A review of common drawbacks inherent to Co-57 sheet sources

Objectives Gamma cameras must be tested for uniformity on a daily basis prior to performing clinical exams. In most nuclear medicine practices, Co-57 sheet sources are typically used to acquire extrinsic uniformity floods so that the integrity of the collimator can be assessed along with the gamma camera performance. These Co-57 sources are convenient and provide a consistent, uniform “flood” of radioactivity over the useful field of view (UFOV) of the camera. However, technologists should be aware of potential drawbacks associated with Co-57 sheet sources.

Methods New Co-57 sheet sources usually have small levels of high energy contaminants present along with the Co-57 isotope. These high energy contaminants (Co-56 and Co-58) will cause artifacts and non-uniform quality control (QC) images if the sheet source is placed directly on the face of a low energy collimator. The artifacts from these contaminants can be minimized by increasing the distance between the sheet source and the collimator. Due to the fact that Co-56 (T ½ = 77 days) and Co-58 (T ½ = 71 days) have shorter half-lives than Co-57 (T ½ = 272 days), the effects from these high energy contaminants are gradually reduced as the sheet source ages. Another issue that technologists should be aware of, although relatively uncommon, is the possibility of receiving a sheet source with a non-uniform distribution of Co-57 throughout the source. Therefore, the uniformity of a sheet source should be verified by acquiring 2.5 million count images with the source positioned in four different orientations. Those four images are then summed together and the resulting image should be compared to an image acquired with the source positioned in a single orientation for 10 million counts. If discrepancies in the integral uniformity measurements are noted between the summed image and the 10 million count image, it is most likely due to a non-uniform sheet source.

Results The high energy contaminants found in new Co-57 sheet sources will cause artifacts on QC images if the source is placed directly on top of a low energy collimator. These artifacts can be minimized by positioning the source a few inches away from the collimator to improve image uniformity (Figure 1). In our nuclear medicine practice, we have discovered that some sheet sources have a non-uniform distribution of the Co-57 isotope throughout the source. We have also found that there can be considerable variation in sheet source uniformity between manufacturers. One sheet source that we received showed a significant, non-uniform distribution of Co-57, in a diagonal pattern, throughout the source (Figure 2), while a second Co-57 sheet source had a focal area of decreased activity at the edge of the source (Figure 3).

Conclusions Even though Co-57 sheet sources are convenient to use for daily QC testing of nuclear medicine gamma cameras, there are some inherent drawbacks to these sources that a technologist should be cognizant of. New sheet sources typically contain high energy contaminants and they can occasionally have a non-uniform distribution of the Co-57. Either of these occurrences can falsely elevate the integral uniformity of QC images and possibly result in failed QC tests. These failures could, in turn, result in delays to the clinical workload if additional time is spent repeating the QC tests or submitting unnecessary service calls. $$graphic_9631D652-1038-4640-B20E-D4E8E616F57E$$