Characterizing gamma camera collimators for sentinel node detection: the effects of septal penetration

Abstract

Our aim was to evaluate the effects of septal penetration on the identification of sentinel lymph nodes and the visualization of lymphatic tracts. These nodes may take up less than 1% of activity injected at the tumour site and provide relatively few counts. Evaluation was performed of cast and foil collimators from a Siemens eCam and an Elscint SP6, respectively. A phantom represented the injection sites (1, 3 and 5 cm) and enabled imaging of point source ‘nodes’ at different distances and angles. At 1% of injected activity, the node was identified at ≥1.4 cm from the centre of a 1 cm lesion, 0.1% at ≥2.0 cm, using the poorest foil collimator. In patient studies, lymphatic tracts visualized immediately after subcutaneous injection showed activity of up to 2 kBq cm−1 and the poorest collimator gave similar count density in the star artifact at distances of up to 2 cm from the injection site. In conclusion, the star artifact from a poor collimator may be visually distracting but this is only significant for nodes accumulating less than 0.5% of injected activity. However, the star artifact may be of similar or greater magnitude than lymphatic tract activity within 2 cm of an injection site. Septal penetration may affect lymphatic tract visualization more than nodal identification.