Interstitial magnetic thermotherapy dosimetry based on shear wave magnetomotive optical coherence elastography

Abstract

While magnetic thermoseeds are often utilized in interstitial magnetic thermotherapy (iMT) to enable localized tumor ablation, we propose to extend their use as the perturbative source in magnetomotive optical coherence elastography (MM-OCE) so that the heat-induced elasticity alterations can be 'theranostically' probed. MM-OCE measurements were found to agree with indentation results. Tissue stiffening was visualized on iMT-treated porcine liver and canine soft tissue sarcoma specimens, where histology confirmed thermal damages. Additionally, the elasticity was found to increase exponentially and linearly with the conventional thermal dosage metrics and the deposited thermal energy, respectively. Collectively, a physiologically-meaningful, MM-OCE-based iMT dosimetry is feasible.

Fig. 1

Schematics and experimental setup of MM-OCE and interstitial magnetic thermotherapy (iMT). (a) Schematics of shear-wave MM-OCE. (b) Illustration of the iMT coil and (c) photograph of the iMT system. (d) (Left) illustration of the placement of the magnetic thermoseed in tissues and (right) photograph of the magnetic thermoseed.

Fig. 2

Representative PDMS phantom results. (a) Space-time plots, (b) reconstructed Young’s modulus ($E$) maps, (c) structural OCT images, and (d) shear wave propagation (full video shown in Visualization 1 at 120 fps) of the (I) uniformly stiff, (II) uniformly soft, and (III) heterogeneous soft-stiff (left-right) samples.

Fig. 3

Comparison between MM-OCE and indentation measurements performed on homogeneous PDMS phantoms. (a) Young’s modulus values obtained by both methods showed a good linear correlation (Pearson’s r = 0.965) and agreement (slope = 1.047, with a negligible offset = −1.325 kPa) with each other. (b) Bland-Altman plot also showed a good agreement between the two measurements (mean difference = 0.558 kPa). Sample size N = 16.

Fig. 4

Representative results of (a) iMT-treated and (b) non-treated porcine liver specimens. (I) Photographs obtained (left) before and (right) after treatment; thermal images acquired at the (left) 0th and (right) 4th min of the treatment. (II) Structural OCT images and (III) reconstructed Young’s modulus ($E$) maps obtained (left) before and (right) after iMT treatment. White arrows in (I) and (II) indicate the locations of the magnetic thermoseed. (IV) (Left) Post-treatment Masson Trichrome-stained histology and (right) a zoomed-in area. Collagen is stained blue. The ablation zone was delineated with the dashed line. (V) Shear wave propagation captured at different temporal instants were also visualized (full video shown in Visualization 2 at 120 fps).

Fig. 5

Correlation between (a) Young’s modulus ratio ($E$ratio) and CEM43 (Pearson’s r = 0.799), and (b) $E$ ratio and squared magnetic field strength (${\left|H_0\right|}^2$) (Pearson’s r = 0.938) in porcine liver specimens. Note that outliers (indicated in gray) were excluded from the fitting and ‘ + ’ denotes the non-treated tissues. The outliers were defined as the datapoints which deviated from the baseline model by 1.5 standard deviations. Sample size of the non-treated and treated samples were 4 and 23, respectively.

Fig. 6

Representative results of a canine soft tissue sarcoma (STS) specimen (a) before treatment, (b) after 1st iMT, and (c) after 2nd iMT. (I) Structural OCT images and (II) reconstructed Young’s modulus ($E$) maps. (III) Photographs obtained (left) before and (right) after each treatment; thermal images acquired at the (left) 0th and (right) 4th min of each treatment. White arrows in (I) and (III) indicate the locations of the magnetic thermoseed. (IV) (Bottom) Post-treatment Masson Trichrome-stained histology and (top) a zoomed-in area. Collagen is stained blue. The ablation zone was delineated with the dashed line. (V) Shear wave propagation captured at three temporal instants were also visualized (full video shown in Visualization 3 at 120 fps).