Effect of Photopeak and Window Setting on Image Interpretation for Sentinel Lymph Node Imaging with a Medium Energy Collimator

  • Mr Dave Francia, Nuclear Medicine Department, Royal North Shore Hospital, Sydney, Australia, Australia
  • Ms Elizabeth Bailey, Nuclear Medicine Department, Royal North Shore Hospital, Sydney, Australia, Australia
  • A/Prof Dale Bailey, Nuclear Medicine Department, Royal North Shore Hospital, Sydney, Australia, Australia
  • A/Prof George Bautovich, Nuclear Medicine Department, Royal North Shore Hospital, Sydney, Australia, Australia
  • Dr Robert Cooper, Nuclear Medicine Department, Royal North Shore Hospital, Sydney, Australia, Australia

    • Background: High residual activity at injection site can produce star artifacts and a wide dynamic display range which can interfere with visualization of sentinel lymph nodes (SLNs) behind or close to injection site.

    Aim: To determine the ideal photopeak and window size to use for SLN imaging with a medium energy (ME) collimator to improve visualization of SLNs close to injection site.

    Methods: Standard protocol modified to include concurrent planar image acquisition using multiple energy peaks of 140keV ± 10% and 146keV ± 5% on Phillips SKYLight dual-head gamma camera. Images acquired using ME collimator, 256 x 256 matrix, 5 minutes per image in anterior and lateral projections. Images eviewed by 2 experienced nuclear medicine physicians randomized by acquisition photopeak. Observers assessed number SLNs identified, total number seen, whether injection site interfered with image interpretation and confidence score to assess image quality. ROI automatically generated using 10%, 30% and 50% threshold for both 140keV and 146keV images using Multimodality (HERMES Medical Systems) software to measure difference in size of injection site.

    Results: Total 17 patients studied with more SLNs identified using the offset peak in 29% of cases (5/17). Reports suggested that injection site interfered with image interpretation in 34% cases using conventional peak. Size of injection site was smaller in all studies using offset peak with 33% reduction in counts.

    Conclusion: Results indicate that, in this small number of cases, using an offset photopeak and narrow window aids image interpretation due to decreased injection site and more SLNs identified.