diagnostic studies. Extrapolation of these findings to lower doses, however, suggested that some canine neoplasms or developmental disorders might be related to diagnostic radiation delivered during gestation especially at the time of parturition. (Collaborative Radiological Health Laboratory, Colorado State University, Fort Collins, Colorado 80523)
Upton, et al (1953) observed lens changes in mice after an exposure of 15 R and thus documented the risk of not shielding the eyes during radiographic techniques. Later Upton, et al (1956) found that the lens of man is more refractory to ionizing radiation than that of laboratory animals. Merriam and Socht (1957) reported cataracts in adults after a lens exposure of about 200 R and noted that the lens in adults is less sensitive to radiation than in chickens. The latency period between exposure and the detection of radiation cataracts varies with dose and may exceed 20 years.
In addition to the effect on the patient being examined, it is obviously important to quiz all persons assisting in the radiographic study so that no pregnant women are permitted to assist in the examination. Also, all young people under the age of 18 years of age should be excluded from participating in the radiographic examinations.
Genetic effects of radiation occur as a result of injury to the genes of reproductive cells of the exposed individual. This effect is often referred to as "gene mutation" which may have the inclusive meaning of all changes in the hereditary material capable of altering the individual phenotype whether or not chromosomal aberration is involved. The mutation may be a lethal type or may be only a visible point mutation. The mutation may be a recessive change and may not be responsible for death of an individual for many generations after its production.
It has not been possible to make a simple noncontroversial statement concerning the implications for man of the genetic effects of radiation.
One view begins with the geneticist, Haldane's opinion, that even our natural or spontaneous mutation rates are due to natural or background radioactivity. There is general agreement that genetic radiation effects have no threshold. The dose-rate relationship for man has been estimated by extrapolation from data on the fruit fly, mice and dog. Consequently, estimates of the dose causing doubling of the mutation rate range from 5 to 150 r with most estimates being of 80 r. The manner in which mutation rate might increase would depend upon the pattern of inheritance, and whether the radiation exposure was for a short interval or continuing exposure. At present, it is estimated that 2% of live-births in the U.S. have defects which are of simple genetic origin. Many do not impair function of the individual. Some, such as mental deficiency, are enormous burdens. These burdens of deleterious mutants are not easily lost, but perpetuate frequently "silently" through many generations.
Theoretically, there is no radiation level without increase in frequency of mutation. The maximum permissible exposure appears to have an effect comparable to that of unavoidable natural exposure (background radiation).
Film badges. The film badge is the most commonly used radiation detection device today. It provides a reasonably accurate means of determining doses from beta, gamma, and x-radiation. Most film badges consist of a plastic holder containing radiation-sensitive film, usually of dental film size or 35-mm photographic film. The film badge also contains a variety of filters used to absorb radiations of varying energies.
The variety of filters placed at different points on the film badge allows identification of a specified type and energy of the radiation producing the exposure. There is also an area on the film badge that has no filter and is not covered by even the plastic holder. Beta as well as very weak energy gamma radiation can be detected in this area.
Films are developed and then evaluated by measuring the density of the blackening on the film. These measurements are compared to standard films that have been exposed to known radiation doses. Generally, film badges are capable of measuring doses from 10 mR to 500 R. However, sensitivity has been reported to the 6 mR level.
The film badges are usually worn on the pocket, lapel or collar. The same film may be worn for a week, or more commonly a month. The length of time depends on the sensitivity of the film and the amount of radiation to which the radiation worker is exposed.
Radiation monitoring badges are available in several forms: l) rings, 2I clips, and 3) wrist badges. The badge may be color coded so that it is easy to determine whether an employee is wearing the correct badge for the current monitoring period. Following submission of the exposed films , the company will respond with a routine exposure report or an emergency report if exposure levels are unduly high. The report gives you information on the dose for the current period, quarter to date, year to date, lifetime total and remaining permissible dose. Unit price usually depends on the number of film packets per shipment and ranges from $5.40 to less than $3.00 per film (Fig. 23-2).
