The Truth About Diagnostic X-Ray Safety
We are all exposed to small amounts of radiation daily from soil, rocks, building materials, air, water, and cosmic radiation. These types of radiation are called naturally occurring background radiation. Radiation can be measured in many ways. Measurements can be used to estimate the radiation dose deposited in the whole body or to an individual organ. Because every animal patient is different in size and shape, different X-ray settings must be used to accommodate these differences. One way of looking at doses from X-ray examinations is to compare effective radiation dose estimates from different sources using millisievert units (mSv) (from the American College of Radiologists and the Radiological Society of North America). In addition, the radiation used in diagnostic X-rays and CT scans has been compared to our daily exposure of background radiation. This approach often over estimates exposure risk as this refers to whole body dose which is not truly comparable to studies which image only a portion of the body. However, the following comparison may be helpful in understanding relative radiation doses to an animal during a diagnostic radiograph (x-ray) or CT scan.
What are the risks from medical radiation?
There is no conclusive evidence that directly links radiation from diagnostic X-rays to the development of cancer later in life. However, some studies of large populations exposed to radiation have demonstrated slight increases in cancer risk even at low levels of radiation exposure. Veterinarians work to minimize the risks by utilizing x-rays when there is a clear medical benefit, using the lowest amount of radiation for adequate imaging based on size of the animal, imaging only the indicated area, avoiding multiple scans, and utilizing alternative diagnostic studies (such as ultrasound or MRI) when possible.
What are the uses of x-ray in pregnant dogs?
A single abdominal x-ray is an important tool to diagnose pregnancy in dogs. To achieve the diagnosis of pregnancy by x-ray, skeletal mineralization of the puppies must have occurred. Prior to mineralization, the enlarged uterus of pregnancy is indistinguishable from that seen with uterine disease, such as pyometra. The danger of ionizing radiation is extremely low with this one time diagnostic x-ray late in gestation as mentioned above. The exposure risk to the developing puppies is also proportional to the dose received and their gestational age with developing puppies being most sensitive to the effects of ionizing radiation during organogenesis, which occurs during the first half of pregnancy (prior to 35 days of gestation). Late in gestation, risk of radiation damage to the developing puppies is no greater than for the mother (Dam). Fetal skeletons are visible from 45 days of gestation and beyond. Because the normal bitch may stand for breeding for a wide window of time around ovulation, if ovulation timing was not performed, this window may range from 43 to 54 days post-breeding. The closer to the whelping date, the more easily identifiable the puppies are, making accuracy higher and risks lower with this diagnostic tool. In the last trimester of pregnancy, x-ray has been reported to be 100% accurate for pregnancy diagnosis. X-ray is not as accurate an indicator of fetal viability as is ultrasonography, and so these two diagnostic tests may be combined based on the recommendation of your reproductive veterinarian. Late gestation x-ray is the most accurate modality for assessment of puppy number or expected litter size, with a reported accuracy of 93%. Degree of mineralization of the puppies can be also used to assess gestational age and roughly predict whelping date, although gestational aging is more accurate with ultrasound. In addition, x-ray can assist with determination of obvious fetal-maternal disproportion which may increase the risk for difficult birth (dystocia) and assist with the decision to pursue and elective Caesarian section.
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