Positron lymphography: multimodal, high-resolution, dynamic mapping and resection of lymph nodes after intradermal injection of 18F-FDG

Abstract

The lymphatic system plays a critical role in the maintenance of healthy tissues. Its function is an important indicator of the presence and extent of disease. In oncology, metastatic spread to local lymph nodes (LNs) is a strong predictor of poor outcome. Clinical methods for the visualization of LNs involve regional injection and tracking of (99m)Tc-sulfur colloid ((99m)Tc-SC) along with absorbent dyes. Intraoperatively, these techniques suffer from the requirement of administration of multiple contrast media ((99m)Tc-SC and isosulfan blue), unwieldy γ-probes, and a short effective surgical window for dyes. Preclinically, imaging of transport through the lymphatics is further hindered by the resolution of lymphoscintigraphy and SPECT. We investigated multimodal imaging in animal models using intradermal administration of (18)F-FDG for combined diagnostic and intraoperative use. PET visualizes LNs with high sensitivity and resolution and low background. Cerenkov radiation (CR) from (18)F-FDG was evaluated to optically guide surgical resection of LNs.

Methods: Imaging of (18)F-FDG uptake used PET and sensitive luminescent imaging equipment (for CR). Dynamic PET was performed in both sexes and multiple strains (NCr Nude, C57BL/6, and Nu/Nu) of mice. Biodistribution confirmed the uptake of (18)F-FDG and was compared with that of (99m)Tc-SC. Verification of uptake and the ability to use (18)F-FDG CR to guide nodal removal were confirmed histologically.

Results: Intradermal injection of (18)F-FDG clearly revealed lymphatic vessels and LNs by PET. Dynamic imaging revealed rapid and sustained labeling of these structures. Biodistribution of the radiotracer confirmed the active transport of radioglucose in the lymphatics to the local LNs and over time into the general circulation. (18)F-FDG also enabled visualization of LNs through CR, even before surgically revealing the site, and guided LN resection.

Conclusion: Intradermal (18)F-FDG can enhance the preclinical investigation of the lymphatics through dynamic, high-resolution, and quantitative tomographic imaging. Clinically, combined PET/Cerenkov imaging has significant potential as a single-dose, dual-modality tracer for diagnostics (PET/CT) and guided resection of LNs (Cerenkov optical).

Figure 1

Three-dimensionally rendered positron lymphography images demonstrate high-resolution mapping using intradermal administration of ¹⁸F-FDG to a male nu/nu mouse at multiple angles (A–C). Radiotracer is transported from site of injection through lymphatic system, enabling visualization of lymph flow. Delineation of LN draining injection site is clearly distinguished (B; insert is without fused CT). At 10 min after injection, we can identify lymphatic vessels in tail leading first to sacral nodes (1). Tracer then moves proximally to caudal (2) and then mesenteric nodes (3). Gradient arrow indicates direction of flow. Three-dimensionally rendered PET data are generated from weighted average of intensities from all 2-dimensional slices. As such, it is strictly semiquantitative; color bar indicates range of intensities. Max = maximum; Min = minimum.

Figure 2

Dynamic imaging of progress of lymphatic transport of small-molecule radiotracer (5-min intervals). Within minutes, ¹⁸F-FDG has begun to be cleared from injection site through lymphatic system. Transport through and uptake at LN makes these features obviously apparent. Background tissue uptake of tracer builds slowly over approximately 30–35 min, via lymphatic clearance and diffusion of the small molecule. However, clearly identifiable presence of nodes even at late time points is significant, enabling ready identification of these nodes at 1 h after injection. Detailed imaging of transport immediately after injection is shown in Figure 3. MIP = maximum-intensity projection.

Figure 3

Quantitative and dynamic imaging of lymphatic ¹⁸F-FDG transport. (A) Axial, coronal, and sagittal views of dynamic imaging (30-s frames starting at time of injection) of ¹⁸F-FDG transport in lymphatics. Acquisition of rapidity of flow through vessels and uptake into nodes, with both high spatial and longitudinal resolution, is an advantage of this technique. (B) PET information is inherently quantitative, providing activity concentration information. Here, regions of interest were drawn around LN to assess lymphatic function by computing uptake over time (over linear regions of curve). For right sacral, left sacral, caudal, and mesenteric nodes, this yields 6.47, 6.57, 5.67, and 1.13 Bq/s, respectively.

Figure 4

Biodistribution of intradermally injected small-molecule tracer. (A) Bar chart showing %ID distribution of locally administered ¹⁸F-FDG in major organs and LNs at 10 and 60 min after injection. Radiotracer slowly leaves dermal site of tail to circulation, renal excretion, and accumulation in bladder. (B) Normalizing dose distribution for mass of tissue (%ID/g) demonstrates draining-lymph–specific transit of tracer. Retained radiotracer at injection site continues to be carried through lymphatics from injection site to draining nodes, enabling continued visualization. Left and right plots have different scales to show organ and nodal %ID/g, respectively.

Figure 5

Contrast ratio of ¹⁸F-FDG to blood and muscle. (A) Ratios of %ID/g from draining LN and blood are high, affording clear distinction between lymphatics and surrounding tissues. (B) Contrast derived from %ID/g between LN and muscle enables clear delineation of nodes. LNs not in the draining path from the injection site (inguinal and axillary) do not produce significant ratios with respect to background. From 10 to 60 min, ratio of LN to muscle drops as conscious and ambulatory mice increase muscle uptake. Inguinal (and axillary) uptake is low because these nodes are not within drainage of injection site, which drains to deep pelvic nodes

Figure 6

Cerenkov-guided surgical resection of ¹⁸F-FDG–bearing LN, 10 min after injection, with surgical validation. (A) Lateral-tail intradermal injection yields greater uptake in 1 sacral node, seen with just skin removed. PET image is included as Supplemental Figure 2. (B) CR guides resection of node, magnified in inset. (C) Systemic administration of ¹⁸F-FDG (retroorbital injection) does not enable identification of nodes, even after surgical exposure. Signal from renal clearance can be seen. (D) Immunohistologic verification of Cerenkov-guided excised tissues. Resected node as indicated by CR, along with surrounding connective tissue, was stained with hematoxylin and eosin, control hamster IgG, and podoplanin. The stromal region of node is indicated by dense hematoxylin and eosin and positive podoplanin staining. Dotted line indicates region of magnification (included as Supplemental Fig. 4). Scale bar, 1 mm. H&E = hematoxylin and eosin.