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Film |
gamma beta thermal netron fast neutron |
0.001 to 10000 rem |
20 Kev for gamma rays , 200 kev for beta rays |
1.Inexpensive2. Give estimate of integrated dose3. Provides permanent record |
1. A moderate directional dependence2. Strong energy dependance for low energy X-rays3. False readings produced by heat ,Pressure, and certain vapors.4. Information not immediately availabel. |
Pocket Ionization Chambers |
gamma beta minus gamma thermal neutron fast nertron minusgammaX-rays |
0.001 to2000R |
30 Kev for gamma rays, 20 Kev for fast neutron |
1.Yield fairly accurate informaiton2. Small size3. informatin available immediately4. Reasonably uniform in response to radiation in the energy range of 50 Kev tp 2 mev.5. Require little maintenance6. Reusable |
1. There is no permanent record2. Frequent reading , tabulatin , and rechargingmay be required.3. Subject to accidental discharge (through shock and sometimes, electrical leakage)4. Range of measurement is limited full scale ranges from 0.2 to 20100 R available5. Economical only for long-term use |
TLD |
gamma beta |
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20 Kev |
1. indefiniteshelf life with supplied as the useful range2.Small size3.Small energy dependence4. Reusable5. Inexpensive6.Give estimate of grated dose over long periods. |
1. limited TLD systems service.2. Cancellation of dose upon reading3. Dose range depends on the senditivity of the reader4. Radiations detected depend on type of TL material |
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Pocket ionization chambers. Another method of measuring radiation exposure is with an ionization chamber. Ionization in air discharges the charged electrodes, and the amount of discharge is read on a meter in roentgens. The chamber can then be charged again. They have the advantage of giving an immediate reading in milliroentgens (Fig. 23-2a).
The disadvantage of these chambers is that while they are fairly accurate, they are subject to accidental discharge following being dropped. Another problem is the lack of any permanent record of the radiation exposure. The greatest advantage is the immediate availability of the level of radiation received (Table 23-3).
Thermoluminescent dosimetry (TLD). Thermoluminescent dosimetry is a method of radiation detection that is rapidly gaining acceptance as a personnel monitoring device. Materials used for thermoluminescence are primarily calcium fluoride and lithium fluoride. These materials, when exposed to ionizing radiation, absorb the energies released in the material. This energy is liberated only on subsequent heating of the material. As the temperature reaches a characteristic value, the energy is released in the form of light, and this light is analyzed for exposure intensities-hence the name thermoluminescence.
Thermoluminescent dosimetry offers a variety of uses in the radiation medicine field. In addition to personnel monitoring applications, it is used in the measurement of doses in tissues surrounding a therapeutic tissue implant source. The thermoluminescent method is more sensitive than the film badge.
Personnel exposure meter. A recently described personnel-exposure meter has been described that offers immediate exposure rate readout on a LCD display in mR/hr. In addition, an integrated exposure in mR can be displayed. A sound-alarm indicates pre-selected thresh old limits of 2, 8, 32, and 129 mR/hr.
The meter is obvious value in that personnel can immediately be advised by alarm of a thresholdlimit as well as know an exposure rate or an integrated exposure level at any time. The value of such a unit in teaching and guaranteeing use of radiation protection procedures is obvious (Schraub et al, 1983).
The concept of a maximum permissible exposure was discussed by R.S. stone in his Carman Lecture in 1952 in which he traced the history of our growing awareness of radiation consequences and maximum permissible exposure. As shown below, in the chronological table, the permissible level of exposure has been progressively reduced (Table 23-5). Some of the reasons for justifiable caution are
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Maximum permissible doses (MPD) are of great interest to the radiographer since this is the dose equivalent that persons are allowed to receive in a stated period of time. It is this dose that helps to determine whether procedures and equipment in use are adequate to provide the degree of protection to the worker that is necessary. The exposure for whole body, gonads, blood-forming organs, and lens of the eye is:
The question is often raised as to whether the film badge or ionization chamber should be worn under the lead apron or on the outside of the apron. The lens of the eye is considered a critical organ; therefore, it is necessary to determine the measurement to the head and eyes and the device should be placed outside of the apron and at the neckline position. There are situations where it may be desirable to know the whole body exposure under the lead apron plus the exposure to the head and eyes, and the use of two measuring devices can be considered.
Exposure of patients for medical and dental purposes is not included in the maximum permissible dose equivalent. Risk to individuals exposed to the MPD is considered to be very small; however, risk increases gradually with the dose received. It is only a guide to be followed in determining if protection offered workers in radiation areas is as adequate as it should be.
