Limited-angle TOF-PET for intraoperative surgical applications: latest results

Abstract

Intraoperative Gamma probes and near infrared probes are the current gold standard modalities for sentinel lymph node identification and tumor removal in cancer patients. However, they do not meet with <5% false negative rates (FNR) requirement, a key metric suggested by American Society of Clinical Oncology. We are aiming to reduce FNR by using time of flight (TOF) PET detector technology in limited angle geometry by using only two detector heads in coincidence where one small-area detector is placed above the patient and the other with larger-area right under the patient bed. To explore the capabilities of this concept we used two Hamamatsu TOF PET detector modules (C13500-4075YC-12) featuring a 12×12 array of 4.14×4.14×20 mm3 LYSO:Ce crystal pixels with 4.2 mm pitch one-one coupled with MPPC pixels. Detector coincidence timing resolution (CTR) was 271 ps. We 3D printed a lesion phantom containing spheres with 2–10 mm in diameter representing lymph nodes and placed the phantom inside a 10-litre background water phantom. Experimental results show that with sub-minute data acquisition, 6 mm diameter spheres can be identified in the image when a lesion phantom with sphere to background activity ratio of 10:1 is used. With the encouraging results we further simulated detectors with different parameters to investigate the effect of detector depth of interaction, CTR, and pixel dimension on lesion detectability. The results show that 6 mm diameter with 60 seconds acquisition are resolved in 25 cm deep background water phantom with 10:1 activity ratio. As expected, the image quality improves as the CTR improves and with decreasing background water phantom depth. In 12 and 5 cm deep water phantom, 4 mm diameter spheres can be resolved both with 100 and 200 ps CTR. With the results presented here we conclude limited angle TOF PET is a major step forward for intraoperative applications in that improved lesion detectability is beyond what the conventional gamma- and NIR-based probes could achieve.