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Reducing Exposure from ⁵⁷Co Sources During Breast Lymphoscintigraphy by Optimizing Energy Windows and Other Suggested Enhancements of Acquisition and the Display of Images

Borys R. Krynyckyi, MD, Suzana Sata, CNMT, Ian Zolty, CNMT, Chun K. Kim, MD and Karin Kneaurek, PhD

Division of Nuclear Medicine, Department of Radiology, The Mount Sinai School of Medicine, New York, New York

We set out to measure the reduction in exposure attained by using a weak ⁵⁷Co sheet source with optimal energy windows. Methods: Two groups of 10 lymphoscintigraphy studies were analyzed. Group 1 consisted of 10 studies obtained with a stronger source of ⁵⁷Co, 59 MBq (1.6 mCi) at the time of data acquisition, with transmission images acquired at 3 energy windows of 115–129, 130–134, and 135–150 keV. Group 2 consisted of 10 studies with a weaker sheet source of ⁵⁷Co, 11 MBq (0.3 mCi). Transmission images were acquired at 3 energy windows of 112–132, 130–134, and 135–150 keV. Same-sized regions of interest (ROIs) were drawn on the patient’s torso (PT) and on the nonattenuated image of the transmission source itself (TS), all 1-min images. The counts in each ROI obtained over 1 min and the ratios between the TS ROI and the PT ROI were calculated for all of the energies. Dosimetry calculations based on measured exposure rates and the activity of the sheet sources were used to calculate the patient equivalent dose at 30 cm. Results: For the ⁵⁷Co energy window, group 1 had an average ROI count of 1,955 in the TS region and 135 counts in the PT region. The average ratio of TS/PT was 15.4. Similarly, group 2 had an average ROI count of 646.4 in the TS region and 91.2 counts in the PT region. The average ratio of TS/PT was 8.6. The relative "outlining performance," when comparing the ⁵⁷Co and ^99mTc windows, showed an average improvement when using the ⁵⁷Co window of 4.4 and 5.8 times for group 1 and group 2, respectively (TS/PT at ⁵⁷Co window)/(TS/PT at ^99mTc window). Estimates of the patient equivalent dose per study were 2.30 µSv for the stronger ⁵⁷Co flood source and 0.46 µSv for the weaker ⁵⁷Co flood source, a 5-fold reduction in equivalent dose. Technologists received less than half of the above doses. Conclusion: Use of expanded, separate energy windows optimized for the primary 122-keV photon of ⁵⁷Co greatly improves transmission scan image quality compared with the standard 140-keV ^99mTc windows used for the delineation of the sentinel node. This markedly reduces exposure for all, by allowing the use of a weaker source, and can save time.

Key Words: breast; lymphoscintigraphy; ⁵⁷Co; exposure; energy windows; display; shield; arm position

This article has been cited by other articles:

				S H Kim, J Shim, C K Kim, J Machac, and B R Krynyckyi Reverse echelon node and a lymphatic ectasia in the same patient during breast lymphoscintigraphy: the importance of injection and imaging technique Br. J. Radiol., December 1, 2004; 77(924): 1053 - 1056. [Abstract] [Full Text] [PDF]