Background Radiation. Background radiation is the inherent radiation above which a radiation level must be measured or evaluated. Examination of levels of exposure received by radiation workers must be made with this concept in mind. The radiation background at any point on earth is usually understood to include natural background (from cosmic radiation and naturally occurring terrestrial radioactive materials), displaced terrestrial radioactive materials (such as radioactive materials in building and paving materials), and man-made radiation sources(such as fallout deposits from atomic weapons or nearby radioactive storage sources). Altitude is important in determining cosmic radiation and it should be appreciated that the cosmic radiation measured in Denver, Colorado is about two and one-half times as much as that measured in New York City (Table 23-7).
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Rollins |
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Muscheller, Sievert |
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Solomon |
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Dutch Board |
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Barclay, cox |
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Kaye |
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Advisory Comm.,USA |
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Failla |
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Advisory Comm,.USA |
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International Commissionq |
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Advisory Comm., USA |
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Stenstrom |
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Averages have been estimated for the U.S. population that suggest the composition of our total radiation dose from background radiation (environmental, medical, occupational, and miscellaneous). When you try to relate those figures to yourself, remember individual variation such as altitude and days of sunlight, and how many air plane flights you have made recently. Cosmic radiation dose to the whole body during a round trip flight to Washington, D.C. from the West Coast of the United States at 10,000 meters (35,000 feet) is 3-5 millirem (.003-.005 rem) (Table 23-8).
Exposure from diagnostic radiographic examinations contributes heavily in the estimated radiation dose received by the average American. These exposures can be calculated either as whole body exposure or as gonadal exposures. Average exposures received during several types of radiographic examinations are listed (Table 23-9). These can be compared with the level of background exposure received.
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Maximum accumulated |
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rem |
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For Restricated Workers (controlled areas) |
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Whole body, grounds, blood forming organs,lens of eyeSkin of whole bodyHands and forearms, head, neck feet, and ankles |
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Normal population
regions 75-100 millirem (0.1 rem)
Brazil
Volcanic areas 1600 millirem (1.6 rem)
Monazite sand areas 500 millirem (0.5 rem)
India (Kerala)
Monazite sand areas 1300 millirem (1.3 rem)
France
Granite areas 19 millirem (0.02 rem)
Environmental Millirem
Cosmic rays---------- 44
Rocks, soils, building materials ----------40
Inside body ------------18
Global fallout ------------4
Nuclear power in -------------1970 0.003
Nuclear power in 2000 (est.) --------------0.425
Subtotal -------------------------------------------106
Medical
Diagnostic x-rays-------------------- 72
Radiopharmaceuticals -------------1
Subtotal -------------------------------73
Occupational
X-ray specialists, uranium miners, nuclear workers-----------------0.8
Miscellaneous
TV, Consumer products, air travel ------------------------------------------2.6
TOTAL ----------------------------------------------------------------182.4(0.18 rem)
Area Irradiated Gonad Dose (mrads)
Male Female
Chest------------------------------------------ 0.1-6 0.1-15
Stomach (barium series) ---------------3-123 6-1108
Lumbar spine------------------------------- 25-1310 20-2530
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Frames per Secong |
Exposure Rate (R/min.) |
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Cine radiography is the exposure of 16, 35, 70 or 100 mm square film frames with the image taken from the output phosphor of an image intensifier that results in the production of a film that can be reevaluated diagnostically after the study. The exposure rates to the patient during the production of the film are listed (Table 23-10).
Surface doses of radiation received during a diagnostic study vary with the type of exposure used. Exposures made with higher kVp settings and appropriate lowering of mAs settings deliver a lower exposure to the patient. Added filtration also lower patient exposure. In addition, it is apparent that under almost all circumstances the exposure level to the patient is higher when using a stationary anode tube than a rotating anode tube. This is because the quality of the beam produced with the stationary anode tube has a higher percentage of low energy photons present. Note that by adding only 3 mmAl filtration, this low energy segment of the beam generated by the stationary anode has been eliminated. The radiation intensity at the 50 kVp settings is close for both stationary and rotating anode tubes. This is because of the narrow range of photon energies that fall below the 50 kVp maximum and that energy level high enough to penetrate the tube and housing (Table 23-11).
Estimates of radiation levels encountered by people in various aspects of daily life have been made (Table 23-12). In noting the wide range of doses, remember that some exposures are confined to limited areas of the body while others are whole body exposures. Also, some exposures are received in a short time while others are received over longer periods of time up to one year.
