Quantitative investigation of a novel small field of view hybrid gamma camera (HGC) capability for sentinel lymph node detection

Abstract

Objective: The hybrid gamma camera (HGC) has been developed to enhance the localization of radiopharmaceutical uptake in targeted tissues during surgical procedures such as sentinel lymph node (SLN) biopsy. To assess the capability of the HGC, a lymph node contrast (LNC) phantom was constructed to simulate medical scenarios of varying radioactivity concentrations and SLN size.

Methods: The phantom was constructed using two clear acrylic glass plates. The SLNs were simulated by circular wells of diameters ranging from 10 to 2.5 mm (16 wells in total) in 1 plate. The second plate contains four larger rectangular wells to simulate tissue background activity surrounding the SLNs. The activity used to simulate each SLN ranged between 4 and 0.025 MBq. The activity concentration ratio between the background and the activity injected in the SLNs was 1 : 10. The LNC phantom was placed at different depths of scattering material ranging between 5 and 40 mm. The collimator-to-source distance was 120 mm. Image acquisition times ranged from 60 to 240 s.

Results: Contrast-to-noise ratio analysis and full-width-at-half-maximum (FWHM) measurements of the simulated SLNs were carried out for the images obtained. Over the range of activities used, the HGC detected between 87.5 and 100% of the SLNs through 20 mm of scattering material and 75-93.75% of the SLNs through 40 mm of scattering material. The FWHM of the detected SLNs ranged between 11.93 and 14.70 mm.

Conclusion: The HGC is capable of detecting low accumulation of activity in small SLNs, indicating its usefulness as an intraoperative imaging system during surgical SLN procedures.

Advances in knowledge: This study investigates the capability of a novel small-field-of-view (SFOV) HGC to detect low activity uptake in small SLNs. The phantom and procedure described are inexpensive and could be easily replicated and applied to any SFOV camera, to provide a comparison between systems with clinically relevant results.

Figure 1.

Photograph of the hybrid gamma camera.

Figure 2.

Schematic and photographs of the lymph node contrast (LNC) phantom: (a) a diagram of the LNC phantom dimensions and (b) a representation of the two Perspex plates used to construct the phantom.

Figure 3.

Illustration of node image size analysis. FWHM, full width at half maximum.

Figure 4.

Gamma image for the lymph node contrast phantom while the simulated sentinel lymph nodes were located beneath 25 mm of scattering material; the hybrid gamma camera, while acquiring this image, was fitted with the 1500-μm-thick CsI(Tl) scintillator and the 1.0-mm pinhole collimator, and the acquisition time was 240 s. Circle (A): a node containing 0.05 MBq; circle (B): a node containing 0.025 MBq. The contrast-to-noise ratio values for node (A) and node (B) are 6.02 and 3.13, respectively.

Figure 5.

Full-width-at-half-maximum (FWHM) recorded values for the largest and smallest simulated sentinel lymph nodes (SLNs) (i.e. 2.5 and 10 mm in diameter); the simulated SLNs were imaged using the hybrid gamma camera fitted with 600-μm-thick and 1500-μm-thick CsI(Tl) scintillators and 0.5-mm-diameter and 1.0-mm-diameter pinhole collimators. The total imaging distance was 120 mm and the acquisition time was 240 s.

Figure 6.

Graphs showing the relationship between the simulated sentinel lymph node (SLN) depths and the visibility (i.e. detection rate): these graphs represent the ability of the hybrid gamma camera fitted with the 600-μm-thick CsI(Tl) scintillator and two different pinhole collimators (i.e. 0.5-mm and 1.0-mm diameter) to detect the simulated SLNs at different threshold values (3 and 5).

Figure 7.

Graphs showing the relationship between the simulated sentinel lymph node (SLN) depths and the visibility (i.e. detection rate): these graphs represent the ability of the hybrid gamma camera fitted with the 1500-μm-thick CsI(Tl) scintillator and two different pinhole collimators (i.e. 0.5-mm and 1.0-mm diameter) to detect the simulated SLNs at different threshold values (3 and 5).