Answer to Question #8079 Submitted to "Ask the Experts"

According to NUREG-1507, it is possible to relate exposure rate and count rate analytically. What does this mean? Is there any calculation or measurement to relate the exposure rate (Geiger-Mueller counter) and count rate (gamma camera) of a radionuclide, for example ^99mTc? Does activity from ^99mTc also influence the relationship between those two?

Depending on the source and detector characteristics, it is possible to correlate exposure rate with source activity and with detector count rate. Perhaps the simplest case is when a small dimension source of a particular radionuclide of known activity emits isotropic gamma radiation, and the dose point of interest is at a reasonable distance from the source (distance large compared to source and detector dimensions, but small enough so that appreciable photon attenuation does not occur). It is then possible to calculate the expected exposure rate at the point. You can find this described in a document on the Health Physics Society (HPS) website. For more complicated geometries of source and/or detector and for cases where photon attenuation cannot be ignored, calculations may be possible but may become rather difficult.

The correlation between exposure rate and detector count rate may, in some cases, be made through analytical calculation, but is usually more easily accomplished through direct measurement in which the detector is exposed in the gamma radiation field of known exposure rate and the count rate determined. For many commonly used GM (Geiger-Mueller) detectors, the count rate per unit exposure rate ranges from about 15 to 70 cps (nC s^-1)^-1. Naturally, this response factor will vary with specific detector characteristics. Additionally, such a response factor may not be energy independent, especially if the GM detector is not an energy-compensated type. You can find some related information among the answers to questions that have been asked on the HPS Ask the Experts website—e.g., see Question 4148.

The response of a gamma camera to radiation from a radionuclide such as ^99mTc is more complex. The detector is a high-efficiency solid detector, such as NaI(Tl), and the radiation it views is restricted by the collimation devices used on the camera. Additionally, the radiation is emanating from various depths within the body, depending on tissue(s) being imaged, and attenuation within the body becomes significant.

The count rate that might apply to the gamma camera may not easily be correlated with exposure rate that might be estimated using a GM detector. One could remove the gamma camera and place the GM detector at about the same location that the face of the active volume of the gamma camera had been and measure exposure rate. This value would be affected by photons reaching the detector from various possible directions, depending on the distribution of activity within the patient. This would allow a correlation between approximate exposure rate and gamma camera count rate for that situation.

If a different diagnostic test was performed in which the activity distribution changed, the same correlation would not necessarily hold true. Any correlation with activity in the body or tissue would be even more difficult considering the attenuation effects that would prevail. For a small source outside the body, one could evaluate the count rate on the gamma camera and the GM exposure rate at the same distance and make correlations of count rate and activity with exposure rate, but this would not be particularly useful when the ^99mTc is distributed within a patient.

Regarding your last question, it is true that, for a given patient undergoing a specific diagnostic test, the amount of ^99mTc activity affects both the count rate (on GM and a gamma camera) and the GM exposure rate in a similar fashion. For example, if the activity increased by a factor of two, the observed count rates and exposure rate would also increase by a factor of two.

Hope this is helpful.

George Chabot, PhD, CHP