Abstract
This study aimed to evaluate the usefulness of the subtraction method for improving sentinel lymph node (SLN) visibility by reducing scattering near the injection site. Images of two phantoms for the injection site and SLNs built using an original design were simultaneously acquired using a dual-head camera equipped with a low-energy high-resolution collimator on the lower detector (posterior view) and a low-energy general-purpose collimator on the upper detector (anterior view). Subtraction method images were created by subtracting the posterior view from the anterior view, the latter of which was designated as the conventional method. Image contrast was calculated from the counts of regions of interest placed on the two phantoms of the injection site and SLNs. SLNs visibility to a distance from the injection site and a radioactivity ratio based on the injection site (15MBq) was evaluated by image contrast and visual interpretation. The best improvement in contrast occurred at a distance of 20 pixels (1.08mm/pixel) from the injection site, and improved further as the lymph node radioactivity was smaller. The SLN's visibility corresponding to a distance of 20 pixels improved significantly (p<0.001), from 1/2560 of radioactivity at the injection site (approximately 6kBq) to 1/640 (approximately 23kBq), and the SLN was only detectable using the subtraction method. The SLN (1/5120, approximately 3kBq) was difficult to detect even with the subtraction method, whereas the SLN with a ratio ≥1/320 (approximately 46kBq) was easily detected even with the conventional method. These visibilities did not differ significantly between the two methods (p=0.16 and >0.32, respectively). The subtraction method could detect SLNs near the tumor on clinical images. The subtraction method improved SLN visibility near the injection site by reducing scattering from the injection site. Furthermore, an advantage of the subtraction method is that it does not require additional imaging, because the posterior view is obtained simultaneously and utilized.
Highlights
The presence of cancer cells in the sentinel lymph node (SLN) is an important factor used in cancer staging [1]
With respect to the deterioration of visibility caused by subtraction processing, we evaluated whether the subtraction method would deteriorate MT phantom lymph nodes (MTLNs) visibility relative to the conventional method with regard to the count ratio
In terms of visual evaluation, we evaluated the detectable MTLNs using multi-lymph node (MT) phantom images created by adding the counts and noise from BG regions of interest (ROI) at a distance of 20 pixels from combination phantom injection site (CBIS) using ImageJ software (US National Institutes of Health, Bethesda, MD, USA) [19]
Summary
The presence of cancer cells in the sentinel lymph node (SLN) is an important factor used in cancer staging [1]. Most facilities use preoperative lymphoscintigraphy to localize SLNs in patients with breast cancer. Lymphoscintigraphy reveals drainage from the tumor to axillary lymph nodes and accurately localizes SLNs within the body, providing surgical assistance [2,3,4,5,6]. Lymphoscintigraphy is the standard means of evaluating SLNs from tumors. A large amount of radioactivity is retained at the injection site; the target SLN is small in size and retains only a small amount of radioactivity [4]. Radioactive scattering may cover SLNs near the injection site on images
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