Transmission Imaging in Lymphoscintigraphy with a 153Gd Flood Source

Lymphoscintigraphy uses intradermal or interstitial injections of 99mTc-labeled tracers to produce images of focal lymph nodes. Because there is little or no anatomic information in the 99mTc images, a 57Co flood source is sometimes used to provide transmission data along with the emission data. The anatomic shadow from the transmission scan generally improves interpretation and surgical planning. However, the 57Co transmission photons contribute to background on the 99mTc images, reducing contrast and signal-to-noise ratio (SNR). SNR is related to lesion detection, and some lymph nodes that would be detected in an emission-only scan might not be detected if acquired with a 57Co flood source. An alternative to a 57Co flood source is a 153Gd flood source, which has primary photon emissions well below the 99mTc emission window, allowing the shadow to be acquired in a separate transmission window. Significantly smaller crosstalk from 153Gd should improve SNR and therefore would be expected to improve lymph node detection. We hypothesized that the use of a 153Gd flood source would reduce background and improve SNR for these studies. Methods: Phantom studies simulating lymphoscintigraphy were performed to compare performance with a 153Gd flood source, a 57Co flood source, and no flood source. SNR in the 99mTc emission images was measured using a water phantom to simulate patient body and point sources of various activities to simulate nodes and injection site. The encouraging phantom studies prompted use of the 153Gd flood source in routine clinical breast lymphoscintigraphy, melanoma lymphoscintigraphy, and lymphedema studies. Because emission and transmission data were acquired in separate energy windows, fused planar images of emission and transmission data were available to the physician. Results: SNR was highest with no flood source and was lowest with the 57Co flood source by a significant margin. SNR with the 153Gd flood source was similar to that with no flood source on the anterior (transmission) view. SNR was reduced somewhat in the posterior (nontransmission) view because of attenuation of signal by the flood source itself. Minor crosstalk in the 99mTc window was observed with the 153Gd flood source, attributed to simultaneous detection of x-ray photons and gamma-photons. This crosstalk was reduced by introducing thin metal filters to absorb most x-ray photons, at the expense of more attenuation in the posterior view. Unlike with the 57Co flood source, a usable posterior view (with anatomic shadow derived from the anterior view) was generated with the 153Gd flood source. Clinical lymphoscintigraphy images with the 153Gd flood source were of high quality. Interpretation was aided by the ability to control image mixing and brightness and contrast of separate color scales. Conclusion: By producing fused images with reduced crosstalk and improved image quality, a 153Gd flood source offers advantages over a conventional 57Co flood source for anatomic shadowing in lymphoscintigraphy. Lymph nodes in emission images have higher SNR, indicating a likely improvement in clinical lesion detection. Separate emission and transmission images provide additional flexibility in image display during interpretation.