Answer to Question #8034 Submitted to "Ask the Experts"

If we inject a patient with 3 mCi of ^99mTc DTPA (diethylene triamine pentaacetic acid) and put him under a gamma camera and take counts over a kidney region at different times, how can we convert this count to activity (e.g., Ci or Bq)? After we convert, we can then draw functions of activity vs. time—how can we integrate this curve to calculate residence time to calculate internal dose? If we want to instead calculate internal dose to the liver, are there any differences in method?

Quantification of nuclear medicine images is fairly complex, and there are a number of possible methods for doing this. If you are taking planar images, which I think you are, there are a number of software programs (e.g., the freely available ImageJ from the  National Institutes of Health, or other commercial packages) that allow you to draw regions of interest (ROIs) over anatomical regions and extract the counts. The counts can then be converted to absolute values of activity or percent of administered activity.

The most popular method for this is the Geometric Mean method, which is well described in the journal article "Techniques for Quantitative Radiopharmaceutical Biodistribution Data Acquisition and Analysis for Use in Human Radiation Dose Estimates" (Siegel J, Thomas S, Stubbs J, Stabin M, Hays M, Koral K, Robertson J, Howell R, Wessels B, Fisher D, Weber D, Brill A. MIRD Pamphlet No 16- J Nucl Med 40:37S-61S, 1999) as well as in my textbook (Stabin M., Fundamentals of Nuclear Medicine Dosimetry, Springer 2008).

The book also describes in detail how to then fit the data to functions and integrate them by various methods to obtain the area under the curve (one expression of this is "residence time") and calculate absorbed dose to any organ of the body. I promise you that I am not just promoting this book because it is mine—I know of no other resource presently that so directly addresses the questions you are asking. You can see a table of contents at http://www.doseinfo-radar.com/TOC2.doc.

Michael G. Stabin, PhD, CHP