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Optimal ⁵⁷Co Flood Source Activity and Acquisition Time for Lymphoscintigraphy Localization Images

Division of Diagnostic Imaging, Department of Imaging Physics, University of Texas, M.D. Anderson Cancer Center, Houston, Texas

Correspondence: For correspondence or reprints contact: Martha V. Mar, University of Texas, M.D. Anderson Cancer Center, 1515 Holcombe Blvd., Unit 1352, P.O. Box 83, Houston, TX 77030-1439. E-mail: mmar{at}di.mdacc.tmc.edu

We evaluated different ⁵⁷Co flood source activities and acquisition times to obtain an optimal localization image for breast lymphoscintigraphy that would adequately outline the body and allow detection of nodes seen on the emission scan while minimizing unnecessary radiation exposure to the patient. Methods: An anthropomorphic thorax breast phantom representing an average-size patient was used to simulate nodes on a breast lymphoscintigraphy scan. The activities in the nodes at the time of acquisition ranged from 37 to 185 kBq (1–5 µCi). Four experiments were performed, consisting of 10-min emission and 3-min localization images. Anterior, posterior, and right and left lateral views of the thorax phantom were acquired, using each of 5 different ⁵⁷Co flood sources with activities ranging from 37 to 269 MBq (1.0–7.26 mCi). Ten 1-min localization images for each source were acquired and compared for quality. Three-minute localization images for 2 phantom thicknesses of 10 and 20 cm were acquired to determine the contrast-to-noise ratio for each ⁵⁷Co source. The total exposure was measured using an ion chamber survey meter. Results: All sources allowed visualization of the lymphatic nodes in acquisitions as short as 3 min. Images using the 126-MBq (3.41-mCi) source demonstrated an adequate body outline along with visualization of all nodes seen on the emission image. The 37-MBq (1.0-mCi) source did not provide sufficient definition of the body outline, whereas the hotter sources decreased node visualization by increasing the background around the nodes at the same time that they increased the patient exposure. Node activity of 37 kBq (1 µCi) became undetectable on the anterior localization images yet was still visible on the lateral image because of greater attenuation of ⁵⁷Co photons. The estimated dose rate from the ⁵⁷Co sheet sources was 0.641 µSv/MBq/h. Conclusion: Acquiring a 3-min localization scan using a 126-MBq (3.41-mCi) source provided the best combination of clear-body outline and visualization of all nodes seen on the emission image. The estimated dose to the patient from the 126-MBq (3.41-mCi) sheet source was very low (8.7 µSv for unilateral and 13.1 µSv for bilateral). Node detectability decreased in localization images acquired using ⁵⁷Co sources of higher activity. This effect would be more pronounced in lymphoscintigrams of thin patients compared with those of patients of average thickness.

Key Words: breast; localization; lymphoscintigraphy; ⁵⁷Co flood source