Patient Exposure. Most of our concern with radiation safety in Veterinary radiography is with the owner, technician, helper or veterinarian. There is in addition a bit of information reported concerning the exposure to the animal patient as well. The following typical exposures were measured with 2 mm Al filtration added. Without the added filters, the doses would be increased approximately 4 times. In diagnostic exposures in small animal practice an approximate entrance skin dose of 100mR in the primary beam was measured at 80 cm focal object distance (70kVp), 10 mAs). The scatter level adjacent to the body of the patient was 5n mR at the table edge. Levels were also measured in large animal radiography with the level of radiation measured in the primary beam at 75 cm focal-object distance being 50 mR per exposure (80 kVp, 5 mAs). The level of scatter measured adjacent to the cassette was from 2 to 10 mR (Corwin, L.A. personal correspondence). Routine radiographic procedures were evaluated in Beagle dog phantoms at 100 cm focal-film distance using thermolucent dosimetry (TLD) techniques and indicated that the entrance surface exposure was 12 to 65 mR per exposure when using a range of 60 to 92 kVp and 5 to 50 mAs(Barnes and Osborn, 1972).
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Flight from Los Angeles
to Paris (cosmic rays)... 4.8 millirems
Chest X-ray (1 film)...... 22 millirems
Contamination
measured one-half mile from Three
Mile Island during nuclear accident -----------(1980) 83 millirems
Apollo X astronauts on moon flight
9 cosmic rays).......... 480 millirems
Dental x-ray (whole mouth)...... 910 millirems
Dose on Three Mile Island site
during accident (1980) ------------1,100 millirems
Breast mammography (1 film)..... 1,500 millirems
Current yearly occupational-exposure
limit................. 5,000 millirems
Fallout in St.George, Utah, from
1953 atomic-bomb test------------ 6,000 millirems
Barium enema.... 8,000 millirems
Heart catheterization
(before bypass surgery).... 45,000 millirems
Pacemaker insertion with
fluoroscopy...... 132,000 millirems
Radiation treatment for Hodgkin's
disease------------ 4,500,000 millirems
Radiation treatment for bone cancer .... 6,000,000 millirems
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The first consideration in radiation safety is to completely eliminate as many people as possible from the examination area. This includes all pregnant women, individuals under 18 years of age, and anyone not required for the examination. Next the examination must be planned in such a way that as few people as possible are required to be in the room during the exposures. this is done by using anesthesia or tranquilization of the patient and use of positioning devices, restraining devices, and cassette holders (Fig. 23-3).
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Next consider increasing the distance of all individuals in the room from the primary beam. This can be done by restraining devices and cassette holders that permit anyone still required in the examination room to position themselves as far as possible from the primary beam (Figs. 23-3, 23-4, 23-5 and 23-6).
Next consider the use of protective clothing. All people within the x-ray room must wear protective aprons when assisting in a diagnostic study. Aprons should be 0.5 mm lead equivalent determined at a level of 85 kVp. If hands are placed within 1 meter of the primary beam, protective gloves must be worn (Figs. 23-7 and 23-8). These gloves should also be 0.5 mm lead equivalent determined at a level of 85 kVp. Often it is possible to shield personnel in the room from scatter by use of mobile lead shield or by lead shielding suspended from the tube housing. Use of this type of protective device can often cut radiation exposure markedly. The use of a control booth offers maximum protection and all personnel should be behind this shield during the exposure if the examination permits (Fig.23-9).
For many years, the only alternative to lead glass for shielding windows has been plate glass. Trout et al, (197400 found that the thickness of plate glass required to shield single-phase x-ray equipment was approximately the same as the required thickness of concrete. For three-phase x-ray equipment, additional material would be required for the same kVp setting. Lead acrylic has added a third alternative to lead glass and plate glass for the design of shielding windows. The cost of shielding windows is shown (Table 23-12a) when the most readily available thicknesses of lead glass, plate glass, and lead acrylic are utilized. The plate glass is constructed of more than one layer of 12 mm thick glass.
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The cost of primary barriers may show little price differences, however, use of lead acrylic or plate glass can result in lower cost. If only a 0.5 mm lead-equivalent is needed, the cost of the lead acrylic is only 52%, and plate glass is only 36%, of the cost of standard lead glass. For 0.25 mm lead-equivalent protection, the cost savings is even greater.
The lead acrylic has another advantage in that it is more suitable for mobile or custom shaped shields because the same lead-equivalent is lighter in weight and more easily machined.
It is recommended that lead glass or lead acrylic be used for 1.0-1.7 mm lead-equivalent and that lead acrylic be used for 0.5-1.0 mm lead-equivalent windows(Aldrich and Andrew, 1983).
