Histology-validated electromagnetic characterization of ex-vivo ovine lung tissue for microwave-based medical applications

Abstract

Microwave thermal ablation is an established therapeutic technique for treating malignant tissue in various organs. Its success greatly depends on the knowledge of dielectric properties of the targeted tissue and on how they change during the treatment. Innovation in lung navigation has recently increased the clinical interest in the transbronchial microwave ablation treatment of lung cancer. However, lung tissue is not largely characterized, thus its dielectric properties investigation prior and post ablation is key. In this work, dielectric properties of ex-vivo ovine lung parenchyma untreated and ablated at 2.45 GHz were recorded in the 0.5-8 GHz frequency range. The measured dielectric properties were fitted to 2-pole Cole-Cole relaxation model and the obtained model parameters were compared. Based on observed changes in the model parameters, the physical changes of the tissue post-ablation were discussed and validated through histology analysis. Additionally, to investigate the link of achieved results with the rate of heating, another two sets of samples, originating from both ovine and porcine tissues, were heated with a microwave oven for different times and at different powers. Dielectric properties were measured in the same frequency range. It was found that lung tissue experiences a different behavior according to heating rates: its dielectric properties increase post-ablation while a decrease is found for low rates of heating. It is hypothesized, and validated by histology, that during ablation, although the tissue is losing water, the air cavities deform, lowering air content and increasing the resulting tissue properties.

Figure 1

Sketch of the ablation setup.

Figure 2

Comparison between the portion of the tissue used for dielectric measurements (left) and for TTC staining (right).

Figure 3

Relative change of dielectric properties of points in the ablated area: (a) Group 1, (b) Group 2 and (c) Group 3.

Figure 4

Comparison of dielectric properties of non-ablated (“out”) and ablated (“abl”) tissue, averaged across: (a) all samples regardless of the inflation state, (b) all deflated samples and (c) all inflated samples.

Figure 5

Comparison of gross and histology appearance of ablated and not ablated ovine lung tissue (the histological samples were obtained from the same ablated sample). An inflated sample (L6L2) is reported in the left column and a deflated sample (L4L2) in the right column. The top panels (A,B) show the gross image of the ablated sample. In these pictures, the areas sampled for histology, containing the measurement points, are marked and color coded. The histology appearance of those sampled areas are reported in (C–G) with ×0.7  magnification H&E: (C) contains the measurement point 1, (E) contains point 2 and 3, (G) point 4 of L6L2; (B) contains the measurement point 1 and 2, and paned (D) point 3 of L4L2. For the samples in contact with the antenna the position of the microwave (MW) applicator is shown. Then, in (C–G), symbols are used to mark specific areas reported magnified ×10  in Fig. 6.

Figure 6

×10  magnification H&E images of the three areas identified by the histology analysis are reported: zone i with dense collapsed tissue (A,B), zone ii with less dense altered tissue with increased inflammatory cellularity and oedema (C,D) and zone iii of non-ablated tissue (E,F). (A,C,E) Obtained by an inflated sample (‘L6L2’), while (B,D,F) from a deflated sample (‘L4L2’). The symbols marks in the panels allow to locate the magnified areas in Fig. 5.

Figure 7

Averaged dielectric properties of samples measured in laboratories in Italy and Ireland: (a) without uncertainity; (b) with uncertainity; (c) legend.

Figure 8

Measured dielectric properties of the porcine sample MWO heated several times for 45–90 s: (a) without uncertainty; (b) with uncertainty; (c) legend.; curves marked as ‘def’ and ‘inf’ represent properties of deflated and inflated lung measured at body temperature^,.