Evaluation of a ¹⁵³Gd flood source for anatomic shadowing in scintigraphy scans

Objectives In certain scintigraphy studies where anatomic reference is desired, a ⁵⁷Co flood source often is introduced to acquire an anatomic shadow along with the emission image. The 136 keV photons from the ⁵⁷Co source produce the shadow in the 140 keV ^99mTc window. However this method is not ideal since crosstalk degrades lesion detection. A ¹⁵³Gd flood source was investigated as an alternative, with the goal of improving lesion detection.

Methods The ¹⁵³Gd source emits 100 keV photons which do not appear in the 140 keV window; thus emission data and transmission data are acquired in separate windows. Phantoms with ^99mTc point sources of various activities were imaged to quantify signal versus background counts using different flood sources (⁵⁷Co, ¹⁵³Gd, or none). Some crosstalk was observed with the ¹⁵³Gd source and was attributed to simultaneous detection of 100 keV photons and x-ray photons (~40 keV) from ¹⁵³Gd decay. Use of brass filters to suppress x-ray detection also was studied.

Results Lesion signal-to-noise ratio (SNR) was significantly higher with the ¹⁵³Gd flood source than with the ⁵⁷Co flood source in all phantom configurations studied. Lesion SNR with the ¹⁵³Gd flood source was improved slightly further when introducing filters to attenuate x-rays from ¹⁵³Gd decay. Compared to the case of no flood source, lesion SNR with the ¹⁵³Gd flood source was similar or slightly lower. Clinical images (breast lymphoscintigraphy and wholebody lymphedema scans) using the ¹⁵³Gd source demonstrated the ability to visualize low-intensity lesions without significant background.

Conclusions Using the ¹⁵³Gd flood source, co-registered functional and anatomic images are produced for both anterior and posterior views. Lesion detection is improved compared to using a ⁵⁷Co flood source for anatomic shadowing.