A slightly different technique of operator protection can be considered for a stationary unit with the advent of lighter weight shielding materials. A recently described system utilized lead-impregnated, transparent, lead acrylic sheets(0.32 mm lead equivalent) suspended by light weight link chain from a continuous track mounted in the ceiling above the x-ray table. The suspended sheets measured 39 cm by 91 cm and weighed 3.6 kg each. The screens were easily moved to positions at the ends or on the side of the table (Fig. 23-9). Shielding factors ranged from 16 to greater than 150 (Van Hise and Schuchman, 1984). Use of such a system would markedly lower exposure levels to the head and neck and would permit operators to work without the inconvenience of wearing lead aprons.
Patient Examination Films retaken General comment
__________________________________________________________________
Breed Age Sex Condition View No.ofretaken Reason for retake
retaken
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Beam control is an easy way to decrease the size of the primary x-ray beam and thus lower the area exposed to the primary x-ray beam. The collimator should have an illuminated field so that the size of the area exposed can always be smaller than the cassette size.
Even at the high kilovoltage setting common in modern radiographic technique, the x-raybeam contains some x-rays of low penetrating power that have no value in most radiographic examinations. A large portion of these useless, soft, long wavelength x-rays can be filtered out of the beam if the tube aperture is covered with at least 2.5 mm of aluminum or equivalent material.
Proper film exposure is not always considered a radiation protection principle. You should select focal-film distance, beam filter, and film-screen systems with the thought of lowering the radiation level required to make the exposure as small as possible and still produce a diagnostic radiograph. High kVp technique permits you to lower mAs settings and therefore decrease radiation levels.
The last consideration in radiation protection is to plan the examination carefully. Position the animal correctly, select the correct machine settings, center the x-ray beam and choose the correct size of cassette.
Radiation level in primary beam of diagnostic x-ray unit at table top (mr/mAs measured at 80 kVp).
Radiation level in primary beam of Nuoroscopic x-ray unit at table top (measured at 1/2 mA tube current).
Radiation level of secondary radiation of fluoroscopic x-ray unit measured at 1/2 mA tube current at various locations using 0.5 mm Pb equivalent drapes.
Radiation level of secondary radiation of diagnostic x-ray unit (mr/exposure measured at various machine settings).
l) Side of table (25 cm distance)&emdash;eye level&emdash;10
2) Side of table (25 cm distance)&emdash;gonad level&emdash;10
3) Side of table (25 cm distance( e)&emdash;eye level&emdash;30
4) End of table (l m distance)&emdash;eye level&emdash;10
5) End of table (l m distance)&emdash;eye level&emdash;30
6) End of table (1 m distance)&emdash;gonad level&emdash;10
7) End of table (1 m distance)&emdash;gonad level&emdash;30
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is one of the biggest causes of excessive radiation exposure to personnel.There may be value in maintaining a repeat-film log to determine the cause for retakes. This log can become a method to decrease the number of retakes and serve as a technique for radiation protection (Table 23-13).
Proper film processing is another basic principle that influences radiation levels to your patients and assistants. The time and temperature of processing can be used to maximize reduction of silver halide crystals to silver particles. Unfortunately automatic processing tends to use high temperatures and fast developing times which decrease the efficiency of the chemical conversion in the film.
Maintenance Checklist for Radiation Safety Items. Radiation safety devices require some maintenance and care if they are to remain in good condition and provide the degree of protection that you assume they do. The only thing worse than not practicing radiation safety is to assume that you are doing everything possible to shield yourself and those working with you and doing so with faulty equipment. The checklist below will assist you in the care of and evaluation of the radiation safety items in your practice.
Hang aprons without creases
(Fig. 23-12)-----------------------------------------------After use
Store gloves so glove liners
(Fig 23-12) can dry------------------------------------After use
Change film badges---------------------------------Monthly (or as established by practice)
Check collimator light for
accuracy---------------------------------------------------Monthly
Process dental film mounted on
tube housing---------------------------------------------Monthly
Radiograph aprons and gloves
for defects (Fig. 23-13)---------------------------------Quarterly
Aprons can be evaluated for defects by placing the middle of the apron over a single 35 x 43 cm (14 x 17 inch) cassette. If the apron is long it may be necessary to use 2 cassettes and slightly overlap the fields of exposure. Exposures should be in the range of 5-10 mAs at 90 kVp at 100 cm FFD. Settings of this magnitude ensure that x-rays will penetrate any break in the lead and cause black exposed areas on the film.
