Answer to Question #9782 Submitted to "Ask the Experts"

Wikipedia says the unit Gy is J kg^-1.Does this mean a mouse exposed to 1 Gy receives the same amount of radiation per mass as an adult human who weighs 200 pounds exposed to the 1 Gy? He might be exposed to greater joules, but his larger mass counteracts this, giving him the same absorbed dose as a mouse?

I noticed that some new CT (computerized tomography) scanners have a low-dose pediatric setting that has a CTDI_vol ~ 4mGy for an abdominal/pelvic scan.

The dose from an adult scan of the same region is ~ 25mGy.

I'm using CTDI_vol because in all the reports by the International Commission on Radiological Protection it is stated that it is a close approximation of "absorbed dose."

Understandably, a larger patient would require more joules of energy to visualize his greater mass on a CT. But his larger mass in kg should balance out so that he could still receive a safer "pediatric" dose and still produce good images.

My question is, if they can produce good images in children with only ~4mGy why can't they do the same for adults and reduce their radiation exposure?

I know you physicists like math, so to I can put the same question in a mathematical perspective:

A child who weighs 35 kg, exposed to 4mGy CT receives 140 joules to her body.

An adult who weighs 77 kg, exposed to a CT with CTDI_vol of 25 mGy, would receive 1,925 joules.

That is 13.75 times as much energy deposited in his body, even when we account for his much larger size.

So why do they expose adults to so much more radiation when it is possible to use less and still obtain quality images, as they do with a "pediatric" setting?"

Many of your inferences about the definition of absorbed dose, often expressed in units of Gy or mGy are correct. Two tissues or bodies, each receiving an absorbed dose, averaged throughout the tissues or bodies, of 1 Gy, do receive an average of 1 J kg^-1 of tissue mass or body mass. If the body that received such a dose is that of a 35 g (0.035 kg) mouse, the total energy deposited throughout the mouse’s body mass would have been 0.035 J, and if the body had been that of an 91 kg (200 pound) man, the total deposited energy would have been 91 J. Dose is an intensive quantity, and it is the magnitude of energy deposited per unit mass that is important in determining effects. This idea of intensity as the effects-governing factor is pervasive with respect to many other quantities as well. For example if we quickly supply 1,000 calories of energy to one liter of water we would expect the temperature of the water to increase by 1 degree centigrade, but if we quickly supplied the same 1,000 calories of heat to 1 cc of water we would expect the water to vaporize very quickly.

An important consideration in performing medically useful CT scans is that the x rays used in the scan are attenuated, more or less exponentially as they traverse the thickness of tissues in their path. If the volume of the target object is large, the projected x-ray pathlengths through the body are commensurately large and, in order to obtain statistically reliable data, the integrated intensity may have to be greater than might be the case when the target object is smaller. Thus, it is appropriate to reduce pediatric doses by using techniques that favor lower values of CTDI_vol compared to what would be used with larger adult patients. We should keep in mind, however, that the CTDI_vol values are not specific to a particular patient. They are based on the assumed use of phantoms of specific dimensions for specific portions of patients (e.g., head or body) and for patients of different sizes (e.g., child vs. adult). The individual patient, whether child or adult, may have dimensions that vary appreciably from that assumed for the specified CTDT_vol values, and corrections to the CTDI_vol number may be necessary to get the best estimate of absorbed dose to the patient.

As you are likely aware, there have been numerous instances in which the CTDI_vol values for an adult have been used in estimating doses to children, and such estimations may underestimate doses by more than a factor of two because of the lower attenuation of radiation in the child, thus leading to higher doses throughout the irradiated tissues. As you noted, the pediatric setting for a CT abdominal scan may indicate a CTDI_vol value of 4 mGy. If the adult settings had been used for the machine, the value may have been about three times as great. Because of the smaller pathlengths traversed by the x rays in the child, the intensity of x rays may be reduced considerably compared to an adult, and suitable images may still be obtained.

Regarding your last example of a 35 kg child and a 77 kg adult, your general conclusions are correct, although the absolute values of the energies in joules that you calculated should be reduced by a factor of 1,000 (you were using mGy not Gy, and 1 mGy is only 0.001 J kg^-1). Your factor of 13.75 for estimated energy deposited is still correct, but keep in mind that the estimated adult-to-child dose ratio is 25 mGy/4mGy = 6.25. This value is still significant but more than a factor of two less than the factor for total energy deposited, and it is the dose value that is an indicator of potential harm. Also, the value of 25 mGy for a 77 kg adult seems rather on the high side for an abdominal CT scan.

I guess the bottom line is that you can’t get something for nothing, and with the smaller child we can, and should, use reduced x-ray intensities compared to what are necessary for a larger adult in order to obtain acceptable images.

We should all be concerned with minimizing unnecessary doses to patients and, to the extent practicable, we should be implementing procedures that are consistent with that goal. Using appropriate pediatric settings for CT scans is an important step in that direction. Thanks for your question.

George Chabot, PhD, CHP

Editor's Note: The Health Physics Society is a member of the Image Gently Alliance. See http://hps.org/media/documents/image-gently-press-release.pdf. This campaign emphasizes CT dose reduction in children.