Hybrid Modality Fusion of Planar Scintigrams and CT Topograms to Localize Sentinel Lymph Nodes in Breast Lymphoscintigraphy: Technical Description and Phantom Studies

Lymphoscintigraphy is a nuclear medicine procedure that is used to detect sentinel lymph nodes (SLNs). This project sought to investigate fusion of planar scintigrams with CT topograms as a means of improving the anatomic reference for the SLN localization. Heretofore, the most common lymphoscintigraphy localization method has been backlighting with a 57Co sheet source. Currently, the most precise method of localization through hybrid SPECT/CT increases the patient absorbed dose by a factor of 34 to 585 (depending on the specific CT technique factors) over the conventional 57Co backlighting. The new approach described herein also uses a SPECT/CT scanner, which provides mechanically aligned planar scintigram and CT topogram data sets, but only increases the dose by a factor of two over that from 57Co backlighting. Planar nuclear medicine image fusion with CT topograms has been proven feasible and offers a clinically suitable compromise between improved anatomic details and minimally increased radiation dose. 1. Introduction An essential step in the procedure for sentinel lymph node (SLN) biopsy is to locate the first-echelon node of the drainage basin. Lymphoscintigraphy is a minimally invasive diagnostic tool for mapping the SLN. After injecting the patient with sulfur colloid radiolabeled with in locations proximal to a tumor site and waiting for lymphatic drainage to occur, the lymphatic drainage pathway from the primary injection site may be imaged with a gamma camera [1]. Interpretation of lymphoscintigrams is hindered by the absence of anatomical landmarks in the scintigraphy image. To partly cope with this limitation, it is customary to acquire additional transmission images via a backlighting 57Co sheet source in order to facilitate the anatomic localization of the nodes seen on the scintigraphy image. While the backlighting imaging technique creates an outline of the patient’s body, it depicts no internal landmarks. The introduction of hybrid single photon emission computed tomography with computed tomography (SPECT/CT) systems into clinical practice presents an opportunity to improve the spatial localization in lymphoscintigraphy [2]. Improvement in the localization technique could potentially help the surgeon prepare for sentinel node biopsy (SNB) operations, for example, when SLNs lie in the internal mammary chain where bony anatomy necessitates pinpointing the location of a node before incision. Single photon emission computed tomography and computed tomography image fusion (known as SPECT/CT) provides accurate three-dimensional (3D)