Gloves require slightly greater exposures because they are double thickness as they lay on the cassette during the exposure. Usually 10-20 mAs at 90 kVp will be adequate exposure at 100 cm FFD to penetrate a break in either side of the glove and cause a black exposed shadow on the radiograph (Fig 23-13).
There are many handbooks, reports and regulations that contain information concerning radiation protection to be provided, exposure levels that may not be exceeded, and reporting and licensing procedures. Some of this information is in the form of suggestions while some is in the form of State code. Some of the information only makes recommendations while some is a requirement.
The National Council on Radiation Protection and Measurements has produced several reports relating to use of diagnostic x-ray. NCRP Report No. 33 discusses the use of x-ray up to 10 meV in energy in medical practice. NCRP Report No. 34 discusses protection devices. NCRP Report No. 35 discusses dental applications of diagnostic x-ray. NCRP Report No. 36 was issued in 1970 and is entitled " Radiation Protection in Veterinary Medicine."It is principally concerned with the protection of persons who may be exposed to radiation emitted by x-ray equipment and sealed radioactive sources used in the practice of veterinary medicine. The report presents recommendations pertaining to equipment design and use, structural shielding design and evaluation, operating procedures, radiation protection surveys, and working conditions. The report also includes comments that explain the rationale and intent of certain recommendations and performance standards. In the past, the general recommendations set out in the NCRP reports on medical radiation protection were intended to meet the needs of veterinary medicine. Now, however, the Council has decided that a more useful treatment results from the formulation of recommendations aimed directly at veterinary practice.
Fluoroscopy is the direct examination of the patient by utilizing the primary beam directed through the animal onto a fluoroscopic screen. the fluorescence can be observed directly or can be intensified and viewed by use of an image intensifier. Because of the high levels of radiation to both patient and technician, special safety rules are necessary. Never use fluoroscopy when radiography can be used because less detail is appreciated and fluoroscopy produces no permanent record. Insure use of lead protective apron, gloves and shields (Fig 23-10).
If you do an examination with a fluoroscopic screen:
If you do an examination with either fluoroscopic screen or image intensifier:
NCRP Report No.36 specifically outlines when radiation protection surveys should be made. Relative to equipment, the report makes recommendations concerning the tube housing aluminum filtration, types of collimation and centering devices. A special section discusses radiography with portable and mobile diagnostic equipment.
Copies of NCRP Report No. 36 pertaining to Radiation Protection in Veterinary Medicine should be ordered from NCRP Publications, P.O. Box 30175, Washington, D.C. 20014
Individual states publish radiation control regulations that are reprinted from the State code. These have the status such that a person who violates any provision of the regulation is guilty of a misdemeanor. This is in sharp contrast to the NCRP reports which are recommendations only. The State regulations include information concerning the licensing of x-ray machines, the license fee, and procedure to be followed in obtaining licensing. Occupational dose limits are usually based on the recommendations by the NCRP but may be different in a particular state. The State regulations specify the conditions under which personnel monitoring equipment must be used. Equipment requirements are spelled out in detail and refer to types of tube housing, collimation, switches and shielding. the types of records to be maintained by the veterinarian are specifically outlined as well as the instruction and demonstrations to be given workers pertaining to the fundamentals of radiation safety, radiation instrumentation and radiographic equipment.
Responsibility for radiation emitting medical devices is now assigned to the Bureau of Radiological Health of the Food and Drug Administration. The Bureau of Radiological Health is the lead bureau in control of most medical devices subject to the Radiation Control Act, such as x-ray machines, medical lasers and microwave and acoustic devices. In addition the Bureau of Radiological Health is the lead agency for cobalt teletherapy units and brachytherapy units as well as nuclear medicine scanners, image receptors and film processors . The Bureau of Radiological Health has responsibility for assuring safety and effectiveness of these devices as well as having to present radiation safety programs. Safety programs must also address potential electrical, thermal and mechanical hazards.
One Federal regulation deals only with the manufacture of x-ray equipment but does not apply to equipment to be used in veterinary medicine (21 CFR; 1020.30 Diagnostic Equipment, 1020.31 Radiography, 1020.32 Fluoroscopy) . These regulations went into effect August 1, 1974 and state that all components of the x-ray equipment built after that date must be certified according to the regulation. The new regulations deal with tube housing, collimators, controls that provide reproducibility of exposures, controls that provide linearity of exposures so that equal but different combinations of mAs will produce the same film density, table top filtration, and a PBL (positive beam limitation) device that connects the bucky tray and collimator so that the field of exposure is automatically limited to the size of the cassette.
