Reducing exposure from 57Co sources during breast lymphoscintigraphy by optimizing energy windows and other suggested enhancements of acquisition and the display of images.
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 microSv for the stronger 57Co flood source and 0.46 microSv for the weaker 57Co flood source, a 5-fold reduction in equivalent dose. Technologists received less than half of the above doses.


CONCLUSIONS
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.
Krynyckyi
Borys R
BR
Division of Nuclear Medicine, Department of Radiology, The Mount Sinai School of Medicine, New York, New York 10029-6574, USA. syrob@msn.com
Sata
Suzana
S
Zolty
Ian
I
Kim
Chun K
CK
Knesaurek
Karin
K
D001940:Breast / Q000000981:diagnostic imaging*; D001943:Breast Neoplasms / Q000175:diagnosis*; D003037:Cobalt Radioisotopes; D005260:Female; D015902:Gamma Cameras; D005720:Gamma Rays; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D008198:Lymph Nodes / Q000000981:diagnostic imaging*; D008327:Mammography / Q000379:methods*; D011829:Radiation Dosage; D012815:Signal Processing, Computer-Assisted; D013671:Technetium Tc 99m Sulfur Colloid; D015899:Tomography, Emission-Computed, Single-Photon
D003037:Cobalt Radioisotopes; D013671:Technetium Tc 99m Sulfur Colloid