Augmented reality for sentinel lymph node biopsy

Abstract

Introduction: Sentinel lymph node biopsy for oral and oropharyngeal squamous cell carcinoma is a well-established staging method. One variation is to inject a radioactive tracer near the primary tumor of the patient. After a few minutes, audio feedback from an external hand-held [Formula: see text]-detection probe can monitor the uptake into the lymphatic system. Such probes place a high cognitive load on the surgeon during the biopsy, as they require the simultaneous use of both hands and the skills necessary to correlate the audio signal with the location of tracer accumulation in the lymph nodes. Therefore, an augmented reality (AR) approach to directly visualize and thus discriminate nearby lymph nodes would greatly reduce the surgeons' cognitive load.

Materials and methods: We present a proof of concept of an AR approach for sentinel lymph node biopsy by ex vivo experiments. The 3D position of the radioactive [Formula: see text]-sources is reconstructed from a single [Formula: see text]-image, acquired by a stationary table-attached multi-pinhole [Formula: see text]-detector. The position of the sources is then visualized using Microsoft's HoloLens. We further investigate the performance of our SLNF algorithm for a single source, two sources, and two sources with a hot background.

Results: In our ex vivo experiments, a single [Formula: see text]-source and its AR representation show good correlation with known locations, with a maximum error of 4.47 mm. The SLNF algorithm performs well when only one source is reconstructed, with a maximum error of 7.77 mm. For the more challenging case to reconstruct two sources, the errors vary between 2.23 mm and 75.92 mm.

Conclusion: This proof of concept shows promising results in reconstructing and displaying one [Formula: see text]-source. Two simultaneously recorded sources are more challenging and require further algorithmic optimization.

Keywords: AR; Augmented reality; Gamma camera; Gamma detector; Inverse problem; Nuclear medicine; SNB; Sentinel lymph node biopsy.

Fig. 1

Pipeline: overview of our method. From the $\gamma$-source to the AR headset

Fig. 2

$\gamma$-Image of a single $\gamma$-source. Circular $\gamma$-photon accumulations are clearly discernible. These are produced by the multi-pinhole collimator pattern. The vertical line is a sensor artifact

Algorithm 1

SLNF: finding the position of the radioactive ^99mTc source.

Fig. 3

Calibration: a Detecting the QR marker $Q{R}_1$ to determine the liminal space $T_1$ (scene anchoring), followed by detecting $Q{R}_2$ to yield $T_2$. b $T_3^{\prime }$ is the mapping from $T_1$ to $T_2$. c Adapting $T_3^{\prime }$ to yield $T_3$ (dotted arrows). d Augmentation (boxed): applying $T_3$ to display the augmentation in the correct location (i.e., source (x, y, z)). $Q{R}_2$ is only needed during calibration

Fig. 4

Setup of measurement space (Experiments 1, 2, and 3), view from above. Wooden block for the vials, their source positions (tuples (x, y, z), [ mm]) with the origin in the middle of the collimator plate (coordinate system with axes x, y, z drawn), and their associated index $\overline{)1}$ to $\overline{)9}$; the $\gamma$-detector is to the right (schematics)

Fig. 5

Assessment of the x-coordinate. a The center of the ground truth cluster is anchored on the red line with its origin in the top left of the wooden block (white arrow). b The red cluster is compared against the same reference line. The two clusters have a projected distance of approximately 61 mm along the x-axis from the reference point in the top left

Fig. 6

Assessment of the y-coordinate. a The center of the ground truth cluster is anchored on the black reference line of the wooden block top edge (white arrow). b The red cluster is compared against the same reference line. The two clusters have a projected distance of approximately 58 mm along the y-axis to the reference line

Fig. 7

Assessment of the z-coordinate. a The center of the ground truth cluster is anchored on the red reference line with its origin at the collimator site (white arrow). b The red cluster is compared against the same reference line. The two clusters have a projected distance from the collimator of approximately 138 mm along the z-axis