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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.

Source Estimated effective dose (mSv) Comparable to natural background radiation
Natural background radiation
3 mSv/yr
 

Airline passenger (cross country)

0.04 mSv

~ 4 days

Chest or Abdominal X-ray (single view)

up to 0.01 mSv

~ 1 day

Head CT

up to 2 mSv

Up to 8 months

Abdominal CT

up to 5 mSv

Up to 20 months

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.

Helpful web sites

References

  • American Academy for Pediatrics, What Pediatricians Should Know about Medical Radiation Safety. http://www2.aap.org/sections/radiology/RadiologyPediatricianPage.pdf
  • Amis ES, Jr., Butler PF, Applegate KE, et al. American College of Radiology white paper on radiation dose in medicine. Journal of the American College of Radiology 2007; 4:272-284.
  • Cardis E, Vrijheid M, Blettner M, et al. The 15-country collaborative study of cancer risk among radiation workers in the nuclear industry: Estimates of radiation-related cancer risks. Radiation Research 2007; 167:396-416.
  • Frush DP, Donnelly LF, Rosen NS. Computed tomography and radiation risks: What pediatric health care providers should know. Pediatrics 2003; 112:951-957.
  • Goske MJ, Applegate KE, Boylan J, et al. The 'Image Gently' Campaign: increasing CT radiation dose awareness through a national education and awareness program. Pediatric Radiology 2008; 38:265-269. www.imagegently.org
  • NAS. Health risks from exposure to low levels of ionizing radiation: BEIR VII phase 2. Washington DC: NationalAcademy of Sciences, 2005.
  • Miles K., Imaging pregnant dogs and cats. Comp Cont Ed 1995;17:1217–26.
  • Root-Kustritz, M, Pregnancy diagnosis and abnormalities of pregnancy in the dog, Theriogenology 64 (2005) 755–765.
 

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