The strength of each of the 2 sources in this example = 20 mg Radium equivalent of cesium = 50.6 mCi =1.87x109Bq.
Using physical information about each source and anatomic information from the CT scan, a computer program calculated and displayed the seven colored "isodose lines."
Isodose lines connect points receiving an equal radiation dose or equal dose rates. They are similar to isobars and isotherms on weather maps.
The seven "isodose lines" on this CT slice connect points with equal dose rates in units of cGy/hour.
Not shown in this graphic are the estimated dose rates on the skin and a few millimeters below the skin surface, where the dose rate could be as high as 1200 cGy/min.
Dose rate (cGy/hr)
These Cs-137 sources emit radiation in three dimensions, not just in 2 dimensions as shown on a single CT slice. Note that radiation is also emitted into air (not shown).
Connecting all points of equal dose or dose/rate in the 3-dimensional space surrounding a source actually produces an isodose volume or isodose cloud (not shown). The shape of an isodose cloud depends on factors listed below; it may not necessarily be uniform or cylindrical in shape.
Other key concepts
Radiation dose decreases as the distance from the source increases (inverse square law).
Radiation dose also decreases progressively due to attenuation in tissue.
Since dose is not uniform throughout the irradiated tissue, it is imprecise to describe the radiation dose absorbed as a single number.
Doses can be expressed in many ways, including the following
An absorbed dose at a specific point in air, in tissue, on an isodose line, or on the surface or interior of an isodose volume
A dose rate, i.e., dose/unit time (e.g. 400 cGy/hour as shown by the white line in the diagram above).