Shooting the "STAR"

Objectives: Utility of medium-energy collimator in head & neck Tc-99m labeled lymphoscinitigraphy

Introduction: Head & neck lymphoscintigraphy is commonly performed with Tc-99m labeled compounds (filtered sulfur colloid or lymphoseek). Due to the low energy of Technetium (140 Kev), imaging is usually performed with low energy collimators as endorsed in the practice guidelines for lymphoscintigraphy and sentinel lymph node biopsy in melanoma by EANM and SNMMI.¹ However, this often results in a star shaped artifact due to septal penetration, which can mask nearby sentinel nodes. This can be dealt with by shielding with lead on planar images, but such shielding is not feasible with SPECT imaging. The star artifact turns into a cone shaped artifact in SPECT images and can result in non-visualization of nodes near the injection site. The solution is to use a medium energy collimator for the SPECT acquisition. Although the spatial resolution and count sensitivity are slightly worse, the septal penetration is completely eliminated.

Results: In this exhibit, we describe a case where a low energy collimator was inadvertently used, followed by repeat imaging with a medium energy collimator. The sentinel lymph node was completely hidden when using the low-energy collimator, but was clearly seen with the medium energy collimator SPECT images.

Conclusions: Medium-energy collimator imaging is essential for SPECT imaging in sentinel node lymphoscintigraphy. The septal penetration artifacts seen with low energy collimation can result in non-visualization of the sentinel node and a high rate of false negative studies. Reference: 1. Bluemel C, Herrmann K, Giammarile F et al. EANM practice guidelines for lymphoscintigraphy and sentinel lymph node biopsy in melanoma. Eur J Nucl Med Mol Imaging. 2015 Oct;42(11):1750-1766.

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