Every veterinary practice using diagnostic radiology should have copies of NVRP Report No. 36, "Radiation Protection in Veterinary Medicine," and the appropriate regulation from the State code. Often the Department of Public Health administers the regulations for the State. Learn the location of this office and the people responsible for radiation control. In most instances, they are only interested in helping you to achieve a level of exposure for the personnel working in diagnostic radiology that is within the regulations. They can be called on for advice and assistance to help solve your particular problems.
Radiation Safety Log&emdash;A book or log can be maintained that contains information on the dates, results and circumstances relative to all tests made on x-ray generating equipment and accessory safety equipment. All monitoring or dosimetry results can be recorded. This log might include:
The occupational Safety and Health Administration (OSHA) has published
rules requiring employers to allow an employee to review and copy medical and exposure records relevant to the employee. The employees have access to records of "exposure to toxic substances or harmful physical agents." Veterinary facilities are affected if they monitor employees' x-radiation exposure. The final regulations were published in the Federal Register on May 23, 1980; they were effective on Aug. 21, 1980.
These rules do not require an employer to establish a system of monitoring or record keeping. They also do not require any particular format or frequency of gathering such information. The rules do require that an employer allow any employee (or formal employee) or his representative to review and copy most medical records or exposure records.
The regulation directs the employer to make any such record available to the employee, even if the record was compiled years before this regulation was promulgated. Any records of exposure monitoring must be maintained and made available, generally, for a period of 30 years . An employee 's medical records must be kept for the period of employment plus 30 years. The regulations also state explicitly that an OSHA inspector has the right of access to these records.
Governmental institutions may further complicate the problem of radiation safety since it is within the power of the Food and Drug Administration through the Bureau of Radiological Health to promulgate guidelines concerning techniques used in radiography. These may not always consider the effect on the amount of diagnostic information that can be gained from the examination. For example, consider a possible ruling that would recommend the use of non-calcium tungstate intensifying screens without considering the effect on the resulting radiograph. I do not believe that it helps to hide behind the fact that these rulings may be only for human radiology in the beginning. Later they will come to affect those of us in veterinary radiography as well. The ruling that no person may be within the room during a radiographic study has already caused considerable problems in certain states.
A general concern about the potential problems of radiation exposure is healthy and will ultimately be helpful. The availability of only one side of the story whether because of the news media or because of our inability to offer the other side instills fear into the minds of the public instead of offering them complete factual data. It is our obligation to be able to correctly offer all sides of the story and to accurately measure the advantages of lowering exposure to the patient and those assisting with the radiographic examination against the possibility of loss in the diagnostic quality of the examination.
Checklist for Unusual Radiation Exposure. If anyone in the radiology service receives an unusual radiation exposure measurement on a film badge or pocket decimeter the following checklist may be of value in attempting to trace the cause of the exposure.
One badge or decimeter has an exposure of more than 10 times usual level. This may often be a false reading and the following should be explored:
One or more badges or dosimeters have an exposure of more than 2 times but less than 10 times the usual level. This suggests that the magnitude of change is more believable and the reading is probably accurate and a change in working habits should be considered before looking for reasons for a false reading. Consider the following:
Future Possibilities of Exposure Reduction. Even if we follow all of the safety rules we will still have personnel who are receiving a measurable amount of radiation exposure. It is therefore necessary to continue to examine techniques and new equipment that will enable us to further decrease these exposure levels. Possible new techniques being explored include:
The advantages of the use of carbon filter reinforced plastic in the cassette front and table top that will decrease x-ray attenuation are now available. The use of a carbon conjugate takes advantage of the transmissivity of carbon that has not been possible before development of a form that would support body weight. Radiation exposure was reduced in a trial by 29.82% with no loss in diagnostic quality in the radiography. KVp changes are more sensitive with the use of carbon. In addition, it is expected that there will be additional savings in terms of x-ray tube replacement due to the 30-50% reduction in mA settings which are required (Phillips et al., 1979). Another study showed reduction in patient exposure ranging from 28% at 60 kVp to 17% at 120 kVp using carbon-fiber cassette fronts (Schmidt et al, 1983).
