Multimodal sentinel lymph node mapping with single-photon emission computed tomography (SPECT)/computed tomography (CT) and photoacoustic tomography

Abstract

The identification of cancer cells in the lymph nodes surrounding a tumor is important in establishing a prognosis. Optical detection techniques such as fluorescence and photoacoustic tomography (PAT) have been reported in preclinical studies for noninvasive sentinel lymph node (SLN) mapping. A method for validation of these techniques is needed for clinical trials. We report the use of a multimodal optical-radionuclear contrast agent as a validation tool for PAT in a preclinical model. Methylene blue (MB) was radiolabeled with (125)I for multimodal SLN mapping and used in conjunction with MB to assess the feasibility of multimodal SLN mapping in a rat model by PAT and single-photon emission computed tomography (SPECT). MB provided sufficient contrast for identifying SLNs noninvasively with a PAT system adapted from a clinical ultrasound imaging system. The signal location was corroborated by SPECT using (125)I labeled MB. The translation of PAT into the clinic can be facilitated by a direct comparison with established imaging methods using a clinically relevant dual SPECT and photoacoustic imaging agent. The new high-resolution PAT is a promising technology for the sensitive and accurate SLN detection in cancer patients.

FIGURE 1

Structures of methylene blue and iodo-methylene blue (top). Reversed phase radio-chromatogram of ¹²⁵I-iodo-methylene blue (bottom).

FIGURE 2

PAT imaging of lymph nodes after injection of MB. A) Control PAT image obtained before MB injection. B) PAT image taken at 10 minutes post-injection. C) Overlaid PAT (pseudo color) and US (gray scale) image. PAT, photoacoustic tomography; US, ultrasound imaging; MB, methylene blue; and SLN, sentinel lymph node.

FIGURE 3

SPECT/CT projection images of rat acquired 1 h after subcutaneous injection of ¹²⁵I-MB in the left forepaw. * indicates location of injection and crosshairs indicate location of signal from axillary lymph node. The red arrow indicates the contralateral lymph node in the transverse view. The lymph node was detected about 3 mm below the surface of the skin, correlating well with the PAT finding.

FIGURE 4

Post-mortem optical and planar scintigraphy of rat 2 h after subcutaneous injection of ¹²⁵I-MB in the left forepaw. A) The MB can be clearly seen as blue coloring in the forepaw near the site of injection (*) and at the first lymph node after removal of overlying skin and hair. The lymph tracts leading to the lymph node can be seen as well (arrows). B) Planar scintigraphy corresponds with blue color with high intensity from the left paw and lymph node. C) After dissection, the first lymph node is distinctly blue from dye accumulation while the following nodes are free of the dye. D) Ex vivo scintigraphy confirmed that the first lymph node was the source of the signal.